1 Structure and Function Neuroanatomy and Research Methods
Electrical Storm Sam had been feeling a bit odd all day; he thought perhaps he was coming down with a bug. But when he collapsed unconscious to the floor of the lunchroom at work and began twitching and jerking, it was clear that he had a much bigger problem than the flu. Sam was having a seizure, a type of uncontrollable convulsion that he'd never had before. By the time Sam arrived at the hospital, the seizure had stopped, and although he was confused and slow to respond to commands, he didn't seem to be in distress. But when Sam smiled at Dr. Cheng, the attending neurologist, and offered to shake her hand, Dr. Cheng ordered immediate brain scans: Sam could offer only half a smile, because only the left side of his face was working, and he was unable to grip Dr. Cheng's hand at all.
How can an understanding of the pathways between brain and body provide clues about Sam's problem? We now know quite a bit about the neural organization of basic functions, but the control of complex cognition remains a tantalizing mystery. However, the advent of sophisticated brain-imaging technology has invigorated the search for answers to fundamental questions about brain organization: Does each brain region control a specific behavior, or is the pattern of connections within the brain more important? Do some regions of the brain act as general purpose information processors? Is everybody's brain organized in the same way?
Almost everything about us--our thoughts, feelings, and behavior, however serious or silly--is the product of a knobbly three-pound organ that, despite its unremarkable appearance, is the most complicated object in the known universe. In this chapter we'll have a look at the structure of the brain, first at the cellular level and then zooming out to survey the brain's larger-scale anatomical landscape. We'll also take a brief look at the elements of research design that all neuroscientists must consider when designing their studies, and some of the amazing technologies that allow researchers to probe the mysteries of the cells and structures that make up the mind's machine.
1.1 The Nervous System Is Made of Specialized Cells
The first part of the chapter is concerned with the cells of the nervous system. By the end of the section, you should be able to: 1.1.1 Name and describe the general functions of the four main parts of a neuron. 1.1.2 Classify neurons according to both structure and function. 1.1.3 Outline the key components of a synapse and the major steps in neurotransmission. 1.1.4 Describe the four principal types of glial cells and their basic functions.
neuron Also called nerve cell. The basic unit of the nervous system, each composed of receptive extensions called dendrites, an integrating cell body, a conducting axon, and a transmitting axon terminal. glial cells Also called glia. Nonneuronal brain cells that provide structural, nutritional, and other types of support to the brain. synapse The cellular location at which information is transmitted from a neuron to another cell.
All of your organs and muscles are in communication with the nervous system, which, like all other living tissue, is made up of highly specialized cells. The most important of these are the neurons (or nerve cells), arranged into the circuits that underlie all forms of behavior, from simple reflexes to complex cognition. Each neuron receives inputs from many other cells, integrates those inputs, and then distributes the processed information to other neurons. Your brain contains 80-90 billion of these tiny cellular computers (Herculano-Houzel, 2012), working together to process vast amounts of information with apparent ease. An even larger number of glial cells (sometimes called just glia) are found in the human brain, mostly providing a variety of support functions but also participating in information processing. Because neurons are larger and produce readily measured electrical signals, we know much more about them than about glial cells. An important early controversy in neuroscience concerned the functional independence of individual neurons: Was each neuron a discrete component? Or were the cells of the nervous system fused together into larger functional units, like continuous circuits? Through painstaking study of the fine details of individual neurons, the celebrated Spanish anatomist Santiago Ramón y Cajal (1852-1934) was able to show that although neurons come very close together, they are not quite continuous with one another. Ramón y Cajal and his contemporaries established what came to be known as the neuron doctrine: (1) neurons and other cells of the brain are structurally, metabolically, and functionally independent, and (2) information is transmitted from neuron to neuron across tiny gaps, later named synapses. It's impossible to measure exactly how many synapses there are in the brain, but scientists think there may be as many as 1015 (a quadrillion) synapses. That's a number too huge for most of us to comprehend: if you gathered a quadrillion grains of sand, each a millimeter in diameter, they would fill a cube that is longer on each side than an American football field--well over a million cubic yards, or 750,000 cubic meters (in more familiar units that's about 260 million gallons of sand, or 750 million liters)! These vast networks of connections are responsible for all of our achievements.
Nineteenth-Century Drawings of Neurons Santiago Ramón y Cajal created detailed drawings of the many types of neurons found in the brain (labeled here by Cajal with lowercase letters). Based on his studies of neurons, Cajal and his collaborators proposed that neurons are discrete cells that communicate via tiny contacts, which were later named synapses.
The neuron has four principal divisions Although neurons come in hundreds of different shapes and sizes, they all share certain features. Like other cells of the body, a neuron contains genes encoded in DNA inside a cell nucleus, as well as a wide assortment of organelles performing basic functions like producing energy (the mitochondria) to power cellular operations, or translating genetic instructions (the ribosomes) into the specialized proteins essential for the structure and functioning of the neuron (consult the Appendix if you need a refresher on cell biology). But neurons also share a set of unique, highly
Input zone, where neurons collect and process information, either from the environment or from other cells Integration zone, where the decision to produce a neural signal is made
Information from other neurons is passed to the dendrites and cell body via synapses. Some neurons receive only a few synaptic inputs; other receive thousands.
Conduction zone, where information can be electrically transmitted over great distances
Each axon terminal synapses onto another cell in order to transmit information.
Output zone, where the neuron transfers information to other cells
FIGURE 1.1 The Major Parts of the Neuron
specialized components that allow them to collect input signals from multiple sources, process and combine this information, and distribute the results of this processing to other cells. These information-processing features, illustrated in FIGURE 1.1, can be viewed as belonging to four functional zones: 1. Input zone At cellular extensions called dendrites (from the Greek dendron, "tree"), neurons receive information via synapses from other neurons. Some Watsnoen/uBrorenesdlhovaeve dendrites that are elaborately branched, providing room for many The Mind's Machine Founsdyatnioanps soef Bs.raDineannddrBiteehasvmiora4ye be covered in dendritic spines, small projections from the surface of the dendrite that add additional space for synapses. MM4e_01.01 08/14/20 2. Integration zone In addition to receiving additional synaptic inputs, the neuron's cell body (or soma, plural somata) integrates (combines) the information that has been received to determine whether or not to send a signal of its own. 3. Conduction zone A single extension, the axon (or nerve fiber), carries the neuron's own electrical signals away from the cell body. Toward its end, the axon may split into multiple branches called axon collaterals. 4. Output zone Specialized swellings at the ends of the axon, called axon terminals (or synaptic boutons), transmit the neuron's signals across synapses to other cells.
input zone The part of a neuron that receives information from other neurons or from specialized sensory structures. dendrite An extension of the cell body that receives information from other neurons. integration zone The part of a neuron that initiates neural electrical activity. cell body Also called soma. The region of a neuron that is defined by the presence of the cell nucleus. conduction zone The part of a neuron--typically the axon--over which the action potential is actively propagated. axon Also called nerve fiber. A single extension from the nerve cell that carries action potentials from the cell body toward the axon terminals. axon collateral A branch of an axon. output zone The part of a neuron at which the cell sends information to another cell. axon terminal Also called synaptic bouton. The end of an axon or axon collateral, which forms a synapse onto a neuron or other target cell and thus serves as the output zone.
Life size: The adult human brain, on average, is 15 cm from front to back.
Life size: The cortex of the human brain is about 3 mm thick.
Life size: The tiny black dot in the center of this circle is how the largest nerve cell bodies appear to the naked eye when stained.
Magni cation ×100 (102): Large nerve cell bodies are about 100 µm (0.1 mm) in diameter.
Magni cation ×1000 (103): Large axons and dendrites are about 10 µm (0.01 mm) in diameter.
Parts of neurons Magni cation ×104: A synaptic ending is about 1 µm in diameter.
Magni cation ×105: The synaptic cleft between neurons is about 20 nm across.
Magni cation ×106: A neuronal membrane is about 5 nm thick.
Magni cation ×107: The diameter of an ion channel is about 0.5 nm.
FIGURE 1.2 Sizes of Some Neural Structures and the Units of Measure and Magnification Used in Studying Them
Watson/Breedlove The Mind's Machine Foundations of Brain and Behavior 4e MM4e_01.02 07/07/20
FIGURE 1.3 Neurons Are Classified into Three Principal Types
Integration zone, where the decision to produce a neural signal is made Conduction zone, where information can be transmitted over great distances
View Activity 1.1: Major Components and Classifications of Neurons
Output zone, where the neuron transfers information to other cells
The tremendous diversity of sizes, and shapes, and functions of neurons reflects their differing processing functions. For example, motor neurons (also called motoneurons) are large with long axons reaching out to synapse on muscles, causing muscular contractions. As their name implies, sensory neurons are specialized to gather sensory information, and they take many different shapes depending on whether they detect light or sound or touch and so on. Most of the neurons in the brain are interneurons, which analyze information gathered from one set of neurons and communicate with others. The axons of interneurons may measure only a few micrometers (µm; a micrometer is a millionth of a meter), while motor neurons and sensory neurons Watson/Breedlove TmheayMhinadv'seMaaxcohninsea meter or more in length, conveying information to and from the Fmouonsdtatdioinsstaonf BtrpaianratnsdoBfethhaveiobro4de y. In general, larger neurons tend to have more-complex inputs and outputs, cover greater distances, and/or convey information more rapidly MM4e_01.03 09/02/20 than smaller neurons. The relative sizes of neural structures that we will be discussing throughout the book are illustrated in FIGURE 1.2. In addition to size, neuroscientists classify neurons into three general categories of shape, each specialized for a particular kind of information processing (FIGURE 1.3): 1. Multipolar neurons have many dendrites and a single axon. They are the most common type of neuron. 2. Bipolar neurons have a single dendrite at one end of the cell and a single axon at the other end. Bipolar neurons are especially common in sensory systems, such as vision.
motor neuron Also called motoneuron. A neuron that transmits neural messages to muscles (or glands). sensory neuron A nerve cell that is directly affected by changes in the environment, such as light, odor, or touch. interneuron A nerve cell that is neither a sensory neuron nor a motor neuron. Interneurons receive input from and send output to other neurons. multipolar neuron A nerve cell that has many dendrites and a single axon. bipolar neuron A nerve cell that has a single dendrite at one end and a single axon at the other end.
unipolar neuron Also called monopolar neuron. A nerve cell with a single branch that leaves the cell body and then extends in two directions; one end is the input zone, and the other end is the output zone. presynaptic Referring to the "transmitting" side of a synapse. postsynaptic Referring to the region of a synapse that receives and responds to neurotransmitter. presynaptic membrane The specialized membrane on the axon terminal of a nerve cell that transmits information by releasing neurotransmitter. synaptic cleft The space between the presynaptic and postsynaptic neurons at a synapse. postsynaptic membrane The specialized membrane on the surface of a neuron that receives information by responding to neurotransmitter from a presynaptic neuron. synaptic vesicle A small, spherical structure that contains molecules of neurotransmitter. neurotransmitter Also called synaptic transmitter, chemical transmitter, or simply transmitter. The chemical released from the presynaptic axon terminal that serves as the basis of communication between neurons. neurotransmitter receptor Also called simply receptor. A specialized protein that selectively senses and reacts to molecules of a corresponding neurotransmitter or hormone. neuroplasticity Also called neural plasticity. The ability of the nervous system to change in response to experience or the environment. axon hillock The cone-shaped area on the cell body from which the axon originates. innervate To provide neural input to.
3. Unipolar neurons (also called monopolar neurons) have a single extension (or process), usually thought of as an axon, that branches in two directions after leaving the cell body. One end is the input zone with branches like dendrites; the other, the output zone with terminals. Unipolar neurons transmit touch information from the body into the spinal cord. In all three types of neurons, the dendrites comprise the input zone. In multipolar and bipolar neurons, the cell body also receives synaptic inputs, so it is also part of the input zone. We'll return to a discussion of some of the techniques used to visualize neurons later in the chapter. Information is transmitted through synapses A neuron's dendrites reflect the complexity of the inputs that are received. Some simple neurons have just a couple of short dendritic branches, while others have huge and complex dendritic trees (or arbors) receiving many thousands of synaptic contacts from other neurons. At each synapse, information is transmitted from an axon terminal of a presynaptic neuron to the receptive surface of a postsynaptic neuron (FIGURE 1.4A). A synapse can be divided into three principal components (FIGURE 1.4B): 1. The specialized presynaptic membrane of the axon terminal of the presynaptic (i.e., transmitting) neuron 2. The synaptic cleft, a gap of about 20-40 nanometers (nm; billionths of a meter) that separates the presynaptic and postsynaptic neurons 3. The specialized postsynaptic membrane on the dendrite or cell body of the postsynaptic (i.e., receiving) neuron Presynaptic axon terminals contain many tiny hollow spheres called synaptic vesicles. Each synaptic vesicle contains molecules of neurotransmitter, the special chemical with which a presynaptic neuron communicates with postsynaptic cells. This communication starts when, in response to electrical activity in the axon, synaptic vesicles fuse to the presynaptic membrane and then rupture, releasing their payload of neurotransmitter molecules into the synaptic cleft (see Figure 1.4B). After crossing the cleft, the released neurotransmitter molecules interact with matching neurotransmitter receptors that stud the postsynaptic membrane. The receptors capture and react to molecules of the neurotransmitter, altering the level of excitation of the postsynaptic neuron. This action affects the likelihood that the postsynaptic neuron will in turn release its own neurotransmitter from its axon terminals. Molecules of neurotransmitter generally do not enter the postsynaptic neuron; they simply bind to the outside of the receptors momentarily to induce a response, and then detach and diffuse away. The configuration of synapses on a neuron's dendrites and cell body is constantly changing--synapses come and go, dendrites change their shapes, dendritic spines wax and wane--in response to new patterns of synaptic activity and the formation of new neural circuits. We use the general term neuroplasticity to refer to this capacity for continual remodeling of the connections between neurons. We will take a much more detailed look at neurotransmission in Chapter 2 and Chapter 3. The axon integrates and then transmits information Most neurons feature a distinctive cone-shaped enlargement on the cell body called an axon hillock ("little hill"), from which the neuron's axon extends. The axon hillock has unique properties that allow it to gather and integrate the information arriving from the synapses on the dendrites and cell body. As we will discuss in more detail later, this process of integration determines when the neuron will produce neural signals of its own. The neuron's output information, encoded in a stream of electrical impulses, then races down the axon toward the targets that the neuron is said to innervate.
Axon terminals typically form synapses on the cell body or dendrites of a postsynaptic neuron. (A) Presynaptic neuron
Presynaptic axon terminal Mitochondrion Synaptic vesicles Presynaptic membrane Neurotransmitter molecules Synaptic cleft Postsynaptic membrane Dendritic spine
Information ows through a synapse from the presynaptic membrane across a gap called the synaptic cleft to the postsynaptic membrane.
FIGURE 1.4 Synapses
On dendrites, synapses may form on dendritic spines or on the dendrite's shaft.
The axon is a hollow tube, and various important substances, such as enzymes and structural proteins, are conveyed through the interior of the axon from the cell body, where they are produced, to the axon terminals, where they are used. This axonal transport works in both directions: anterograde transport moves materials toward the axon terminals, and retrograde transport moves used materials back to the cell body for recycling. So, it's important to understand that the axon has two quite different functions: the rapid transmission of electrical signals along the outer mWeatmsobnr/aBnreeed(lloikvee a wire), and the much slower transportation of substances within The Mind's Machine tFhouenadxatoionn,stoof Barnaidn farnodmBehtahveioar x4eon terminals (like a pipe). GMlMia4el_c01e.0ll4s p08r/o2t1e/c20t and assist neurons Early neuroscientists had little regard for glial cells, viewing them as a mere filler holding the neurons together (in Greek glia means "glue"). But we now know that glial cells are much more important than that. Glial cells directly affect neuronal processes by providing neurons with raw materials, chemical signals, and specialized structural components.
axonal transport The transportation of materials from the neuronal cell body toward the axon terminals, and from the axon terminals back toward the cell body.
Micrograph A left courtesy of Mark Ellisman and the Natl. Ctr. Microsc. Imag. Res; right © ISM/CTµ-UCBL/Medical Images
activity and regulate adjacent capillaries
Extensions of oligodendrocytes form myelin
(A) wrapping (blue) on axons (yellow). (B) with more energy when they are active.
FIGURE 1.5 Glial Cells
The myelin sheath around a myelinated axon has many layers.
Activated microglial cells surround and break down any debris that forms, especially after damage to the brain. Microglia engulf and destroy debris
oligodendrocyte A type of glial cell that forms myelin in the central nervous system. Schwann cell A type of glial cell that forms myelin in the peripheral nervous system. mWyatesloinn/ BrTeheedlfoavttey insulation around an The Mind's Machine axon, formed by glial cells. This sheath Foundations of Brain and Behavior 4e boosts the speed at which nerve impulses aMreMc4oen_0d1u.0c5ted.07/07/20 node of Ranvier A gap between successive segments of the myelin sheath where the axon membrane is exposed. astrocyte A star-shaped glial cell with numerous processes (extensions) that run in all directions. microglial cells Also called microglia. Extremely small motile glial cells that remove cellular debris from injured or dead cells.
There are more glial cells than neurons in the brain (Herculano-Houzel, 2014), but in contrast to the hundreds of types of neurons that have been identified, there are just four main kinds of glial cells (FIGURE 1.5). Two of these four types of glia-- oligodendrocytes and Schwann cells--wrap around successive segments of axons to insulate them with a fatty substance called myelin. These myelin sheaths give an axon the appearance of a string of elongated slender beads. Between adjacent beads, small uninsulated patches of axonal membrane, called nodes of Ranvier, remain exposed (FIGURE 1.5A). Within the brain and spinal cord, myelination is provided by the oligodendrocytes, each cell typically supplying myelin beads to several nearby axons (also illustrated in Figure 1.5A). In the rest of the body, it is Schwann cells that do the ensheathing, with each Schwann cell wrapping itself around a segment of one axon to provide a single bead of myelin. But whether it is provided by oligodendrocytes or by Schwann cells, myelination has the same result: a large increase in the speed with which electrical signals pass down the axon, jumping from one node of Ranvier to the next. (In Chapter 2 we discuss the diverse abnormalities that arise when the myelin insulation is compromised in the disease multiple sclerosis [MS].) The other two types of glial cells--astrocytes and microglial cells--perform more diverse functions in the brain. Astrocytes (from the Greek astron, "star," for their usual shape) weave around and between neurons with tentacle-like extensions (FIGURE 1.5B). Some astrocytes stretch between neurons and fine blood vessels, controlling local blood flow to increase the amount of blood reaching more-active brain regions (Schummers et al., 2008). Astrocytes help to form the tough outer membranes that swaddle the brain, and they also secrete chemical signals that affect synaptic transmission and the formation of synapses (Perea et al., 2009; Eroglu and Barres, 2010). In contrast, microglial cells (or microglia) are tiny and mobile (FIGURE 1.5C). Their
primary job appears to be to contain and clean up sites of injury (S. A. Wolf et al., 2017). However, astrocytes and microglia may also worsen some problems, such as harmful swelling (edema) following brain injury, and degenerative processes like Alzheimer's disease (Chapter 4) and Parkinson's disease (Chapter 11) (W. S. Chung et al., 2015; Liddelow et al., 2017). Supported and influenced by glial cells, and sharing information through synapses, neurons form the vast ensembles of information-processing circuits that give the brain its visible form. Powerful anatomical and genetic initiatives, such as the Allen Institute brain-mapping project (www.brain-map.org), are creating detailed maps of the cellular composition of the major divisions of the nervous system. These major divisions are our next topic.
1. What are the four "zones" common to all neurons, and what are their functions? 2. Compare and contrast axonal signal transmission and axonal transport. 3. Describe the three main components of the synapse. What are some of the specialized structures found on each side of the synapse? 4. What are the names and general functions of the four types of glial cells? 5. What special properties does myelin have?
1.2The Nervous System Extends throughout the Body
The second part of the chapter surveys the major components of the nervous system. By the end of the section, you should be able to: 1.2.1 Explain what a nerve is, and distinguish between somatic and autonomic nerves. 1.2.2 Identify the cranial and spinal nerves by name and function. 1.2.3 Describe the general functions of the two divisions of the autonomic nervous system. 1.2.4 Name the main anatomical structures that make up the two cerebral hemispheres. 1.2.5 Summarize the anatomical conventions used to describe locations and projections within the nervous system, and distinguish between gray matter and white matter structures. 1.2.6 Describe the fetal development of the brain, and catalog the major adult brain divisions that arise from each fetal region. Neuronal cell bodies, dendrites, axons, and glial cells mass together to form the tissues that define the gross neuroanatomy of the nervous system--the neural structures that are visible to the unaided eye (in this context gross means "large," not "yucky," but you may feel otherwise). The gross view of the entire human nervous system presented in FIGURE 1.6 reveals the basic division between the central nervous system (CNS; consisting of the brain and spinal cord) and the peripheral nervous system (everything else). Let's take a closer look at the anatomical organization of these systems. The peripheral nervous system has two divisions The peripheral nervous system consists of nerves--collections of axons bundled together--that extend throughout the body. Some nerves, called motor nerves,
gross neuroanatomy Anatomical features of the nervous system that are apparent to the naked eye. central nervous system (CNS) The portion of the nervous system that includes the brain and the spinal cord. peripheral nervous system The portion of the nervous system that includes all the nerves and neurons outside the brain and spinal cord. nerve A collection of axons bundled together outside the central nervous system. motor nerve A nerve that transmits information from the central nervous system to the muscles and glands.
From Gray's Anatomy, 35th ed., Figure 2.9, page 807. © Elsevier, 1973. Dissection by M. C. E. Hutchinson, photograph by Kevin Fitzpatrick, Guy's Hospital Medical School, London
26CHAPTER1 (A) Here, a modern view of the central nervous system (CNS) is superimposed on the peripheral nervous system as drawn by Andreas Vesalius (1514-1564). The peripheral nervous system connects the body to the CNS.
Central nervous system Peripheral nervous system
The brain and spinal cord together form the central nervous system. The solid part of the spinal cord ends in the middle of the lower back...
FIGURE 1.6 The Central and Peripheral Nervous Systems
...and below this point a spray of bers called the cauda equina (Latin for "horse's tail") continues downward inside the spinal column.
Watson/Breedlove sTehnesMoirnyd'nseMrvacehinAe nerve that conveys iFnofournmdaattiioonns ofrfoBmraitnheanbdoBdeyhatvoiotrhe4ecentral nervous system. sMoMm4aet_ic01n.0e6rvo0u7/s0s7/y2s0tem A part of the peripheral nervous system that supplies neural connections mostly to the skeletal muscles and sensory systems of the body. It consists of cranial nerves and spinal nerves. autonomic nervous system A part of the peripheral nervous system that provides the main neural connections to the internal organs. cranial nerve A nerve that is connected directly to the brain.
transmit information from the spinal cord and brain to muscles and glands; others, called sensory nerves, convey information from the body to the CNS. The various nerves of the body are divided into two distinct systems: 1. The somatic nervous system, which consists of nerves that interconnect the brain and the major muscles and sensory systems of the body 2. The autonomic nervous system, which consists of nerves that connect primarily to the viscera (internal organs) THE SOMATIC NERVOUS SYSTEM Taking its name from the Latin word for "body"--soma--the somatic nervous system is the main pathway through which the brain controls movement and receives sensory information from the body and from the sensory organs of the head. The nerves that make up the somatic nervous system form two anatomical groups: the cranial nerves and the spinal nerves. We each have 12 pairs (left and right) of cranial nerves that arise from the brain and innervate the head, neck, and visceral organs directly, without ever joining the spinal cord. As you can see in FIGURE 1.7, some of these nerves are exclusively sensory: the olfactory (I) nerves transmit information about smell, the optic (II) nerves carry visual information from the eyes, and the vestibulocochlear (VIII) nerves convey information about hearing and balance. Five pairs of cranial nerves are exclusively motor pathways from the brain: the oculomotor (III), trochlear (IV), and abducens (VI) nerves innervate muscles to move the eyes; the spinal accessory (XI) nerves control
I Olfactory Smell Sensory Motor XII Hypoglossal Tongue muscles
V Trigeminal Face, sinuses, teeth Jaw muscles
XI Spinal accessory Neck muscles X Vagus Information from internal organs Internal organs
VII Facial Tongue, soft palate Facial muscles, salivary glands, tear glands
IX Glossopharyngeal Taste and other mouth sensations
VIII Vestibulocochlear Inner ear: hearing and balance
FIGURE 1.7 The Cranial Nerves
neck muscles; and the hypoglossal (XII) nerves control the tongue. The remaining cra- nial nerves have both sensory and motor functions. The trigeminal (V) nerves, for ex- ample, transmit facial sensation through some axons but control the chewing muscles tWhartosoung/hBroetehdelorvaexons. The facial (VII) nerves control facial muscles and receive some The Mind's Machine tFaoustnedasteionnssaoftiBornai,naannddBtehheavgiolro4sesopharyngeal (IX) nerves receive additional taste sensations and sensations from the throat and also control the muscles there. The vagus (X) nMeMrv4ee_e0x1t.0e7nds07f/a0r7f/r2o0m the head, running to the heart, liver, and intestines, and other organs. Its long, convoluted route is the reason for its name, which is Latin for "wan- dering." The vagus is the primary route by which the brain both controls and receives information from many visceral organs, and it participates in such varied functions as sweating, digestion, and heart rate (Shaffer et al., 2014). Along the length of the spinal cord, an additional 31 pairs of spinal nerves--again, one member of each pair serves each side of the body--emerge through regularly
View Activity 1.2: The Cranial Nerves spinal nerve A nerve that emerges from the spinal cord.
28CHAPTER1 FIGURE 1.8 The Spinal Cord and Spinal Nerves
These cross sections were taken from different levels of the spinal cord as indicated.
The spinal column runs from the base of the brain to the coccyx (tailbone); a pair of nerves emerges from each segment.
The gray matter in the center of the spinal cord contains interneurons and the motor neurons that send axons to the muscles. The surrounding white matter consists of myelinated axons communicating up and down the spinal cord.
The membranes (meninges) that surround the spinal cord
The spinal cord is enclosed in bone (vertebrae) and three layers of membranes (the meninges).
View Activity 1.3: Gross Anatomy of the Spinal Cord cervical Referring to the topmost eight segments of the spinal cord, in the neck region. thoracic Referring to the 12 spinal sWegamtsoenn/tsBrbeeeldolwovtehe cervical (neck) pTohretioMninodf'sthMeascphiinnael cord, in the torso. Foundations of Brain and Behavior 4e lumbar Referring to the five spinal segmMeMnt4sei_n01th.0e8up0p7e/r0p7a/r2t0of the lower back. sacral Referring to the five spinal segments in the lower part of the lower back. coccygeal Referring to the lowest spinal vertebra (the coccyx, or "tailbone"). sympathetic nervous system The part of the autonomic nervous system that generally prepares the body for action. parasympathetic nervous system The part of the autonomic nervous system that generally prepares the body to relax and recuperate.
spaced openings along both sides of the backbone (FIGURE 1.8). Each spinal nerve is made up of a group of motor fibers, projecting from the ventral (front) part of the spinal cord to the organs and muscles, and a group of sensory fibers that enter the dorsal (rear) part of the spinal cord. Spinal nerves are named according to the segments of the spinal cord to which they are connected. There are 8 cervical (neck), 12 thoracic (torso), 5 lumbar (lower back), 5 sacral (pelvic), and 1 coccygeal (bottom) spinal segments. The name of each spinal nerve reflects the position of the spinal cord segment to which it is connected; for example, the nerve connected to the 12th thoracic segment is called T12, the nerve connected to the 7th cervical segment is called C7, and so on. After leaving the spinal cord, axons from the spinal nerves spread out in the body and may merge with axons from different spinal nerves to form the various peripheral nerves. THE AUTONOMIC NERVOUS SYSTEM Although it is "autonomous" in the sense that we have little conscious, voluntary control over its actions, the autonomic nervous system is the brain's main system for controlling the organs of the body. The activity of our organs is determined by a balance between the two major divisions of the autonomic nervous system--called the sympathetic and parasympathetic nervous systems--that act more or less in opposition to each other (FIGURE 1.9). Axons of the sympathetic nervous system exit from the middle parts of the spinal cord, travel a short distance, and then innervate the sympathetic ganglia (small clusters of neurons found outside the CNS), which run in two chains along the spinal column, one on each side (see Figure 1.9 left). Axons from the sympathetic ganglia then spread throughout the body, innervating all the major organ systems. In general, sympathetic innervation prepares the body for immediate action: blood pressure increases, the pupils of the eyes widen, the heart quickens, and so on. This set of reactions is sometimes called the fight-or-flight response.
The sympathetic nervous system consists of the sympathetic ganglia and axons that lead to the organs. It helps prepare the body for " ght or ight."
FIGURE 1.9 The Autonomic Nervous System
Sympathetic division Dilates (opens) pupil
Parasympathetic division Constricts pupil
The parasympathetic nervous system arises from both the brain and the sacral parts of the spinal cord. It helps prepare the body to "rest and digest."
Cranial Cervical (8 segments) Thoracic (12 segments) Lumbar (5 segments)
Stimulates glucose production and release
The sympathetic and parasympathetic projections use different neurotransmitters (see Chapter 3) and have opposing effects on the organs they innervate, allowing precise control.
Sacral (5 segments) Coccygeal (1 segment) Sympathetic chain
Adrenal gland Stimulates secretion of epinephrine and norepinephrine
Constricts blood vessels in skin Relaxes bladder
Dilates blood vessels in intestines Dilates blood vessels in skin
Contracts bladder Stimulates penile erection and clitoral engorgement
Noradrenergic neurons Cholinergic neurons Cell body
Postganglionic Preganglionic Postganglionic
In contrast to the effects of sympathetic activity, the parasympathetic nervous system generally helps the body to relax, recuperate, and prepare for future action-- sometimes called the rest-and-digest response. Anatomically, nerves of the parasympaWthaetstoicn/sByrseteedmlovoeriginate in the brainstem (above the sympathetic nerves) and in the The Mind's Machine Foundations of Brain and Behavior 4e MM4e_01.09 08/14/20
View Activity 1.4: Concept Matching: Sympathetic vs. Parasympathetic
sagittal plane The plane that divides the body or brain into right and left portions. coronal plane Also called frontal plane or transverse plane. The plane that divides the body or brain into front and back parts. horizontal plane The plane that divides the body or brain into upper and lower parts. medial In anatomy, toward the middle.. lateral In anatomy, toward one side. ipsilateral In anatomy, pertaining to a location on the same side of the body. contralateral In anatomy, pertaining to a location on the opposite side of the body. superior In anatomy, above. inferior In anatomy, below. basal "Toward the base" or "toward the bottom" of a structure. anterior Also called rostral. In anatomy, toward the head end of an organism. posterior Also called caudal. In anatomy, toward the tail end of an organism. proximal In anatomy, near the trunk or center of an organism. Compare distal. distal In anatomy, toward the periphery of an organism or toward the end of a limb. afferent Carrying action potentials toward the brain, or toward one region of interest from another region of interest. efferent Carrying action potentials away from the brain, or away from one region of interest toward another region of interest. dorsal In anatomy, toward the back of the body or the top of the brain. ventral In anatomy, toward the belly or front of the body, or the bottom of the brain. gray matter Areas of the brain that are dominated by cell bodies and are devoid of myelin. Gray matter mostly receives and processes information. white matter A light-colored layer of tissue, consisting mostly of myelin-sheathed axons, that lies underneath the gray matter of the cortex. White matter mostly transmits information. cerebral hemisphere One of the two halves--right or left--of the forebrain.
sacral spinal cord (below the sympathetic nerves), which explains the name: the Greek para means "around" (see Figure 1.9 right). Compared with sympathetic nerves, parasympathetic nerves travel a longer distance before terminating in parasympathetic ganglia, clusters of neurons that are usually located close to the organs they serve. The sympathetic and parasympathetic systems have very different effects on individual organs because the organs receive different neurotransmitters from the two opposing systems (norepinephrine from sympathetic nerves and acetylcholine from parasympathetic nerves; see Chapter 2 and Figure 11.19). The balance between the two systems determines the state of the internal organs at any given moment. So, for example, when parasympathetic activity predominates, heart rate slows, blood pressure drops, and digestive processes are activated. As the brain causes the balance of autonomic activity to become predominantly sympathetic, opposite effects are seen: increased heart rate and blood pressure, inhibited digestion, and so on. This tension between parasympathetic and sympathetic activity ensures that the individual is appropriately prepared for current circumstances. The central nervous system consists of the brain and spinal cord The spinal cord funnels sensory information from the body up to the brain and conveys the brain's motor commands out to the body. The spinal cord also contains circuits that perform local processing and control simple units of behavior, such as reflexes. We will discuss other aspects of the spinal cord in later chapters, so for now let's focus on the anatomy of the executive portion of the CNS: the brain. ANATOMICAL CONVENTIONS FOR DESCRIBING THE ANATOMY OF THE BRAIN Because the nervous system is a three-dimensional structure, two-dimensional illustrations and diagrams cannot represent it completely. Anatomists use standard terminology to help identify structures, locations, and directions in the brain. It's a bit of a chore, but learning the anatomical lingo now will make later discussions of brain organization much easier to follow. As illustrated in FIGURE 1.10, the brain is usually visualized in one of three main planes to obtain a two-dimensional section from this three-dimensional object. The plane that divides the brain into right and left portions is called the sagittal plane. The plane that divides front (anterior) from back (posterior) is called the coronal plane (also known as the frontal plane), and the horizontal plane divides between upper and lower parts. In addition to the three planes of dissection, locations in the nervous system are described using directional terms. Medial means "toward the middle," whereas lateral means "toward the side." Ipsilateral means "on the same side," as opposed to contralateral, which means "on the opposite side." These terms are all relative, as are the terms superior ("above"), inferior ("below"), and basal ("toward the bottom"). Locations toward the front of the brain are anterior or rostral, locations toward the rear are posterior or caudal (from the Latin cauda, "tail"). Proximal means "near" and distal means "far" or "toward the end of a limb." And a nerve or pathway is afferent if it carries information into a region that we're interested in, and it's efferent if it carries information away from the region of interest (a handy way to remember this is that efferents exit but afferents arrive, relative to the region of interest). Lastly (phew!), dorsal means "toward the back," and ventral means "toward the belly." Most people have heard brain tissue referred to as gray matter. When you cut into a brain, you see that the outer layers of the cortex have a darker grayish shade (see Figure 1.10). This is because they contain a preponderance of neuronal cell bodies and dendrites. In contrast, the underlying white matter gets its snowy appearance from the whitish fatty myelin that insulates many axons. So, a simple view is that gray matter mostly receives and processes information, while white matter mostly transmits information. THE OUTER SURFACE OF THE BRAIN On average, the human brain weighs only 1,400 grams (about 3 pounds), accounting for just 2% of the average body weight. Put your two fists together and you get a sense of the size of the two cerebral hemispheres--
FIGURE 1.10 Terms for Describing Anatomical Locations
The lighter-colored interior is white matter, packed with the fatty myelin that surrounds axons sending information in and out of the cortex. Dorsal Coronal plane
Gray matter consists of cell bodies that form the outer layers of the cortex and nuclei within the brain.
Photographs courtesy of S. Mark Williams and Dale Purves, Duke University Medical Center
smaller than most people expect. But what the brain lacks in size and weight it makes
up for in intricacy. One obvious complication of the brain is its lumpy, convoluted sur-
face--the result of elaborate folding of a thick sheet of tissue, mostly the dendrites, cell
bodies, and axonal projections of neurons, called the cerebral cortex (or sometimes
just cortex). The resultant ridges of tissue, called gyri (singular gyrus), are separated from
each other by crevices called sulci (singular sulcus). Folding up the tissue in this way
greatly increases the amount of cortex that can be crammed into the confines of the
Wskautslol,na/nBdreaedbloouvet two-thirds of the cerebral cortex is hidden in the depths of these folds. TThheeMpiantdt'esrMnaochf ifnoelding is not random; in fact, it is similar enough between brains that
Fwoeuncdaantionnsaomf BertahineanvdarBieohuavsiogry4rei and sulci and group them together into lobes.
distinguish four major cortical regions of each cerebral hemisphere: the frontal, parietal,
temporal, and occipital lobes (FIGURE 1.11). In some cases, the boundaries between
adjacent lobes are very clear; for example, the Sylvian fissure (or lateral sulcus) divides
the temporal lobe from other regions of the hemisphere. The central sulcus provides
a distinct landmark dividing the frontal and parietal lobes. The physical boundaries
between the occipital lobe and the temporal and parietal lobes are less obvious, but the
lobes are quite different with regard to the functions they perform.
The cortex is the seat of complex cognition. Depending on the specific regions
affected, cortical damage can cause symptoms ranging from impairments of move-
ment or body sensation; through speech errors, memory problems, and personality
changes; to many kinds of visual impairments. In people with undamaged brains, the
four lobes of the cortex are continually communicating and collaborating in order to
produce the seamless control of complex behavior that distinguishes us as individuals.
Ventral cerebral cortex Also called simply cortex. The outer covering of the cerebral hemispheres, which consists largely of nerve cell bodies and their branches. gyrus A ridged or raised portion of the cortical surface. sulcus A crevice or valley of the cortical surface. frontal lobe The most anterior portion of the cerebral cortex. parietal lobe The large region of cortex lying between the frontal and occipital lobes in each cerebral hemisphere. temporal lobe The large lateral region of cortex in each cerebral hemisphere. It is continuous with the parietal lobe posteriorly and separated from the frontal lobe by the Sylvian fissure. occipital lobe A large region of cortex that covers much of the posterior part of each cerebral hemisphere. Sylvian fissure Also called lateral sulcus. A deep fissure that demarcates the temporal lobe. central sulcus A fissure that divides the frontal lobe from the parietal lobe.
FIGURE 1.11 The Human Brain Has Four Distinct Lobes
View Activity 1.5: Gross Anatomy of the Human Brain: Lateral View
corpus callosum The main band of axons that connects the two cerebral hemispheres. postcentral gyrus The strip of parietal cortex, just posterior to (behind) the central sulcus, that receives somatosensory information from the entire body. precentral gyrus The strip of frontal cortex, just anterior to (in front of) the central sulcus, that is crucial for motor control. neural tube An embryonic structure with subdivisions that correspond to the future forebrain, midbrain, and hindbrain. forebrain The frontal division the neural tube, containing the cerebral hemispheres, the thalamus, and the hypothalamus. midbrain The middle division of the brain. hindbrain The rear division of the brain, which in the mature vertebrate contains the cerebellum, pons, and medulla. telencephalon The anterior part of the fetal forebrain, which will become the cerebral hemispheres in the adult brain. diencephalon The posterior part of the fetal forebrain, which will become the thalamus and hypothalamus in the adult brain. brainstem The region of the brain that consists of the midbrain, the pons, and the medulla.
Furthermore, hundreds of millions of axons connect the left and right hemispheres via the corpus callosum, allowing the brain to act as a single entity during complex processing. Some life-sustaining functions--heart rate and respiration, reflexes, balance, and the like--are governed by lower, subcortical brain regions. The sense of touch is mediated by a strip of parietal cortex just behind the central sulcus called the postcentral gyrus (see Figure 1.11), so it is often referred to as the primary somatosensory cortex. In front of the central sulcus, the precentral gyrus--primary motor cortex--of the frontal lobe is crucial for motor control. As we will see in later chapters, both gyri exhibit somatotopic organization, which means that they precisely map the various parts of the contralateral side of the body (Penfield and Rasmussen, 1950). The occipital lobes are crucial for vision, and the temporal lobes receive auditory inputs and help in memory formation. But each lobe of the brain also performs a wide variety of other high-level functions. These will be major topics in later chapters. Watson/Breedlove TDheEMVEinLdO'sPMMacEhNinTe OF SUBDIVISIONS WITHIN THE BRAIN It can be difficult to underFsotuanndadtitohnseoof Brirgaiinnaondf sBoehmaveioor f4ethe regional names applied to the adult human brain. For MeMxa4me_p0l1e.1,1par0t7o/f07t/h2e0brain closest to the back of the head is anatomically identified as part of the forebrain. Why? The key to understanding this confusing terminology is to consider how the gross anatomy of the brain develops early in life. In a very young embryo of any vertebrate, the CNS looks like a tube (Chapter 4). The walls of this neural tube are made of cells, and the interior is filled with fluid. A few weeks after conception, the human neural tube begins to show three separate swellings at the head end (FIGURE 1.12A): the forebrain, the midbrain, and the hindbrain; the remainder of the neural tube eventually forms the spinal cord. By about 50 days, the fetal forebrain features two clear subdivisions. At the very front is the telencephalon (from the Greek encephalon, "brain"), which will become the cerebral hemispheres (consisting of cortex plus some deeper structures). The other part of the forebrain is the diencephalon, which will go on to become the thalamus and the hypothalamus, two of the many subcortical structures of the forebrain.
Similarly, the hindbrain further develops into several large structures: the cerebellum, pons, and medulla. The term brainstem usually refers to the midbrain, pons, and medulla combined (some scientists include the diencephalon too). FIGURES 1.12B and C show the positions of these structures and their relative sizes in the adult human brain. Even when the brain achieves its adult form, it is still a fluid-filled tube, but a tube of very complicated shape. The main sections of the brain can be subdivided in turn. We can work our way from the largest, most general divisions of the nervous system on the left of the schematic in Figure 1.12B to more-specific ones on the right. Within and between the major brain regions are collections of neurons called nuclei (singular nucleus) and bundles of axons called tracts. Recall that outside the CNS, collections of neurons are called ganglia, and bundles of axons are called nerves. Unfortunately, the word nucleus can mean either "a collection of neurons in the CNS" or "the spherical DNA-containing organelle within a single cell." You must rely on the context to understand which meaning is intended. Because brain tracts and nuclei are the same in different individuals, and often the same in different species, they have names too (many, many names). You are probably more interested in the functions of all these parts of the brain than in their names, but as we noted earlier, each region serves more than one function, and
nucleus Here, a collection of neuronal cell bodies within the central nervous system (e.g., the caudate nucleus). tract A bundle of axons found within the central nervous system. View Activity 1.6: The Developing Brain See Video 1.3: Brain Development
(A) Development of the human brain Neural tube
About 50 days after conception, the ve main divisions of the brain are visible.
Telencephalon (cerebral hemispheres) Diencephalon Midbrain
Cortex Basal ganglia Limbic system Thalamus Hypothalamus
Sympathetic division Parasympathetic division
FIGURE 1.12 Divisions of the Human Nervous System in the Embryo and the Adult
34CHAPTER1 View Activity 1.7: The Basal Ganglia and Activity 1.8: The Limbic System
our knowledge of the functional organization of the brain is continually being updated with new research findings. So, with that caution in mind, we'll briefly survey the functions of specific brain structures next, leaving the detailed discussion for later chapters. 1. Name and briefly describe the major divisions of the peripheral nervous system. What general function does each part perform? 2. Briefly sketch and describe the anatomical organization of the cranial nerves. How many nerves are there? Now do the same for the spinal nerves. 3. Give some examples of how each division of the autonomic nervous system affects organs of the body. 4. Why does the cortex look so lumpy on the outside? 5. What is special about the pre- and postcentral gyri? 6. Review the fetal development of the brain, and the major divisions of the brain that arise from the earlier fetal form of the nervous system.
FIGURE 1.13 Layers of the Cerebral Cortex (After P. Rakic, in F. O. Schmitt and F. G. Worden, 1979. The Neurosciences: Fourth Study Program. MIT Press: Cambridge, MA.)
I II III The six layers of cortex can be distinguished with stains that reveal all cell bodies... IV Cell body V VI
1.3The Brain Shows Regional Specialization of Functions
Functional neuroanatomy connects behaviors to brain regions. By the end of this section, you should be able to: 1.3.1 Describe the cellular organization of the cortex. 1.3.2 Identify the major components of the basal ganglia and limbic system, and state some of the behavioral functions of each. 1.3.3 Name the major divisions of the brainstem and midbrain, and identify key functions performed by each.
Vertebrates are bilaterally symmetrical: our bodies have mirror-image left and right sides. The brain is no exception, and almost all the structures of the brain also come in twos. One important principle of the vertebrate brain is that each side of the brain generally controls the contralateral side of the body. The right side of the brain thus controls movement of the left side of the body and receives left-sided sensory information. Likewise, the left side of the brain monitors and controls the right side of the body. In Chapter 15 we'll learn about how the two cerebral hemispheres interact, but for now let's review the various components of the brain and their functions.
...or with stains that reveal a few neurons in their entirety. One pyramidal neuron
The cerebral cortex performs complex cognitive processing Neuroscientists are only just beginning to understand how the structures and functions of the cerebral cortex accomplish the feats of human cognition. If the human cortex were unfolded, it would occupy an area of about 2,000 square centimeters (315 square inches)--more than 3 times the area of this book's front cover. How are all those millions of cells arranged? Cortical neurons make up six distinct layers, as shown in FIGURE 1.13. Each cortical layer has a unique appearance because it consists of either a band of similar neurons, or a particular pattern of dendrites or axons.
For example, the outermost layer, layer I, is distinct because it has few cell bodies, while layers V and VI stand out because of their many neurons with large cell bodies. The most prominent kind of neuron in the cerebral cortex--the pyramidal cell--usually has its pyramid-shaped cell body in layer III or V. In some regions of the cerebral cortex, neurons are organized into regular columns, perpendicular to the layers, that seem to serve as information-processing units (Horton and Adams, 2005). These cortical columns extend through the entire thickness of the cortex, from the white matter to the surface. Within each column, most of the synaptic interconnections of neurons are vertical, although there are some horizontal connections as well (Mountcastle, 1979; Sakmann, 2017). Important nuclei are hidden beneath the cerebral cortex Buried within the cerebral hemispheres are several large gray matter structures, richly connected to each other and to other brain regions and contributing to a wide variety of behaviors. One prominent cluster--the basal ganglia, consisting primarily of the caudate nucleus, the putamen, and the globus pallidus (FIGURE 1.14A)--plays a critical role in the control of movement (see Chapter 5). Curving through each hemisphere, alongside the basal ganglia, lies a loose network of structures called the limbic system (identified in FIGURE 1.14B) that is involved in emotion and learning. The amygdala is a limbic structure involved in emotional regulation (see Chapter 11) and the perception of odor (see Chapter 6). The hippocampus and fornix are important for learning and memory (see Chapter 13). A strip of cortex atop the corpus callosum in each hemisphere, called the cingulate gyrus, is implicated in many cognitive functions, including the direction of attention (see Chapter 14), and the olfactory bulb processes the sense of smell. Other limbic structures near the base of the brain, especially the hypothalamus, help to govern motivated behaviors, like sex and aggression, and to regulate the hormonal systems of the body. Toward the medial (middle) and basal (bottom) aspects of the forebrain are found the thalamus and the hypothalamus (the latter means simply "under thalamus"). You can see both the hypothalamus and thalamus in Figure 1.14B and Figure 1.15A. The thalamus is the brain's traffic cop, directing virtually all incoming sensory information
pyramidal cell A type of large nerve cell that has a roughly pyramid-shaped cell body and is found in the cerebral cortex. cortical column One of the vertical columns that constitute the basic organization of the cerebral cortex. basal ganglia A group of forebrain nuclei, including the caudate nucleus, globus pallidus, and putamen, found deep within the cerebral hemispheres. limbic system A loosely defined, widespread group of brain nuclei that innervate each other and form a network. amygdala A group of nuclei in the medial anterior part of the temporal lobe. hippocampus A medial temporal lobe structure that is important for learning and memory. fornix A fiber tract that extends from the hippocampus to the mammillary body. cingulate gyrus A strip of cortex, found in the frontal and parietal midline, that is part of the limbic system and is implicated in many cognitive functions. olfactory bulb An anterior projection of the brain that terminates in the upper nasal passages and provides the primary inputs for the sense of smell. thalamus Paired structures to either side of the third ventricle that direct the flow of sensory information to and from the cortex. hypothalamus Part of the diencephalon, lying ventral to the thalamus.
Putamen Globus pallidus Amygdala Subthalamic nucleus FIGURE 1.14 Two Important Brain Systems
tectum The dorsal portion of the midbrain, consisting of the inferior and superior colliculi. superior colliculi Paired gray matter structures of the dorsal midbrain that process visual information. inferior colliculi Paired gray matter structures of the dorsal midbrain that process auditory information. tegmentum The main body of the midbrain, containing the substantia nigra, periaqueductal gray, part of the reticular formation, and multiple fiber tracts. substantia nigra A brainstem structure that innervates the basal ganglia and is a major source of dopaminergic projections. periaqueductal gray A midbrain region involved in pain perception. reticular formation An extensive region of the brainstem, extending from the medulla through the thalamus, that is involved in sleep and arousal. cerebellum A structure located at the back of the brain, dorsal to the pons, that is involved in the central regulation of movement and in some forms of learning. View Activity 1.9: Gross Anatomy of the Human Brain: Midsagittal View and Activity 1.10: Gross Anatomy of the Human Brain: Basal View
to the appropriate regions of the cortex for further processing, and receiving instructions back from the cortex about which sensory information is to be transmitted. The small but mighty hypothalamus has a much different role: it is packed with discrete nuclei involved in many vital functions, such as hunger, thirst, temperature regulation, sex, and many more. Furthermore, because the hypothalamus also controls the pituitary gland, it serves as the brain's main interface with the hormonal systems of the body. We'll encounter the hypothalamus again in several later chapters. The midbrain has sensory and motor components Compared with the forebrain and hindbrain, the midbrain doesn't encompass a lot of tissue, but that doesn't mean its components are unimportant. The top part of the midbrain, called the tectum (from the Latin for "roof," because it's atop the midbrain), features two pairs of bumps--one pair in each hemisphere--with specific roles in sensory processing. The more rostral bumps are called the superior colliculi (singular colliculus), and they have specific roles in visual processing. The more caudal bumps, called the inferior colliculi (see Figure 1.15A), process information about sound. The main body of the midbrain is called the tegmentum, and it also contains several important structures. The substantia nigra is in many ways a part of the basal ganglia, and loss of its neurons (which normally release the neurotransmitter dopamine within the forebrain) leads to Parkinson's disease, discussed in Chapter 5. The periaqueductal gray is a midbrain structure implicated in the perception of pain (see Chapter 3). The reticular formation (reticular means "netlike") is a loose collection of neurons that are important in a variety of behaviors, including sleep and arousal (see Chapter 10). Multiple large tracts of nerve fibers run in, out, and through the midbrain to connect the brain to the spinal cord. The brainstem controls vital body functions The midsagittal and basal views of the brain in FIGURE 1.15 show the hemispheres of the cerebellum, which is tucked up under the posterior cortex and attached to the dorsal brainstem. Like the cerebral cortex, the cerebellum is highly convoluted, but it is made up of a simpler three-layered tissue instead of the six layers found in the cerebral cortex. The cerebellum has long been known to be crucial for motor coordination and
The four lobes of the cerebral cortex are color coded here as in Figure 1.11. In addition, the cingulate gyrus and brainstem are shaded red and yellow, respectively.
Hypothalamus Cingulate gyrus Corpus callosum
Superior colliculus Inferior colliculus Reticular formation
Pituitary Midbrain Pons Medulla Brainstem
FIGURE 1.15 Midline and Basal Structures of the Brain
Midbrain and brainstem structures are especially evident in this view. Medulla Spinal cord Cerebellum Pons
control, but we now know that it also participates in certain aspects of cognition, including learning. The adjacent pons (from the Latin word for "bridge") contains many nerve fibers and important motor control and sensory nuclei; it is the point of origin for several cranial nerves. The reticular formation, which we first saw in the midbrain, stretches down through the pons and ends in the medulla. The medulla marks the transition from the brain to the spinal cord. In addition to conveying all of the major motor and sensory fibers to and from the body, the medulla contains nuclei that drive such essential processes as respiration and heart rate, so brainstem injuries are often lethal. And like other parts of the brainstem, the medulla gives rise to several cranial nerves. Behaviors and cognitive processes depend on networks of brain regions In order to understand the neural origins of our most complex behaviors and experiences--thought, language, music--it will be necessary to understand how different brain regions with distinct functions collaborate in larger-scale networks. This applies to functional units as small as the individual cortical columns we mentioned earlier and to much larger assemblages of millions of cells making up substantial parts of cortical lobes. Cortical regions communicate with one another via tracts of axons looping through the underlying white matter. Some of these connections are short pathways to nearby cortical regions; others travel longer distances through and between the two cerebral hemispheres and subcortical structures like the basal ganglia. Progress in describing the "connectome" of the human brain (Glasser, Coalson et al., 2016)--the network map that completely describes the functional connections within and between brain regions, based on huge volumes of human and nonhuman animal neuroanatomical data (van Essen and Glasser, 2018; Suárez et al., 2020)--is rapidly transforming the field of behavioral neuroscience.
pons The portion of the brainstem that connects the midbrain to the medulla. medulla The posterior part of the hindbrain, continuous with the spinal cord.
1. How are the cells of the cerebral cortex organized? 2. Name the major components of the basal ganglia and the limbic system. What behaviors especially rely on these systems? 3. What functions are served by the thalamus and hypothalamus? 4. Name and describe the general functions of the major components of the midbrain and hindbrain. 5. Why are injuries to the medulla often fatal? 6. Define the connectome, and discuss its significance for understanding the functioning of the brain.
1.4Specialized Support Systems Protect and Nourish the Brain Next, we consider the specialized structures and fluids that support the operations of the brain. By the end of this section, you should be able to: 1.4.1 Name and describe the meninges, ventricular system, and glymphatic system, and review their clinical significance. 1.4.2 Give an outline of the vascular supply of the brain, and note the signs and symptoms of stroke.
meninges The three protective membranes--dura mater, pia mater, and arachnoid--that surround the brain and spinal cord. dura mater The outermost of the three meninges that surround the brain and spinal cord. pia mater The innermost of the three meninges that surround the brain and spinal cord. arachnoid The thin covering (one of the three meninges) of the brain that lies between the dura mater and the pia mater. cerebrospinal fluid (CSF) The fluid that fills the cerebral ventricles. meningitis An acute inflammation of the meninges, usually caused by a viral or bacterial infection. meningioma A noninvasive tumor of the meninges. ventricular system A system of fluid-filled cavities inside the brain. lateral ventricle A complex C-shaped lateral portion of the ventricular system within each hemisphere of the brain. choroid plexus A specialized membrane lining the ventricles that produces cerebrospinal fluid by filtering blood. third ventricle The midline ventricle that conducts cerebrospinal fluid from the lateral ventricles to the fourth ventricle. fourth ventricle The passageway within the pons that receives cerebrospinal fluid from the third ventricle and releases it to surround the brain and spinal cord.
The brain is relatively soft and easily damaged. It also needs a steady and substantial supply of fuel to maintain normal functioning, and thus keep us alive. Fortunately, the brain is equipped with systems to protect and cushion it and to provide a continual source of energy, nutrients, and important chemicals. The brain floats within layers of membranes Within the bony skull and vertebrae, the brain and spinal cord are swaddled by three protective membranes called meninges (see Figure 1.8). Between a tough outer sheet called the dura mater (in Latin, literally "tough mother") and the delicate pia mater ("tender mother") that adheres tightly to the surface of the brain, a webby substance called the arachnoid ("spiderweb-like") creates a reservoir called the subarachnoid space that suspends the brain in a bath of a watery liquid called cerebrospinal fluid (CSF). The meninges can become inflamed by infections, termed meningitis, or distorted by a hemorrhage; either situation is a medical emergency because the brain is squeezed and impaired. Tumors called meningiomas can form in the meninges and are technically benign, because they don't spread, but any mass that takes up space in the enclosed cranium is far from harmless. The brain relies on two fluids for survival The brain essentially floats in cerebrospinal fluid within the subarachnoid space, cushioning it from minor blows to the head. But CSF has additional important roles, passing into the substance of the brain, conveying nutrients and signaling chemicals, and picking up waste matter for later clearance. Inside the brain is a series of chambers called the cerebral ventricles, which are filled with CSF (FIGURE 1.16). These chambers comprise the ventricular system. Each hemisphere of the brain contains a lateral ventricle extending into all four lobes of the hemisphere. The lateral ventricles are lined with a specialized membrane called the choroid plexus, which produces CSF by filtering blood. The CSF flows from the lateral ventricles into a midline third ventricle (so named because it follows the two lateral ventricles) and continues down a narrow passage (the cerebral aqueduct) to the fourth ventricle, which lies between the cerebellum and the pons. Just below the cerebellum, three small openings allow CSF to exit the ventricular system and circulate over the outer surface of the brain and spinal cord. The CSF is absorbed back into the circulatory system through large veins
The positions of the cerebral ventricles are shown here within an adult brain.
FIGURE 1.16 The Cerebral Ventricles
Third ventricle Choroid plexus CSF Cerebrospinal uid (CSF) is made by the choroid plexus in the lateral ventricles...
Cerebral aqueduct Fourth ventricle ...and exits from the fourth CSF ventricle to surround the brain and spinal cord.
beneath the top of the skull. A problem that blocks the flow of CSF through the ventricular system may result in hydrocephalus, a ballooning of the ventricles as they accumulate fluid, resulting in greatly varying symptoms. Although the brain had long been thought to lack the lymphatic system found in other tissues, the recently discovered glymphatic system (FIGURE 1.17; the name reflects the involvement of glial cells) provides for drainage of waste-bearing CSF-derived fluids from the brain as well as the distribution of various nutrients, immune system components, and signaling substances (Jessen et al., 2015; Mestre et al., 2020). Curiously, glymphatic clearance occurs primarily while we sleep, and
hydrocephalus A ballooning of the ventricles, at the expense of the surrounding brain, which may occur when the circulation of CSF is blocked. glymphatic system A lymphatic system in the brain that participates in removal of wastes and the movement of nutrients and signaling compounds.
Glymphatic drainage, which occurs primarily during sleep, helps clear debris and wastes from the brain, including proteins that have been implicated in Alzheimer's disease.
View Activity 1.11: The Cerebral Ventricles
1 CSF ows from the CSF- lled subarachnoid space into the periarterial space surrounding ne arterioles that penetrate the brain, and is propelled along by the pulsing of the artery walls. Periarterial space 2 CSF enters the brain tissue via specialized channels, called aquaporins, in the end feet of astrocytes surrounding the arterioles, and then ows through the brain, accumulating waste material as it goes. Neuron Astrocyte End foot
Dura mater Pia mater Perivenous space 3 The CSF then drains into the perivascular space surrounding veins in the brain, ultimately draining into the body's lymphatic system. Aquaporins
FIGURE 1.17 The Glymphatic System (After M. Nedergaard and S. A. Goldman. 2016. Sci. Am. 314: 44-49.)
cerebral arteries The three pairs of large arteries within the skull that supply blood to the cerebral cortex. blood-brain barrier The mechanisms that make the movement of substances from blood vessels into cells more difficult in the brain than in other body organs, thus affording the brain greater protection from exposure to some substances found in the blood. stroke Damage to a region of brain tissue that results from the blockage or rupture of vessels that supply blood to that region. transient ischemic attack (TIA) A temporary blood restriction to part of the brain that causes stroke-like symptoms that quickly resolve, serving as a warning of elevated stroke risk.
researchers are working to understand how this system may protect against neurological problems such as Alzheimer's disease, stroke, and multiple sclerosis (M. K. Rasmussen et al., 2018). The second crucial fluid for the brain is, of course, blood. Without a lavish supply of oxygen- and nutrient-rich blood, the tissue of the brain would swiftly die. That's because brain tissue is unusually needy: it accounts for only 2% of the average human body but consumes more than 20% of the body's energy at rest. So the brain is critically dependent on a set of large blood vessels. Blood arrives in the brain via two pairs of arteries: the carotid arteries in the neck, and the vertebral arteries that ascend within each side of the vertebrae of the neck, fusing to form the basilar artery inside the skull. These arteries give rise to a set of three pairs of cerebral arteries that supply the cortex, plus a number of smaller vessels that penetrate and supply other regions of the brain. Fine vessels and capillaries branching off from the arteries deliver nutrients and other substances to brain cells and remove waste products. In contrast to capillaries in the rest of the body, capillaries in the brain are highly resistant to the passage of large molecules across their walls and into neighboring neurons. This blood-brain barrier probably evolved to help protect the brain from infections and blood-borne toxins, but it also makes the delivery of drugs to the brain more difficult. You can learn more about the brain's elaborate vascular system in A STEP FURTHER 1.1, on the website.
SIGNS & SYMPTOMS Stroke The general term stroke applies to a situation in which a clot, a narrowing, or a rupture interrupts the supply of blood to a particular brain region, causing the affected region to stop functioning or die (FIGURE 1.18). Although the exact effects of stroke depend on the region of the brain that is affected, the five most common warning signs are sudden numbness or weakness, altered vision, dizziness, severe headache, and confusion or difficulty speaking. Effective treatments are available to help
restore blood flow and minimize the long-term damage of a stroke, but only if the victim is treated immediately (Albers et al., 2018). Some people experience temporary stroke-like symptoms lasting for a few minutes. Caused by a brief interruption of blood supply to some part of the brain, this transient ischemic attack (from the Greek ischemia, "interrupted blood"), or TIA, is a serious warning sign that a major stroke may be imminent, and it should be treated as a medical emergency.
FIGURE 1.18 Stroke
A hemorrhagic stroke occurs when a rupture in an artery allows blood to leak into the brain. Rupture Anterior cerebral artery Middle cerebral artery Internal carotid artery
In ischemic stroke, clots or other debris prevent blood from reaching a region of the brain, causing it to die. Blockage The effect of a stroke on cognition depends on which artery--and associated brain tissue--is compromised.
1. Name the three meninges, and describe how they're organized. Identify one special characteristic of each. 2. What is CSF? What function does it serve, where does it come from, and where does it go? 3. Describe the ventricular system of the brain. 4. What is the blood-brain barrier? 5. What are the two types of stroke, and what are some common symptoms of a stroke?
1.5Scientists Have Devised Clever Techniques for Studying the Structure and Function of the Nervous System
Our focus now turns to experimental approaches researchers use to probe the nervous system. By the end of this section, you should be able to: 1.5.1 Distinguish between invasive and noninvasive experimental techniques. 1.5.2 Review the techniques for studying the detailed, cellular structure and function of the brain, and review their application in various types of studies. 1.5.3 Summarize the major brain-imaging technologies used to study living human brains, highlighting their differing uses and limitations.
Because it is both fantastically complex and somewhat inaccessible, the brain poses special challenges when it comes to formulating research questions and designing experiments to answer those questions. Researchers have long sought methods that would allow them to study the detailed structure and function of the nervous system, from mapping the molecular components of the various cells that make up the brain to tracking the moment-by-moment activity of large neural networks during the execution of complex behaviors. These techniques vary in terms of their invasiveness: to study the brain at the cellular level, we generally need to work with postmortem tissue samples or biopsies, whereas the larger-scale activity of the brain can be studied using less-invasive functional-imaging technologies. Histological techniques let us view the cells of the nervous system in varying ways Over the last 150 years or so, technical advances in histology--the study of the composition of body tissues--have made it possible to selectively stain different parts of neurons and glia. Nowadays, scientists use specialized staining procedures to study the numbers, shapes, distribution, and interconnections of neurons within targeted regions of the brain. We can group these techniques based on the types of experiments they enable. REGIONAL CELL COUNTS Using Nissl stains, scientists can visualize all of the cell bodies in a tissue section, making it possible to measure the size and number of cell bodies in particular regions (FIGURE 1.19A). INDIVIDUAL CELL SHAPES Mysteriously, and in contrast to Nissl stains, Golgi stains label only a small minority of neurons in a sample, but the affected cells are stained very completely, revealing fine details of cell structure such as the branches of
histology The study of tissue structure. Nissl stain A tissue stain that outlines all cell bodies because the dyes are attracted to RNA, which encircles the nucleus. Golgi stain A tissue stain that completely fills a small proportion of neurons with a dark, silver-based precipitate.
FIGURE 1.19 Histological Methods for Studying Neurons
A Nissl stains label all cell bodies in a region.
B Golgi stains reveal ne details of individual neurons.
Courtesy of Cynthia L. Jordan, Michigan State University
From N. Sunn et al., 2002. P. Natl. Acad. Sci. U.S.A. 99: 3. © National Academy of Sciences, U.S.A.
C Here, immunohistochemistry has revealed cells expressing c-fos, a gene product produced only by recently activated neurons.
D Tract tracers reveal the axonal projections of neurons.
From J. Yuan et al., 2015. Front. Neuroanat. 9: 70, courtesy of Dr. Qingming Luo
E The Brainbow technique vividly illustrates the interconnections of many neurons simultaneously.
autoradiography A staining technique that shows the distribution of radioactive chemicals in tissues. See Box 8.1.
dendrites and axons. Neurons stained with the Golgi method (and similar techniques, such as filling cells with fluorescent dye) stand out in sharp contrast to their unstained neighbors, so Golgi staining is useful for identifying the types and precise shapes of neWuraotsnosn/inBraeerdelogvioe n. (FIGURE 1.19B) The Mind's Machine Foundations of Brain and Behavior 4e EXPRESSION OF CELLULAR PRODUCTS Often, neuroscientists would like to know thMe dMi4set_r0ib1.u1t9ion07o/f0n9/e2u0rons that exhibit a specific property. In autoradiography, for
example, animals are treated with radioactive versions of experimental drugs, and then thin slices of the brain are placed alongside photographic film. Radioactivity emitted by the labeled compound in the tissue "exposes" the emulsion--like light striking film--so the brain essentially takes a picture of itself, highlighting the specific brain regions where the drug has become selectively concentrated. An alternative way to visualize cells that have an attribute in common--termed immunohistochemistry (IHC) (FIGURE 1.19C)--involves creating antibodies against a protein of interest (we can create antibodies to almost any protein). Equipped with colorful labels, these antibodies can selectively seek out and attach themselves to their target proteins within neurons in a brain slice, revealing the distribution of only those neurons that make the target protein. A related procedure called in situ hybridization goes a step further and, using radioactively labeled lengths of nucleic acid (RNA or DNA, see the Appendix), labels only those neurons in which a gene of interest has been turned on. INTERCONNECTIONS BETWEEN NEURONS Many research questions are more concerned with the pattern of connections between neurons than with their cellular structure (FIGURE 1.19D). To accomplish this goal, scientists have developed many sorts of tract tracers, substances that are taken up by neurons and transported over the routes of their axons. Some tract tracers can even jump across synapses, or work their way backward through the length of the neural pathway, leaving visible molecules of label all along the way. In "Brainbow" experiments, inserted genes cause neurons to express fluorescent proteins in hundreds of different hues (FIGURE 1.19E), powerfully aiding the study of interconnections of neurons (Lichtman et al., 2008; Weissman and Pan, 2015). Brain-imaging techniques reveal the structure and function of the living brain How can we study intact and functioning brains? Unlike older, more invasive techniques, modern brain-imaging technology permits study of the brains of living participants, revealing both structure and patterns of activity in the brain. COMPUTERIZED AXIAL TOMOGRAPHY In computerized axial tomography (CAT or CT scans), X-ray energy is used to generate images by moving an X-ray source in steps around the head. At each point, detectors on the opposite side of the head measure the amount of X-ray radiation that is absorbed; this value is proportional to the density of the tissue the X-rays passed through. When this process is repeated from many angles, the results are mathematically combined into a computer-generated anatomical image of the brain based on density (FIGURE 1.20A). CT scans are medium-resolution images, useful for visualizing problems such as strokes, tumors, or cortical shrinkage. Sam, whom we met at the beginning of the chapter, had developed a meningioma that was pressing on and deforming the motor cortex on the left side of his brain. The tumor impaired the functioning of regions of the motor cortex responsible for voluntary control of the muscles of Sam's right arm and the right side of his face, producing his alarming symptoms. Fortunately, his emergency CT scan pinpointed the meningioma, and following surgical removal of the tumor, Sam experienced immediate improvement; he has been healthy ever since. MAGNETIC RESONANCE IMAGING Using magnetic fields and radio waves instead of X-rays, magnetic resonance imaging (MRI) provides higher-resolution images than CT, with fewer damaging effects. For an MRI image of the brain, the person's head is first placed in an extremely powerful magnet that causes all the protons in the brain to line up in parallel, instead of in their usual random orientations (protons are found in the nuclei of atoms; in body tissues, most protons are found within water molecules). Next, the protons are knocked over by a powerful pulse of radio waves. When this pulse is turned off, the protons relax back to their original configuration, emitting radio waves as they go. Detectors surrounding the head measure those radio waves, which differ for tissues of varying densities. This density-based information is
immunohistochemistry (IHC) A histological technique in which labeled antibodies are used to visualize specific proteins within tissues. in situ hybridization A method for detecting particular RNA transcripts in tissue sections by providing a nucleotide probe that is complementary to, and will therefore hybridize with, the transcript of interest. See Box 8.1; Appendix Figure A.4. tract tracer A substance used to visualize the axonal connections of neurons. computerized axial tomography (CAT or CT scans) A noninvasive technique for examining brain structure through computer analysis of X-ray absorption at several positions around the head. magnetic resonance imaging (MRI) A noninvasive brain-imaging technology that uses magnetism and radiofrequency energy to create images of the gross structure of the living brain.
44CHAPTER1 A Computerized tomography (CT) helps us spot problems such as strokes or tumors, like the one evident here.
B Magnetic resonance imaging (MRI) shows great detail, enabling us to see ne structure and recognize subtle changes in the brain.
C Diffusion tensor imaging (DTI) uses MRI technology to visualize water in axons, resulting in images of the axonal interconnections between brain regions.
© Living Art Enterprises, LLC/Science Source © MriMan/Shutterstock.com
D Functional MRI (fMRI) detects small changes in brain metabolism. Changes in brain activity associated with visual or auditory stimuli are highlighted in this example.
E Positron emission tomography (PET) provides a portrait of the brain's activity. Here, PET shows that metabolic activity is diminished in the brain of a person with Alzheimer's disease.
FIGURE 1.20 Visualizing the Living Human Brain
Watson/Breedlove The Mind's Machine Foundations of Brain and Behavior 4e diffusion tensor imaging (DTI) A mMoMdif4ie_d0f1o.2rm0 of07M/R09I/in20which the diffusion of water in a confined space is exploited to produce images of axonal fiber tracts. functional MRI (fMRI) Magnetic resonance imaging that detects changes in blood flow and therefore identifies regions of the brain that are particularly active during a given task.
then used by a computer to create a detailed cross-sectional view of the brain (FIGURE 1.20B) that scientists use to evaluate the size and shape of distinct brain regions. MRI images can also reveal subtle changes in the brain, such as the local loss of myelin that is characteristic of multiple sclerosis. A variant of MRI, called diffusion tensor imaging (DTI), exploits a signal associated with the diffusion of water within axons in order to visualize axonal fiber tracts within the brain. This kind of research, generally known as tractography, is helping us to learn how networks of brain structures work together in various forms of complex cognition and consciousness. FUNCTIONAL BRAIN IMAGING With its ability to image localized changes in the brain's activity, rather than details of its structure, functional MRI (fMRI) has revolutionized cognitive neuroscience. Offering both reasonable speed (temporal resolution) and sharpness (spatial resolution) at the gross anatomical level, fMRI uses rapidly oscillating magnetic fields to detect regional changes in brain metabolism, particularly patterns of oxygen use and blood flow in the most active regions of the brain. Scientists can use fMRI data to create "difference images" of the specific activity of different parts of the brain while people engage in various experimental tasks. Although fMRI cannot resolve the fine cellular structure of the brain and is too slow to track rapid, moment-by-moment changes in the activity of networks of neurons, fMRI combined with conventional anatomical MRI has revealed many important clues about how networks of brain structures collaborate on complex cognitive processes (FIGURES 1.20C and D). An additional consideration is
that, as in other imaging techniques, fMRI imagery is not photographic: it is created by a computer, based on mathematical models. There is concern among researchers that the computer algorithms used in this process may sometimes be misleading (Eklund et al., 2016; Poldrack et al., 2017). Like fMRI, positron emission tomography (PET) depicts the brain's activity during behavioral tasks. Short-lived radioactive chemicals are injected into the bloodstream, and radiation detectors encircling the head map the destination of these chemicals in the brain. A particularly effective strategy is to inject radioactively labeled glucose ("blood sugar") while the person is engaged in a cognitive task of interest to the researcher. Because the radioactive glucose is selectively taken up and used by the most active parts of the brain, a moment-to-moment color-coded portrait of brain activity can be created (FIGURE 1.20E) (P. E. Roland, 1993; Chiaravalloti et al., 2019). Although PET can't match the detailed resolution of fMRI, it tends to be faster and thus better able to track quick changes in brain activity.
Electromagnetic coil In transcranial magnetic stimulation (TMS), magnetic elds induced by electromagnetic coils stimulate neurons of the underlying cortical surface.
Pulsed magnetic eld Stimulated cortical region
FIGURE 1.21 Transcranial Magnetic Stimulation
MAGNETIC STIMULATION AND MAPPING It is a simple matter to pass magnetic fields
into the brain. However, it is technically more challenging to project magnetic fields in a highly focused and precise manner. In transcranial magnetic stimulation (TMS) (FIGURE 1.21), focal magnetic currents are used to briefly stimulate the cortex of alert people directly, without any lasting physical alterations or surgery. Using TMS allows experimenters to map cortical surfaces by activating discrete areas of the brain while simultaneously tracking any resulting changes in behavior, and it can be powerfully
A brain-imaging technology that tracks the metabolism of injected radioactive substances in the brain, in order to map brain activity. transcranial magnetic stimulation (TMS) A noninvasive technique for examining brain function that applies
combined with functional brain-imaging techniques like PET (Tremblay et al., 2020).
strong magnetic fields to stimulate cortical
Not only can magnets stimulate neurons, but neurons also act as tiny electromagnets
neurons in order to identify discrete areas
themselves! In magnetoencephalography (MEG), a large array of ultrasensitive detectors
of the brain that are particularly active
measures the minuscule magnetic fields produced by the electrical activity of cortical neu- during specific behaviors.
rons. This information is used to construct real-time maps of brain activity during oWnagtsooinn/gBreedlomveagnetoencephalography (MEG)
The Mind's MacAhinneoninvasive brain-imaging technology
cognitive processing (FIGURE 1.22). Because MEG can track quick, moment-by-moment
that creates maps of brain activity during
changes in brain activity, it is excellent for studying the rapidly shifting patterns of brain
cognitive tasks by measuring tiny magnet-
activity in cortical circuits that fMRI is too slow to track (Baillet, 2017; Gross, 2019).MM4e_01.21 0ic7/f0ie9l/d2s0produced by active neurons.
Now let's look a little more closely at a process scientists use to distinguish the
brain activity underlying a specific behavior from the background activity of the busy
Magnetoencephalography (MEG) measures the minuscule magnetic elds given off by ensembles of cortical cells during speci c behavioral functions. Here, MEG maps brain activity associated with viewing faces... ...versus viewing nonface objects.
View Animation 1.4: Visualizing the Living Human Brain
MEG images courtesy of Mario Liotti and Anthony Herdman, Simon Fraser University and Down Syndrome Research Foundation
FIGURE 1.22 Animal Magnetism
Subtractive analysis isolates specific brain activity
Modern brain imaging provides dramatic pictures showing the particular brain regions that are activated during specific cognitive processes; there are many such images in this book. But if you do a PET scan of a healthy person, you find that almost all of the brain is active at any given moment (showing that the old notion that "we use only 10% of our brain" is nonsense). How do researchers obtain these highly specific images of brain activity? In order to associate specific brain regions with particular cognitive operations,
researchers developed a sort of algebraic technique, in which activity during one behavioral condition is subtracted from activity during a different condition. So, for example, the data from a control PET scan made while a person was gazing at a blank wall might be subtracted from the data from a PET scan collected while that person studied a complex visual stimulus. Averaged over enough trials, the specific regions that are almost always active during the processing task become apparent, even though, on casual inspection, a single experimental
FIGURE 1.23 Isolating Specific Brain Activity
Hypothesis Brain regions engaged in a specific behavior can be isolated by algebraic means, subtracting resting scans from scans during activity.
scan might not look much different from a single control scan (FIGURE 1.23). It is important to keep in mind that although functional brain images seem unambiguous and easy to label, they are computer-generated composites--not actual brain images--and thus only as accurate as the assumptions and algorithms with which they are created (Racine et al., 2005; Poldrack et al., 2017).
Test Participants are scanned twice--once while looking at a blank screen, and once while looking at test stimuli. The control scan is then subtracted from the test scan.
Result By repeating the process and averaging the results across multiple participants, a stable "difference image" showing activation of just a few brain regions is formed. Averaging such difference images from several people increases confidence that this brain region is indeed involved in that activity.
Difference images from several participants are added together and averaged...
...to arrive at a "mean difference image" that shows the most active brain areas across participants in an experiment.
Conclusion The activated brain regions in the difference image--in this case, in the occipital cortex--are selectively involved in the particular cognitive processing required by the stimulus.
1. Compare and contrast the main methods for producing still images of the structure of the brain. What do you think are some of the advantages and disadvantages of each method? 2. Compare and contrast the main functional-imaging technologies used for visualizing the activity of brain regions. What do you think are some of the advantages and disadvantages of each method? 3. Describe the process that neuroscientists can use to isolate brain activity associated with a specific behavior, as visualized by functional-imaging techniques.
1.6Careful Research Design Is Essential for Progress in Behavioral Neuroscience
In the final section of the chapter, we turn our attention to factors that neuroscientists consider in designing research: both the formal layout of experiments and the theoretical considerations on which research questions are based. Studying this section should prepare you to: 1.6.1 Describe and distinguish between correlational studies and experimental studies--in which either the body is altered and behavior is measured, or behavior is manipulated and bodily changes are measured--and explain how scientists rely on all three types of studies to develop research programs. 1.6.2 Discuss the major theoretical perspectives that inform research in behavioral neuroscience. 1.6.3 Discuss important issues that modern behavioral neuroscience must contend with, such as the use of animals in research, and the replication crisis in behavioral research. 1.6.4 Explain the different levels of analysis that may be focused on by behavioral neuroscientists, and describe how they may relate to one another.
The complexity of human behavior, and the organ by which it is produced, necessitate the use of indirect means to manipulate behavior and the activity of the brain. This complexity also contributes to an emerging crisis in behavioral neuroscience: difficulty in replicating numerous influential earlier findings (De Boeck and Jeon, 2018). These complications underscore the importance of careful research design based on detailed observation, precise control of experimental variables, and selection of appropriate research participants, and they are the impetus driving rapid evolution of research methodology across the behavioral sciences. Three types of study designs probe brain-behavior relationships Behavioral neuroscientists use three general types of studies for research. In an experiment employing somatic intervention (FIGURE 1.24A), we alter a structure or function of the brain or body to see how this alteration changes behavior. In this sort of experiment, the physical alteration is an independent variable (a general term used to describe the manipulated aspect of any experiment), and the behavioral effect is the dependent variable (a general term used to describe the measured consequence of an experimental manipulation). Some examples of somatic intervention experiments include (1) administering a hormone to some animals, but not others, and comparing their sexual behavior; (2) electrically stimulating a specific brain region and measuring alterations in movement; and (3) destroying a specific region in the brain and observing subsequent changes in sleep patterns. In each case, the behavioral measurements
somatic intervention An approach to finding relations between body variables and behavioral variables that involves manipulating body structure or function and looking for resultant changes in behavior. See Figure 1.24. Compare behavioral intervention. independent variable The factor that is manipulated by an experimenter. dependent variable The factor that an experimenter measures to monitor a change in response to manipulations of an independent variable.
(A) Somatic intervention: Manipulating the body may affect behavior.
(B) Behavioral intervention: Experience affects the body (including the brain).
(C) Correlation: Body and behavioral measures covary.
Somatic interventions Example: Administer a hormone Example: Stimulate brain region electrically Example: Cut connections between parts of nervous system
Behaviors affected Strength of mating behavior Movement toward goal object Recognition of stimulus
Somatic effects Changes in hormone levels Changes in electrical activity of brain Anatomical changes in nerve cells
Behavioral interventions Example: Put male in presence of female Example: Present a visual stimulus Example: Give training
Hormone levels Correlations Strength of mating behavior
(D) Behavioral neuroscience seeks to understand all these relationships. Somatic variables
FIGURE 1.24 Three Main Approaches to Studying the Neuroscience of Behavior
Somatic intervention Correlations Behavioral variables Behavioral intervention
control group In research, a group of individuals that are identical to those in an experimental (or test) group in every way except that they do not receive the experimental treatment or manipulation. The experimental group is then compared with the control group to assess the effect of the treatment. Wwiatthsoinn-/pBarreteidcliopvaents experiment TAhneeMxpinedri'ms Menatchininwehich the same set Foundations of Brain and Behavior 4e of individuals is compared before and MaftMer4ae_n0e1x.2p4erim08e/n1t7a/l 2m0anipulation. The experimental group thus serves as its own control group. between-participants experiment An experiment in which an experimental group of individuals is compared with a control group of individuals that have been treated identically in every way except that they haven't received the experimental manipulation. behavioral intervention An approach to finding relations between body variables and behavioral variables that involves intervening in the behavior of an organism and looking for resultant changes in body structure or function.
follow the bodily intervention; furthermore, in each case the behavioral measurements are compared with those of a control group. In a within-participants experiment, the control group is simply the same individuals, tested before the somatic intervention occurs. In a between-participants experiment, the experimental group of individuals is compared with a different group of individuals who are treated identically in every way except that they don't receive the somatic intervention. The approach opposite to somatic intervention is behavioral intervention (FIGURE 1.24B). In this approach the scientist alters or controls the behavior of an organism and looks for resulting changes in body structure or function. Here, behavior is the independent variable, and change in the body is the dependent variable. A few examples include (1) allowing adults of each sex to interact and then measuring their hormone levels, (2) having a person perform a cognitive task while in a brain scanner and then measuring changes in activity in specific regions of the brain, and (3) training an animal to fear a previously neutral stimulus and then observing electrical changes in the brain that may encode the newly learned association. As with somatic intervention, these experimental approaches may employ either within-group or between-groups designs. The third type of study is correlation (FIGURE 1.24C), which measures how closely changes in one variable are associated with changes in another variable. Two examples of correlational studies include (1) observing the extent to which memory ability is associated with the size of a certain brain structure, and (2) noting that increases in a certain hormone are accompanied by increases in aggressive behavior. Note that while this type of study tells us if the measured variables are associated in some way, it can't tell us which causes the other. We can't tell, for example, whether the hormones cause the aggression or aggression increases the hormones. But even though it can't establish causality, correlational research can help researchers identify which things are linked, directly or indirectly, and thus it helps us to develop hypotheses that can be tested experimentally using behavioral and somatic interventions.
Combining these three approaches yields the circle diagram of FIGURE 1.24D, showing how the three types of studies complement each other. It also underscores that the effects of brain and behavior are reciprocal: each affects the other in an ongoing cycle. Animal research is an essential part of life sciences research, including behavioral neuroscience Human beings' involvement and concern with other species predates recorded history; early humans had to study animal behavior and physiology in order to escape some species and hunt others. To study the biological bases of behavior inevitably requires research on animals of other species, as well as on human beings. Psychology students usually underestimate the contributions of animal research to psychology because the most widely used introductory psychology textbooks often present major findings from animal research as if they were obtained with human participants (Domjan and Purdy, 1995). A vocal minority of people believe that research with animals, even if it does lead to lasting benefits, is unethical. Others argue that animal research is acceptable only when it produces immediate and measurable benefits. The potential cost in taking this perspective lies in the fact that we have no way of predicting which experiments will lead to a breakthrough. The whole point of studying the unknown is that it is unknown; there is a long history of chance observation, based on the steady accumulation of basic knowledge, leading to unexpected benefits. There's no denying that animal research can cause stress and discomfort, and researchers have a strong ethical obligation to hold pain and stress to the absolute minimum levels possible. Animal research has itself provided us with the drugs and techniques that make most research painless for lab animals, while also leading to improved veterinary care for our animal companions (Sunstein and Nussbaum, 2004), and researchers are ethically bound to continually refine lab practices, with animal well-being a primary concern. Researchers are also bound by animal protection legislation and are subject to continual administrative oversight to ensure adherence to nationally mandated animal care policies that emphasize the use of as few animals as possible without jeopardizing research integrity, as well as the use of the simplest species that can answer the questions under study. As human beings with the full range of emotions and empathetic feelings toward animals, we all wish there were an alternative to the use of animals in research. But if we want to understand how the nervous system works, we have to actually study it, in detail. The life sciences would slow to a crawl without the basic knowledge that we derive from studying animals. How do similarities and differences among people and animals fit into behavioral neuroscience? Each person is in some ways like all other people, in some ways like some other people, and in some ways like no other person. As shown in FIGURE 1.25, we can extend this observation to the much broader range of animal life. The electrical messages used by nerve cells (see Chapter 2) are essentially the same in a jellyfish, a cockroach, and a human being, and many species employ identical hormones. These characteristics are said to be conserved, meaning that they first arose in a shared ancestor. But mere similarity of a feature between species does not guarantee that the feature came from a common ancestral species. Our eyes resemble those of octopuses, but certain key differences reveal that their eyes and our eyes evolved separately. With respect to each biological property, researchers must determine how animals are identical and how they are different. When we seek animal models for studying human behavior or biological processes, we must ask the following question: Does the proposed animal model really have some things in common with the process at work in humans? In later chapters we will see many cases in which it does, but even within the same species, individuals differ from one another: cat from cat, blue jay from blue jay, and person from person. Behavioral neuroscience seeks to understand individual differences as well as similarities.
correlation The tendency of two measures to vary in concert, such that a change in one measure is matched by a change in the other. causality The relation of cause and effect, such that we can conclude that an experimental manipulation has specifically caused an observed result. conserved In the context of evolution, referring to a trait that is passed on from a common ancestor to two or more descendant species. Animal Research is Crucial In studying something as complicated as the mammalian brain, there is often no option but to study the brains of lab animals, the vast majority of which are rats and mice. Much of the research you will read about in this book has relied on studies of animals. Very high standards of care are provided to research animals, as a result of extensive legislative requirements, the need to protect the integrity of research, and the researchers' ethical and empathetic concern for their research subjects.
Each person has some characteristics shared by...
All animals use DNA to store genetic information.
All vertebrates have a backbone and spinal cord.
All primates have a relatively large, complex brain, and many primates have a hand with an opposable thumb.
All humans use symbolic language to communicate with each other.
Behavioral neuroscientists use several levels of analysis
A final consideration that researchers must weigh in designing ex-
Some people like to eat beets (no one knows why).
periments is the level of complexity at which to work. Even the most complex behavior could, in theory, be understood at the level of cellular activity or even lower, at the level of biochemistry and molecular interactions. This idea, that we can understand complex systems by
No two people, even identical twins, are alike in each and every way, as individual experiences leave their unique stamp
dissecting their simpler constituent parts, is known as reductionism. But we wouldn't get very far if we set out to explain, say, the use of grammar in terms of chemical reactions; the behavior is so complex that an explanation at the molecular level would involve a vast amount of data. So instead, the reductionist approach aims to identify levels of analysis that are just simple enough that they allow us to
make rapid progress on the more complex phenomena under study.
Finding explanations for behavior often requires several levels of
FIGURE 1.25 We Are All Alike, and We Are All Different
analysis, ranging from social interactions, to brain systems, to circuits and single nerve cells and their even simpler, molecular constituents
Naturally, different problems are carried to different levels of analysis, and fruit-
ful work is often being done simultaneously by different workers at several levels. For
example, in their research on visual perception, some cognitive psychologists care-
reductionism ThFeousncdiaetniotinfiscofsBtrraatiengayndofBehavior 4e
fully analyze behavior. They try to determine how the eyes move while looking at a
smaller parts in ordMerMto4eu_n0d1.e2r5stan0d7/i0t.9/20
visual pattern, or how the contrast among parts of the pattern determines its visibility.
Meanwhile, other behavioral neuroscientists study the differences in visual abilities among species and try to determine the adaptive significance of these differences. For example, how is the presence (or absence) of color vision related to the lifestyle of a species? At the same time, other investigators trace out brain structures and networks involved in different visual tasks. Still other scientists try to understand the electrical and chemical events that occur in the brain during vision. Some people doubt whether our "merely human" brains will ever be able to understand something as complicated as the human brain, to a level where we can fully explain mysterious properties like consciousness, identity, or the perception of free will. Nevertheless, the gains we are making in understanding how the brain works--the subject of this book--bring us closer to that goal every day.
level of analysis The scope of an experimental approach. A scientist may try to understand behavior by monitoring molecules, nerve cells, brain regions, or social environments or using some combination of these levels of analysis.
1. What are the three general forms of research studies in behavioral neuroscience? What is the issue of "causality"? How do the three research perspectives inform and shape one another? 2. Define independent variable, dependent variable, control group, within-participants experiment, and between-participants experiment. 3. Consider both sides of the debate over animal research, weighing the pros and cons of the "for" and "against" positions. How do you think animal use should be regulated? 4. What is the general principle behind reductionism? How does this influence the level of analysis at which a researcher works? For that matter, what is meant by "level of analysis"?
Social level: Individuals behaving in social interaction
Organ level: Brain, spinal cord, peripheral nerves, and eyes
Neural systems level: Eyes and visual brain regions Brain region level: Visual cortex
FIGURE 1.26 Levels of Analysis in Behavioral Neuroscience
Recommended Reading Bausell, R. B. (2015). The Design and Conduct of Meaningful Experiments Involving Human Participants: 25 Scientific Principles. New York, NY: Oxford University Press. Blumenfeld, H. (2021). Neuroanatomy through Clinical Cases (3rd ed.). Sunderland, MA: Oxford University Press/Sinauer. Huettel, S. A., Song, A. W., and McCarthy, G. (2014). Functional Magnetic Resonance Imaging (3rd ed.). Sunderland, MA: Oxford University Press/Sinauer. Papanicolau, A. C. (Ed.). (2017). The Oxford Handbook of Functional Brain Imaging in Neuropsychology and Cognitive Neurosciences. Oxford, UK: Oxford University Press. Schoonover, C. (2010). Portraits of the Mind: Visualizing the Brain from Antiquity to the 21st Century. New York, NY: Abrams. Swanson, L. W., Newman, E., Araque, A., and Dubinsky, J. M. (2017). The Beautiful Brain: The Drawings of Santiago Ramón y Cajal. New York, NY: Abrams. Vanderah, T., and Gould, D. J. (2020). Nolte's The Human Brain: An Introduction to Its Functional Anatomy (8th ed.). New York, NY: Elsevier.
You should be able to relate each summary to the adjacent illustration, including structures and processes. The online version of this Visual Summary includes links to figures, animations, and activities that will help you consolidate the material.
1 Neurons (nerve cells) are the basic units of the nervous system. The typical neuron has four main parts: (1) dendrites receive information; (2) the cell body (soma) integrates the information; (3) an axon carries impulses from the neuron; and (4) axon terminals transmit the neuron's signals to other cells. Neurons almost universally feature an input zone, an integration zone, a conduction zone, and an output zone. Review Figure 1.1, Animation 1.2
3 Neurons make functional contacts with other cells at specialized junctions called synapses. By changing their shape or function in response to experiences, synapses exhibit neuroplasticity. At most synapses a chemical neurotransmitter released from the presynaptic membrane diffuses across the synaptic cleft and binds to special neurotransmitter receptor molecules in the postsynaptic membrane. Review Figure 1.4
Input zone Integration zone Conduction zone Output zone
2 Neurons take numerous forms and may be multipolar, bipolar, or unipolar. The exact shape of a neuron is determined by the type of information it gathers, the processing it performs, and the destination to which it transmits the processed information. Thus each neuron gathers, analyzes, and transmits information. Review Figure 1.3, Activity 1.1 4 Glial cells provide support to neurons, form myelin, maintain the brain, and contribute to neural activity. Review Figure 1.5
5 To the naked eye, the nervous system of vertebrates is divided into the central nervous system (CNS--the brain and spinal cord) and the peripheral nervous system. The peripheral nervous system, in turn, consists of two parts: the somatic nervous system and the autonomic nervous system. Twelve pairs of cranial nerves, which make up one part of the somatic nervous system, arise from the brain to directly take in information or send out commands to the body, mostly the head and neck. Review Figures 1.6 and 1.7, Activity 1.2
6 A second major part of the somatic nervous system consists of the 31 pairs of spinal nerves, spaced through the cervical, thoracic, lumbar, sacral, and coccygeal segments of the spinal cord. Review Figure 1.8, Activity 1.3
7 The autonomic nervous system consists of the sympathetic nervous system (which tends to ready the body for immediate action) and the parasympathetic nervous system (which tends to have an effect opposite to that of the sympathetic system). We cannot consciously control autonomic activity. Review Figure 1.9, Activity 1.4
8 The human brain is dominated by the cerebral hemispheres, which include the frontal, parietal, temporal, and occipital lobes. The outermost parts of the cerebral hemispheres are known as cerebral cortex, or simply cortex. Review Figure 1.10 and 1.11, Activity 1.5
9 The major divisions of the brain are established during fetal development. These include the forebrain (cortex and embedded structures); the thalamus and hypothalamus, together called the diencephalon; the brainstem components--midbrain, pons, and medulla; and the cerebellum atop the pons. Review Figure 1.12, Video 1.3, Activity 1.6
11 Important subcortical systems include the basal ganglia, which regulate movement; the limbic system, which controls emotional behaviors; and the cerebellum, which aids motor control. Review Figures 1.14 and 1.15, Activities 1.7-1.10 13 The brain requires a constant supply of blood, via the cerebral arteries, to fuel the activity of neurons. An interruption in blood flow, caused by a blockage or hemorrhage, is called a stroke. The effect of a stroke is determined by its size and location in the brain. Review Figure 1.18
15 Behavioral neuroscientists balance three general research perspectives-- correlation, somatic intervention, and behavioral intervention--in designing their research. Review Figure 1.24
10 The cerebral cortex is an extensive sheet of folded tissue. The six-layered cerebral cortex is responsible for higher-order functions such as vision, language, and memory. Review Figure 1.13 12 The brain and spinal cord, surrounded and protected by the three meninges-- dura mater, pia mater, and arachnoid-- float in cerebrospinal fluid (CSF), which is produced in the lateral ventricles and exits the ventricles to surround the brain. Glymphatic drainage, which occurs mostly during sleep, helps clear harmful wastes from the brain. Review Figures 1.16 and 1.17, Activity 1.11 14 Studies of the structure and function of the brain rely on histological techniques, to reveal cellular features, and brain-imaging technology to visualize the living brain. Structural-imaging technologies like computerized axial tomography (CT or CAT) and magnetic resonance imaging (MRI) provide high-resolution images of the structure of the brain. Functional-imaging technologies, such as positron emission tomography (PET), functional MRI (fMRI), and magnetoencephalography (MEG), provide maps of brain activity during behavior. Review Figures 1.19-1.23, Animation 1.4 16 Research in behavioral neuroscience is conducted at levels of analysis ranging from molecular events to the functioning of the entire brain and complex social situations. In many cases, details of neural function can only be revealed through studies of nonhuman animals. Review Figures 1.25 and 1.26
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