Ch 3Resting Membrane Potential
Ion concentrations (typical mammalian neuron):
- Inside: K⁺ ~140 mM, Na⁺ ~10 mM, Cl⁻ ~10 mM, Ca²⁺ ~100 nM
- Outside: K⁺ ~5 mM, Na⁺ ~145 mM, Cl⁻ ~110 mM, Ca²⁺ ~1.2 mM
The Na⁺/K⁺-ATPase maintains these gradients — pumps 3 Na⁺ out + 2 K⁺ in per ATP. Net export of one positive charge per cycle = electrogenic, contributes ~−5 to −10 mV directly. Without this pump, gradients would equilibrate within minutes.
The Nernst equation calculates equilibrium potential for ONE ion based on concentration gradient:
E_ion = (RT/zF) ln([ion]_outside / [ion]_inside)
At 37 °C: E_K ≈ −85 mV, E_Na ≈ +60 mV, E_Cl ≈ −65 mV, E_Ca ≈ +120 mV.
The Goldman-Hodgkin-Katz (GHK) equation extends Nernst to multiple ions weighted by permeability:
V_m = (RT/F) ln[(P_K[K]_o + P_Na[Na]_o + P_Cl[Cl]_i) / (P_K[K]_i + P_Na[Na]_i + P_Cl[Cl]_o)]
Why is V_rest close to E_K? At rest, the membrane has many open K⁺ leak channels + few open Na⁺ channels. K⁺ permeability dominates, so V_m drifts toward E_K. Real V_rest ~−65 mV is slightly more positive than E_K because of small but non-zero Na⁺ leak.
Ch 4The Action Potential
- Threshold (~−55 mV): Na_v opening exceeds K leak → positive feedback → commitment.
- Rising phase: rapid Na⁺ influx → V_m rises toward E_Na (+60 mV). Net inward current.
- Overshoot: peaks ~+30 to +40 mV (doesn't quite reach E_Na due to delayed K⁺ activation).
- Falling phase: Na_v inactivates (ball-and-chain blocks pore) + delayed-rectifier K⁺ channels open → K⁺ efflux → repolarization.
- Afterhyperpolarization (AHP): K⁺ channels still open beyond rest → V_m undershoots E_K briefly.
- Recovery: K⁺ channels close, Na_v de-inactivates (returns to closed-resting state). Ready for next AP.
Voltage-gated Na⁺ channel — three states:
- Closed (resting): voltage sensor in resting position, gate closed.
- Open (activated): depolarization moves voltage sensor → gate opens → Na⁺ flows.
- Inactivated: ball-and-chain (intracellular loop III-IV) swings up + plugs the channel from inside. Cannot reopen until V_m returns to rest.
Inactivation explains the absolute refractory period (~1 ms): no AP possible regardless of stimulus while Na_v is inactivated. The relative refractory period (~2-4 ms) follows: some Na_v de-inactivated but K⁺ channels still open + AHP — stronger stimulus needed.
Voltage-gated K⁺ channel (delayed rectifier): activates more slowly than Na_v, NO fast inactivation, repolarizes. Tetraethylammonium (TEA) blocks K⁺ channels.
Saltatory conduction in myelinated axons: AP "jumps" between unmyelinated nodes of Ranvier. Myelin (oligodendrocytes in CNS, Schwann cells in PNS) increases membrane resistance + reduces capacitance → AP regenerates only at nodes where Na_v is densely clustered. ~10-50× faster than continuous conduction in unmyelinated axons of same diameter. Demyelination (MS, Guillain-Barré) → conduction failure.
Pharmacology: Tetrodotoxin (TTX) from pufferfish blocks Na_v from outside → no AP. Saxitoxin (red tide dinoflagellates) similar. Local anesthetics (lidocaine, novocaine) block Na_v from inside (use-dependent block of firing axons).
Ch 5Synaptic Transmission
Electrical synapses: gap junctions (connexons) between cells. Bidirectional, fast, no synaptic delay. Coupling for synchronized firing in some networks (inferior olive).
- AP arrives at axon terminal, depolarizes membrane.
- Voltage-gated Ca²⁺ channels (Cav2.1, Cav2.2) open.
- Ca²⁺ enters terminal (rises from ~100 nM to ~50-100 μM in microdomains).
- Ca²⁺ binds C2 domains of synaptotagmin (vesicle Ca²⁺ sensor).
- Triggers conformational change in already-assembled SNARE complex: v-SNARE synaptobrevin (VAMP) + t-SNAREs syntaxin + SNAP-25 in 4-helix bundle.
- Membranes pulled into apposition; fusion pore opens.
- NT released into 20-50 nm cleft.
- NT diffuses to postsynaptic receptors.
- Receptors gate ions (ionotropic, ms timescale) or activate G proteins (metabotropic, seconds).
- Termination: reuptake by transporters (DAT, SERT, NET, GAT, EAAT), enzymatic breakdown (AChE for ACh), or diffusion.
- Vesicles recycled via clathrin endocytosis. NSF + α-SNAP disassemble SNAREs (ATP-dependent) for reuse.
Quantal release (Katz, Miledi 1965): NT is released in fixed packets ("quanta") corresponding to single vesicles. EPSP amplitude histogram is multi-peaked at integer multiples of unitary EPSP.
EPSP (excitatory postsynaptic potential): depolarizing — typically glutamate at AMPA/NMDA receptors → Na⁺ + K⁺ influx (and Ca²⁺ for NMDA). IPSP (inhibitory postsynaptic potential): hyperpolarizing — typically GABA at GABA_A → Cl⁻ influx, or activation of GIRK K⁺ channels via GABA_B/Gβγ.
Spatial summation: multiple synapses simultaneously sum at soma. Temporal summation: rapid repeated input from one synapse sums before previous EPSP decays. Both push V_m toward AP threshold at the axon hillock.
NMDA receptor — coincidence detector:
- At rest, Mg²⁺ blocks the NMDA pore.
- Glutamate alone → no current (Mg²⁺ blocks).
- Depolarization (e.g., from AMPA + temporal summation) → expels Mg²⁺.
- Now glutamate + open NMDA → Ca²⁺ + Na⁺ influx.
- "Coincidence" of pre + post activity = Hebbian "fire together, wire together."
- Glycine (or D-serine) is a co-agonist required at the NMDA glycine site.
LTP at hippocampal CA1 (Bliss + Lomo 1973): tetanic stimulation produces sustained synaptic strengthening lasting hours-days. NMDA-dependent: tetanus → AMPA depolarizes postsynaptic cell → Mg²⁺ unblock → NMDA Ca²⁺ influx → CaMKII activates → AMPA receptor insertion + phosphorylation → larger EPSP for same glutamate. Cellular substrate of declarative memory.
LTD: opposite — modest Ca²⁺ rise → phosphatases (PP1, PP2A) → AMPA internalization. Required for forgetting + circuit refinement.
Ch 13Glia + Neuroinflammation — CHIVERO FOCUS
Four glial cell types in CNS:
- Astrocytes: GFAP-positive, neuroectoderm-derived, star-shaped. Functions: K⁺ buffering (Kir4.1 channels), glutamate uptake (EAAT1/GLAST + EAAT2/GLT-1), tripartite synapse (release gliotransmitters: glutamate, ATP, D-serine), BBB end-feet (regulate blood flow + barrier integrity), lactate shuttle to neurons (via MCTs), glutamate-glutamine cycle (recycle Glu via glutamine synthetase).
- Oligodendrocytes: CNS myelinator. One oligodendrocyte myelinates ~30 axon segments. (Schwann cells in PNS — one cell, one segment.) Damaged in MS.
- Microglia: yolk-sac-derived (NOT bone marrow), distinct from infiltrating monocytes. Iba1+. Resident immune cells of CNS. Phagocytose debris, dying cells, complement-tagged synapses (developmental + Alzheimer's pruning).
- Ependymal cells: ciliated cells lining ventricles + central canal. Choroid plexus cells produce CSF (~500 mL/day in adults).
Blood-brain barrier (BBB): tight junctions (claudin-5, occludin, ZO-1) between brain capillary endothelial cells + astrocyte end-feet + pericytes. Excludes most polar/large molecules. Lipid-soluble drugs cross easily; selective transport for glucose (GLUT1), amino acids (LAT1), iron (transferrin receptor).
Microglial activation states
Spectrum, not strict binary:
- "M1" pro-inflammatory: TNFα, IL-1β, IL-6, IL-12, IL-18, ROS, NO. Triggered by LPS, IFNγ, DAMPs (HIV-Tat, Aβ, α-synuclein).
- "M2" anti-inflammatory / restorative: IL-10, TGF-β, arginase-1. Triggered by IL-4, IL-13. Promotes tissue repair + myelin debris clearance.
NLRP3 inflammasome — CHIVERO research target
Signal 1 — Priming:
- Microglia detects DAMP/PAMP (HIV-Tat, LPS, ATP via P2X7, Aβ, α-synuclein).
- TLR4/MD2 (or IL-1R, TNFR) activates MyD88/IRAK/TRAF6 cascade.
- NF-κB translocates to nucleus.
- Transcribes pro-IL-1β, pro-IL-18, NLRP3 itself, gasdermin D.
- Cell now "primed" — has all components but NLRP3 not yet assembled.
Signal 2 — Activation:
- Second hit: K⁺ efflux (P2X7 opens), ROS, lysosomal damage (uric acid crystals), ATP release.
- NLRP3 oligomerizes via NACHT domain.
- NLRP3's PYD domain recruits ASC adapter via PYD-PYD interaction.
- ASC oligomerizes; CARD domain recruits procaspase-1 via CARD-CARD.
- Procaspase-1 self-cleaves → active caspase-1.
- Caspase-1 cleaves: pro-IL-1β → mature IL-1β; pro-IL-18 → mature IL-18; gasdermin D → GSDMD-N.
- GSDMD-N inserts into plasma membrane → forms ~10-nm pores.
- Pores release cytokines + cause membrane rupture → pyroptosis (lytic inflammatory cell death).
- Released IL-1β + IL-18 amplify neuroinflammation in surrounding tissue.
HIV-Tat in CNS — Chivero focus
HIV-1 produces Tat (Trans-activator of transcription) — a small RNA-binding viral protein essential for viral gene expression. Even on suppressive ART, latent reservoirs continue to produce some Tat. Tat:
- Crosses the BBB (cell-penetrating peptide property).
- Activates microglia via TLR4 → primes NLRP3 (Signal 1).
- Interacts with NMDA receptors → Ca²⁺ overload + excitotoxicity in neurons.
- Synergizes with cocaine + methamphetamine: meth induces oxidative stress + K⁺ efflux → provides Signal 2 → fully activates NLRP3 inflammasome in primed microglia.
- Drives HAND (HIV-Associated Neurocognitive Disorders) — affects ~30-50% of HIV patients despite ART.
Methamphetamine + microglia (Chivero focus)
Meth crosses BBB (lipophilic), enters dopaminergic neurons via DAT (reverses to release DA), and:
- Induces oxidative stress (DA auto-oxidation → quinones + ROS).
- Depletes ATP → K⁺ efflux + lysosomal damage.
- Activates NLRP3 inflammasome in microglia.
- Triggers IL-1β release → recruits more microglia + amplifies inflammation.
- Synergizes with HIV-Tat in dual-exposed patients (common in HIV+ stimulant-using populations).
Astrocyte tripartite synapse
Astrocyte processes ensheath synapses on both sides. Functions:
- Glutamate uptake via EAAT1/GLAST + EAAT2/GLT-1 (~90% of synaptic glutamate). Failure → excitotoxicity + neuronal death (ALS, ischemia).
- Glutamate-glutamine cycle: astrocyte glutamine synthetase converts Glu → Gln → exported → neurons re-convert via PAG (phosphate-activated glutaminase).
- K⁺ buffering: Kir4.1 channels take up extracellular K⁺ that rises during AP firing.
- Gliotransmission: regulated release of glutamate, ATP, D-serine via vesicular or non-vesicular mechanisms.
Reactive astrocytosis
After CNS injury or chronic disease, astrocytes upregulate GFAP, hypertrophy, and form a glial scar. Two phenotypes (Liddelow + Barres 2017):
- A1 (neurotoxic): induced by activated microglia (IL-1α + TNF + C1q). Loses synaptic + phagocytic functions; can KILL neurons via secreted factor. Found in AD, PD, ALS, MS.
- A2 (neuroprotective): induced by ischemia. Releases trophic factors + thrombospondin to support recovery.
Microglial synaptic pruning
Microglia phagocytose synapses tagged with complement (C1q, C3). Critical in development (refining circuits — visual cortex eye-specific layers). Aberrant in:
- Alzheimer's: complement-mediated synapse loss correlates with cognitive decline. Anti-C1q therapeutic strategies in trials.
- Schizophrenia: excessive pruning during adolescence (C4 risk variants).
Ch 14Pharmacology + Drugs of Abuse
Mesolimbic DA reward pathway: VTA → nucleus accumbens (NAc) + PFC + amygdala. DA release in NAc signals "this is rewarding, repeat it." Activated by food, sex, social bonding, AND every addictive drug.
Drug mechanisms (key ones for NEUR 1520):
- Cocaine: BLOCKS DAT, NET, SERT (monoamine reuptake inhibitor) → ↑ extracellular DA/NE/5-HT during normal firing. Strong reinforcer via NAc DA.
- Amphetamine + methamphetamine: REVERSE DAT (efflux) + enter vesicles via VMAT2 → displace DA into cytosol → DA exported via reversed DAT. Massive elevation. Meth is more lipophilic + neurotoxic. Chivero focus.
- Opioids: μ-receptor agonists (heroin, morphine, fentanyl, oxycodone). Inhibitory GPCRs → presynaptic Ca²⁺↓ + postsynaptic K⁺↑ → reduced firing. Analgesia, euphoria, respiratory depression. Cause overdose deaths via respiratory depression at brainstem.
- Alcohol: enhances GABA_A (positive allosteric) + inhibits NMDA. Sedation. Chronic withdrawal → glutamate hyperactivity → seizure risk (delirium tremens).
- Nicotine: nicotinic ACh receptor agonist (α4β2 in VTA). Activates VTA DA neurons → NAc reward. Highly addictive due to fast pharmacokinetics.
- Cannabis (THC): CB1 receptor agonist; presynaptic; reduces NT release. Affects memory (hippocampus CB1), motor (basal ganglia), reward.
- Hallucinogens (LSD, psilocybin): 5-HT2A agonists. Distort perception via modulation of cortical layer V pyramidal cells.
- Benzodiazepines: GABA_A positive allosteric modulators. Bind allosteric site → increase Cl⁻ flux when GABA bound → enhanced inhibition. Anxiolytic, sedative, anticonvulsant.
- Barbiturates: also GABA_A PAMs but at higher dose can directly open Cl⁻ channel (no GABA needed) → narrower therapeutic index → easier overdose.
Tolerance: reduced response after repeated exposure. Pharmacodynamic (receptor downregulation) + pharmacokinetic (CYP450 induction). Sensitization: increased response with repeated psychomotor stimulant exposure (locomotor activity in rodents).
Addiction circuits: VTA → NAc + amygdala + PFC. Loss of top-down PFC control + amygdala dysregulation + sensitized DA response = addiction. Modern view emphasizes anti-reward systems (CRF, dynorphin) in withdrawal.
Methods in neuroscience
- EEG: scalp electrodes record summed dendritic potentials. Excellent temporal (ms), poor spatial. Frequency bands: δ (deep sleep), θ (drowsy, REM), α (relaxed eyes-closed), β (active), γ (cognition).
- fMRI: BOLD (blood-oxygenation-level dependent) signal as proxy for neural activity. Good spatial (~2 mm), poor temporal (~seconds). Indirect.
- PET: radioactive tracer (e.g., [¹⁸F]FDG for glucose metabolism, [¹¹C]raclopride for D2 occupancy). Spatial similar to fMRI but radiation dose limits use.
- Patch clamp: glass pipette suction onto cell membrane. Single-channel mode (gigaohm seal) records individual channel openings. Whole-cell mode measures aggregate currents. Sakmann + Neher 1991 Nobel.
- Optogenetics: express channelrhodopsin (ChR2 — blue-light-gated cation channel → excitation) or halorhodopsin (NpHR — yellow-light-gated Cl⁻ pump → inhibition). Genetically targeted to specific neuron types. ms timescale control. Boyden + Deisseroth 2005+.
- Chemogenetics (DREADDs): designer Gq (hM3Dq, excite) or Gi (hM4Di, inhibit) GPCRs activated by clozapine-N-oxide. Minute timescale.
- Calcium imaging: GCaMP fluorescent Ca²⁺ indicator + two-photon or confocal microscopy. Reports activity of many neurons over time but slower than electrophysiology (ms-seconds).
- Single-cell RNA-seq: reveals neuronal cell-type diversity (>100 cortical types in mouse Allen atlas). Drives cell-type-specific tools.