Cellular & Synaptic Basis of Neural Signaling

Introduction: Fundamentals of Neuronal Excitability

• Neurons encode and transmit information via electrical signals generated by ion flow across the plasma membrane.
  • Ion flow occurs only through membrane-spanning proteins ➜ ion channels.
• Two broad ion-channel classes
  • Nongated (leak) channels
  • Permanently open at rest.
  • Establish and maintain the resting membrane potential (RMP).
  • Gated channels
  • Transition between closed ↔ open states when triggered by
    • Changes in membrane voltage (voltage-gated)
    • Binding of extracellular ligands (ligand-gated)
    • Binding of intracellular messengers (second-messenger-gated)

Action Potentials & Signal Transmission (Overview)

• Voltage-gated Na⁺ channels (Nav)
  • Open within microseconds of membrane depolarization.
  • Produce an all-or-none regenerative spike (action potential, AP).
  • Permit long-distance propagation with little decrement.
• Saltatory conduction ("jumping" propagation)
  • Occurs in myelinated axons.
  • APs regenerate only at Nodes of Ranvier where Nav density is highest.
  • Boosts conduction velocity & metabolic efficiency.
• Voltage-gated Ca²⁺ channels (Cav)
  • Depolarization at presynaptic terminals ➜ Cav open ➜ Ca²⁺ influx.
  • Intracellular Ca²⁺ rise triggers synaptic vesicle fusion and neurotransmitter (NT) release.

Neurotransmitter Signaling: Receptor Categories

• Ligand-gated ion channels (LGICs / ionotropic)
  • Binding of NT directly gates the pore ➜ rapid (millisecond) responses.
• G-protein-coupled receptors (GPCRs / metabotropic)
  • NT binding activates heterotrimeric G-proteins.
  • Second-messenger pathways indirectly modulate ion channel gating or cell biochemistry ➜ slower (hundreds of ms to minutes) but versatile modulation.

Principles of Cellular Electrophysiology: Resting Membrane Potential (RMP)

• Dominant role of K⁺ permeability
  • Membrane at rest is primarily permeable to K⁺ via nongated leak channels.
  • Intracellular $[K^+]<em>i \approx 100\ \text{mM}$ vs. extracellular $[K^+]</em>o = 2\text{–}6\ \text{mM}$ ➜ chemical gradient drives K⁺ efflux.
  • Fixed intracellular anions (proteins, organic acids) create an electrical gradient pulling K⁺ inward.
• Equilibrium (Nernst) potential
  • Voltage at which electrical & chemical forces balance and net ion flux = 0.
  • For K⁺: $E_K \approx -90\ \text{mV}$ under typical neuronal ionic composition.
• Na⁺/K⁺-ATPase (sodium–potassium pump)
  • Exchanges $3\,\text{Na}^+<em>{\text{out}} / 2\,\text{K}^+</em>{\text{in}}$ per ATP hydrolyzed.
  • Electrogenic: extrudes more positive charge than it imports ➜ contributes a few mV to RMP.
  • Consumes ~40 % of total cerebral O₂ to restore gradients after activity.
• Functional significance of channel opening
  • Opening extra K⁺ channels ➜ membrane moves toward $-96\ \text{mV}$ (hyperpolarization).
  • Opening Na⁺ or Ca²⁺ channels ➜ depolarization toward positive potentials.

Cell Membrane as an Electrical Circuit

• Components
  • Resistors = ion channels (variable resistance; conductance $g = 1/R$).
  • Capacitor = lipid bilayer (membrane capacitance $C_m$ stores charge).
  • Batteries = ionic concentration gradients (Nernst potentials).
• Capacitive current (Icap)
  • When Vm changes, current first charges the capacitor before ions flow through resistors.

Active Membrane Properties: Action Potentials

1. Threshold & Initiation

• Depolarization activates voltage-gated Na⁺ channels.
• Threshold: Vm at which inward $I_{Na}$ > total outward leak currents; typically $-45\text{ to }-30\ \text{mV}$.
• Positive feedback loop: $\uparrow Na^+$ influx ➜ $\uparrow$ depolarization ➜ more Na⁺ channels open ➜ rapid rising phase.

2. Why Vm never reaches $E_{Na}$ (≈ $+66\ \text{mV}$)

• Continuous leak currents oppose depolarization.
• Fast Nav inactivation halts Na⁺ entry within ~1 ms.
• Concurrent opening of voltage-gated K⁺ channels drives Vm back toward $E_K$.

3. Phases of the AP

• Depolarization (rising): explosive Na⁺ influx, Vm approaches 0 (overshoot to slightly + values).
• Peak/overshoot: Vm > 0 but < $E_{Na}$.
• Repolarization: Nav inactivation + Kv opening ➜ Vm moves negative.
• Undershoot / after-hyperpolarization (AHP): Kv remain open; Vm transiently overshoots RMP toward $E_K$.

4. Refractory periods

• Absolute: Nav fully inactivated ➜ impossible to trigger new AP.
• Relative: Kv still open ➜ stronger stimulus required.

5. Role in Neurotransmitter Release

• AP invasion of axon terminals opens Cav ➜ Ca²⁺-dependent exocytosis of synaptic vesicles.

Action Potential Conduction in Axons

1. Initiation site

• Axon hillock / initial segment (~50 µm from soma) has lowest threshold due to high Nav density.

2. Passive vs. Active conduction

• Passive spread (electrotonic) decays with distance: characteristic of dendrites.
• Axons employ active regeneration (sequential Nav opening) to avoid decrement.

3. Myelinated vs. Unmyelinated fibers

• Myelinated
  • Myelin ↑ membrane resistance + ↓ capacitance ➜ faster charging of nodes.
  • Saltatory conduction; velocities up to $\sim 100\ \text{m/s}$ (large diameters).
• Unmyelinated
  • Continuous AP regeneration along entire membrane.
  • Slower (≈ $0.3\ \text{m/s}$ in thin axons).

4. Saltatory conduction details

• Passive internodal current leaps to next node almost instantaneously.
• Node Nav regenerate full-amplitude AP ➜ minimal signal loss.

5. Clinical relevance

• Multiple Sclerosis (CNS) & Guillain-Barré Syndrome (PNS): immune demyelination ➜ conduction slowing/block → neurologic deficits.

Ion Channels and Their Roles in Neurophysiology

1. Voltage-gated ion channel families

• Na⁺, K⁺, Ca²⁺, Cl⁻ channels + some nonselective cation channels (e.g., glutamate/acetylcholine receptors) exclude Cl⁻.

2. Sodium (Nav) channels

• Produce AP upstroke, synchronize neural networks.
• Structure: one α (pore) + two β subunits (modulatory).
• Isoforms: Nav1.1–1.3, 1.6–1.9 (neuronal); Nav1.4 (muscle); Nav1.5 (heart).
• Toxins/drugs
  • Tetrodotoxin (TTX), saxitoxin (STX): pore blockers.
  • Scorpion/anemone toxins: alter gating.
  • Local anesthetics & anticonvulsants (lidocaine, carbamazepine, phenytoin): use-dependent block.
  • TTX-resistant channels in nociceptors influence pain perception.

3. Potassium (Kv, Kir, etc.) channels

• Shape AP repolarization, set RMP, control firing patterns.
• Subtypes & functions
  • Voltage-gated Kv1–Kv12 ➜ AP repolarization.
  • Delayed rectifiers ➜ late repolarization.
  • BK/SK (Ca²⁺-activated) ➜ AHP, adaptation.
  • A-type ➜ interspike interval regulation.
  • M-channels (KCNQ2/3) ➜ mutations → epilepsy, arrhythmia.
  • Inward rectifiers (Kir) ➜ K⁺ buffering.
  • HCN (hyperpolarization-activated cyclic-nucleotide-gated) ➜ pacemaker currents.

4. Calcium (Cav) channels

• Couple depolarization to neurotransmitter release, gene transcription, plasticity.
• Low-voltage-activated T-type (Cav3.1–3.3) ➜ oscillations, burst firing.
• High-voltage-activated (HVA)
  • L-type (Cav1.1–1.4): slow inactivation; blocked by dihydropyridines.
  • N-type (Cav2.2): synaptic release; ω-conotoxin sensitive (analgesia target).
  • P/Q-type (Cav2.1): cerebellar; CACNA1A mutations → familial hemiplegic migraine.
  • R-type (Cav2.3): excitatory transmission.

5. Chloride channels

• Maintain inhibitory tone, stabilize membrane potential.
• Voltage-gated ClCs, CFTR, and ligand-gated GABA_A/GlyR.
• Mutation in ClC-1 ➜ myotonia congenita (muscle hyperexcitability).

6. Other channels

• CNG (cyclic nucleotide-gated): vision, olfaction.
• TRP: temperature, pain, touch.
• VDAC (mitochondrial): apoptosis regulation, metabolite exchange.

Neurotransmitters & Ion Channels

1. Classes of neurotransmitters

• Low-molecular-weight amines
  • Excitatory: glutamate, acetylcholine (ACh).
  • Inhibitory: GABA, glycine.
  • Modulatory biogenic amines: dopamine, norepinephrine (NE), epinephrine, serotonin (5-HT), histamine.
• Purines: ATP, adenosine.
• Neuropeptides: vasopressin, cholecystokinin, etc.
• Co-release allows synergistic/antagonistic effects at synapses.

2. Mechanisms of action

• Ionotropic (LGIC) ➜ fast (ms) EPSPs/IPSPs.
• Metabotropic (GPCR) ➜ slow modulatory effects (hundreds ms–min).
• Excitatory NTs open cation channels (Na⁺, K⁺, Ca²⁺) ➜ depolarization.
• Inhibitory NTs open Cl⁻ channels ➜ hyperpolarization.

3. Conductance logic

• Excitation: drives Vm toward 0 mV via nonselective cation conductance.
• Inhibition: Cl⁻ conductance clamps Vm near $E_{Cl}$ (often −70 mV) or hyperpolarizes.
• Developmental switch: early high $[Cl^-]<em>i$ makes GABA depolarizing; maturation of KCC2 transporter lowers $[Cl^-]</em>i$ ➜ GABA becomes inhibitory.

4. Receptor families

• Cys-loop LGICs: nAChR, GABA_A, GlyR, 5-HT3 – pentameric, extracellular disulfide loop.
• Ionotropic glutamate receptors: AMPA (fast EPSCs), NMDA (voltage + ligand gating; plasticity), Kainate.
• P2X (ATP-gated): Ca²⁺/Na⁺-permeable trimers.
• Metabotropic: 7-TM GPCRs (mGluRs, muscarinic AChR, monoamine receptors).

5. Functional & clinical notes

• Imbalance of excitatory vs. inhibitory transmission underlies numerous disorders.
  • Epilepsy: hypo-GABAergic or hyper-glutamatergic states.
  • Schizophrenia: NMDA receptor hypofunction.
  • Anxiety: benzodiazepines enhance GABA_A.
  • Parkinson’s: dopaminergic degeneration.

Clinical Aspects of Ion Channels & Neural Signaling

1. Oscillatory firing & behavioral states

• Brain rhythms (δ, θ, α, β, γ bands) emerge from intrinsic conductances & network interactions.
• Thalamic & inferior olive pacemaker neurons rely on LVA (T-type) Ca²⁺ currents.
• Absence epilepsy: 3 Hz spike-wave ➜ ethosuximide blocks T-type; lamotrigine augments HCN currents.
• Burst firing common in peptide-releasing neurons.
• Ketamine
  • Induces 1–3 Hz retrosplenial oscillations ➜ dissociative state.
  • Elevates γ oscillations ➜ linked to rapid antidepressant action.

2. Excitation/Inhibition (E/I) balance in psychiatry

• Hypothesis: shift toward excitation or reduced inhibition disrupts cortical synchrony ➜ mental illness.
• Schizophrenia & autism
  • Enhanced glutamatergic drive or reduced PV⁺ interneuron-mediated inhibition.
  • Resultant γ-oscillation defects correlate with cognitive symptoms.
• Optogenetics confirms that restoring GABAergic tone ameliorates behaviors.

3. Ion channelopathies

• Epilepsy
  • BFNC: KCNQ2/3 (Kv7.2/7.3) mutations.
  • GEFS+: Nav mutations.
• Calcium channel disorders
  • CACNA1A mutations ➜ familial hemiplegic migraine, episodic ataxia-2, SCA-6.
• Pain syndromes
  • SCN9A (Nav1.7) gain-of-function: PEPD (extreme pain).
  • Loss-of-function: congenital insensitivity to pain.
• Psychiatric channelopathies
  • KCNH2 (Kv11.1) polymorphism ➜ schizophrenia cognitive deficits.
  • Cav3.2 mutations ➜ autism spectrum.
  • CHRNA5 variant ➜ nicotine dependence.
• Autoimmune
  • Anti-NMDAR encephalitis: antibodies reduce NMDAR function ➜ psychosis, seizures.
  • Potential therapies: NMDAR enhancers.

4. Homeostatic plasticity & treatment

• Hebbian plasticity: activity-dependent LTP/LTD.
• Homeostatic plasticity: global scaling to stabilize activity.
• Metaplasticity: history-dependent plasticity threshold.
• Ketamine: enhances synaptic strength via non-Hebbian, homeostatic mechanisms.
• Lithium: may reduce network activity through synaptic scaling.

5. Brain stimulation therapies

• Electroconvulsive therapy (ECT): generalized seizure resets excitability; ultrabrief pulses minimize memory effects.
• Deep brain stimulation (DBS): >100 Hz pulses modulate pathological circuits (e.g., Parkinson’s, OCD).
• Repetitive transcranial magnetic stimulation (rTMS)
  • 10–20 Hz excitatory ➜ left DLPFC (depression).
  • 1 Hz inhibitory ➜ right DLPFC.
• Vagus nerve stimulation (VNS): delayed antidepressant/antiepileptic effects—likely homeostatic.
• Future
  • Optogenetics: light-activated channels (e.g., channelrhodopsin) for millisecond precision control.
  • Chemogenetics: engineered GPCRs (DREADDs) or ion channels activated by inert ligands.