Membrane Potentials - Charge Matters
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Resting Membrane Potential (RMP): Potential across cell membrane at rest. Neuron RMP ~-70mV (inside negative).
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Ionic Basis:
- Unequal ion distribution (Na⁺, K⁺, Cl⁻, Anions⁻).
- Selective membrane permeability (dominant K⁺ leak at rest).
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Key Players:
- K⁺ efflux: Primary RMP determinant due to high resting K⁺ permeability.
- Na⁺/K⁺ ATPase pump: Electrogenic (3 Na⁺ out / 2 K⁺ in), maintains concentration gradients.
⭐ The Na+/K+ ATPase pump contributes to RMP by pumping 3 Na+ ions out for every 2 K+ ions in, making the inside more negative and maintaining concentration gradients.
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Equilibrium Potential ($E_{ion}$): Potential at which net ion flow for a specific ion is zero.
- Nernst Equation: $E_{ion} = (RT/zF) \ln([ion]{out}/[ion]{in})$.
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Goldman-Hodgkin-Katz (GHK) Equation: Calculates overall RMP considering relative permeabilities ($P$) of multiple key ions (K⁺, Na⁺, Cl⁻).
- $V_m = (RT/F) \ln((P_K[K^+]{out} + P{Na}[Na^+]{out} + P{Cl}[Cl^-]{in}) / (P_K[K^+]{in} + P_{Na}[Na^+]{in} + P{Cl}[Cl^-]_{out}))$.
Action Potentials - Spark of Life
- Rapid, all-or-none electrical signals in excitable cells. Triggered when stimulus reaches threshold potential (approx. -55mV).
- Phases & Ionic Basis:
- Depolarization: Rapid Na$^+$ influx (voltage-gated Na$^+$ channels open).
- Repolarization: K$^+$ efflux (voltage-gated K$^+$ channels open, Na$^+$ channels inactivate).
- Hyperpolarization: K$^+$ channels close slowly; membrane potential briefly dips below RMP.
- Refractory Periods:
- Absolute (ARP): No new AP possible. 📌 "Absolutely No AP".
- Relative (RRP): New AP with stronger stimulus. 📌 "Relatively Stronger Stimulus for AP".
⭐ Tetrodotoxin (TTX) selectively blocks voltage-gated Na$^+$ channels, preventing action potential generation.
Nerve Conduction & Synapses - Message Relays
- Conduction: AP propagation.
- Myelinated: Saltatory (Nodes of Ranvier), ↑speed. 📌 SALTy Dog: Saves ATP, Large diameter, Temperature dependent, Myelinated.
- Unmyelinated: Continuous, ↓speed.
- Factors: ↑Diameter, Myelin → ↑Velocity.
⭐ Myelination ↑conduction velocity (insulation, saltatory at Nodes of Ranvier).
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Synapses: Signal junctions.
- Chemical: Neurotransmitters (e.g., ACh); unidirectional; synaptic delay (~0.5 ms). AP → Ca²⁺ influx → NT release → PSP.
- Electrical: Gap junctions; rapid; bidirectional.
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Erlanger-Gasser Nerve Fibers:
Type Myelin Dia (µm) Vel (m/s) Key Function(s) Aα Yes 12-20 70-120 Proprioception, Motor Aβ Yes 5-12 30-70 Touch, Pressure Aγ Yes 3-6 15-30 Muscle spindle motor Aδ Yes 2-5 12-30 Fast pain, Cold B Yes <3 3-15 Preganglionic autonomic C No 0.4-1.2 0.5-2 Slow pain, Warmth, Postganglionic
Clinical Electrophysiology - Diagnostic Sparks
Clinical electrophysiology uses electrical recordings to diagnose neurological disorders.
| Test | Records | Key Uses & Examples | Characteristic Findings |
|---|---|---|---|
| EMG | Muscle electrical activity (insertional, spontaneous, MUAPs) | Myopathy, neuropathy, NMJ disorders (Myasthenia Gravis, LEMS) | Fibrillations, +ve sharp waves (denervation); Small, short, polyphasic MUAPs (myopathy); Decrement on RNS (MG) |
| NCS | Nerve signal speed (CV) & strength (CMAP/SNAP amplitude, latency) | Peripheral neuropathy (CTS, GBS), radiculopathy, plexopathy, nerve trauma | ↓ CV, ↑ distal latency (demyelination); ↓ amplitude (axonal loss); Conduction block |
| EEG | Brain's spontaneous electrical activity via scalp electrodes | Seizures/epilepsy, encephalopathy, sleep disorders, brain death determination | Spike-wave discharges (epilepsy); Diffuse slowing (encephalopathy); Specific sleep stage patterns |
| EPs | Nervous system electrical responses to sensory stimuli (Visual, Auditory, Somatosensory) | MS (VEP for optic neuritis), hearing loss (BAEP), spinal cord/brainstem lesions (SSEP) | ↑ Latency, ↓ amplitude of recorded potentials, indicating slowed or impaired conduction |
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⭐ In Myasthenia Gravis, repetitive nerve stimulation (RNS) during EMG shows a characteristic decremental response (>10% decrease in amplitude of CMAP).
High‑Yield Points - ⚡ Biggest Takeaways
- Nernst equation calculates equilibrium potential for a single ion.
- Goldman-Hodgkin-Katz (GHK) equation determines Resting Membrane Potential (RMP) considering multiple ion permeabilities.
- Patch-clamp technique measures current through single ion channels.
- EEG records brain's electrical activity; identifies seizure patterns.
- EMG assesses muscle electrical activity; differentiates myopathy from neuropathy.
- Nerve Conduction Velocity (NCV) measures action potential speed; reduced in demyelination.
- Evoked Potentials (EPs) assess neural pathways via sensory stimuli responses.
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