Electrophysiological Measurements

Membrane Potentials - Charge Matters

• Resting Membrane Potential (RMP): Potential across cell membrane at rest. Neuron RMP ~-70mV (inside negative).

• Ionic Basis:

  • Unequal ion distribution (Na⁺, K⁺, Cl⁻, Anions⁻).
  • Selective membrane permeability (dominant K⁺ leak at rest).

• 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.

• Equilibrium Potential ($E_{ion}$): Potential at which net ion flow for a specific ion is zero.

  • Nernst Equation: $E_{ion} = (RT/zF) \ln([ion]{out}/[ion]{in})$.

• 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).

• 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.

• 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.

⭐ 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.