Bioelectric Phenomena

Resting Membrane Potential - Cell's Electric Chill

• Electrical potential difference across a non-excited cell membrane; inside negative.
• Typical: -70 mV to -90 mV (neurons).
• Key Determinants:
  • High K⁺ permeability (K⁺ efflux): Primary driver. K⁺ Nernst Potential ($E_K$) approx. -90 mV.
    • Nernst Eq: $E_{ion} = (61.5/z) \log ([ion]{out}/[ion]{in})$
  • Na⁺/K⁺ ATPase: 3 Na⁺ out / 2 K⁺ in (electrogenic). 📌 "PUMPKIN": Pump K⁺ IN.
  • Intracellular anionic proteins (A⁻).
• Goldman-Hodgkin-Katz (GHK) equation: Calculates RMP using multiple ion permeabilities.

⭐ The Na+/K+ ATPase pump contributes significantly to RMP by pumping 3 Na+ ions out for every 2 K+ ions in, making the inside more negative (electrogenic pump, contributes ~-4mV).

Action Potential - Nerve's Electric Spike

• Rapid, transient, 'all-or-none' electrical spike in excitable cells (neurons, muscle).
• Triggered when stimulus depolarizes membrane to threshold potential (approx. -55 mV).

Phases & Ionic Basis:

• Depolarization:
  • Voltage-gated Na+ channels (VGSCs) open rapidly → Na+ influx.
  • Membrane potential reverses (overshoot, e.g., to +30 mV).
• Repolarization:
  • VGSCs inactivate.
  • Voltage-gated K+ channels (VGKCs) open → K+ efflux.
• Hyperpolarization (Afterpotential):
  • VGKCs close slowly → RMP transiently more negative (undershoot).

Properties:

• All-or-None Law: AP fires at full amplitude if threshold is met, or not at all.
• Refractory Periods:
  • Absolute (ARP): No new AP. VGSCs inactivated.
  • Relative (RRP): Stronger stimulus needed. Some VGSCs recovered, K+ efflux continues.
• 📌 Mnemonic: "Salt (Na+) IN for Depolarization, Potato (K+) OUT for Repolarization."

⭐ Tetrodotoxin (TTX) from pufferfish selectively blocks voltage-gated Na+ channels, preventing action potential generation.

Key Channel Blockers:

• Voltage-gated Na+ channels:
  • Tetrodotoxin (TTX)
  • Lidocaine (local anesthetic)
• Voltage-gated K+ channels:
  • Tetraethylammonium (TEA)

Synaptic & Receptor Potentials - Signals Across Gaps

• Synaptic Potentials: Graded postsynaptic Vm changes.
  • EPSP (Excitatory Postsynaptic Potential): Depolarization ($Na^+$ influx via glutamate). Moves Vm to threshold.
  • IPSP (Inhibitory Postsynaptic Potential): Hyperpolarization ($Cl^-$ influx / $K^+$ efflux via GABA, glycine). Moves Vm from threshold.
  • Summation:
    • Temporal: Single presynaptic neuron, rapid stimuli.
    • Spatial: Multiple presynaptic neurons, simultaneous stimuli.
• Receptor/Generator Potentials: Graded sensory receptor potentials.
  • Stimulus alters ion permeability.
  • Amplitude ~ stimulus intensity.
  • Triggers AP if threshold reached.
  • Adaptation: ↓ response to sustained stimulus.

⭐ Temporal summation involves multiple stimuli from a single presynaptic neuron close in time, while spatial summation involves stimuli from multiple presynaptic neurons at the same time.

Clinical Correlations - Bioelectric Insights

• Diagnostic tools utilize body's bioelectric signals.
• ECG (Electrocardiogram):
  • Assesses heart's electrical activity.
  • Key for arrhythmias, ischemia detection.
• EEG (Electroencephalogram):
  • Records brain's electrical rhythms.
  • Aids in epilepsy, sleep disorder diagnosis.
• EMG (Electromyogram) & NCS (Nerve Conduction Studies):
  • Assess nerve & muscle electrical activity.
  • Crucial for diagnosing neuropathies, myopathies.

⭐ Channelopathies are diseases caused by disturbed function of ion channel subunits or the proteins that regulate them, e.g., Liddle's syndrome (ENaC).

High‑Yield Points - ⚡ Biggest Takeaways

• RMP: Primarily due to K+ efflux; maintained by Na+/K+ ATPase.
• Action Potential: All-or-none; Na+ influx (depolarization), K+ efflux (repolarization).
• Nernst potential for single ion; Goldman equation for RMP with multiple ions.
• Chronaxie: Excitability measure; time for 2x Rheobase current to cause excitation.
• Saltatory conduction: Myelinated nerves; AP jumps Nodes of Ranvier, ↑ conduction velocity.
• Donnan equilibrium: Explains effect of non-diffusible ions on ion distribution.