Define action potential. Describe its phases and ionic basis. Add a note on the refractory period.

Definition

Phases

Ionic Basis

Stimulus → Membrane depolarises to firing level (−55 mV) → Voltage-gated Na+ channels open → Na+ influx → further depolarisation → positive feedback → spike to +35 mV → Na+ channels inactivate; K+ channels open → K+ efflux → repolarisation

• Na+ channel has two gates — an activation gate (outer, opens fast) and an inactivation gate (inner, closes slowly). This explains the refractory period
• Total duration in nerve 1–2 ms; in skeletal muscle 2–4 ms; in cardiac muscle 200–300 ms
• Actual ion movement is minute — 1 in 100,000 ions; the Na+–K+ pump restores it later

Properties of Action Potential

• All-or-none law — a subthreshold stimulus produces no response; at or above threshold the response is maximal and of constant amplitude, regardless of stimulus strength
• Propagated without decrement — amplitude does not fall along the fibre
• Self-propagating — local circuit currents depolarise the adjacent segment
• Refractory period follows
• Stimulus strength is coded by frequency, not amplitude

Refractory Period

• Significance — limits the maximum firing frequency (about 2500/s in nerve) and ensures one-way conduction
• In cardiac muscle the long refractory period prevents tetanus — essential for the pumping action

Electrotonic Potential and Local Response

• Subthreshold stimulus → local, graded, non-propagated change
• Catelectrotonus — depolarisation at the cathode → ↑ excitability
• Anelectrotonus — hyperpolarisation at the anode → ↓ excitability
• Local response — at 7–15% below threshold; graded and can summate

Strength–duration Curve

• Chronaxie is inversely proportional to excitability — a short chronaxie means a highly excitable tissue
• Chronaxie values — nerve 0.0001–0.001 s; skeletal muscle 0.001–0.01 s; cardiac muscle 0.001–0.003 s
• Denervated muscle has a greatly prolonged chronaxie — used to detect denervation

Compound Action Potential

• Recorded from a whole nerve trunk, not a single fibre
• Does not obey the all-or-none law — amplitude increases with stimulus strength as more fibres are recruited
• Shows multiple peaks (A, B, C waves) because fibres conduct at different velocities

Applied Aspects

• Local anaesthetics (lignocaine) → block voltage-gated Na+ channels from within → no action potential → loss of sensation. Small unmyelinated pain fibres are blocked first
• Tetrodotoxin (puffer fish) → blocks Na+ channels; tetraethylammonium blocks K+ channels
• Hypocalcaemia → ↑ Na+ permeability → firing level approaches RMP → spontaneous discharge → tetany
• Hyperkalaemia → sustained depolarisation → Na+ channels inactivated → cardiac arrest
• Multiple sclerosis → demyelination → conduction block or slowing → visual and motor deficits
• Epilepsy → abnormal synchronous high-frequency discharge of cortical neurones; phenytoin acts by prolonging Na+ channel inactivation
• Hyperventilation → alkalosis → ↓ ionised calcium → tetany with carpopedal spasm