4. The Nerve Impulse

Resting Potential (−70 mV)

• At rest, the membrane is polarised: more Na⁺ (and Ca²⁺, Cl⁻) outside, more K⁺ inside; large anions keep the inside negative.
• Leakage channels and the Na⁺/K⁺ pump maintain this resting potential.

Action Potential

• A stimulus reaching threshold (about −55 mV) opens voltage-gated Na⁺ channels → Na⁺ rushes in → depolarisation (−70 mV to +30/+40 mV).
• At the peak, Na⁺ channels inactivate and voltage-gated K⁺ channels open → K⁺ leaves → repolarisation, with a brief hyperpolarisation (~−90 mV) before returning to rest.
• Follows the All-or-Nothing Law: below threshold, no impulse is generated; at or above threshold, a full, identical impulse is always produced.

Refractory Period

• Absolute refractory period: no stimulus (however strong) can trigger a new impulse — Na⁺ channels are inactivated.
• Relative refractory period: only a stronger-than-normal stimulus can trigger a new impulse.
• Significance: ensures one-way propagation, limits impulse frequency, and keeps separate impulses distinct.

Speed of Conduction

• Faster with a larger axon diameter and with myelination. Myelinated axons show saltatory conduction (impulse jumps node to node); unmyelinated axons show slower continuous conduction.