Transmission of Impulses

The transmission of a nerve impulse unfolds in two connected phases: the resting membrane potential, which is the baseline state of a neuron that is not currently signalling, and the action membrane potential, the brief electrical event that is the impulse itself. In the resting state, the inside of the neuron is negatively charged relative to the outside. This happens because the axolemma is far more permeable to K+ than to Na+ and is essentially impermeable to the large negatively charged protein ions trapped inside — so K+ constantly leaks outward faster than Na+ can leak inward, leaving the interior net negative. This concentration gradient (high K+ and protein inside, high Na+ outside) is actively maintained against its natural tendency to dissipate by the ATP-driven sodium-potassium pump, which continuously exchanges 3 Na+ ions out for every 2 K+ ions it brings in. The resulting resting potential in a neuron typically sits around -70 mV (ranging roughly -40 mV to -90 mV), with the minus sign simply indicating that the inside is negative relative to the outside; a membrane in this state is described as polarized.

An action potential is triggered when a stimulus is strong enough to open the sodium voltage-gate, letting Na+ rush into the axoplasm faster than K+ can leave — flipping the membrane's charge so the inside briefly turns positive relative to the outside. This flip is called depolarization. If enough Na+ has entered to push the potential to around -55 mV — the threshold potential — the neuron fires; a stimulus that reaches this threshold is a threshold stimulus, and because the response either fires fully or not at all (never partially), this is called the all-or-none principle. Once triggered, the rapid Na+ influx drives the membrane potential up to roughly +45 mV, the spike potential. The sodium gate then closes and the potassium gate opens, letting K+ flow back out and pulling the potential back down toward resting levels — this falling phase is repolarization. Sometimes the potassium gates ('lazy gates', which close slowly even once the threshold has passed) let too much K+ leave, pushing the potential briefly more negative than the normal resting value (down toward about -90 mV) before it settles back — this overshoot is called hyperpolarization, and during it the sodium gate stays firmly closed. Conduction speed itsel …