Q.The action potential occurs in response to a threshold stimulus; but not at sub threshold stimuli. What is the name of the principle involved?

A nerve impulse is a travelling electrical signal made possible by an unequal distribution of ions across the axolemma — high K+ and negatively charged proteins inside the axon, high Na+ outside — actively maintained by the ATP-driven sodium-potassium pump (3 Na+ out for every 2 K+ in). At rest, the membrane is far more permeable to K+ than Na+, so the inside stays negative relative to the outside (typically around -70 mV); this baseline is the resting potential, and the membrane is described as polarized. A strong enough stimulus opens voltage-gated Na+ channels, letting Na+ rush in and flip the membrane's charge — depolarization — and once this reaches the threshold potential (~-55 mV) the neuron fires completely (the all-or-none principle), spiking to roughly +45 mV. The Na+ gate then closes and the K+ gate opens, letting K+ flow back out and pulling the potential back down — repolarization — and because the K+ gates ('lazy gates') close slowly, the potential often b …