What are the events that take place during depolarisation of the membrane​

The depolarization, also called the rising phase, is caused when positively charged sodium ions (Na+) suddenly rush through open voltage-gated sodium channels into a neuron. As additional sodium rushes in, the membrane potential actually reverses its polarity. During this change of polarity the membrane actually develops a positive value for a moment (+40 millivolts).

The repolarization or falling phase is caused by the slow closing of sodium channels and the opening of voltage-gated potassium channels. As a result, the membrane permeability to sodium declines to resting levels. As the sodium ion entry declines, the slow voltage-gated potassium channels open and potassium ions rush out of the cell. This expulsion acts to restore the localized negative membrane potential of the cell.

Hyperpolarization is a phase where some potassium channels remain open and sodium channels reset. A period of increased potassium permeability results in excessive potassium efflux before the potassium channels close. This results in hyperpolarization as seen in a slight dip following the spike.

The propagation of action potential is independent of stimulus strength but dependent on refractory periods. The period from the opening of the sodium channels until the sodium channels begin to reset is called the absolute refractory period. During this period, the neuron cannot respond to another stimulus, no matter how strong.

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