Voltage–dependent ion channels

Neurons are excitable, i.e. they can produce action potentials, because they have in their plasma membrane a class of transmembrane proteins called voltage-dependent ion channels. Voltage-dependent channels are preferentially selective for one of three ions, Na⁺, K⁺ or Ca²⁺. Voltage-dependent sodium channels (VDSCs) are large glycoproteins that span the full thickness of the plasma membrane of most excitable cells. The combined effect of sodium influx through a few hundred VDSCs will produce the upstroke of the spike that is the depolarizing phase of the action potential. Excitable cells have a great variety of distinct types of potassium channel each with their own particular properties. One type, the delayed outward rectifier is involved in the repolarization phase of the action potential. The ion fluxes that underlie neuron action potentials were discovered in the early 1950s using a technique called voltage clamping. It measures the currents that flow across an excitable cell membrane at a fixed potential.