Introduction
The membrane current at the time of excitation is the flow of ions through the membrane channels during an action potential, a short, rapid rise and fall of the membrane potential that follows a fixed pattern. It occurs in excitable cells (neurons, muscle cells and some endocrine cells): in neurons it carries signals along the axon to the synapses, and in muscle cells it is the first step towards contraction.
Resting state and stimuli
A membrane potential (Eₘ) can be recorded in all excitable tissues: nerve tissue, skeletal muscle and smooth muscle. At physiological rest they have a constant negative value, the resting potential (Eₚ). This equilibrium state is kept by the membrane's greater permeability to K+ than to Na+ or Cl-.
The resting state can be changed by chemical, electrical or mechanical stimuli applied to the membrane (as happens, for instance, in muscle contraction). Two ways are possible:
- Hyperpolarisation: the stimulus makes the membrane potential more negative than the resting potential.
- Depolarisation: the potential rises from Eₚ towards 0 mV; this is what happens in muscle contraction.