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Zusammenfassung:
Many physiological processes are controlled by changes in membrane permeability. Inflow of sodium or calcium ions into a living cell can, for instance, trigger a nerve action potential, elicit muscle contraction, or lead to hormone release. These permeability changes occur in so-called ionic channels which open and close in response to chemical and electrical stimuli. The ion flow through individual channels can be recorded as an electrical signal. The typical signal is a rectangular current pulse of 1 to 2 pA (1-2 × 10−12A) amplitude and of 1 to 10 msec duration. Three examples of channels and their unitary currents are discussed: 1) The gramicidin channel, a model channel of known structure, which can be incorporated into artificial membranes; 2) the acetylcholine- activated channel, which is the best-characterized channel in biological membranes; and 3) the electrically activated Na channel, which is responsible for the nerve action potential.
