Myogenic contraction

Myogenic contraction refers to a contraction initiated by the myocyte cell itself instead of an outside occurrence or stimulus such as nerve innervation. Most often observed (although not necessarily restricted to) in smaller resistance arteries, this 'basal' tone may be useful in the regulation of organ blood flow and peripheral resistance, as it positions a vessel in a preconstricted state that allows other factors to induce additional constriction or dilation to increase or decrease blood flow.

Mechanisms

Unstable Membrane Potentials

Many cells have resting membrane potentials that are unstable. ility is usually due to ion channels in the cell membrane that spontaneously open and close (e.g. I_f channels in cardiac pacemaker cells). When the membrane potential reaches depolarization threshold an action potential (AP) is fired, excitation-contraction coupling initiates and the myocyte contracts.

Slow wave potentials

Slow wave potential are unstable resting membrane potentials that continuously cycle through depolarization- and repolarization phases. However, not every cycle reaches depolarization threshold and thus an action potential (AP) will not always fire. Owing to temporal summation (depolarization potentials spaced closely together in time so that they summate), however, cell membrane depolarization will periodically reach depolarization threshold and an action potential will fire, triggering contraction of the myocyte.

Pacemaker potentials

Pacemaker potentials are unstable cell membrane potentials that reach depolarization threshold with every depolarization/repolarization cycle. This results in AP's being fired according to a set rhythm. Cardiac pacemaker cells, a type of cardiac myocyte in the SA node of heart, are an example of cells with a pacemaker potential.

Stretch

This mechanism involves the opening of mechanically-gated Ca²⁺ channels when some myocytes are stretched. The resulting influx of Ca²⁺ ions lead to the initiation of excitation-contraction coupling and thus contraction of the myocyte.