A.4.8. The Smooth Muscle

To understand the major differences in action potentials and contractions in skeletal and smooth muscles.

Different Types of Smooth Muscle Action Potentials


There are many types of smooth muscles in the body and they all display different types of (action) potentials and contractions.
Some smooth muscles, such as the smooth muscles in the walls of blood vessels such as the arteries and the veins do not even display an action potential at all! But they do show slow depolarization and repolarizations, the level of which is determined by the action of local nerve endings.
Other smooth muscles resemble the heart, such as the stomach, with a pacemaker region, an action potential that propagates in the wall of the organ and a resulting contraction. Note that the stomach action potential lasts much longer than the cardiac action potential (5-10x).
Smooth muscle action potentials in the gastrointestinal system often last very long. In the small intestine they can have a plateau that lasts for many seconds. Note here that it is the "spikes" that induce the contraction, not the action potential itself. These spikes occur in the plateau phase of the action potential.
Other smooth muscles, such as the uterus and the bladder, show very brief action potentials, also called “spikes”. Often, these spikes occur in bursts and will lead to summation of contractions (temporal summation).
Four examples of smooth muscles action potentials and contractions
The major ion channels in smooth muscles are Calcium-channels (for influx and depolarization) and Potassium-channels (for efflux and repolarization). Sodium and sodium channels do occur in smooth muscles but do not seem to be very important.
Please note that this is only a very brief introduction in smooth muscles. A much more elaborate presentation will be given at a later time.

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A.4.8. The Smooth Muscle

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