Smooth Muscles


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


A. 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 the arteries and the veins do not 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 resulting contraction. Note that the stomach action potential lasts much longer than the cardiac action potential (5-10x).


Smooth muscles 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 pregnant uterus, show very brief action potentials, also called “spikes”. Often, these spikes occur in bursts and will lead to summation of contractions (temporal summation).


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.




B. Some additional notes relating to smooth muscles:


Smooth muscles are called “smooth” because there is no striation visible under the microscope (in contrast to skeletal and cardiac muscles).


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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