noun, plural: involuntary muscles
A muscle that contracts without conscious control
A muscle may act ordinarily either under the control of the will or without conscious control. Muscles that can be controlled at will are referred to as voluntary muscles. Those that are not under the control of the will (volition) are called involuntary muscles. Examples of involuntary muscles are the smooth muscles, which are muscles lacking striations when viewed under a microscope. This is why involuntary muscles are sometimes called non-striated or un-striped muscles. The smooth muscles are found lining the internal organs (such as esophagus, stomach, intestines, etc.) and blood vessels. These muscles contract slowly and rhythmically as opposed to skeletal muscles that contract relatively faster (under the control of the will). Another example of involuntary muscle is the cardiac muscle. The latter is the muscle of the heart. The cardiac muscle has striations when viewed under the microscope but its contractions are not under the control of the will. The part of the peripheral nervous system associated with the involuntary action of the smooth and cardiac muscles is the autonomic nervous system. The autonomic nervous system supplies stimulation to these involuntary muscles.