Involuntary muscle

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Involuntary muscles are smooth, unicycle nucleated, non-branching muscles that are not directly controllable at will. These muscles are not as free as the voluntary muscles such as those in the arms, legs, fingers, toes, etc.

Cardiac muscle: a type of striated muscle exclusively in the heart, can be called an involuntary muscle, although it may be classified separately due to its structural differences. It is controlled by nerve impulses produced by a natural pacemaker called the sinoatrial node, the rate of which is controlled by the sympathetic and parasympathetic nervous system.

Smooth muscles, a type of non-striated muscle, include the radially arranged iris muscles, the digestive system, reproductive system, major blood vessels, the skin, and internal organs. These are also controlled by the autonomic nervous system, particularly the vagus nerve (in the case of smooth muscles lining the digestive system).

Involuntary muscles are muscles controlled only by the central nervous system, and (in some cases) by hormones. These cannot be controlled consciously. The pupils of the eye are an example of this as they contract quickly when the eye is exposed to bright light.

Many involuntary muscles are controlled by spinal nerve centres, with no direct involvement of the brain at all. These muscles react to certain external or internal stimuli, at certain thresholds.

Skeletal muscle is under the classification of conscious or voluntary muscle. These muscles are striated. They also carry on working throughout the whole time one is alive.