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Involuntary muscles are muscles that are not controllable consciously, and instead contract due to unconscious impulses sent by the autonomic nervous system or certain specialized cells or hormones. Both smooth muscle and cardiac muscle can be classified as involuntary muscles. Smooth muscle is comprised of spindle-shaped cells that have no striations and is found in numerous locations throughout the human body. Cardiac muscle is striated rather than smooth, and is found only within the walls of the heart.
Smooth muscles are involuntary muscles composed of thick and thin protein filaments that are homologous to the organelles known as myofibrils found in skeletal muscles. The thin filaments are composed of a globular protein called actin, while the thick ones are made up of a motor protein called myosin. Smooth muscles require extracellular calcium ions to contract: the ions activate a nucleotide called Adenosine triphosphate (ATP), which then activates the myosin filaments. The myosin filaments attach to the actin filaments in a process known as the crossbridge cycle, which causes the thick and thin filaments to slide over each other and contract. When the myosin filaments release the actin filaments, the muscle relaxes.
There are many types of smooth muscle in the human body. Smooth involuntary muscles control the iris of the eye, which contracts and expands involuntarily in accordance with changing light levels. The process of peristalsis, which creates a wave that pushes food through the esophagus and small intestine, is also controlled by involuntary muscles. Smooth muscle can also be found in the respiratory tract, the reproductive systems of both women and men, the ciliary muscle of the eye, and the urinary bladder. Most blood and lymphatic vessels in the human body are lined with smooth muscle cells, allowing them to constrict and dilate.
Cardiac muscle is also sometimes considered to be involuntary muscle. However, cardiac muscle shares features of both smooth and skeletal muscle tissue. It is striated like skeletal muscle tissue, but its contractions are involuntary, like those of smooth muscle tissue. Cardiac muscle is unique in that is particularly invulnerable to fatigue.
Contractions of cardiac muscles are controlled by the nerve impulses delivered by a group of cells located in the right atrium of the heart called the sinoatrial node. These contractions push blood through the four chambers of the human heart, the atria and ventricles. They also move blood throughout the veins and arteries of the circulatory system. These contractions, like those of smooth muscles, are initiated by calcium ions that come from outside the muscle cell.