Muscular tissues can be classified into:
- Smooth, non-striated or involuntary muscles.
- Cardiac muscle or myocardium.
- Skeletal, striated or voluntary muscles.
Skeletal muscle comes in different shapes and sizes and allows movement of the body and body parts. These muscles have the longest fibers. Skeletal muscles have striations and can be controlled voluntarily. Skeletal muscles are able to contract very rapidly but they tire easily; they must also rest after relatively short periods of activity otherwise physical damage and muscle fatigue will occur. Skeletal muscles can exert tremendous power and are remarkably adaptable. Skeletal muscle is attached to the skeleton. The movement of these muscles also cannot be controlled by will.
Smooth muscle tissue is found in the walls of the hollow visceral organs such as the respiratory passages, intestinal tract and urinary bladder. This is not striated and is involuntary. Contractions of smooth muscles are slow and sustained. The contraction of most of these muscles forces fluid and other substances through the internal body channels. Smooth muscle contractions are involuntary movements triggered by impulses that travel through the autonomic nervous system to the smooth muscle tissue. The arrangement of cells within smooth muscle tissue allows for contraction and relaxation with great elasticity. The smooth muscle in the walls of organs like the urinary bladder and the uterus allow those organs to expand and relax as needed. The smooth muscle of the alimentary canal (the digestive tract) facilitates the peristaltic waves that move swallowed food and nutrients. In the eye, smooth muscle changes the shape of the lens to bring objects into focus. Artery walls include smooth muscle that relaxes and contracts to move blood through the body.
Cardiac muscle makes up most of the heart walls. The muscles are striated and are involuntary – we have almost no conscious control over how our heart beats. The heart wall is composed of three layers. The middle layer, the myocardium, is responsible for the heart’s pumping action. Cardiac muscle found only in the myocardium, contracts in response to signals from the cardiac conduction system to make the heartbeat. Cardiac muscle is made from cells called cardiocytes. Cardiac muscle cells usually contract at a steady pace set by the heart’s pacemaker cells, though nerves can influence them to speed up or slow down. The cardiac muscle tissue forms many branches, which then recombine to form a continuous sheet. This enables the tissue to contract as a unit and improves its efficiency. Cardiac muscle never rests – to do so would be fatal. Contractions of cardiac muscle move blood through the heart and around the body.