Physiology, Muscle Contraction

The physiological concept of muscle contraction is based on two variables: length and tension. In physiology, muscle shortening and muscle contraction are not synonymous. Tension within the muscle can be produced without changes in the length of the muscle, as when holding a dumbbell in the same position or holding a sleeping child in your arms. Upon termination of muscle contraction, muscle relaxation occurs, which is the return of muscle fibers to a low-tension state.  Mammals have three types of muscles: skeletal, cardiac, and smooth. Skeletal muscles are attached to bones and give the body structure and strength. Cardiac muscle comprises the walls of the heart, allowing blood to be pumped through the vasculature. Smooth muscle is found throughout the blood vessels, gastrointestinal (GI) tract, bronchioles, uterus, and bladder. Muscle contraction throughout the human body can be broken down based on muscle subtype specialization. In general, muscle fibers are classified into two large categories: striated muscle fibers and smooth muscle fibers. Striated muscle fibers contain actin and myosin filaments that power contraction and are organized into repeating arrays, called sarcomeres, with a striated microscopic appearance. Cardiac muscle tissue is a striated muscle fiber under involuntary control by the body's autonomic nervous system (ANS). Skeletal muscle tissue is a striated muscle fiber under voluntary control. Smooth muscle fibers do not contain sarcomeres but use actin and myosin contraction to constrict blood vessels and move the contents of hollow organs in the body. These fibers are under involuntary control by reflexes and the body's ANS.