Muscle

Muscle, tissue or organ of the animal body characterized by the ability to contract, usually in response to a stimulus from the nervous system. The basic unit of all muscle is the myofibril, a minute, threadlike structure composed of complex proteins. Each muscle cell, or fiber, contains several myofibrils, which are composed of regularly arranged myofilaments of two types, thick and thin. Each thick myofilament contains several hundred molecules of the protein myosin. Thin filaments contain two strands of the protein actin. The myofibrils are made up of alternating rows of thick and thin myofilaments with their ends interleaved. During muscular contractions, these interdigitated rows of filaments slide along each other by means of cross bridges that act as ratchets. The energy for this motion is generated by densely packed mitochondria that surround the myofibrils.

Three types of muscular tissue are recognized:

1. smooth muscle -> Human smooth muscle is composed of slender, spindle-shaped cells, each with a single nucleus. Smooth muscle cells contract in rhythmic waves to propel food through the digestive tract and provide tension in the urinary bladder, blood vessels, uterus, and other internal organs.
2. skeletal or striated mucle tissue -> Skeletal muscle enables the voluntary movement of bones. Skeletal muscle consists of densely packed groups of elongated cells known as muscle fibers. Within these fibers, the alternation of thick and thin myofilaments gives skeletal muscles a striated, or striped, appearance.
3. cardiac muscle -> Cardiac muscle, found only in the heart, drives blood through the circulatory system. Cardiac muscle cells connect to each other by specialized junctions called intercalated disks. Without a constant supply of oxygen, cardiac muscle will die, and heart attacks occur from the damage caused by insufficient blood supply to cardiac muscle.