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: smooth, skeletal, and cardiac.
Visceral, or involuntary, muscle is composed of spindle-shaped cells, each having a central nucleus. The cells have no cross
striations, although they do exhibit faint longitudinal striations. Stimuli for the contractions of smooth muscles are mediated by
the autonomic nervous system. Smooth muscle is found in the skin, internal organs, reproductive system, major blood vessels,
and excretory system.
SKELETAL, OR STRIATED, MUSCLE TISSUE
This type of muscle is composed of long fibers surrounded by a membranous sheath, the sarcolemma. The fibers are elongated,
sausage-shaped cells containing many nuclei and clearly display longitudinal and cross striations. Skeletal muscle is supplied
with nerves from the central nervous system, and because it is partly under conscious control, it is also called voluntary muscle.
Most skeletal muscle is attached to portions of the skeleton by connective-tissue attachments called tendons. Contractions of