Muscle

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Introduction

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.

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Smooth Muscle

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.

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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 skeletal muscle serve to move the various bones and cartilages of the skeleton. Skeletal muscle forms most of the underlying flesh of vertebrates.

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IV

Cardiac Muscle

This muscle tissue composes most of the vertebrate heart. The cells, which show both longitudinal and imperfect cross striations, differ from skeletal muscle primarily in having centrally placed nuclei and in the branching and interconnecting of fibers. Cardiac muscle is not under voluntary control. It is supplied with nerves from the autonomic nervous system, but autonomic impulses merely speed or slow its action and are not responsible for the continuous rhythmic contraction characteristic of living cardiac muscle. The mechanism of cardiac contraction is not yet understood.