MUSCULAR SYSTEM

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Muscular System

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.

Anterior Muscles

This illustration shows the body’s complex network of skeletal muscles. Skeletal muscles attach to bones of the skeleton and control all voluntary movements. A skeletal muscle crosses a joint between two bones and is attached to each bone either directly or by means of a tendon or a fibrous sheet or band called a fascia. Bones move when muscles contract, or shorten, across the joint.

The size of a muscle depends on the function it performs. Where dexterity is required, as in the fingers, the muscles are typically very small. Where strength is needed, as in the thigh, the muscles are large.

Posterior Muscles

This illustration shows the body’s complex network of skeletal muscles. Skeletal muscles attach to bones of the skeleton and control all voluntary movements. A skeletal muscle crosses a joint between two bones and is attached to each bone either directly or by means of a tendon or a fibrous sheet or band called a fascia. Bones move when muscles contract, or shorten, across the joint.

The size of a muscle depends on the function it performs. Where dexterity is required, as in the fingers, the muscles are typically very small. Where strength is needed, as in the thigh, the muscles are large.