32 BONES AND MUSCLES

HUMAN BODY I 33

Muscular Fiber fiber is the long, thin cell that, when organized by the hundreds into groups called fascicles, constitutes the muscles. It is shaped like an elongated cylinder. The amount of fiber present varies according to the function accomplished by each muscle. Fibers are classified as white, which contract readily for actions that require force and power, and red, which perform slow contractions in movements of force and sustained traction. Each muscle fiber contains in its structure numerous filaments called myofibers. Myofibers, in turn, have two classes of protein filaments: myosin, also called thick filaments, and actin, or thin filaments. Both kinds of fibers are arranged in tiny matrices called sarcomeres.

A

MYOSIN AND ACTIN FILAMENTS The actin and myosin filaments overlap each other to cause muscular contraction.

CONNECTED FILAMENTS Actin and myosin are linked through these filaments.

Relaxation
The order to contract given by the nervous system ceases, and the muscle fibers return to a position of rest. This happens to all muscles, regardless of the duration of contraction.

CAPILLARIES These bring blood to the muscle fibers.

FASCICLE Each of the hundreds of fiber bundles that make up one muscle

SARCOMERE Each small internal cylinder of the myofibril, consisting of actin and myosin MUSCLE FIBER

THE HEAD OF A MOLECULE The head of a myosin molecule extends. It makes contact with the actin, and the myocin and actin overlap each other, producing a muscular contraction.

Contraction
Z BAND marks the boundary between sarcomeres. The nervous system orders the muscle fibers, no matter which type, to shorten. In order to create muscle contraction, calcium is released within the muscle cell, which allows the actin and the myosin to come together and overlap each other.

Specialization
The quantity of muscle fiber varies according to the size and function of the muscle. Also, the same