Contraction of muscles

The units of skeletal muscle are the muscle fibers. The fibers are arranged in bundles which are known as FASCICULI. Each fiber is in receipt of a single terminal branch of a nerve (Neuron). This one nerve may branch and innervate (connect to) many muscle fibers (Neuromuscular Junction). Collectively, this single nerve plus all the muscle fibers it attaches to are called a MOTOR UNIT. When sufficient stimulation occurs, chemical and physiological events take place and a muscle contraction occurs. As you can see, one single nerve may cause the stimulation of up to several hundred skeletal muscle fibers.

The Smaller the Motor Unit, i.e., the smaller the number of muscle fibers innervated by 1 Neuron, the MORE PRECISE control of muscular action which can occur.

For example, for the muscles controlling eye movement, a motor unit may include only 6-12 muscle fibers, where in one of the large muscles of an extremity, the motor unit may consist of upwards of 2000 muscle fibers per 1 Neuron.

In skeletal muscles, there is an "All or None" phenomena, whereby a muscle cell will contract to its fullest extent when it is stimulated adequately; it never partially contracts. However, as the book discusses on P. 165, there may be different degrees of contraction/shortening, to the Whole Muscle, meaning the Speed of contraction or the strength of contraction is determined by how many of the individual muscle cells/fibers are stimulated fully.

MUSCLE TONE

Healthy muscle then, is characterized by active contraction in response to the reaction of the Nervous System to the environment. This produces a constant tension in the muscle for long periods of time, known as MUSCLE TONE. This tone is due to the constant steady contraction and relaxation of different muscle fibers in individual muscles that help to maintain the Metabolism of muscle cells.

Skeletal muscle is particularly suited for the regulation of the body's INTERNAL ENVIRONMENT and disturbances in its function often result in impaired HOMEOSTASIS: the tendency of a system, especially the physiological system of higher animals, to maintain internal stability, owing to the coordinated response of its parts to any situation or stimulus tending to disturb its normal condition or function.