Hinkle, C. Z. (1997). Fundamentals of anatomy and movement. St. Louis, MO: Mosby.

Components of muscle. A muscle fiber consists of many myofibrils, which are tiny thread-like structures. A single muscle fiber is wrapped in a thin connective tissue layer called the endomysium. Many muscle fibers are bundled together by another layer of connective tissue called the perimysium. That forms a fascicle. Most skeletal muscles consist of several fascicles. A group of fascicles is wrapped together by the epimysium to form a muscle.

Innervation of muscle fibers (contraction). Skeletal muscles receive impulses from motor neurons, which are branches of large nerves that lie outside the spinal cord and are part of the peripheral nervous system. When a muscle is at rest, the sarcolemma carries a natural negative polarity; it is polarized. Muscle stimulation causes a fiber to become positively charged; it is depolarized. Inside each fiber is a network of channels called the sarcoplasmic reticulum. That network contains high concentrations of positively charged calcium ions. Depolarization releases calcium into the sarcomere and triggers the myosin and actin to respond to one another by sliding together in contraction. The response is instant and total within the fiber, the "all-or-none" response of every myofibril within the entire muscle fiber. A muscle fiber is the smallest controllable part of a muscle.

Fibers in muscles. All muscles consist of both slow- and fast-twitch fibers. The ratio of each fiber type in a muscle is determined by the function of the muscle. The delicate muscles of the face and eyes have many fast-twitch fibers since they perform quick movements followed by periods of inactivity. They are not expected to contract constantly over long periods of time. In contrast, the muscles of the lower extremity resist fatigue by having larger numbers of slow-twitch fibers that allow them to perform for extended periods under a workload.

Muscle functions. Muscles whose origins and insertion lie within a single plane will more likely have very simple movement patterns, such as only flexion and extension or abduction and adduction. However, if a muscle belly and its tendon run in an oblique direction or warp around a shaft or joint, there may be several different motions available through the joint. Rotational movements are possible and result from muscles with indirect paths.

Muscle spindles in skeletal muscles constantly relate information to the central nervous system about current muscle length, contraction speed, and stretch.

Golgi tendon organs are specialized receptors in tendons near where they attach to the muscles. They are afferent neurons and collagen fiber groups. In some areas, terminal branches of sensory axons are wrapped around groups of collagen fibers. When the tendon is stretched, the terminal branches become compressed triggering an impulse along the axon and returning afferent information on the degree of strain on the tendon back to the CNS. Not all collagen tissue has afferent innervation.

Proprioception (joint posture sense). This sense results from the integration of information from muscle spindles, golgi tendon organs, and joint receptors. Joint receptors are afferent components located within the synovial joint capsules and ligaments.

Reflex arc. The body has the capacity to react to stimuli without conscious thought or higher brain function. This is accomplished by a stimulus to an afferent (sensory) neuron at the periphery being carried to the posterior horn of the spine. The stimulus then moves via an association neuron to an effector (motor) neuron in the anterior horn and back to the effector (usually a skeletal muscle). This is a protective capacity because it allows actions to be taken to avoid injury, for example, withdrawing a hand from a hot stove.

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