Organization of Muscle Fibers in a Muscle

Groups of 100 to 150 muscle fibers are bound together with connective tissue to form a unit known as a fasciculus. Groups of fasciculi are bound together into still larger units which in turn are bound together to form the muscle itself; it is likewise invested with a connective tissue sheath. At each end of the muscle this connective tissue merges with tile tendon bundles which attach to the bone.

Nerve and Blood Supply of Skeletal Muscles

The sarcolemma of each muscle fiber insulates it from adjacent fibers, so that excitalion of one fiber does not directly affect neighboring fibers. Hence, each muscle fiber must be supplied with a separate motor nerve twig. A motor nerve supplying a muscle is composed of numerous nerve fibers, each originating from a separate nerve cell in the spinal cord. In the substance of the muscle each nerve fiber breaks up into many branches and each branch penetrates the sarcolemma of a single muscle fiber to terminate in a specialized mass of protoplasm known as a motor end plate. When a single motor nerve cell in the spinal cord discharges impulses to a muscle, all the muscle fibers supplied by branches of tile corresponding nerve fiber are stimulated and contract together. This group of muscle fibers, which forms the smallest functional unit under normal conditions, is believed to correspond to the fasciculus mentioned above. A single motor nerve cell in the spinal cord together with its nerve fiber and the group of muscle fibers supplied by its branches form the basic neuromuscular unit commonly referred to as the motor unit.

In addition to the motor nerve supply, skeletal muscles are also supplied with sensory nerve endings, some of which lie between the groups of muscle fibers while others are associated with muscle tendons. These sensory nerve endings are stimulated by changes in tension in the muscle (contraction, relaxation, stretching) and send impulses to the central nervous system. These impulses play an important role in maintaining muscle tone, in informing the central nervous system of the position of parts of the body with respect to one another and in adjusting the rate and extent of muscle movements.

Each muscle receives blood through one or more arteries. These arteries break up into profuse capillary networks in the connective tissue surrounding each muscle fiber. Under resting conditions many of these capillaries are closed, but they open when the muscle undergoes vigorous contraction. In this way the blood supply to a muscle is adjusted in accordance with the degree of muscular activity.