To understand the importance of the gamma efferent system, one should recognize that 31 percent of all the motor nerve fibers to the muscle are the small type A gamma efferent fibers rather than large type A alpha motor fibers. Whenever signals are transmitted from the motor cortex or from any other area of the brain to the alpha motor neurons, in most instances the gamma motor neurons are stimulated simultaneously, an effect called coactivation of the alpha and gamma motor neurons. This effect causes both the extrafusal skeletal muscle fibers and the muscle spindle intrafusal muscle fibers to contract at the same time.
The purpose of contracting the muscle spindle intrafusal fibers at the same time that the large skeletal muscle fibers contract is twofold: First, it keeps the length of the receptor portion of the muscle spindle from changing during the course of the whole muscle contraction. Therefore, coactivation keeps the muscle spindle reflex from opposing the muscle contraction. Second, it maintains the proper damping function of the muscle spindle, regardless of any change in muscle length. For instance, if the muscle spindle did not contract and relax along with the large muscle fibers, the receptor portion of the spindle would sometimes be flail and sometimes be overstretched, in neither instance operating under optimal conditions for spindle function.
Brain Areas for Control of the Gamma Motor System
The gamma efferent system is excited specifically by signals from the bulboreticular facilitatory region of the brain stem and, secondarily, by impulses transmitted into the bulboreticular area from (1) the cerebellum, (2) the basal ganglia, and (3) the cerebral cortex.
Little is known about the precise mechanisms of control of the gamma efferent system. However, because the bulboreticular facilitatory area is particularly con cerned with antigravity contractions, and because the antigravity muscles have an especially high density of muscle spindles, emphasis is given to the importance of the gamma efferent mechanism for damping the move ments of the different body parts during walking and running.
The Muscle Spindle System Stabilizes Body Position During Tense Action. One of the most important functions of the muscle spindle system is to stabilize body position during tense motor action. To perform this function, the bulboreticular facilitatory region and its allied areas of the brain stem transmit excitatory signals through the gamma nerve fibers to the intrafusal muscle fibers of the muscle spindles. This action shortens the ends of the spindles and stretches the central receptor regions, thus increasing their signal output. However, if the spindles on both sides of each joint are activated at the same time, reflex excitation of the skeletal muscles on both sides of the joint also increases, producing tight, tense muscles opposing each other at the joint. The net effect is that the position of the joint becomes strongly stabilized, and any force that tends to move the joint from its current position is opposed by highly sensitized stretch reflexes operating on both sides of the joint.
Any time a person must perform a muscle function that requires a high degree of delicate and exact positioning, excitation of the appropriate muscle spindles by signals from the bulboreticular facilitatory region of the brain stem stabilizes the positions of the major joints. This stabilization aids tremendously in performing the additional detailed voluntary movements (of fingers or other body parts) required for intricate motor procedures.
