Proprioceptive Reflex Control of Muscle Tension

When muscle spindles respond to stretching or to a change in the length of a given muscle, there is increased activity in the Ia afferent fibers, which then directly stimulates the alpha motor neurons supplying that particular muscle. (The same Ia fibers inhibit antagonist muscles through interneuron connections.) By contrast, activity in Ib afferent fibers caused by Golgi tendon organs, which respond to muscle tension, stimulates spinal interneurons to inhibit the alpha motor neurons supplying a particular muscle.

Passive Stretch. When the muscle is passively lengthened, both extrafusal and intrafusal fibers are stretched (A). The muscle spindles are activated, causing a volley of activity in group Ia and group II (not shown) fibers; this provokes reflex excitation of alpha motor neurons, thus stimu-lating the extrafusal fibers to contract and oppose the applied force. Golgi tendon organs, which respond poorly to passive stretch, do not discharge under these circumstances. The more rapid or intense the stretching and the change in length, the more rapid or intense the contraction, an example of which is the knee jerk. Thus the spinal stretch reflex enables the muscle to perform like a spring; if either the afferent or the efferent limb of the nerve supply is damaged, such action is not possible.

Active Contraction. The role of spinal reflexes during active contraction of a muscle is shown in B and C (see Plate 6-13). A situation in which there is higher stimulation of alpha motor neurons only (B), brings about the contraction of the extrafusal fibers, which leads to a shortening of the muscle overall and a slackening of the intrafusal fibers. This results in a termination of activity in the muscle spindles and Ia fibers. The increase in muscle tension, however, is sufficient to activate the Golgi tendon organs and the Ib afferent fibers that attempt to inhibit the alpha motor neurons via interneurons. Sufficient Ib inhibition will lead to relaxation or cessation of muscle contraction.

In a normal situation during voluntary contraction of a muscle (C), commands from the brain excite both alpha and gamma motor neurons, resulting in the stimulation and shortening of both extrafusal and intrafusal fibers. The muscle spindles are activated and produce a discharge of Ia fibers, which thus reinforce the higher stimulation of the alpha motor neurons. This reinforced motor neuron activity increases the springlike tension of the contracting muscle and helps it adjust to changes in the load. The activated Ib afferent fibers from the Golgi tendon organs oppose the alpha motor neurons through a feedback mechanism that reduces tension and causes relaxation if the load becomes too great. The role of this “force feedback” mechanism is not well understood.

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