The second type of specialized sensory receptor found in muscle tissue is the Golgi tendon organ. Its function is to signal the amount of tension generated by muscle contraction. The end organ is composed of braided collagen fibers within a capsule approximately 1 mm in length and 0.1 mm in diameter (Fig. 3). It is innervated by a free nerve ending classified as Ib, slightly smaller than Ia fibers of the muscle spindle. Golgi tendon organs are located at junctions between muscles and their tendinous insertion points. As the muscle contracts, it stretches the capsule of the Golgi tendon organ and causes the Ib fiber to discharge. Information about the amount of tension generated is fed back to interneurons in the spinal cord, forming a polysynaptic feedback loop that inhibits the activity of homologous alpha motor neuron pools. This reflex is called the reverse myotactic reflex, and it prevents the muscle from extreme contraction that could be damaging.
In summary, the muscle spindle and the Golgi tendon organ provide counterbalanced systems for setting overall muscle tone. Group Ia and II afferents from muscle spindles carry information about the static length of the muscle and its rate of change during contraction and relaxation. Because the muscle spindle is connected to the main muscle fibers in parallel, passive stretching or relaxation lengthens the spindle, excites the sensory end organ, and increases the firing rate of Ia and II fibers. Through a monosynaptic feedback pathway (the myotac-tic reflex), Ia fibers excite alpha motor neurons and cause muscles to contract after they are passively stretched.
In contrast, the Golgi tendon organ is connected to the muscle in series, so that contraction of the muscle causes stretching of the nerve ending and an increased firing rate in the Ib fiber. Through a polysynaptic feedback pathway (the reverse myotactic reflex), Ib fibers inhibit alpha motor neurons and cause muscles to relax after they contract. Both pathways offer protection to the body and to the muscle tissue itself, particularly at extremes of contraction or stretch. At lesser loads, both reflex pathways continually modulate activity of the alpha motor neuron, increasing or decreasing its tendency to fire in response to changes in ongoing muscle activity.
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