Golgi Tendon Organ Helps Control Muscle Tension. The Golgi tendon organ, shown in Figure 1, is an encapsulated sensory receptor through which muscle tendon fibers pass. About 10 to 15 muscle fibers are usually connected to each Golgi tendon organ, and the organ is stimulated when this small bundle of muscle fibers is “tensed” by contracting or stretching the muscle. Thus, the major difference in excitation of the Golgi tendon organ versus the muscle spindle is that the spindle detects muscle length and changes in muscle length, whereas the tendon organ detects muscle tension as reflected by the tension in itself.
Fig1. Golgi tendon reflex. Excessive tension of the muscle stimulates sensory receptors in the Golgi tendon organ. Signals from the receptors are transmitted through a sensory afferent nerve fiber that excites an inhibitory interneuron in the spinal cord, inhibiting anterior motor neuron activity, causing muscle relaxation, and protecting the muscle against excessive tension.
The tendon organ, like the primary receptor of the muscle spindle, has both a dynamic response and a static response, reacting intensely when the muscle tension suddenly increases (the dynamic response) but settling down within a fraction of a second to a lower level of steady state firing that is almost directly proportional to the muscle tension (the static response). Thus, Golgi tendon organs provide the nervous system with instantaneous information on the degree of tension in each small segment of each muscle.
Transmission of Impulses from the Tendon Organ Into the Central Nervous System. Signals from the tendon organ are transmitted through large, rapidly conducting type Ib nerve fibers that average 16 micro meters in diameter, only slightly smaller than those from the primary endings of the muscle spindle. These fibers, like those from the primary spindle endings, transmit signals both into local areas of the cord and, after synapsing in a dorsal horn of the cord, through long fiber pathways such as the spinocerebellar tracts into the cerebellum and through still other tracts to the cerebral cortex. The local cord signal excites a single inhibitory interneuron that inhibits the anterior motor neuron. This local circuit directly inhibits the individual muscle without affecting adjacent muscles. The relation between signals to the brain and function of the cerebellum and other parts of the brain for muscle control is discussed in Chapter 57.
The Tendon Reflex Prevents Excessive Tension on the Muscle. When the Golgi tendon organs of a muscle tendon are stimulated by increased tension in the connecting muscle, signals are transmitted to the spinal cord to cause reflex effects in the respective muscle. This reflex is entirely inhibitory. Thus, this reflex provides a negative feedback mechanism that prevents the development of too much tension on the muscle.
When tension on the muscle—and therefore on the tendon—becomes extreme, the inhibitory effect from the tendon organ can be so great that it leads to a sudden reaction in the spinal cord that causes instantaneous relaxation of the entire muscle. This effect is called the lengthening reaction; it is probably a protective mechanism to prevent tearing of the muscle or avulsion of the tendon from its attachments to the bone.
Possible Role of the Tendon Reflex to Equalize Contractile Force Among the Muscle Fibers. Another likely function of the Golgi tendon reflex is to equalize contractile forces of the separate muscle fibers. That is, the fibers that exert excess tension become inhibited by the reflex, whereas those that exert too little tension become more excited because of the absence of reflex inhibition. This phenomenon spreads the muscle load over all the fibers and prevents damage in isolated areas of a muscle where small numbers of fibers might be overloaded.
