Golgi Tendon Organs
The human skeletal muscle system is critical for movement and locomotion. In order for this system to function properly, the relationship between various muscles at all the joints of the body must be maintained so that one muscle group does not overpower others or prevent other muscle groups needed for movement from functioning. This interplay is accomplished by the Golgi tendon organs found throughout the skeletal system in humans. This structure is important in preventing hyperextension or hypercontraction of muscles and allows for the movement associated with joint motions.
The anatomy of the Golgi tendon organ (GTO) is complex and multifaceted. It requires a very integrated interplay between the muscles and tendons of a joint as well as the nervous system via the spinal cord. In summary, the role of the GTO is to signal the nervous system how strong a muscle is in contracting and then to let the nervous system send a signal back to the muscle to regulate its continued contraction and relaxation as well as to regulate the muscles that are antagonistic or agonistic to the contracting muscle. For example, in order to flex the arm, the bicep muscle must contract, while at the same time, the tricep muscle on the back of the arm must relax and stretch. The GTO is integral in this coordinated movement.
A more detailed look at the physiology of the GTO reveals this interplay. The body of the GTO is composed of collagen (a protein) and is embedded within the tendon connecting the muscle to the bone associated with a specific muscle and joint. By being embedded in the tendon, the GTO is able to sense the strength of a muscle contraction as the muscle is stimulated to shorten. What is unique is that the GTO is able to respond in a graded way depending on the speed of muscle contraction as well as the strength of the contraction. The more rapidly a muscle contracts and the harder it contracts, the tighter the tendon becomes and the more simulated the GTO becomes. This results in action potentials being sent to the spinal cord (the intensity of the action potentials is directly proportional to the speed and strength of the muscle contraction), which then determines how to further regulate muscle contraction. Should the speed or intensity of the muscle contraction be too great, the GTO signals the spinal cord to send action potentials to the primary muscle as well as the surrounding agonistic and antagonistic muscle to relax or lengthen and protect the joint (https://www.youtube.com/watch?v=7T4NI_2qDEM).
The Tendon Reflex
The tendon reflex, one that many people are familiar with (knee reflex), is a way to test and determine how well the nervous system is working, and how well the reflex arc is functioning. The tendon reflex is very different from what is called the stretch reflex, which regulates muscle contraction. The tendon reflex is a feedback mechanism that causes muscle relaxation before damage is done to overcontracted or stretched muscles. This reflex arc has a very specific pathway. Once tension is applied to the tendon, the GTO is stimulated and sends signals to the spinal cord. Through a series of neurotransmitters including glutamate and glycine, action potentials are sent back to the tendon/muscle, causing relaxation.
While there are not a significant number of diseases that affect the GTO, the obvious, most significant issue involving the GTO is muscle damage as a result of the GTO failing to prevent hyperextension or overcontraction of the muscle. As stated earlier, the role of the GTO is to inhibit further tension if a muscle becomes overstimulated. Failure of the GTO to prevent this overstimulation could result in tearing of the muscle fibers.
A second disease that has been shown to affect the GTO function of some individuals is Parkinson’s disease. In this disease the effectiveness of the GTO in preventing overcontraction or overstimulation is decreased and could be a contributing component to the tremor and rigidity often seen in this disease process.
Charles A. Ferguson
See also: Meissner’s Corpuscles; Pacinian Corpuscles; Reflex
Moore, J. C. (1984). The Golgi tendon organ: A review and update. American Journal of Occupational Therapy, 38(4), 227—236.
Prochazka, A., D. Gillard, & D. J. Bennett. (1997). Positive force feedback control of muscles. Journal of Neurophysiology, 77(6), 3226—3236.