Golgi tendon organs are proprioceptors responsible for collecting and transmitting data relating to the tension developed by the muscles.
Located at the level of the muscle-tendon junction, where the extrafusal fibers continue with the tendon ones, they are involved in the genesis of the so-called inverse myotatic reflex: when the muscles are contracted, especially if in an isometric way, the Golgi organs detect the degree of developed tension, triggering a reflex that leads to muscle relaxation. By doing so, they protect the fibers from damage from over-shrinking.
Beyond this classic physiological role, which is nowadays downsized, we must not forget that the Golgi muscle tendon organs are also sensitive to reduced tensions and remain active at all levels of muscle shortening. Therefore, they do not act as a simple "emergency" mechanism, but as a "detection control unit", useful for informing the CNS on the degree of tension developed during movements.
Anatomy and physiology
Golgi muscle tendon organs are located in the region of transition between striated muscle fibers and tendon fibers. At this level, they are arranged in series with the contractile elements, in the form of spindle structures, a few millimeters long, formed by fibrous strings, which are inserted with one end into the tendon and with the other into the muscular region close to the muscle-tendon border. The entire structure is enveloped in a capsule of rigid connective tissue.
The extrafusal fibers with which each organ of the Golgi draws relationships generally belong to several motor units and are numerically between 10 and 20.
The sensory innervation of the Golgi tendon organs is made up of a bundle of nerve fibers that are distributed around the fibrous strings. These terminations are richly myelinated (they lose this coating when they enter the tendon organ), have a large diameter, belong to the class of nerve fibers Ib and are therefore endowed with a very high conduction speed. Unlike neuromuscular spindles, the Golgi organs are devoid of motor innervation.
As mentioned, muscle contraction subjects the tendons to a certain degree of stretching, which is transmitted to the Golgi tendon muscle organs. It is believed that this traction causes the approaching of the fibrous strings, increasing the pressure on the sensory terminations Ib and inducing them to discharge. This succession of impulses is sent to the spinal cord, where it excites inhibitory interneurons which suppress the alpha motor neurons responsible for innervating the same muscle from which the signal originated. As a result, muscle contraction slows or ceases in response to stimuli from the Golgi musculotendinous organs.
The whole process is called an inverse myotatic reflex, or a switchblade, and has a dual function: on the one hand it is useful for avoiding tendon injuries caused by a too violent contraction and on the other it protects the muscles from damage associated with the abrupt decrease. of the applied load.
The inverse myotatic reflex, triggered by the Golgi muscle tendon organs, therefore opposes excessive shortening, as opposed to the normal myotatic reflex, triggered by the muscle spindles, which opposes excessive stretching of the muscle.
Muscle tendon organs are not all the same, but they exist with more or less marked degrees of rigidity. The less rigid ones are activated by lower tensions than those with higher stiffness. In any case, it seems that their role in the appearance of the inverse myotatic reflex is much more marginal than what is reported by the classic physiology texts. All this due to the "non-linear" behaviors that emerged from some studies, in which the frequency of discharge of the Golgi organs proved not to be proportional to the degree of muscle tension applied. On the other hand, these proprioceptors, being activated even by minimal tensions, would have the task of informing the central nervous system about the state of active contraction of the muscles. If a group of fibers contracts with a higher intensity than the others, the organ Golgi tendon muscle intervenes indirectly by relaxing them and decreasing its inhibitory effect on those contracted to a lesser extent.The result is a more fluid and harmonious movement already at low levels of muscle tension.
