Golgi tendon organ
Proprioceptive sensory receptor organ
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Through Skeletal muscle, Reflex arc and Series and parallel circuits
It represents the sensory leg of the Golgi tendon reflex arc.
The Golgi tendon organ is one of several eponymous terms named after the Italian physician Camillo Golgi.
Structure
The body of the Golgi tendon organ is made up of braided strands of collagen (intrafusal fasciculi) that are less compact than elsewhere in the tendon and are encapsulated. The capsule is connected in series (along a single path) with a group of muscle fibers (10-20 fibers) at one end, and merge into the tendon proper at the other. Each capsule is about 1 mm long, has a diameter of about 0.1 mm.
One or more fast-conducting type Ib sensory nerve fibers penetrate the capsule and lose their medullary sheaths, branch, intertwine with the collagen fibers, and terminate as flattened leaf-like endings between the collagen strands (see figure).
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Function
When the muscle generates force, the sensory terminals of the Ib afferent axons are compressed and become deformed which causes the opening of stretch-sensitive cation channels, depolarizing the axon and causing it to fire nerve impulses. The action potential frequency encodes the force being developed by the muscle fibers associated with the Golgi tendon organ. The average level of activity in a tendon organ population is representative of the whole muscle force.
Integrative function
The Ib sensory feedback generates stretch reflexes and supraspinal responses which control muscle contraction. Ib afferents synapse with interneurons in the spinal cord that also project to the brain cerebellum and cerebral cortex. The Golgi tendon reflex assists in regulating muscle contraction force. It is associated with the Ib. Tendon organs signal muscle force through the entire physiological range, not only at high strain.
During locomotion, Ib input excites rather than inhibits motoneurons of the receptor-bearing muscles, and it affects the timing of the transitions between the stance and swing phases of locomotion. The switch to autogenic excitation is a form of positive feedback.
The ascending or afferent pathways to the cerebellum are the dorsal and ventral spinocerebellar tracts. They are involved in the cerebellar regulation of movement.
History
Until 1967, it was believed that Golgi tendon organs had a high threshold, only becoming active at high muscle forces. Consequently, it was thought that tendon organ input caused "weightlifting failure" through the clasp-knife reflex, which protected the muscle and tendons from excessive force. However, the underlying premise was shown to be incorrect by James Houk and Elwood Henneman in 1967.