The Golgi organ
(also called Golgi tendon organ
, neurotendinous organ or neurotendinous spindle), is a proprioceptive sensory receptor
organ that is located at the insertion of skeletal muscle fibres
into the tendons
of skeletal muscle
The Golgi organ should not be confused with the Golgi Apparatus, which is an organelle in the eukaryotic cell, or the Golgi stain, which is an histologic stain for neuron cell bodies.
The body of the organ is made up of strands of collagen
that are connected at one end to the muscle fibres and at the other merge into the tendon proper. Each tendon organ is innervated by a single afferent type Ib sensory fiber
that branches and terminates as spiral endings around the collagen strands. The Ib afferent axon is a large diameter, myelinated
axon. Each neurotendinous spindle is enclosed in a fibrous capsule
which contains a number of enlarged tendon fasciculi (intrafusal fasciculi). One or more nerve fibres perforate the side of the capsule and lose their medullary sheaths; the axis-cylinders subdivide and end between the tendon fibers in irregular disks or varicosities (see figure)...
During muscle tension the strands of collagen are stretched as the muscle length changes (concentric or eccentric contractions). This stretching deforms the terminals of the Ib afferent axon, opening stretch-sensitive cation channels. As a result, the axon is depolarized and fires nerve impulses up to the central nervous system via the spinal cord. The action potential frequency signals the force being developed within the muscle.
This sensory feedback plays an important role in spinal reflexes and in the central control of muscle contraction. Specifically, it is postulated that because a Golgi tendon organ exists in serial connection with muscle fibers, it can measure the tension that each muscle contraction builds up. The Ib afferent axon synapses with interneurons within the spinal cord and also relays information to the brain. One of the main spinal reflexes receiving an input from the Ib afferent is the autogenic inhibition reflex, which is involved with the regulation of the force profile of on-going muscle contractions. In other words, the golgi tendon organ will cause "weightlifting failure" to protect the muscle and tendons from excessive force.
The ascending or afferent pathways to the cerebellum are the dorsal and ventral spinocerebellar tracts and are involved in the cerebellar regulation of movement.
It was once believed that Golgi tendon organ
s were responsible for the clasp-knife reflex
observed in spinal cord
-injured patients. This theory has been rejected in favor of one that explains the reflex with free nerve endings