The Golgi organ is located where the muscle fibers of the skeletal muscles meet the tendons. Tendons are the structures that connect the muscle fibers to the bones. The Golgi tendon organ is made up of strands of collagen and it also contains nerve tissue. It is also known as tendon organ, neurotendinous organ, or neurotendinous spindle. It is a proprioceptive sensory receptor organ. It was named after Camillo Golgi, an Italian physician, who made numerous discoveries related to the function of the human nervous system, in the late 19th and early 20th centuries. This structure should not be confused with Golgi apparatus, which is an organelle present in the eukaryotic cell, or the Golgi stain, which is a histological stain that is used in neuron cell bodies.
This structure is made up of strands of collagen which are connected at one end to the muscle fibers, and at the other end merge with the tendon proper. Each tendon organ is innervated by a single afferent sensory fiber, which branches and terminates as spiral endings around the collagen strands. The afferent axon is a myelinated axon which has a large diameter. Each neurotendinous spindle is enclosed in a fibrous capsule that contains a number of enlarged tendon fasciculi, which is, intrafusal fasciculi. One or more nerve fibers perforate the side of the capsule and lose their medullary sheaths, also, the axis-cylinders subdivide and terminate between the tendon fibers in irregular discs or varicosities.
When the muscle generates force, its sensory terminals get compressed. This stretching tends to deform the terminals of the afferent axon, opening stretch-sensitive cation channels. As a result, the axon is depolarized and fires nerve impulses, which are propagated to the spinal cord. This action potential frequency signals the force being developed by the ten to twenty motor units within the muscle. This is actually representative of the entire muscle force.
The sensory feedback tends to generate spinal reflexes and supraspinal responses which control the extent of muscle contraction. The afferent synapses with interneurons within the spinal cord, that also project to the brain cerebellum and the cerebral cortex. One of the main spinal reflexes associated with this afferent nerve is the autogenic inhibition reflex, which helps regulate the force of muscle contraction. Initially, it was thought that tendon organs had a high threshold, hence, only become active when there are high muscle forces. Eventually, it was thought that tendon organ input caused a kind of 'weightlifting failure' through the clasp-knife reflex, hence, protecting the muscle and tendons from excessive force. During locomotion, the afferent input excites motoneurons of the receptor-bearing muscles, and it also affects the timing of the transitions between the stance and swing phases of locomotion. Thus, the switch to autogenic excitation is a form of positive feedback.
The action of the Golgi organ particularly interests weightlifters, who want to steadily increase the amount of weight that they can handle, while muscle building. Some of them say that, forcing the muscles to do additional repetitions with the help of very heavy weights, could train the muscles to fail in future lift attempts. However, others claim that this is not likely because of the way the Golgi organ responds to muscle tension, and reports this information to the brain. It is claimed that, muscle stretching would also pull on the tendons and stimulate this organ's afferent. Most of the force of a stretch is absorbed by the muscle itself, so a muscle contraction is actually a much better stimulus for the Golgi tendon.
This is an important structure that is present at the junction of the muscles and tendons, as it is a proprioceptor that is sensitive to changes in muscle tension, and the rate of change of muscle tension.