- Golgi tendon organ
Golgi tendon organ Organ of Golgi (neurotendinous spindle) from the human tendo calcaneus. Latin organum sensorium tendinis Gray's subject #233 1061 Code TH H3.03.00.0.00024
The Golgi organ (also called Golgi tendon organ, tendon organ, neurotendinous organ or neurotendinous spindle), is a proprioceptive sensory receptor organ that is located at the insertion of skeletal muscle fibers into the tendons of skeletal muscle. It provides the sensory component of the Golgi tendon reflex.
The body of the organ is made up of strands of collagen that are connected at one end to the muscle fibers 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).
When the muscle generates force, the sensory terminals are compressed. This stretching deforms the terminals of the Ib afferent axon, opening stretch-sensitive cation channels. As a result, the Ib axon is depolarized and fires nerve impulses that are propagated to the spinal cord. The action potential frequency signals the force being developed by the 10 to 20 motor units within the muscle. This is representative of whole muscle force 
The Ib sensory feedback generates spinal reflexes and supraspinal responses which control muscle contraction. Ib afferents synapses with interneurons within the spinal cord that also project to the brain cerebellum and cerebral cortex. One of the main spinal reflexes associated with Ib afferent activity is the autogenic inhibition reflex, which helps regulate the force of muscle contractions. 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 also affects the timing of the transitions between the stance and swing phases of locomotion. The switch to autogenetic excitation is a form of positive feedback.
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.
- Golgi–Mazzoni corpuscles
- ^ a b Prochazka, A.; Gorassini, M. (1998). "Ensemble firing of muscle afferents recorded during normal locomotion in cats". J Physiology 507 (1): 293–304. doi:10.1111/j.1469-7793.1998.293bu.x. PMC 2230769. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2230769.
- ^ Stephens, J. A.; Reinking, R. M.; Stuart, D. G. (1975). "Tendon organs of cat medial gastrocnemius: responses to active and passive forces as a function of muscle length". J Neurophysiology 38 (5): 1217–1231. PMID 1177014. http://jn.physiology.org/content/38/5/1217.
- ^ Conway, B. A.; Hultborn, H.; Kiehn, O. (1987). "Proprioceptive input resets central locomotor rhythm in the spinal cat". Experimental Brain Research 68 (3): 643–656. doi:10.1007/BF00249807. PMID 3691733.
- ^ Prochazka, A.; Gillard, D.; Bennett, D. J. (1997). "Positive Force Feedback Control of Muscles". J Neurophysiol 77 (6): 3226–3236. PMID 9212270. http://jn.physiology.org/content/77/6/3226.
- ^ Houk, J.; Henneman, E. (1967). "Responses of Golgi tendon organs to active contractions of the soleus muscle of the cat". J Neurophysiology 30 (3): 466–481. PMID 6037588. http://jn.physiology.org/content/30/3/466.
- Golgi+tendon+organ at eMedicine Dictionary
Nervous system, receptors: somatosensory system (GA 10.1059) Medial lemniscus
Touch/mechanoreceptors: Lamellar/Pacinian corpuscles – vibration • Tactile/Meissner's corpuscles – light touch • Merkel's discs – pressure • Bulbous/Ruffini endings - stretch • Free nerve endings – pain • Hair cells • Baroreceptor
Spinothalamic tract Histology: nervous tissue (TA A14, GA 9.849, TH H2.00.06, H3.11) CNSGeneralOther PNSGeneral Neurons/
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