Gamma motoneuron
2199417
206826965
2008-04-20T04:35:36Z
90.197.24.144
/* Function */
[[Image:MuscleSpindle.svg|right|264px|thumbnail|A [[muscle spindle]], with γ motor and Ia sensory fibers]]
'''Gamma motoneurons''' ('''γ-motoneurons'''), also called '''gamma motor neurons''', are a component of the [[fusimotor system]], the system by which the [[central nervous system]] controls [[muscle spindle]] sensitivity. The fusimotor system refers to the combination of [[muscle spindle]]s and γ-motoneurons. The function of the [[muscle spindle]] is to provide [[proprioceptive]] feedback for the movement, position and extension of [[muscle]]s.
γ-Motoneurons are located in the [[brainstem]] and [[spinal cord]] and are smaller than their [[α-motoneuron]] counterparts, which are responsible for controlling [[skeletal muscle]].
==Conduction velocity==
The [[axon]] from a γ-motoneuron is [[myelinated]] and has a slower conduction velocity than α-motoneuron axons. The typical conduction velocity for a γ-motoneuron axon is in the region of 4 to 24 ms<sup>-1</sup>.<ref>Andrew BL, Part NJ (1972) Properties of fast and slow motor units in hind limb and tail muscles of the rat. Q J Exp Physiol Cogn Med Sci 57:213-225.</ref><ref>Russell NJ (1980) Axonal conduction velocity changes following muscle tenotomy or deafferentation during development in the rat. J Physiol 298:347-360.</ref>
==Function==
γ-Motoneurons regulate the gain of the [[stretch reflex]] by adjusting the level of [[tension]] in the [[intrafusal muscle fiber]]s of the muscle spindle. This mechanism sets the baseline level of activity in α-motoneurons and helps to regulate muscle length and tone. For example, stimulation of a γ-Motoneuron from higher centers contracts the ends of the intrafusal fibres and consequently stretches the middle part of the muscle (where [[1a sensory neuron]]s are located). These afferent neurons are therefore innervated and go on to synapse with alpha-motoneurons.
==Populations of neurons==
There are two distinct populations of γ-motoneuron: dynamic γ-motoneurons and static γ-motoneurons.
* Dynamic γ-motoneurons have axons that innervate only dynamic [[nuclear bag fibre]]s (bag<sub>1</sub>) .
* Static γ-motoneurons innervate both [[nuclear chain fibre]]s and static nuclear bag (bag<sub>2</sub>) fibres.
==Effects of nuclear chain fibers==
The effect of [[nuclear chain fibre]]s on primary endings is to drive the discharge up to a [[frequency]] of around 60 Hz in a linear fashion, above which the discharge can become irregular. The activities of bag<sub>2</sub> fibres show an initial sharp peak in discharge, which gets less as the receptor adapts. Bag<sub>2</sub> fibres also reduce the dynamic sensitivity of the [[Ia afferent]] and sometimes also reduce the length sensitivity. Activation of bag<sub>1</sub> fibres has the effect of increasing both the length sensitivity and the dynamic sensitivity of the primary ending.<ref name="Boyd">Boyd I (1980) The action of the three types of intrafusal fibre in isolated cat muscle spindles on the dynamic and length sensitivities of primary and secondary sensory endings. In: Muscle Receptors and Movement (Taylor A, Prochazka A, eds), pp 17 - 32. London: MacMillan.</ref>
It is believed that the [[secondary sensory endings]] serve to measure length and [[muscle contraction]]s of [[nuclear chain fibre]]s at the pole via the static γ-motoneurons both excite the ending and increase its length sensitivity. Bag<sub>1</sub> and bag<sub>2</sub> fibres receive very little innervation from secondary endings, and activation of these fibres has a minimal effect on the discharge of the secondary ending.<ref name="Boyd"/>
==References==
<references/>
==See also==
* [[Intrafusal muscle fibre]]
==External links==
* {{Dorlands|m_20|12542886}}
* http://www.lib.mcg.edu/edu/eshuphysio/program/section8/8ch3/s8ch3_20.htm
* {{MeshName|Motor+Neurons,+Gamma}}
{{Nervous tissue}}
[[Category:Efferent neurons]]
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[[fi:Gammamotoneuroni]]