Gamma motoneuron

Neuron: Gamma motorneuron
A muscle spindle, with γ motor and Ia sensory fibers
NeuroLex ID	sao1438006234
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Gamma motoneurons (γ-motoneurons), also called gamma motor neurons, are the efferent component of the fusimotor system, the system by which the central nervous system controls and modifies muscle spindle sensitivity. The fusimotor system refers to the combination of muscle spindles and γ-motoneurons. The function of the muscle spindle is to provide proprioceptive feedback for the movement, position and extension of muscles.

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 m/s.^[1][2]

Function

Gamma-motoneurons regulate the gain of the stretch reflex by adjusting the level of tension in the intrafusal muscle fibers 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 spindle. This part of the spindle is innervated by type Ia sensory fiber that go on to synapse with alpha-motoneurons, completing the gamma-loop.

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 fibres (bag₁) .
• Static γ-motoneurons innervate both nuclear chain fibres and static nuclear bag (bag₂) fibres.

Effects of nuclear chain fibers

The effect of nuclear chain fibres 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 bag₂ fibres show an initial sharp peak in discharge, which gets less as the receptor adapts. Bag₂ fibres also reduce the dynamic sensitivity of the Ia afferent and sometimes also reduce the length sensitivity. Activation of bag₁ fibres has the effect of increasing both the length sensitivity and the dynamic sensitivity of the primary ending.^[3]

It is believed that the secondary sensory endings serve to measure length and muscle contractions of nuclear chain fibres at the pole via the static γ-motoneurons both excite the ending and increase its length sensitivity. Bag₁ and bag₂ fibres receive very little innervation from secondary endings, and activation of these fibres has a minimal effect on the discharge of the secondary ending.^[3]

References

1. ^ 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.
2. ^ Russell NJ (1980) Axonal conduction velocity changes following muscle tenotomy or deafferentation during development in the rat. J Physiol 298:347-360.
3. ^ ^{a b} 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.

See also

• Alpha motor neuron
• Beta motor neuron
• Muscle spindle
• Intrafusal muscle fibre
• Extrafusal muscle fiber

External links

• http://www.lib.mcg.edu/edu/eshuphysio/program/section8/8ch3/s8ch3_20.htm
• MeSH Motor+Neurons,+Gamma
• NIF Search - Gamma Motor Neuron via the Neuroscience Information Framework