Gamma motoneurons
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γ-motoneurons (gamma-motoneurons or motor neurons) are a component of the fusimotor system, the system by which the CNS controls muscle spindle sensitivity. The fusimotor system refers to the combination of muscle spindles and γ-motoneurons.
γ-motoneurons are located in the ventral horn and are smaller than their counterparts, α-motoneurons which are responsible for controlling skeletal muscle. The axon from a γ-motoneuron are myelinated and have a slower conduction velocity than α-axons. The typical conduction velocity for a γ-axon is in the region of 4 to 24ms-1 (Andrew and Part, 1972; Russell, 1980).
Gamma motor neurons 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 motor neurons and helps to regulate muscle length and tone.
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 (bag1) fibres and static γ-motoneurons (γs) innervate both nuclear chain fibres and static nuclear bag (bag2) fibres.
The effect of nuclear chain fibres on primary endings is to drive the discharge up to a frequency of around 60Hz 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 (Boyd, 1980).
It is believed that the secondary sensory endings serve to measure length and 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 (Boyd, 1980).
[edit] References
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.
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.
Russell NJ (1980) Axonal conduction velocity changes following muscle tenotomy or deafferentation during development in the rat. J Physiol 298:347-360.
