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. A diagram showing the CNS: 1. ... A muscle spindle is a specialized muscle structure innervated by both sensory and motor neuron axons. ... A muscle spindle is a specialized muscle structure innervated by both sensory and motor neuron axons. ...

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). The anterior horn is the anterior division of the lateral ventricle of the brain. ... In vertebrates, motoneurons (also called motor neurons) are efferent neurons that originate in the spinal cord and synapse with muscle fibers to facilitate muscle contraction and with muscle spindles to modify proprioceptive sensitivity. ... A top-down view of skeletal muscle Skeletal muscle is a type of striated muscle, attached to the skeleton. ... An axon, or nerve fiber, is a long slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neurons cell body or soma. ...

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). Sine waves of various frequencies; the lower waves have higher frequencies than those above. ...

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). The word contraction when used alone, has several possible meanings in the English language. ...

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.