Gamma motoneuron



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 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 ms-1.

Function
γ-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.

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.

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.