The term motor-sensory unit (Figure 93.1) refers to the lower motor neuron, the muscle fibers it innervates, and the muscle sensory apparatus. The muscle sensory apparatus consists of the gamma motor neurons and their axons, the intrafusal muscle fibers , and the sensory fibers that carry information from the intrafusal muscle fibers to the alpha motor neurons.
Figure 93.1.— Schematic representation of the motor-sensory unit. Arrows indicate direction of normal signal conduction. AMN: alpha motor neuron; GMN: gamma motor neuron; DGC: dorsal ganglion cell; EFMF: extrafusal motor fiber (muscle fiber); IFMF: intrafusal motor fiber (muscle spindle).
function of the gamma motor neuron is to contract the polar regions of
the intrafusal fiber, thus opening the sodium stretch gated channels in
the central region of the intrafusal fibers. The opening of these channels
leads to the production of an action patential that is conducted by the
Ia fibers to the spinal cord and ultimately reach the alpha motor neuron
and produce a muscle contraction. The function of the motor-sensory apparatus
is to keep the muscle tone fixed at different muscle lengths.
Figure 93.2.— Schematic representation of the
alpha motor neuron and its relation to the Renshaw cell. Arrows indicate
direction of normal signal conduction. AMN: alpha motor neuron; RC: Renshaw