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).

The 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.
The alpha motor neuron is influenced by structures “below it” and “above it.” The structures that influence the alpha motor neuron from “below” are the muscle sensory apparatus and the Renshaw cells (Figure 93.2). The Renshaw cells are neurons that are in close proximity to the alpha motor neuron.

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 cell.

The Renshaw cells receive excitatory signals from the alpha motor neuron by a collateral axon of the alpha motor neuron and send inhibitory signals back to the alpha motor neuron.