Gamma Motoneuron - Muscle Tone

Muscle Tone

Although muscles can be in a relaxed state, muscles have a general resting level of tension. This is termed muscle tone and is maintained by the motor neurons innervating the muscle. Its purpose is to maintain posture and assist in quicker movements, since if muscles were completely loose, then more neuronal firing would need to take place. In a situation where quick movements need to be made such as moving out of the way from a bicycle rider, the overall time would be greater if muscles contraction were not kept at some level.

The amount of tension in the muscles depend primarily on the resting level discharge of alpha motor neurons. It is mainly the Ia spindle afferents that aid in this process. Not only are the alpha motor neurons involved in muscle tone; gamma motor neurons are also involved through the gamma efferent system's action on intrafusal muscle fibers. These intrafusal muscle fibers control the resting level of the Ia afferent pathway, which in turn creates a steady level of alpha neuron activity. Therefore, when gamma motor neurons act on the intrafusal muscle fibers it can increase alpha neuron firing due to sensory input from the Ia sensory fibers.

Muscle tone can also be due to tonic discharge of gamma motor neurons. The activation to these neurons are mostly from the descending fibers of the facilitatory reticular formation. This leads to the stretching of muscle spindle, activation of alpha motor neurons, and finally a partially contracted muscle. The cerebellum is the alpha-gamma motor neuron linkage. Therefore, with the cerebellum the muscle tension is maintained via alpha motor neurons as well as the gamma motor neurons. However, in the absence of this brain structure, the tension is maintained only from alpha motor neurons.