gammamotorneuroner

Gammamotorneuroner, also known as gamma motor neurons, are a class of lower motor neurons that innervate the intrafusal muscle fibers within muscle spindles. Their cell bodies reside in the ventral horn of the spinal cord and their myelinated axons project to the spindle, where they cause a small contraction of the polar regions of the intrafusal fibers. This keeps the spindle taut and sensitive to stretch during muscle contraction. Gamma neurons are smaller than alpha motor neurons and include static and dynamic subtypes that modulate the responsiveness of the spindle’s sensory endings.

Anatomy and connections: Gamma motor neurons receive input from descending pathways in the brainstem and cortex, as well as from interneurons and proprioceptive feedback. They are part of the fusimotor system and participate in alpha–gamma coactivation, a coordinated activation of both gamma and alpha motor neurons during voluntary movement to prevent spindle slack and maintain accurate proprioceptive signaling.

Function: The gamma system adjusts spindle sensitivity to changes in muscle length. Static gamma modulation influences responses related to overall length, while dynamic gamma affects responses to the speed of stretch. By shaping the activity of primary and secondary spindle endings, gamma motoneurons influence the magnitude of the stretch reflex and the regulation of muscle tone in coordination with alpha motor neurons.

Clinical relevance: Altered gamma motor neuron activity can affect reflex gain and proprioception. In certain motor disorders, changes in fusimotor drive contribute to hyperreflexia or altered muscle tone. Studying gamma motoneurons helps clarify mechanisms of motor control, sensorimotor integration, and proprioceptive function.