Fusimotorisk

Fusimotorisk, also known as the fusimotor system or gamma motor system, refers to the neural mechanism controlling the sensitivity of muscle spindles, which are specialized proprioceptive receptors located within skeletal muscles. This system plays a crucial role in fine-tuning motor control and reflex responses by adjusting the stretch sensitivity of muscle spindles.

The fusimotor system consists of gamma motor neurons, which innervate the intrafusal muscle fibers within muscle spindles. These neurons are distinct from alpha motor neurons, which control the contraction of extrafusal muscle fibers responsible for actual movement. Gamma motor neurons regulate the tension in the intrafusal fibers, thereby influencing the discharge rate of spindle afferents (sensory neurons) that signal muscle length and rate of change to the central nervous system.

The primary function of the fusimotor system is to ensure that muscle spindles remain sensitive to stretch even when the extrafusal muscle fibers contract. This prevents the spindles from becoming "unloaded" and ensures accurate feedback for reflexes such as the stretch reflex (myotatic reflex). The system also contributes to voluntary movements by modulating spindle sensitivity in response to descending commands from the brain, allowing for precise adjustments in muscle tone and coordination.

Dysfunction or abnormalities in the fusimotor system can lead to motor control disorders, affecting posture, balance, and voluntary movements. Research into the fusimotor system has implications for understanding neurological conditions such as spasticity, ataxia, and other movement disorders, as well as for developing therapeutic interventions.