Muscle afferent activity and its central projection in man.

Muscle is a complex sensory organ as well as a contractile apparatus, and disease processes that produce muscle weakness and wasting will affect the sensory information transmitted by receptors in muscle. In addition, one of the sensory structures in muscle, the muscle spindle, receives a motor innervation, the gamma-efferent or fusimotor system, with which the brain can alter the feedback that it receives from muscle. Muscle spindle activity forms the afferent limb of spinal reflexes, such as the tendon jerk, and long-loop reflexes that traverse supraspinal reflex pathways. Muscle spindle activity constitutes the major afferent cue for kinaesthetic sensations and contributes to updating the centrally generated programme for movement. This paper reviews briefly some aspects of muscle spindle activity and its fusimotor control as studied in human subjects using microneurography.