Influences on the γ-Muscle Spindle System from Joint Mechanoreceptors

Abstract

The functional role of information mediated by sensory fibres from mechanoreceptors in joint tissue has been a controversial issue for several decades. Until recently it was generally believed that the principal function of joint mechanoreceptors were to initiate reflex activation of α-motoneurones as joint movements approach the limits of the joint’s normal working range, and thereby to protect the joint against hyper-rotations. This view on the functional role of joint receptor afferents was to a large extent based on early findings showing that (1) during passively imposed limb movements, only a very small number of joint mechanoreceptors were active at intermediate joint angles, whereas the majority of joint afferents were most active close to the limits of the physiological working range of the joint, (2) α-motoneurones were influenced by activity in high threshold joint afferents, but not by activity in low threshold joint afferents, and (3) no clear-cut proprioceptive, kinaesthetic or movement deficits were induced after removal of joint receptor feedback. The latter findings together with the observations that muscle vibration could induce illusions of movement in stationary limbs and that muscle receptor afferents had cortical projections made it widely accepted that muscle spindles and Golgi tendon organs were of considerable importance for proprioception, kinaesthesia and reflex regulation of movements, leaving joint receptor afferents to provide nonspecific facilitation of spinal pathways and to initiate joint protective reflexes.