Role of joint afferents in motor control exemplified by effects on reflex pathways from Ib afferents.

Abstract

Intracellular recording from motoneurons to hind limb muscles in the cat was used to investigate the effect of volleys in the posterior nerve to the knee joint on motoneurons and on transmission from Ib afferents. Volleys in the joint nerve facilitate transmission in disynaptic and trisynaptic inhibitory and excitatory reflex pathways from Ib afferents. Facilitation, which appears at a strength of 1.5 times threshold, is evoked, not by Ib afferents which contaminate the joint nerve, but by afferents from joint receptors. The time course of facilitation of the disynaptic Ib IPSP [inhibitory post-synaptic potentials] in motoneurons indicates that these joint afferents have disynaptic connections with the interneurons of the Ib inhibitory pathway. A brief description is given of synaptic effects evoked in motoneurons by graded electrical stimulation of the posterior nerve to the knee joint. At low stimulus strength the effects may be evoked via interneurons of Ib reflex pathways, but some results suggest that other interneuronal paths are utilized as well. Somewhat higher strengths, but occasionally < 2 times threshold, produce later synaptic effects presumably mediated by reflex paths from the flexor reflex afferents; it does not seem likely that the contributory afferents in the joint nerve have nociceptive function. Impulses from joint receptors can influence tension regulation from Golgi tendon organs; if these receptors are activated in the terminal phase of the movement they may contribute a purposeful decrease of tension. Joint receptors may have an important role in motor regulation by their effects on interneurons of the different neuronal systems controlling motoneurons.