The influence of contralateral primary afferents on Ia inhibitory interneurones in humans.

Abstract

SUMMARY1. Contralateral influences on short latency reciprocal inhibition between wrist extensor and flexor muscles were investigated in twenty-two healthy volunteers. Reciprocal inhibition, probably mediated through the Ia inhibitory interneurone, was measured by conditioning the flexor carpi radialis (FCR) H reflex by weak stimulation of the ipsilateral radial nerve. Maximum reciprocal inhibition occurring at a precise delay between conditioning and conditioned stimulations was taken as the test level of inhibition.2. Contralateral median or radial nerves were stimulated at short intervals before the onset of reciprocal inhibition. The latter was increased by 8-6% after median nerve stimulation and decreased by 16-5 % after radial nerve stimulation.3. The contribution of sensory fibres in the two nerves to contralateral effects was investigated by stimulating purely sensory branches of the nerves. No clear modification of the contralateral reciprocal inhibition was observed. The effects produced by mixed nerve stimulation are thus likely to have been mediated by la fibres.4. In three hemiplegic patients where reciprocal inhibition was reduced unilaterally, stimulation on the spastic side produced contralateral effects similar to those observed in normal subjects. This result indicates that contralateral effects are not mediated through the la inhibitory interneurone ipsilateral to the conditioning stimulus.5. Since contralateral effects occur after short delays (2 ms, median nerve; 3 ms, radial nerve), we suggest a functional scheme in which the excitability of la inhibitory interneurones is modified by contralateral primary afferents via the interneurones activated by group I fibres, probably Ia fibres. The short delays indicate that the interneurone transmitting primary afferent influences to the contralateral side is probably excitatory.

Expression of the bilateral deficit during reflexively evoked contractions

“…Other investigations have demonstrated spinal-level cross-inhibition of the homonymous muscle motoneuronal pool (7) or muscle group (23). Cross-inhibition of contralateral agonist and antagonist muscles at the spinal level was also shown in spinal cats (2) and in right hemispastic patients (7). These studies demonstrate the existence of interneurons that may provide intraspinal pathways that could transmit Ia impulses to the Ia inhibitory interneurons of the contralateral side (7).…”

“…Contrary to the work involving isometric contractions, studies involving more complex movement patterns, such as dynamic, multijoint tasks (34-39, 43), generally find a BLD rather than not. If a spinal inhibitory mechanism does have the potential to mediate the BLD, the intensity of such inhibition might be greater during dynamic movements, as previous work (7,23) indicates that afferent input from muscle spindles can elicit a cross-inhibitory effect on homonymous muscles. Because the nuclear bag fibers of the spindle are primarily activated under dynamic conditions, one might speculate that spinal mediation of the BLD is more influential when maximum contractions are performed during dynamic, multijoint movements.…”

Unilateral practice of a ballistic movement causes bilateral increases in performance and corticospinal excitability

“…At a segmental level, movements performed with one limb typically result in depression of the central gain of the Ia afferent reflex pathway (4,6,14). There are also crossed interactions between inhibitory interneurons that receive inputs from primary muscle afferents (8,16, 34).Because high-force contractions promote strong interhemispheric interactions, the spillover hypothesis fits well as a possible explanation for the bilateral improvements in muscular strength that can occur in response to unilateral exercise involving high-force contractions (i.e., strength training). This contralateral strength training effect is a form of cross-transfer that has been known since 1894 (36).…”

“…At a segmental level, movements performed with one limb typically result in depression of the central gain of the Ia afferent reflex pathway (4,6,14). There are also crossed interactions between inhibitory interneurons that receive inputs from primary muscle afferents (8,16,34).…”

Absence of effects of contralateral group I muscle afferents on presynaptic inhibition of Ia terminals in humans and cats

“…For instance, electric stimulation of contralateral group I afferents [Ia fibers from extensor carpi radialis (ECR)] changed the reciprocal inhibition from ECR to the flexor carpi radialis muscle (Delwaide and Pepin 1991). In lower limb experiments, a presynaptic mechanism has been proposed to explain the observed soleus (SO) H-reflex inhibition in response to activation of contralateral group I muscle afferent by either passive leg pedaling movements (Cheng et al 1998) or mechanical Achilles tendon stimulation (Koceja and Kamen 1992).…”