PATTERN OF MONOSYNAPTIC HETERONYMOUS IA CONNECTIONS IN THE HUMAN LOWER-LIMB

Changes in the firing probability of single motor units in response to electrical stimulation of muscle nerves were used to derive the proje ctions of muscle spindle Ia afferents to the motoneurones of various l eg and thigh muscles. Discharges of units in soleus, gastrocnemius med ialis, peroneus brevis, tibialis anterior, quadriceps, biceps femoris and semitendinosus were investigated after stimulation of inferior sol eus, gastrocnemius medialis, superficial peroneal, deep peroneal and f emoral nerves. Homonymous facilitation, occurring at the same latency as the H reflex and therefore attributed to monosynaptic Ia EPSPs, was found in virtually all the sampled units. In many motor nuclei an ear ly facilitation was also evoked by heteronymous low-threshold afferent s. The heteronymous facilitation was considered to be mediated through a monosynaptic pathway when the difference between the central latenc ies of heteronymous and homonymous peaks was not more than 0.2 ms. The heteronymous Ia connections were widely distributed. In particular, m onosynaptic coupling between muscles operating at different joints app ears to be the rule in humans, though it is rare between ankle and kne e muscles in the cat and the baboon.