FORMATION OF TRANSIENT INAPPROPRIATE SENSORIMOTOR SYNAPSES IN DEVELOPING RAT SPINAL-CORDS

The specificity of the convergence of primary afferent projections fro m ankle muscles onto motoneurons that innervate these muscles was stud ied in lumbar spinal cords of embryonic and neonatal rats. The connect ivity pattern was determined for each motoneuron by stimulating nerves from ankle flexor and extensor muscles and recording the synaptic pot entials in identified motoneurons. In mature mammals, muscle spindle a fferents make direct excitatory connections with motoneurons that inne rvate homonymous and synergistic muscles, and with interneurons that i nhibit motoneurons innervating antagonistic muscles. Therefore, approp riate primary afferent-motoneuron connections were identified when sti mulation of homonymous and synergistic muscle nerves evoked monosynapt ic EPSPs. Two criteria were used for identification of EPSPs as monosy naptic potentials: (1) the monosynaptic potentials were evoked at the shortest latency, and (2) they were more resistant to fatigue by repet itive nerve stimulation than the longer-latency, polysynaptic potentia ls. Functionally inappropriate primary afferent-motoneuron contacts we re identified when stimulation of an antagonistic muscle nerve produce d monosynaptic EPSPs instead of polysynaptic IPSPs in homonymous moton eurons. At days 18-21 of gestation, about 30% of motoneurons were inne rvated by primary afferents off antagonist muscles. Such functionally inappropriate synapses persisted at birth, but their percentage was si gnificantly reduced within 3-5 d after birth. The findings suggested t hat in the developing spinal cord of the rat, a significant percentage of motoneurons were initially innervated by inappropriate primary aff erents of antagonistic muscles. The decrease in percentage of such ina ppropriate connections was correlated temporally with the increase in the frequency of spontaneous activity and the onset of myelination.