Formation of transient inappropriate sensorimotor synapses in developing rat spinal cords.

The specificity of the convergence of primary afferent projections from ankle muscles onto motoneurons that innervate these muscles was studied in lumbar spinal cords of embryonic and neonatal rats. The connectivity pattern was determined for each motoneuron by stimulating nerves from ankle flexor and extensor muscles and recording the synaptic potentials in identified motoneurons. In mature mammals, muscle spindle afferents make direct excitatory connections with motoneurons that innervate homonymous and synergistic muscles, and with interneurons that inhibit motoneurons innervating antagonistic muscles. Therefore, appropriate primary afferent-motoneuron connections were identified when stimulation of homonymous and synergistic muscle nerves evoked monosynaptic EPSPs. Two criteria were used for identification of EPSPs as monosynaptic potentials: (1) the monosynaptic potentials were evoked at the shortest latency, and (2) they were more resistant to fatigue by repetitive nerve stimulation than the longer-latency, polysynaptic potentials. Functionally inappropriate primary afferent-motoneuron contacts were identified when stimulation of an antagonistic muscle nerve produced monosynaptic EPSPs instead of polysynaptic IPSPs in homonymous motoneurons. At days 18-21 of gestation, about 30% of motoneurons were innervated by primary afferents of antagonist muscles. Such functionally inappropriate synapses persisted at birth, but their percentage was significantly reduced within 3-5 d after birth. The findings suggested that in the developing spinal cord of the rat, a significant percentage of motoneurons were initially innervated by inappropriate primary afferents of antagonistic muscles. The decrease in percentage of such inappropriate connections was correlated temporally with the increase in the frequency of spontaneous activity and the onset of myelination.