Development of spinal reflex pathways from muscle afferents to motoneurones in chick embryos devoid of descending inputs.

1. The synaptic connections of reflex pathways between hindlimb muscle afferents and motoneurones were investigated in chicken embryos. Descending inputs to the lumbar spinal cord were eliminated by removing two to four segments of the thoracic spinal cord at embryonic day 2 (E2; gap operation). Intracellular recordings from motoneurones innervating the lateral gastrocnemius (LG) muscle were performed in isolated spinal cord preparations of normal and gap-operated embryos at E14-E15. 2. In both normal and gap-operated embryos, homonymous and synergistic muscle nerve stimulation evoked EPSPs in LG motoneurones at a short and fixed latency, suggesting that they were evoked monosynaptically. EPSPs from synergistic muscle afferents were much smaller than those from homonymous muscle afferents. The volleys from the antagonistic muscle nerve evoked IPSPs at a longer central latency than found for EPSPs in both embryos. 3. The maximal amplitudes of homonymous and synergistic EPSPs in gap-operated embryos were 1.3 and 1.7 times, respectively, larger than in normal embryos. Homonymous EPSPs were observed in all LG motoneurones examined, but synergistic EPSPs were more common in gap-operated than in normal embryos. 4. Antagonistic IPSPs were more common in motoneurones of gap-operated embryos than in those of normal embryos. Homonymous and synergistic muscle nerve stimulation also elicited IPSPs in LG motoneurones in both normal and gap-operated embryos. IPSPs evoked both by homonymous and by synergistic muscle nerve stimulation were more common in gap-operated than in normal embryos. 5. The spatial pattern of reflex pathways from hindlimb muscle afferents to motoneurones in chick embryos devoid of both supraspinal and long propriospinal inputs to the lumbar spinal cord is similar to that in normal embryos. However, both mono- and polysynaptic connections in these pathways are enhanced by the blockade of descending projections.