IMMEDIATE INCREASE IN IA-MOTONEURON SYNAPTIC TRANSMISSION CAUDAL TO SPINAL-CORD TRANSECTION

Individual EPSPs recorded from triceps surae motoneurons using the spike-triggered averaging technique were found to be larger in amplitude in cats with acute spinal cord transection than in cats with intact spinal cord. This increase occurred whether transection was at T13 or L5. Very large EPSPs (> 400 μV) only rarely observed in intact preparations were commonly found. In preparations studied before and immediately after spinal cord transection, it was observed that the increase in EPSP amplitude did not occur instantaneously but took a few hours to develop. The average proportion of the motoneuron pool contacted by single Ia afferent fibers (projection frequency) was close to 100%, beginning apparently immediately after transection at either level. This represented an increase in comparison to the 80% projection frequency in intact preparations. We conclude that spinal transection acutely enhances the strength of Ia-motoneuron synaptic transmission. The delayed increase in amplitude suggests the action of nonneural factors. The relatively quick increase in projection frequency indicates a functional change in previously inactive synapses, rather than the growth of new connections. Furthermore, the amplitude and projection frequency changes appear to be independent.