Factors Outside Neuraxis Mediate Acute Increase in EPSP Amplitude Caudal to Spinal Cord Transection

1. This series of experiments was undertaken to explore mechanisms responsible for the elevation in Ia-fiber alpha-motoneuron EPSP amplitude that occurs within hours of a spinal cord transection at a more rostral level. The spike-triggered averaging technique was used to record individual EPSPs produced by single medial gastrocnemius (MC) Ia-fibers in MG motoneurons. 2. In anesthetized cats spinal cord transection at a level (T13) rostral to the recording segment (L7-S1) causes no change in EPSP properties within the first 10-20 min. 3. In cats with chronic spinal cord transection at T13, a second transection at T11 causes enlargement of EPSPs recorded in MG motoneurons within hours. The properties of EPSPs in these doubly transected preparations differ in some details from those observed after a single acute transection. 4. The finding that spinal cord transection at T11 can result in enlarged EPSPs at L7-S1 in preparations previously transected at T13 indicates that spinal injury can influence synaptic transmission via pathways that are not totally contained within the neuraxis. 5. Spinal cord transection consistently yields a greater enlargement of EPSPs recorded in small motoneurons than those in large ones. Large motoneurons participate only when the increase exhibited in the small ones is relatively large