Effects of preventing reinnervation on axotomized spinal motoneurons in the cat. II. Changes in group Ia synaptic function.

1. Composite excitatory postsynaptic potentials (EPSPs) evoked by electrical stimulation of heteronymous group Ia afferents have been studied at various postoperative times in axotomized motoneurons that were denied the opportunity to reinnervate muscle. 2. The medial gastrocnemius (MG) nerve was transected and sutured onto the surface of the normally innervated lateral gastrocnemius (LG) muscle. The denervated MG muscle was excised thereby eliminating access of regenerating MG motor axons to vacant end-plates. 3. The mean amplitude of monosynaptic Ia EPSPs evoked by electrical stimulation of the LG-soleus (LGS) nerve and recorded in axotomized MG motoneurons showed an initial decline at 20 days postoperative (DPO) that was not significant. At 44 DPO, mean amplitude had declined significantly to 43% of the control mean amplitude. At 90 DPO, mean EPSP amplitude was not significantly different from control. At the latest postoperative time (150-180 DPO), mean amplitude was significantly less than the control amplitude. 4. Mean EPSP rise time (time-to-peak) was significantly increased (27%) at the earliest postoperative times (20-44 DPO). At later postoperative times (90-180), mean EPSP rise time was not significantly different from mean control rise time. 5. "Partial responses" superimposed on EPSPs were not observed at any postoperative time. 6. Mean posttetanic potentiation (PTP) of the LGS EPSP was significantly depressed at 20 DPO. At later postoperative times, PTP did not differ significantly from mean control PTP. 7. The possibility is considered that postaxotomy alterations in the electrical properties of motoneurons may explain these complex variations of mean EPSP amplitude and rise time.