Effects of preventing reinnervation on axotomized spinal motoneurons in the cat. I. Motoneuron electrical properties

1. The intent of this study was to determine the effect on the electrical properties of axotomized spinal motoneurons when motor axons are allowed to regenerate but are denied the opportunity to reinnervate muscle. 2. The nerve supplying the medial gastrocnemius (MG) muscle in cats was served close to its entry into the muscle and sutured onto the surface of the lateral gastrocnemius (LG) muscle. The MG muscle was excised to prevent availability of vacant end-plates to the regenerating MG axons. The electrical properties of antidromically identified MG motoneurons were studied using intracellular recording at various postoperative intervals. 3. In 9 of 12 experimental animals, no sign of functional innervation by MG axons of the LG muscle could be detected. In three experimental animals, electrical and contraction activity in the LG muscle was observed following electrical stimulation of the transplanted MG nerve. The observed electrical and contraction activity was, however, negligible compared to the effects of electrical stimulation of the intact LG-soleus nerve. 4. At the earliest postoperative interval studied (20 days), MG motoneuron electrical properties [input resistance, afterhyperpolarization (AHP) duration, conduction velocity, time constant, rheobase current, and sag] exhibited significant changes that were nearly identical to those described for spinal motoneurons following section of ventral roots or motor nerves or in the earliest stages of reinnervation. 5. At the 44-60 day postoperative (DPO) intervals, several motoneuron electrical properties showed signs of recovery to control levels. At 44 DPO, average values of input resistance, time constant, and AHP duration declined from the significant increases observed at 20 DPO and could not be distinguished statistically from control mean values. 6. These indications of an early recovery of normal electrical properties were not sustained. At subsequent postoperative intervals (90, 120, and 150-180 DPO), average values of motoneuron electrical properties tended to be similar to those observed at 20 DPO. 7. Correlations observed among control motoneuron electrical properties were weakened and the pattern of correlation was disrupted at all postoperative intervals. 8. In conjunction with previous results demonstrating recovery of normal electrical properties following reinnervation (Foehring et al. 1986b), our findings suggest that functional contact with muscle is required for the full expression of the normal range of motoneuron electrical properties.(ABSTRACT TRUNCATED AT 400 WORDS)