Properties of self-reinnervated motor units of medial gastrocnemius of cat. II. Axotomized motoneurons and time course of recovery.

This study tested the hypothesis that functional connection to muscle is necessary for expression of normal motoneuron electrical properties. Also examined was the time course of self-reinnervation. Properties of individual medial gastrocnemius (MG) motor units were examined following section and reanastomosis of the MG nerve. Stages examined were 3-5 wk (prior to reinnervation, no-re), 5-6 wk (low-re), 9-10 wk (med-re), and 9 mo (long-re, preceding paper) after nerve section. Motor units were classified on the basis of their mechanical response as type fast twitch, fast fatiguing (FF), fast twitch with intermediate fatigue resistance (FI), fast twitch, fatigue resistant (FR), or slow twitch, fatigue resistant (S) (11, 24). Motoneuron electrical properties were measured. Muscle fibers were classified using histochemical methods as type fast glycolytic (FG), fast oxidative glycolytic (FOG), or slow oxidative (SO) (60). Prior to functional reinnervation, MG motoneurons exhibited increased input resistance, decreased rheobase, decreased rheobase/input resistance, and decreased axonal conduction velocity. There was no change in mean afterhyperpolarization (AHP) half-decay time. Normal relationships between motoneuron electrical properties were lost. These data are consistent with dedifferentiation of motoneuron properties following axotomy (35, 47). At 5-6 wk after reanastomosis, motor-unit tensions were small, and motoneuron membrane electrical properties were unchanged from the no-re stage. There were no differences in motoneuron electrical properties between cells that elicited muscle contraction and those that did not. Motor-unit types were first recognizable at the med-re stage. The proportions of fast and slow motor units were similar to normal MG. Within the fast units, there were fewer type-FF units and more type-FI and type-FR units than normal, reflecting a general increase in fatigue resistance at this stage. Neither motoneuron membrane electrical properties nor muscle contractile properties had reached normal values, although both were changed in that direction from the low-re stage. Normal relationships between muscle properties, between motoneuron properties, and between motoneuron and muscle properties were re-established. The correspondence between motor-unit type and motoneuron type was similar to normal or 9 mo reinnervated MG. Muscle-unit tetanic tensions became larger with time after reinnervation. Most of the increase in muscle tension beyond the med-re stage could be accounted for by increase in muscle fiber area. There was an increased proportion of SO muscle fibers observed in the med-re muscles, as at the long-re stage.(ABSTRACT TRUNCATED AT 400 WORDS)