MOTOR-UNIT RECRUITMENT IN SELF-REINNERVATED MUSCLE

1. Recruitment order of motor units in self-reinnervated medial gastro cnemius (MG) muscles was studied in decerebrate cats 16 mo after surgi cal reunion of the cut MG nerve. Pairs of MG motor units were isolated by dual microelectrode penetration of ventral roots to measure their recruitment sequence during cutaneous reflexes in relation to their ph ysiological properties. 2. Physiological properties of reconstituted m otor units appeared normal, as expected. Also normal were the relation ships among these properties: twitch and tetanic tension tended to inc rease with axonal conduction velocity and decrease with twitch contrac tion time. A small fraction of motor units (10/116) in reinnervated mu scles produced either no measurable tension or unusually large amounts of tension compared with controls. This was the only distinct feature of the sample of reconstituted units. 3. In muscles reinnervated afte r nerve section, stretch was notably ineffective in eliciting reflex c ontraction of MG muscles or their constituent motor units (only 5/116 units). Incomplete recovery from nerve section was probably the cause of this impairment, because stretch reflexes were readily evoked in ad jacent untreated muscles and in one reinnervated MG muscle that was st udied 16 mo after nerve crush. In contrast with the ineffectiveness of muscle stretch, sural nerve stimulation succeeded in recruiting 49/11 6 units, a proportion fairly typical of normal MG muscles. 4. The cont ractions of the first unit recruited in cutaneous reflexes tended to b e slower and less forceful than those of the other unit in a pair. By these measures, recruitment obeyed the size principle. This recruitmen t order with respect to unit contractile properties was not significan tly different (P > 0.05) between untreated and reinnervated muscles bu t was significantly (P < 0.005) different from random order in both gr oups. The same recruitment pattern was observed for pairs of motor uni ts sampled from the muscle reinnervated after nerve crush, whether uni ts were recruited by muscle stretch or sural nerve stimulation. 5. The usual tendency for motor units with slower conduction velocity (CV) t o be recruited in sural nerve reflexes before those with faster CV was not strong in reinnervated muscles. After nerve section the proportio n of units exhibiting the usual recruitment pattern was not significan tly different (P > 0.05) from a random pattern for CV. 6. The central finding is that the normal recruitment patterns recover from nerve inj ury in a muscle that is reinnervated by its original nerve. By contras t, stretch reflexes do not recover well from nerve section, and this d eficiency may contribute to motor disability.