PROPERTIES OF MOTOR UNITS AFTER SELF-REINNERVATION OF THE CAT SUPERIOR OBLIQUE MUSCLE

1. The mechanical properties of motor units of the cat superior obliqu e muscle and axonal conduction velocities of trochlear motoneurons hav e been studied at several postoperative times after intracranial axoto my of the trochlear nerve. 2. Whole muscle twitch forces were generall y within the normal range by similar to 4 mo postoperative, indicating that reinnervation is complete at this time. 3. Among animals studied 3.5-4.5 months after trochlear axotomy, average motor-unit tetanic fo rces were increased by a factor of similar to 2.5 compared with units studied in normal superior oblique muscle. Average motor-unit tetanic forces in animals studied 14.5-23 mo after axotomy were also increased relative to normal, but the difference was not significant. Among all reinnervated motor units, there was a tendency for increased twitch t ime-to-peak relative to control. Reinnervated motor-unit fatigue prope rties were similar to normal. 4. Average trochlear motoneuron conducti on velocities for animals at all postoperative intervals remained sign ificantly lower than the average conduction velocities from three of f our normal animals. 5. Counts of Nissl-stained cell bodies in axotomiz ed and control, contralateral trochlear nuclei showed that some cell l oss had occurred, averaging similar to 17% 3.5-4.5 mo postoperative an d 24% 14.5-23 mo postoperative. Associated with this loss was an incre ase (10%) of axotomized motoneuron soma cross-sectional area. 6. Muscl e fiber cross-sectional areas (CSA) were measured in reinnervated supe rior oblique muscles and compared with CSAs from contralateral, contro l muscles. Average CSA was significantly decreased in all reinnervated muscles, with the relative decreases ranging from similar to 10 to 28 %. 7. The results are discussed in terms of factors that determine mot or-unit force; muscle fiber CSA, specific force, and innervation ratio . We conclude that the increases of average motor-unit force in short- term reinnervated superior oblique muscles are most likely related to polyneuronal innervation of muscle fibers and that the return of these forces to normal levels in long-term muscles is related to synapse el imination. Our results are compared with those of other self-reinnerva tion studies, and the potential role played by the time muscle remains denervated in determining the persistence of polyneuronal innervation after reinnervation is considered.