Properties of Motor Units Following self-Reinnervation of the Cat Superior Oblique Muscle. Waldeck, Robert F., E. Hazel Murphy, and Martin J. Pinter. Department of Anatomy and Neurobiology, Medical College of Pennsylvania, Philadelphia, PA 19129.
APStracts 2:0210N, 1995.
SUMMARY AND CONCLUSIONS
1. The mechanical properties of motor units of the cat superior oblique muscle and axonal conduction velocities of trochlear motoneurons have been studied at several post-operative times following intracranial axotomy of the trochlear nerve. 2. Whole muscle twitch forces were generally within the normal range by about 4 months post-operative, indicating that reinnervation is complete at this time. 3. Among animals studied 3.5-4.5 months following trochlear axotomy, average motor unit tetanic forces were increased by a factor of about 2.5 compared with units studied in normal superior oblique muscle. Average motor unit tetanic forces in animals studied 14.5-23 months following 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 time-to-peak relative to control. Reinnervated motor unit fatigue properties were similar to normal. 4. Average trochlear motoneuron conduction velocities for animals at all post-operative intervals remained significantly lower than the average conduction velocities from 3/4 normal animals. 5. Counts of Nissl- stained cell bodies in axotomized and control, contralateral trochlear nuclei showed that some cell loss had occurred, averaging about 17% 3.5-4.5 months post-operative and 24% 14.5-23 months post-operative. Associated with this loss was an increase (10%) of axotomized motoneuron soma cross-sectional area. 5. Muscle fiber cross-sectional areas (CSA) were measured in reinnervated superior oblique muscles and compared with CSAs from contralateral, control muscles. Average CSA was significantly decreased in all reinnervated muscles, with the relative decreases ranging from about 10% to 28%. 6. The results are discussed in terms of factors that determine motor 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 elimination. Our results are compared with those of other self- reinnervation studies, and the potential role played by the time muscle remains denervated in determining the persistence of polyneuronal innervation following reinnervation is considered. 

Received 30 March 1995; accepted in final form 20 July 1995.
APS Manuscript Number J213-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 30 July 1995.