FAST-TO-SLOW CONVERSION FOLLOWING CHRONIC LOW-FREQUENCY ACTIVATION OF MEDIAL GASTROCNEMIUS MUSCLE IN CATS: I. MUSCLE AND MOTOR UNIT PROPERTIES. Gordon, T., Tyreman, N., Rafuse, V.F. Munson, J.B. Department of Pharmacology, Division of Neuroscience, University of Alberta, Edmonton, CANADA T6G 2S2.
APStracts 4:0038N, 1997.
ABSTRACT
This study of cat medial gastrocnemius (MG) muscle and motor unit (MU) properties tests the hypothesis that the normal ranges of MU contractile force, endurance and speed are directly associated with the amount of neuromuscular activity normally experienced by each MU. We have synchronously activated all MUs in the MG muscle with the same activity (20Hz in a 50% duty cycle) and asked whether conversion of whole muscle contractile properties is associated with loss of the normal heterogeneity in MU properties. Chronically implanted cuff electrodes on the nerve to MG muscle were used for 24 h/d stimulation and for monitoring progressive changes in contractile force, endurance and speed by periodic recording of maximal isometric twitch and tetanic contractions under halothane anesthesia. Chronic low frequency stimulation slowed muscle contractions, made them weaker and increased muscle endurance. The most rapid and least variable response to stimulation was a decline in force output of the muscle and constituent MUs. Fatigue resistance increased more slowly whilst the increase in time to peak force varied most widely between animals and occurred with a longer time course than either force or endurance. Changes in contractile force, endurance and speed of the whole MG muscle accurately reflected changes in the properties of the constituent MUs both in extent and time course. Normally there is a 100-fold range in tetanic force and a 10-fold range in fatigue indices and twitch time to peak force. Following chronic stimulation, the range in these properties was significantly reduced and, even in MU samples from single animals, the range was shown to correspond with the type S MUs of the normal MG. In no case, was the range reduced to less than the type S range. The same results were obtained when the same chronic stimulation pattern of 20Hz/50% duty cycle was imposed on paralysed muscles after hemisection and unilateral deafferentation. The findings that the properties of MUs still varied within the normal range of type S MUs and were still heterogeneous despite a decline in the variance in any one property indicate that the neuromuscular activity can account only in part for the wide range of muscle properties. It is concluded that the normal range of properties within MU types reflects an intrinsic regulation of properties in the multinucleated muscle fibers. 

Received 21 May 1996; accepted in final form 6 January 1997.
APS Manuscript Number J403-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 5 February 1997