FAST-TO-SLOW CONVERSION FOLLOWING CHRONIC LOW-FREQUENCY ACTIVATION OF MEDIAL GASTROCNEMIUS MUSCLE IN CATS. II. MOTONEURON PROPERTIES. JB Munson, RC Foehring, LM Mendell and T Gordon. Department of Neuroscience, University of Florida College of Medicine, Gainesville, FL 32610-0244 USA.
APStracts 4:0039N, 1997.
ABSTRACT
Chronic stimulation (for 2-3 mos) of the medial gastrocnemius (MG) muscle nerve by indwelling electrodes renders the normally heterogeneous MG muscle mechanically and histochemically slow. We tested the hypothesis that motoneurons of MG muscle thus made slow by chronic stimulation would also convert to slow phenotype. Properties of all single muscle units became homogeneously slow (slowly contracting, non-fatiguing, non-sagging contraction during tetanic activation). Motoneuron electrical properties were also modified in the direction of slow, fatigue-resistant motor units. Two separate populations were identified (based on afterhyperpolarization, rheobase and input resistance) that likely correspond to motoneurons that had been type F or type S prior to stimulation. Type-F motoneurons, while modified by chronic stimulation, were not converted to type-S phenotype, in spite of apparent complete conversion of their muscle units to type SO. Muscle units of the former type-F motor units were faster and/or more powerful than those of the former type-S motor units, indicating some intrinsic regulation of motor-unit properties. Experiments in which chronic stimulation was applied to the MG nerve cross-regenerated into skin yielded changes in motoneuron properties similar to those above, suggesting that muscle was not essential for the effects observed. Modulation of group Ia EPSP amplitude during high-frequency trains, which in normal MG motoneurons can be either positive or negative, was negative in 48 of 49 chronically-stimulated motoneurons. Negative modulation is characteristic of EPSPs in motoneurons of most fatigue-resistant motor units. The general hypothesis of a periphery-to-motoneuron retrograde mechanism was supported although the degree of control exerted by the periphery may vary: natural type-SO muscle appears especially competent to modify motoneuron properties. We speculate that activity-dependent regulation of the neurotrophin NT-4/5 in muscle plays an important role in controlling muscle and motoneuron properties. 

Received 21 May 1996; accepted in final form 6 January 1997.
APS Manuscript Number J404-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 5 February 1997