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1 Physiology, and HLHP Research Institute, University of Manitoba, Winnipeg, Canada
2 Physiology, University of Manitoba, Winnipeg, Canada
3 Departments of Physiology and Physical Medicine and Rehabilitation, Northwest University, Medical School, Chicago, Illinois, United States
* To whom correspondence should be addressed. E-mail: gardine2{at}cc.umanitoba.ca.
Evidence is presented that one locus of adaptation in the "neural adaptations to training" is at the level of the alpha-motoneurones. With increased voluntary activity, these neurones show evidence of dendrite restructuring, increased protein synthesis, increased axon transport of proteins, enhanced neuromuscular transmission dynamics, and changes in electrophysiological properties. The latter include hyperpolarization of the resting membrane potential and voltage threshold, increased rate of action potential development, and increased amplitude of the afterhyperpolarization (AHP) following the action potential. Many of these changes demonstrate intensity-related adaptations, and are in the opposite direction under conditions where chronic activity is reduced. A 5-compartment model of rat motoneurones that innervate fast and slow muscle fibers (termed "fast" and "slow" motoneurones in this paper), including 10 active ion conductances, was used to attempt to reproduce exercise training-induced adaptations in electrophysiological properties. The results suggest that adaptations in alpha-motoneurones with exercise training may involve alterations in ion conductances, which may in turn include changes in the gene expression of the ion channel subunits which underlie these conductances. Interestingly, the acute neuromodulatory effects of monoamines on motoneurone properties, which would be a factor during acute exercise as these monoaminergic systems are activated, appear to be in the opposite direction to changes measured in endurance-trained, motoneurones that are at rest. It may be that regular increases in motoneuronal excitability during exercise via these monoaminergic systems in fact render the motoneurones less excitable when at rest. More research is required to establish the relationships between exercise training, resting and exercise motoneurone excitability, ion channel modulation, and the effects of neuromodulators.
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