Adaptations of motoneuron properties after weight-lifting training in rats

Abstract

Resistance training, with repeated short-term and high-intensity exercises, is responsible for an increase in muscle mass and force. The aim of this study was to determine whether such training induces adaptations in the electrophysiological properties of motoneurons innervating the trained muscles and to relate these adaptive changes to previous observations made on motor unit contractile properties. The study was performed on adult male Wistar rats. Animals from the training group were subjected to a 5-wk voluntary progressive weight-lifting program, whereas control rats were restricted to standard cage activity. Intracellular recordings from lumbar spinal motoneurons were made under pentobarbital anesthesia. Membrane properties were measured, and rhythmic firing of motoneurons was analyzed. Strength training evoked adaptive changes in both slow- and fast-type motoneurons, indicating their increased excitability. A shorter spike duration, a higher input resistance, a lower rheobase, a decrease in the minimum current required to evoke rhythmic firing, an increase in the maximum frequencies of the early-state firing (ESF) and the steady-state firing (SSF), and an increase in the respective slopes of the frequency-current (/) relationship were observed in fast motoneurons of the trained group. The increase in the maximum ESF and SSF frequencies and an increase in the SSF / slope were also present in slow motoneurons. Higher maximum firing rates of motoneurons as well as higher discharge frequencies evoked at the same level of intracellular depolarization current imply higher levels of tetanic forces developed by motor units over the operating range of force production after strength training. Neuronal responses to weight-lifting training can be observed in altered properties of both slow and fast motoneurons. Motoneurons of trained animals are more excitable, require lower intracellular currents to evoke rhythmic firing, and have the ability to evoke higher maximum discharge frequencies during repetitive firing.

Effects of acute isometric resistance exercise on cervicomedullary motor evoked potentials

“…Because CMEPs are evoked through stimulation at a subcortical site, their facilitation suggests a change in the spinal cord—either enhanced corticospinal transmission or increased motoneurone excitability. These acute spinal‐level changes in humans are consistent with longer‐term adaptations that have been observed in rats after weeks of resistance exercise …”

“…Support for this mechanism comes from work in both animals and humans. In rats, various changes in the intrinsic properties of motoneurones—such as shorter action potential duration, higher input resistance, decreased minimum current to evoke rhythmic firing—have been reported after 5 weeks of resistance exercise . In humans, the duration of the motoneurone afterhyperpolarization has been reported to decrease after 2 weeks of isometric resistance exercise …”

“…corticospinal, neuromuscular, neurophysiology, specificity, training adaptations that have been observed in rats after weeks of resistance exercise. 8,9 In this study, we expand upon our previous work 6 and explore the roles that enhanced corticospinal transmission and/ or increased motoneurone excitability may play in explaining the concepts of training specificity and transfer. Specificity refers to the phenomenon in which improvements in performance are greatest when measured in the exercise task.…”

“…For a given participant, the 2 sessions were conducted at the same time of day. The median number of days between sessions was 7 (range: [6][7][8][9][10][11][12][13][14].…”

Understanding exercise-dependent plasticity of motoneurons using intracellular and intramuscular approaches

“…A 5-week progressive weight-lifting program requiring animals to lift against resistance to access food, results in more excitable fast MNs (e.g., lower rheobase and increased input resistance), higher maximal firing rates, and increased excitability during rhythmic firing (higher frequency-current slope) in both fast and slow MNs. These adaptations may contribute to increased tetanic force production following resistance training (Krutki et al 2017). Thus, part of the exercise-induced changes in the neuromuscular system that lead to enhanced performance in animals are due to plasticity of the MN.…”