FIRING PROPERTIES OF RAT MOTOR UNITS DURING VOLUNTARY CONTRACTION

The properties of motor units have been well studied in anesthetized rats with current injection. Investigators have collected a large amount of data to support hypotheses about the relationship between firing rates and force. However, a very limited number of studies have been done in awake animals doing voluntary contractions. Because it is suggested that discharge properties of motor units mediated by current injection are different from that with natural synaptic inputs in intact animals, the current study aims to identify differences between the two modalities of activation. We recorded motor unit activity using intramuscular electromyography in left hindlimbs of 5 rats after they underwent behavioral training protocol to perform isometric plantar flexion. We found that the minimum firing rates in intact rats are much lower than those in current injection studies, while the positive relationship between firing rate and force still exists. Possible reasons for the results we observed are discussed, and we proposed some modifications for future studies based on limitations faced in the current one