Evidence Of Self-sustained Motoneuron Firing In Young And Older Adults

Abstract

In many animal models it has been demonstrated that motoneurons can exhibit self-sustained firing behavior in response to a prolonged depolarized state of the cells. Such self-sustained firing has also been reported in human motoneurons. However, the effects of aging on this behavioral phenomenon have not yet been investigated in humans. PURPOSE To investigate the frequency of self-sustained firing behavior in motoneurons of the tibialis anterior muscle (TA) of young and older adults. METHODS Eight young (18–30 years) and eight older (65–79 years) subjects participated in the investigation. Recordings of motor unit activity were obtained from the TA while subjects performed isometric dorsiflexion at 10% MVC. While force was being held constant, a brief 100-Hz vibration was applied to the belly of the muscle to recruit additional motor units. Evidence of self-sustained firing was defined by the continued firing of a newly-recruited motor unit after the removal of vibration, despite a steady firing rate of motor units active before the vibration. RESULTS There were no significant differences in force pre- and post-vibration (p>0.05). In motor units that were active throughout the entire contraction, there were no significant differences in firing rate pre- and post-vibration (p>0.05). There were no significant differences in the frequency of self-sustained firing between young (22% of trials) and older (22% of trials) subjects (p>0.05). CONCLUSIONS Evidence for self-sustained firing in motoneurons of young and older adults was demonstrated. The results suggest that self-sustained firing behavior is a ubiquitous phenomenon that does not seem to be affected by the aging process.