Society For Neuroscience Abstract | 2006

DISCHARGE CHARACTERISTICS OF MOTOR UNITS DURING LONG CONTRACTIONS. MA Pascoe, JA Enoka, and RM Enoka. Dept. of Integrative Physiology, University of Colorado, Boulder, CO USA. When a subject performs a submaximal voluntary contraction for as long as possible, the mean discharge rate of motor units recruited at the onset of the task decreases and the coefficient of variation for discharge rate often increases (Carpentier et al. J Physiol 534: 903, 2001; Mottram et al. J Neurophysiol 93: 1381, 2005). The purpose of the study was to characterize the discharge patterns of motor units when the task was to sustain the discharge at a low rate for as long as possible. The discharge times of single motor units were recorded from the first dorsal interosseus muscle in young adults (28.2 ± 5.1 yr) as they exerted an abduction force (4.3 ± 2.6 % maximum) with the index finger. The task was to maintain the discharge of an isolated single motor unit for as long as possible. Subjects received auditory feedback of the discharge times and were asked to keep the rate relatively constant for the duration of the task. The task was terminated when the motor unit stopped discharging action potentials despite the ability of the subject to sustain the force. The mean task duration was 33.8 ± 18.5 min (range; 12.9 to 61.2 min). The trains of action potentials comprised an average of 18935 ± 10678 interspike intervals. The mean discharge rate was 9.3 ± 1.9 pps and this did not change across the tasks (P = 0.88). Similarly, the mean abduction force did not change during the contraction (P = 0.94). The relative variability (coefficient of variation) of the interspike intervals during the initial 10 % of the task was 23.1 ± 5.9 % and this increased to 31.5 ± 7.0 % during the final 10 % of the task (P = 0.056). All motor units could be recruited within 60 s of recovery. The results indicate that subjects were able to sustain a relatively constant discharge rate based on audio feedback and that the cessation of the discharge was preceded by an increase in discharge rate variability. Supported by NIA AG09000 to RME. Key Words: motor unit, discharge rate, discharge variability, interspike intervals