Society For Neuroscience Abstract | 2005

DISCHARGE CHARACTERISTICS OF MOTOR UNITS IN A HAND MUSCLE OF OLDER ADULTS. BK Barry, MA Pascoe, and RM Enoka. Dept. of Integrative Physiology, University of Colorado, Boulder, CO USA. Single motor units in the first dorsal interosseous of 20 old adults (10 men, 10 women; 77.1 ± 6.9 yr) were tracked during isometric contractions across a range of forces. The discharge of 34 motor units was recorded at 9 (± 3) discrete force targets for 10.1 ± 2.9 s, from recruitment threshold to an average of 17.8 %MVC force above recruitment. These data were compared with a sample of 38 motor units from young adults (25.7 ± 5.7 yr) from a previous study. EMG recordings indicated that the activity of the antagonist muscle, second palmar interosseous, was minimal and consistent across the different target forces. Similar to the young adults, both minimal and peak discharge rates increased with recruitment threshold, but the strength of these relations was notably weaker in old adults. Minimal discharge rates were slightly elevated for old adults (P<0.05), whereas peak discharge rates were lower (P<0.01). Consequently, the range of rate coding within each motor unit for the old adults (6.3 pps) was substantially less than that observed for the young adults (11.1 pps, P<0.01). However, the variability in motor unit discharge was essentially the same for the young and old adults. Relative variability (coefficient of variation) in motor unit discharge was similar at recruitment (old: 24.7, young: 27.1 %, P = 0.21) and declined to almost the same level with an increase in discharge rate (old: 13.4, young: 13.3 %, P = 0.91). The rate of change in discharge rate variability was also essentially the same for young and older adults; the slope of the linear relation between SD ISI and mean ISI was 0.40 for old and 0.39 for young subjects (P = 0.71). At low discharge rates, the discharge rate distributions were skewed with a greater proportion of extra long ISIs, but became normal as discharge rate increased. In old adults the rate coding of motor units is curtailed but variability in motor unit discharge is unaltered. Supported by NIA AG09000 to RME.