Motor unit recruitment patterns in genioglossus and first dorsal interosseous (1102.4)

Abstract

The nervous system regulates force via two means; rate coding and recruitment. In a previous study (Bailey et al. 2007) we reported on rate coding of genioglossus (GG) motor units (MUs) in the context of voluntary tongue movement. Here we focus on MU recruitment and compare GG with first dorsal interosseous (FDI). We recorded single MU action potentials via intramuscular tungsten microelectrodes inserted into the GG and FDI in 9 healthy adults (N=3 men; N=6 women). Recruitment was elicited in FDI via force increments associated with finger abduction (force range 4‐7% of max), while recruitment in the GG was elicited via increments in sound pressure level (loudness range 5‐34% of max) associated with articulation of the vowel /i/. We noted significantly higher firing rates in GG MUs at recruitment relative to FDI (18.9 4.8 vs. 11.0 2.8). Once recruited, GG MUs discharged at significantly higher rates than FDI MUs (20.9 4.7 vs. 12.1 2.7). Moreover, the firing rates of subsequently recruited units was also notably higher for GG than for FDI (21.3 4.8 vs. 12.2 2.7). These findings provide useful insights into motoneuron pools that regulate movements of the tongue in speech and of the hand in grasping. Higher firing rates at recruitment for GG compared to FDI, given comparable pressure/force ranges, suggest hypoglossal motoneurons have a shorter duration after hyperpolarization than spinal motoneurons (Bailey et al. 2007).Grant Funding Source: Supported by the National Institute of Health Grant (DC 009587)