Effect of co-activation of tongue protrudor and retractor muscles on tongue movements and pharyngeal airflow mechanics in the rat

1. The purpose of these experiments was to examine the mechanisms by which either coactivation or independent activation of tongue protrudor and retra ctor muscles influence upper airway flow mechanics. We studied the influenc e of selective hypoglossal (XIIth) nerve stimulation on tongue movements an d flow mechanics in anaesthetized rats that were prepared with an isolated upper airway. In this preparation, both nasal and oral flow pathways are av ailable. 2. Inspiratory flow limitation was achieved by rapidly lowering hypopharyng eal pressure (P-hp) with a vacuum pump, and the maximal rate of flow ((V) o ver dot (I,max)) and the nasopharyngeal pressure associated with flow limit ation (P-crit) were measured. These experimental trials were repeated while nerve branches innervating tongue protrudor (genioglossus; medial XIIth ne rve branch) and retractor (hyoglossus and styloglossus; lateral XIIth nerve branch) muscles were stimulated either simultaneously or independently at frequencies ranging from 20-100 Hz. Go-activating the protrudor and retract or muscles produced tongue retraction, whereas independently activating the genioglossus resulted in tongue protrusion. 3. Go-activation of tongue protrudor and retractor muscles increased (V) ov er dot (I,max) (peak increase 44%, P < 0.05), made P-crit more negative (pe ak decrease of 44%, P< 0.05), and did not change upstream nasopharyngeal re sistance (R-n). Independent protrudor muscle stimulation increased (V) over dot (I,max) (peak increase 61%, P < 0.05), did not change P-crit, and decr eased R-n (peak decrease of 41%, P < 0.05). Independent retractor muscle st imulation did not significantly alter flow mechanics. Changes in P-crit and (V) over dot (I,max) all stimulation frequencies were significantly correl ated during co-activation of protrudor and retractor muscles (r(2) = 0.63, P< 0.05), but not during independent protrudor muscle stimulation (r(2) = 0 .09). 4. These findings indicate that either co-activation of protrudor and retra ctor muscles or independent activation of protrudor muscles can improve upp er airway flow mechanics, although the underlying mechanisms are different. We suggest that co-activation decreases pharyngeal collapsibility but does not dilate the pharyngeal airway. In contrast, unopposed tongue protrusion dilates the oropharynx, but has a minimal effect on pharyngeal airway coll apsibility.