Respiration Physiology (1984) 58, 65-76 Elsevier 65 COMPARISON OF THE RESPONSES OF THE DIAPHRAGM AND UPPER AIRW AY MUSCLES TO CENTRAL STIMULATION OF THE SCIATIC NERVE MUSA A. HAXHIU, ERIK VAN LUNTEREN, JYOTI MITRA, NEIL S. CHERNIACK and KING MAN P. STROHL Department of Medicine, Case Western Reserve University, Cleveland, OH 44106, U.S.A. Abstract. Electrical stimulation of the central end of the sciatic nerve was used to assess the effect of increased somatic sensory input on respiratory muscle electrical activity in anesthetized, spontaneously breathing dogs. Graded electrical stimulation of the sciatic nerve was associated with progressively greater activity of the upper airway dilating muscles (alae nasi, genioglossus, and posterior cricoarytenoid) as weil as the diaphragm. Breathing frequency also increased because of a reduction in inspiratory and expiratory time. After cessation of stimulation of the sciatic nerve, increased activity of ail the muscles studied persisted and only gradually returned to controllevels. The responses to sciatic nerve stimulation were independent of the CO 2 concentration of the inspired gas mixture. At any level of chemical drive electrical stimulation caused greater increases in the electrical activity of upper airway dilating muscles than that of diaphragm. Based on these results, it is concluded that stimulation of sciatic nerve activates upper airway muscles as weil as the diaphragm, and the upper airway muscle activity is augmented to a greater degree than diaphragm activity. It seems possible that somatosensory afferent input produces unequal effects on different respiratory motoneurons. Diaphragm Dog Inspiratory drive Sciatic nerve Somatosensory inputs Upper airway muscle Many of the upper airway muscles respond to the same mechanical and chemical stimuli which affect ventilation, but frequently the upper airway muscle response is quanti- tatively or qualitatively different from that of the diaphragm. For example, hypoxia and hypercapnia increase the activity of the diaphragm and the muscles which seem to dilate the upper airways (e.g. alae nasi, genioglossus, and posterior cricoarytenoid), while baroreceptor stimulation seems to depress the inspiratory activity of all these muscles. However, the etTects on some of the upper airway muscles, particularly the genioglossus, are more marked than the diaphragm (Bartlett, 1979; Strohl et al., 1982; Brouillette and Accepted for publication 10 July 1984 0034-5687/84/$03.00 © 1984 Elsevier Science Publishers B.V.