Electrical stimulation of the anterior ethmoidal nerve produces the divingresponse

Stimulation of the upper respiratory tract usually produces apnea, but it c an also produce a vagally mediated bradycardia and a sympathetically mediat ed increase in peripheral vascular resistance. This cardiorespiratory respo nse, often called the diving response, is usually initiated by nasal stimul ation. The purpose of this research was to investigate the anterior ethmoid al nerve (AEN) that innervates the nasal mucosa of muskrats (Ondatra zibeth icus). Electrical stimulation of the AEN (typically 50 Hz, 100 mu s and 500 mu A) produced immediate and sustained bradycardia and cessation of respir ation similar to that of the diving response. Heart rate (HR) significantly decreased from 264 +/- 18 to 121 +/- 8 bpm, with a concurrent 4.2 +/- 0.9 s apnea, during the 5 s stimulation period. BP decreased from 97.9 +/- 4.8 to 91.2 +/- 6.4 mmHg. Using estimations from (1) cross-sectional areas of A EN trigeminal ganglion cells labeled with WGA-HRP, and (2) electron microsc opic analysis of the AEN, we found that approximately 65% of the AEN is com posed of unmyelinated C-fibers. In addition, 72.4% of myelinated fibers fro m the nerves that innervate the nasal passages were of small diameter (< 6 mu m, presumably A delta fibers). Thus, the AEN of the muskrat contains a h igh concentration of small diameter fibers (89.8%). We conclude that electr ical stimulation of small diameter fibers within the AEN of muskrats can pr oduce the cardiovascular and respiratory responses similar to that of the d iving response. (C) 1999 Elsevier Science B.V. All rights reserved.