Received August 16, 2004
Revised August 27, 2004
Accepted after revision August 27, 2004

¹ Dartmouth Medical School
² University of Bristol

^* To whom correspondence should be addressed. E-mail: walter.m.stjohn{at}dartmouth.edu.

	Abstract

Summary During eupnea, rhythmic motor activities of the hypoglossal, vagal and phrenic nerves are linked temporally. The inspiratory discharges of the hypoglossal and vagus motor neurons commence before the onset of the phrenic burst. The vagus nerve also discharges in expiration. Upon exposure to hypocapnia or hypothermia, the hypoglossal discharge became uncoupled from that of the phrenic nerve. This uncoupling was evidenced by variable times of onset of hypoglossal discharge before or after the onset of phrenic discharge, extra bursts of hypoglossal activity in neural expiration, or complete absence of any hypoglossal discharge during a respiratory cycle. No such changes were found for vagal discharge, which remained linked to the phrenic bursts. Intracellular recordings in the hypoglossal nucleus revealed that all changes in hypoglossal discharge were due to neuronal depolarisation. No evidence of hyperpolarizations was seen that could account for the uncoupling of inspiratory neuronal activities. These results add support to the conclusion that the brainstem control of respiratory-modulated hypoglossal activity differs from control of phrenic and vagal activity. These findings also have implications for any studies in which activity of the hypoglossal nerve is used as the sole index of neural inspiration. Indeed, our results establish that hypoglossal discharge alone is an equivocal index of both rhythm and the pattern of overall ventilatory activity and that this is accentuated by hypercapnia and hypothermia.

Key Words: Breathing, Respiration, Respiratory control

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