The effects of superior laryngeal nerve stimulation on the respiratory rhythm: phase-resetting and aftereffects.

The effect of brief superior laryngeal nerve stimulation on the respiratory rhythm was investigated in midcollicular decerebrate, unanesthetized, artificially ventilated, paralysed, vagotomized and debuffered cats. Stimulus trains (50 ms, 200 Hz) delivered during inspiration (I) with intensities exceeding a threshold value, that was inversely related to the phase of the cycle, terminated I and shortened the following expiration (E) (irreversible I termination). Stimulus trains given during I with intensities just below this threshold value produced a transient suppression of I followed by resumption of activity, resulting in a slight prolongation of both I and the following E (reversible I termination). Stimulation during E produced a phase-dependent prolongation of E, but did not affect the next I. Phase-resetting curves were constructed by measuring the changes in respiratory cycle duration produced by stimuli given at phases throughout the cycle. A single stimulus produced aftereffects that lasted several cycles. The aftereffects were investigated by delivering stimuli at a fixed delay from cycle onset every n cycles (n is an integer). Certain combinations of delay, stimulus intensity, and n, resulted in (1) a variable combination of reversible and irreversible I terminations, rather than a consistent response, and (2) an increase in cycle duration. The stimulus aftereffects, that can last up to 9 cycles, may account for previously described unpredictability in the response of the respiratory oscillator to a given stimulus.