FACIAL COOLING-INDUCED BRADYCARDIA - ATTENUATING EFFECT OF CENTRAL COMMAND AT EXERCISE ONSET

Facial cooling (FC) elicits a marked bradycardia at rest that appears to be reduced during exercise. This study was done to delineate the ef fects of exercise mediated central command from those of muscle affere nt feedback and sympathetic stimulation on the attenuation of the brad ycardic effect of FC during the onset of exercise. Ten healthy subject s (26 +/- 2 yr) were exposed to FC under five different conditions: 1) seated rest on the cycle ergometer, 2) onset of mild exercise (restin g HR + 20 beats . min(-1)), 3) onset of moderate exercise (resting HR + 50 beats . min(-1)), 4) seated rest on the ergometer during electric al stimulation, and 5) sealed rest on the ergometer during a cold imme rsion test (CT) (one hand immersed in an ice slurry at 0 degrees C). T he two exercise intensities were presumed to provide different degrees of central command. Electrical stimulation of the quadriceps was assu med to provide isolated muscle afferent feedback, while the CT served as a sympathetic stimulus. Beat-by-beat data were recorded for HR and mean arterial blood pressure for the duration of each test (50 s), and a rating of perceived pain was taken after each FC. FC elicited signi ficant increases in mean arterial pressure during mild and moderate ex ercise compared with resting control (P < 0.05) and during moderate ex ercise compared to exercise without FC (P < 0.05). Mean decreases in H R during FC were similar for resting control (-12 +/- 3 beats . min(-1 )), electrical stimulation (-10 +/- 3 beats . min(-1)), and CT (-9 +/- 3 beats . min(-1). The HR response to FC during mild exercise (-7 +/- 2 beats . min(-1) was significantly different (P < 0.05) from the res t condition; however, there was no significant bradycardia (-2 +/- 2 b eats . min(-1); P > 0.05) during onset of moderate exercise. These fin dings suggest that the magnitude of cold face-induced bradycardia may be attenuated at exercise onset by neural signals related to the highe r levels of central motor command associated with heavier exercise.