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The effects of mechanical or chemical central command and muscle afferent stimulation on the integrated arterial baroreflex control of the sinus node were analyzed in healthy human males during dynamic exercise. Analysis of neural mechanisms involved in cardiovascular control during exercise indicated the differential modulation of the integrated baroreflex control of the sinus node in response to dynamic exercise. Furthermore, central command and muscle chemoreflex preserved baroreflex sensitivity by resetting the integrated baroreceptor-cardiac response relationship.

Analyses of the physiological association between the spontaneous fluctuations in pulse interval (PI) and arterial pressure to examine the spontaneous baroreflex (BR) modulation of the sinus node in humans during their isometric exercise period reveal that isometric exercise shifts the regression line that links PI to systolic arterial pressure but does not modulate the baroreflex gain that regulates the sinus node rate. The exercise causes a high level 'reset' in the arterial baroreflex in the presence of muscle chemoreflex. The arterial BRs increase arterial pressure during the isometric exercise.

Legramante, J. M., G. Raimondi, C. M. Adreani, S. Sacco, F. Iellamo, G. Peruzzi, and M. P. Kaufman. Group III muscle afferents evoke reflex depressor responses to repetitive muscle contractions in rabbits. Am. J. Physiol. Heart Circ. Physiol. 278: H871-H877, 2000.--Repetitive-twitch contraction of the hindlimb muscles in anesthetized rabbits consistently evokes a reflex depressor response, whereas this type of contraction in anesthetized cats evokes a reflex pressor response in about one-half of the preparations tested. Rapidly conducting group III fibers appear to comprise the afferent arm of the reflex arc, evoking the depressor response to twitch contraction in rabbits because electrical stimulation of their axons reflexly decreases arterial pressure. In contrast, electrical stimulation of the axons of slowly conducting group III and group IV afferents reflexly increases arterial pressure in rabbits. In the present study, we examined the discharge properties of group III and IV muscle afferents and found that the former (i.e., 13 of 20), but not the latter (i.e., 0 of 10), were stimulated by 5 min of repetitive-twitch contraction (1 Hz) of the rabbit triceps surae muscles. Moreover, most of the group III afferents responding to contraction appeared to be mechanically sensitive, discharging in synchrony with the muscle twitch. On average, rapidly conducting group III afferents responded for the 5-min duration of 1-Hz repetitive-twitch contraction, whereas slowly conducting group III afferents responded only for the first 2 min of contraction. We conclude that rapidly conducting; group III afferents, which are mechanically sensitive, are primarily responsible for evoking the reflex depressor response to repetitive-twitch contractions in anesthetized rabbits. dynamic exercise; autonomic nervous system; arterial blood pressure; sensory nerves; somatic afferents; breathing