(laterality in sympathetic action/cardiac reactions/vasomotor nerve response/baroreceptor reflex)

In a study of autonomic reflexes it was found that some produce a generalized, bilaterally uniform response whereas others have an asymmetric or laterality of action. Recordings from vertebral nerve fibers (mainly vasoconstrictors to forelimb muscles), right and left cardiac sympathetics, and renal nerves show that baroreceptors evoke a bilaterally uniform inhibition but chemoreceptors of the carotid sinus and aortic arch initiate a differential discharge. In the chemoreceptor reflex the vagi are activated and bradycardia generally occurs. Vertebral and renal sympathetic fibers increase their activity bilaterally commensurate with the increase in arterial pressure. Sympathetic discharges to the heart, however, are not uniform; they show ipsilateral inhibition and a strong contralateral increase in activity. Stabilization of blood pressure or inactivation of baroreceptors abolishes the ipsilateral inhibition. In isolation, therefore, the chemoreceptor-induced cardiac sympathetic discharge is just quantitatively stronger contralaterally. In the absence of vagi, heart rate changes differ depending on which chemoreceptors are stimulated, because the pacemaker is on the right. Asymmetrical discharges do occur and, in the eventual response to stimulation of chemoreceptors, reflex interactions actually augment the laterality of effects. Peripheral interactions, in the sense that changes effected by one may induce another reflex, are responsible in part for the balances of autonomic activity ultimately seen as the body reacts to stimuli. A long-lasting concern of those who study functions of the autonomic nervous system has been to determine how its central control mechanisms can respond to certain stimuli by producing a differential response whereas other stimuli cause the sympathetic division to discharge as a whole. The responses to baroreceptor and chemoreceptor stimulations provide examples of the two categories of effect, and we thought an analysis of their discharge patterns, particularly those evoked by stimulation of the chemoreceptors, would be instructive. Although it is well known that stimulation of arterial chemoreceptors produces augmentation of respiration and pressor responses, the effects on heart rate are controversial. Some investigators have found that, in anesthetized animals with controlled ventilation, bradycardia is evoked by chemoreceptor stimulation (1-4). Others have reported that a moderate tachycardia is obtained when chemoreceptors are stimulated (5, 6). In previous studies (7, 8) we found that stimulation of the carotid chemoreceptors led to a decrease in cardiac sympathetic discharge but an augmentation in the activity of vertebral nerve fibers that have a vasoconstrictor action on vessels of the forelimb. Similar differential discharges have been recorded by others from splanchnic and cardiac sympathetics during chemoreceptor reflexes (9). In further investigations of the mechanisms whereby differential responses of the sympathetic system are evoked, we have identifed two factors that contribute to this phenomenon. The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S. C. §1734 solely to indicate this fact.