The reflex control and relay to higher brain sites of visceral sensory information within the central nervous system is mediated via discrete sites in the brainstem. Anatomical characterization of these sites in humans has been limited due to the invasive nature of such research. The present study employed 4 Tesla functional magnetic resonance imaging (fMRI) to characterize brainstem sites involved in autonomic control in the human. Eight subjects performed tasks that activate the general visceral (the isometric hand-grip, maximal inspiration, Valsalva maneuver) or special visceral sensory systems (sucrose administration to the tongue). Activation of the nucleus of the solitary tract and parabrachial nucleus was consistently observed with all general visceral tasks. Periaqueductal gray area activation was observed during the maximal inspiration and Valsalva maneuver conditions and raphe activation was present in response to isometric hand-grip and maximal inspiration tasks. The activation in the nucleus of the solitary tract was consistently more rostral in the medulla during sucrose administration than during performance of the other experimental tasks. This finding is consistent with what has been previously demonstrated in animals. This is the first study to image the human brainstem with respect to visceral control and demonstrates the feasibility of using high-resolution fMRI to study the functional organization of the human brainstem.

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Previous evidence indicates that sensitivity of the baroreflex cardiovagal and sympathetic arms is dissociated. In addition, pharmacologic assessment of baroreflex sensitivity (BRS) has revealed that cardiovagal, but not sympathetic, BRS is greater when blood pressure is increasing versus falling. The origin of this hysteresis is unknown. In this study, carotid artery distensibility and absolute distension (diameter) were assessed to test the hypothesis that vessel mechanics in barosensitive regions affect the BRS of cardiovagal, but not sympathetic, outflow. R-R interval (i.e. time between successive R waves), finger arterial blood pressure, muscle sympathetic nerve activity, and carotid artery dimensions (B-mode imaging) were measured during sequential infusions of sodium nitroprusside (SNP) and phenylephrine (PHE). Systolic and diastolic common carotid artery diameters and pulse pressure were recorded to calculate distensibility of this vessel under each drug condition. Cardiovagal BRS was greater when blood pressure was increasing versus decreasing (p < 0.01). Sympathetic BRS was not affected by direction of pressure change. Distensibility did not differ between SNP and PHE injections. However, compared with SNP, infusion of PHE resulted in larger absolute systolic and diastolic carotid diameters (p < 0.001). Therefore, cardiovagal reflex hysteresis was related to drug-induced changes in common carotid artery diameter but not distensibility. The lack of sympathetic hysteresis in this model suggests a relative insensitivity of this baroreflex component to carotid artery dimensions and provides a possible mechanism for the dissociation between cardiovagal and sympathetic BRS.

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