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Forebrain regions associated with postexercise differences in autonomic and cardiovascular function during baroreceptor unloading

¹Neurovascular Research Laboratory, Faculty of Health Sciences and School of Kinesiology, The University of Western Ontario, London, Ontario; ²Advanced Imaging Laboratories, Robarts Research Institute, London, Ontario; and ³Department of Physiology and Pharmacology, The University of Western Ontario, London, Ontario, Canada

Submitted 12 January 2007 ; accepted in final form 4 March 2007

The cortical regions representing peripheral autonomic reactions in humans are poorly understood. This study examined whether changes in forebrain activity were associated with the altered physiological responses to lower body negative pressure (LBNP) following a single bout of dynamic exercise (POST-EX). We hypothesized that, compared with the nonexercised condition (NO-EX), POST-EX would elicit greater reductions in stroke volume (SV) and larger increases in heart rate (HR) and muscle sympathetic nerve activity (MSNA) during LBNP (5, 15, and 35 mmHg). Forebrain neural activity (n = 11) was measured using blood oxygen level-dependent (BOLD) functional magnetic resonance imaging. HR, SV, arterial blood pressure (ABP), and MSNA were collected separately. Compared with NO-EX, baseline ABP was reduced, whereas HR and total vascular conductance (TVC) were elevated in POST-EX (P < 0.05). In both conditions, 5 mmHg LBNP did not elicit a change (from baseline) in any physiological parameter. Compared with NO-EX, 35 mmHg LBNP-mediated decreases in SV and TVC produced greater increases in HR and MSNA during POST-EX (P < 0.05). The right posterior insula and dorsal anterior cingulate cortex demonstrated a larger decrease in BOLD at 5 mmHg LBNP but greater BOLD increase at 15 and 35 mmHg LBNP POST-EX vs. NO-EX (P < 0.005). Conversely, the thalamus and ventral medial prefrontal cortex displayed the opposite BOLD activity pattern (i.e., larger increase at 5 mmHg LBNP but greater decrease at 15 and 35 mmHg LBNP POST-EX vs. NO-EX). Our findings suggest that discrete forebrain regions may be involved with the generation of baroreflex-mediated sympathetic and cardiovascular responses elicited by moderate LBNP.

postexercise recovery; muscle sympathetic nerve activity; cardiovascular regulation; baroreflex; functional magnetic resonance neuroimaging; lower body negative pressure