Work on the central nervous system control of the cardiovascular system has also provided new insights into long standing problems. Most research on both pathophysiological disorders such as hypertension and psychopathological disorders such as anxiety and depression has viewed the cardiovascular and autonomic concomitants of these disorders in terms of sympathetic nervous system activity. However, our work based upon solid theoretical and empirical foundations has sought to re-frame the extant literature in terms of a parasympathetic or vagal deficit (Thayer and Lane, 2000). Individuals with these disorders have been shown to have decreased parasympathetic control of the cardiovascular system. This work has helped to clarify the increased risk of morbidity and mortality associated with these disorders. Recent work has shown how this re-framing may also explicate the gender differences and lateralized nature of affective responding associated with several disorders including depression and heart disease. Using the intracarotid sodium amytal test (ISA) we have recently been able to examine the effects on the neural control of the cardiovascular system of a selective hemispheric inactivation. Individuals undergoing surgery for epilepsy had sequential inactivations of their cerebral hemispheres while cardiac inter-beat intervals were recorded. Because of the relatively large number of patients (73) we were able to examine gender and site of lesion differences that previous studies could not investigate. Relative to pre-inactivation values, inactivation of either hemisphere was associated with an increase in heart rate and a decrease in vagally mediated heart rate variability (HRV). This provided strong evidence for our model, which hypothesized that the cerebral cortex tonically inhibits brainstem sympathoexcitatory circuits, and that this is achieved via parasympathetic pathways. Importantly, heart rate increased more and HRV decreased more when the right cerebral hemisphere was inactivated. This is consistent with the known physiological literature that the right-sided innervations have greater influence on the sino-atrial node of the heart. Individuals with decreased parasympathetic control of the heart show less ability to adapt both physiologically and psychologically to environmental demands leading to a rigid, inflexible response disposition. These findings may help to explain important gender differences and hemispheric differences in risk for morbidity and mortality.
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