between lower-level physical or homeostatic challenges and higher-level psychological and social stressors. Basic homeostatic reflexes, reflexes that keep in balance various critical bodily processes such as blood pressure, body temperature, and blood sugar, are largely hard-wired and organized at relatively low levels of the nervous system, such as the brainstem and spinal cord. An example comes from autonomic nervous system regulation of cardiovascular function. The sympathetic division of the autonomic nervous system is an activational, energy mobilization system that comes into play in the face of adaptive challenges. Sympathetic activation increases heart rate and results in peripheral vasoconstriction, both of which tend to increase blood pressure. In contrast, the parasympathetic division is an energy-conserving, deactivational brake that generally opposes the sympathetic system, yielding decreases in heart rate and blood pressure. The baroreceptor heart rate reflex is a homeostatic reflex that functions to maintain blood pressure within homeostatic limits. Unique pressure sensitive receptors in the heart and large arteries detect changes in blood pressure, and a decrease in blood pressure triggers the baroreceptor heart rate reflex, increasing sympathetic activity and reciprocally decreasing parasympathetic tone. Both effects serve to increase heart rate (and thus cardiac output) and constrict arteries throughout the body, thereby restoring the pressure perturbation. In basic reflexes, the two autonomic branches are generally regulated in this reciprocal fashion, and thus synergistically amplify the effects of the other. This is a useful mechanism to adjust to severe adaptive challenges
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such as a decrease in blood pressure and compromised circulation.
Although this reciprocal mode of regulation of the autonomic branches has considerable utility, and is characteristic of basic reflex organizations, it may not always be optimal. The autonomic nervous system provides the basic support for action and adjustment, and although it figures prominently in survival related functions, it also provides the basic visceral support for emotional and cognitive operations as well. It has long been recognized that cognitively demanding tasks elicit greater autonomic activation than is needed to meet the metabolic demands of the tasks. Moreover, ascending neural signals to the brain from visceral organs such as the heart and blood vessels serve to modulate and regulate cognitive activities (5). The notable early psychologist, William James, proposed that emotion is the experience of somatovisceral sensory feedback. James suggested that we do not run from the bear because we are afraid, but rather we are afraid because we run from the bear (6). Although the strong form of this theory has not been supported, it remains the case that ascending visceral signals can modulate learning, attention, and cortical/cognitive processing (5). The autonomic nervous system is not only for lower level reflexive adjustments. Indeed, it is increasingly recognized that there is a highly complex, even intricate, interaction between the autonomic nervous system and higher level brain structures (e.g., frontal cortex) involved in human behavior. Importantly, these circuits and their interactions with the autonomic nervous system are highly flexible and are not constrained by the simple organization rules that govern basal functions such as homeostasis and
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