Heart Rate Variability: Stress and Psychiatric Conditions

levels of central control. Although Walter Cannon clearly articulated the impact of emotion and other psychological processes on autonomic states (Cannon 1928), he viewed the autonomic nervous system largely as a homeostatic regulatory mechanism (Cannon 1929). Recent research, however, has revealed a re-representation and layering of autonomic functions from the spinal cord to rostral brain networks sufficiently similar reminiscent of those for somatomotor control that they suggest a common evolutionary heritage. Homeostatic functions of the autonomic nervous system are illustrated by the baroreceptor-heart rate reflex, through which a perturbation in blood pressure triggers a tightly coupled, reciprocal change in the activities of the autonomic branches. Postural hypotension, for example, results in a reflexive decrease in parasympathetic control of the heart, together with a reciprocal withdrawal of sympathetic cardiac control, which synergistically increase heart rate (and contractility). Together with vascular components, these changes serve to compensate for the hypotensive perturbation and normalize blood pressure (e.g. Cacioppo et al. 1994). Baroreceptor reflexes are prototypic homeostatic mechanisms, and are organized largely within lower central autonomic substrates at brainstem levels. Through evolutionary development of rostral brain systems, however, these lower autonomic circuits become integrated with higher neural networks (Berntson & Cacioppo 2000). Limbic and forebrain areas implicated in behavioural processes, including the hypothalamus, amygdala and medial prefrontal cortex, have been shown to issue monosynaptic projections to brainstem reflex networks as well as to autonomic source Introduction