Modulation of Pain by the Autonomic Nervous System

There has been increasing evidence to suggest that the ANS may modulate visceral sensory perception, and sympathetically mediated mechanisms are implicated in several chronic pain syndromes (57,58). In addition, there are animal and human data supporting a vagally mediated inhibition of visceral nociceptive sensory inputs (59,60). Iovino et al. used lower body negative pressure (LBNP) to experimentally activate the sympathetic nervous system by inducing venous pooling in the lower extremities, and to determine the effects on the perception of intestinal stimulation (61). Using brief distending stimuli in the intestine, the effect of sympathetic activation by LBNP on sympathetically mediated intestinal relaxation and on vagally mediated gastric relaxation was measured by corresponding barostats. The effect of LBNP on perception of duodenal distension was also compared to the perception of somatic stimulation. It was found that sympathetic activation significantly heightened perception of intestinal distension without modifying perception of somatic stimuli (perception scores increased by 41% and 2%, respectively). Also, the reflex responses to duodenal distension significantly increased during sympathetic activation both in the stomach and in the intestine (relaxation increased by 91% and 69%, respectively, with p < 0.05 for both).

In human studies, the activity of the ANS in response to stress or pain has mainly been determined by analysis of heart rate variability (HRV) or skin conductance. Evaluation of skin conductance via the Galvanic skin response provides a measure of relative sympathetic activation (62), whereas that of HRV allows an approximation of relative sympathetic and parasympathetic dominance, and the extraction of vagal tone (63). The beat-to-beat variability in the heart rate provides a window through which the autonomic input to the sinoatrial node in the heart can be evaluated. Power spectral analysis of HRV reveals a low-frequency band (LF), ranging from 0.05 to 0.15 Hz, which is believed to be largely under sympathetic control. The high-frequency band (HF), ranging from 0.2 to 0.5 Hz, is associated with respiratory sinus arrhythmia and is believed to be under vagal control. Sympathovagal balance has been expressed by the LF/HF ratio as it is said to reflect the relative dominance of efferent modulation of sinoatrial activity (64). Although this technique is not without its criticisms, and these will be discussed later, it may be possible to determine the relative contributions of the ANS components in an individual's response profile to an external stimulus.

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