In response to perceived or experienced change that is considered threatening to the individual, the central nervous system mounts a stereotypic response that decreases the sensitivity to pain, modulates the autonomic nervous system outflow, and activates the hypothalamic-pituitary-adrenal (HPA) axis. This response of the "emotional motor system" may or may not be associated with the conscious experience of feelings of fear or anxiety. Alterations in these response systems (either up- or downregulation) may produce symptoms, such as viscero-somatic hypersensitivity, altered bowel habits, or increased anxiety.