Chronic pain-autonomic interactions.

The nociceptive and the autonomic nervous systems interact at the levels of the periphery, spinal cord, brainstem, and forebrain. Spinal and visceral afferents provide converging information to spinothalamic neurons in the dorsal horn and to neurons of the nucleus tractus solitarius and parabrachial nuclei. These structures project to areas involved in reflex, homeostatic and behavioral control of autonomic outflow and nociception. Clinical and experimental observations on migraine suggest that a general hyperexcitability could develop along nociceptive trigeminal neurons allowing the activation of descending pathways that facilitate pain processing or the suppression of pathways that slow down pain transmission. Slowly progressive dysfunction of central pain systems secondary to the repetition of attacks of the originating headache has been implicated in transforming the process from an episodic to a chronic condition. This concept of an irritable focus in the central nervous system is currently known as "central sensitization" and is a characteristic feature of visceral pain. Considering primary headache as the result of the interactions between the nociceptive and autonomic systems (visceral pain) can help us to understand the complex pathophysiology of these disorders and to explore new treatments.