Relationship between pain and autonomic phenomena in headache and other pain conditions

Involvement of the (efferent) autonomic nervous system in the generation of pain is ongoing matter of debate. Based on clinical and experimental observations, there are good arguments that the sympathetic nervous system may be involved in pain following trauma, with and without nerve lesion, at an extremity, such as in complex regional pain syndrome type I and II. However, the mechanisms involved are in many cases still unclear. In various types of headache there is no convincing evidence that the sympathetic nervous system is involved in the generation of pain, although these pains may be accompanied by considerable autonomic reactions which are dependent on activity in sympathetic neurons. Migraine and headaches with autonomic symptoms are accompanied by autonomic reactions which are dependent on activity in cranial parasympathetic neurons. Whether parasympathetic neurons innervating cranial blood vessels are involved in activation or sensitization of trigemino-vascular afferents is discussed and needs experimental verification.