The human internal environment is regulated by the autonomic nervous system (ANS), endocrine glands and neuropeptides as well as the integrated activity of these three systems. The ANS is divided into thoraco-lumbar (sympathetic) and cranio-sacral (parasympathetic) divisions. They comprise two neurons: pre- and post-ganglionic fibers. The limbic system and hypothalamus are known to regulate the peripheral ANS through descending pathways in the brain stem and spinal cord. Cardinal semiologies of the ANS disorders are classified as follows: cardiovascular (orthostatic hypotension, a variety of syncopes, postprandial hypotension, Raynaud's phenomenon), pupillomotor (paralytic mydriasis, Adie's tonic pupil, Horner's syndrome), urodynamic (urinary retention and incontinence), and sexual (impotence, hyper- and hyposexuality).