The early concepts of the autonomic nervous system date back to the Roman period, during which Galen described the sympathetic chain ganglion and the rami communicantes. Although he hypothesized that this chain originated in the brain and provided sensory function, he thought that the numerous interconnections allowed the spirits to travel between the various organs, maintaining a physiological "sympathy." Much later, in the sixteenth century, Bartholomeo Eustachio investigated the ganglionated nerves but did not contribute further to our understanding of this system. While still believing that this chain descended from the brain, Thomas Willis (1664) placed its origin in the posterior fossa and associated it with involuntary or automatic motion--specifically, with the motion of the heart and respiration. Francois Pourfour du Petit noted that pupil size and the amount of secretions from the eye were altered when the cervical sympathetic nerves were cut, and cast doubt on the sensory function of the sympathetic chain. In 1732, Winslow introduced the term sympathetic nerve, describing great, middle, and small components. In the nineteenth century, a flurry of research on the autonomic nervous system resulted in Walter Gaskell concluding that this system was actually composed of two subsystems. At the turn of the century, the terms preganglionic, postganglionic, and autonomic were first used by John Newport Langley, who theorized that this system contained both peripheral and central components. During this century, further refinements in our understanding of the autonomic nervous system were contributed by Thomas Elliott, Walter Dixon, and Otto Loewi, among others.
In discussing the autonomic nervous system and disorders related to it, a number of common terms require definition. Orthostatic hypotension (Greek, orthos: straight;statikos: causing to stand) refers to decreases of 20 mmHg in systolic blood pressure and 10 mmHg in diastolic pressure after standing. Syncope (Greek, synkope: cessation, pause), which can occur in the presence of autonomic dysfunction, is characterized by a loss of consciousness and postural tone due to a reversible reduction in blood flow to the reticular activating system in the brain stem. When both the sympathetic and parasympathetic neurotransmissions fail, the condition is typically termed a pandysautonomia. When only acetylcholine neurotransmission fails, the term cholinergic dysautonomia is used, whereas the term adrenergic dysautonomia is used to describe isolated failure of noradrenergic neurotransmission.
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