Anatomy of the Autonomic Nervous System

Lee, Won-Taek

Ann Clin Neurophysiol > 2002 ;4(2) > Article

Ann Clin Neurophysiol. 2002; 4(2): 195-203.

Won-Taek Lee

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

ABSTRACT

Autonomic nervous system (ANS) is a part of peripheral and central nervous system primarily concerned with the regulation and control of visceral functions. This system controls visceral activities by regulating the actions of smooth muscles, cardiac muscles, and various exocrine glands. Three subsystems of ANS, the sympathetic, the parasympathetic and the enteric systems are divided by both an anatomical and a functional basis. Sympathetic and parasympathetic efferent system are consist of two neuron chains, preganglionic and postganglionic neurons. Sympathetic preganglionic neurons are located in the intermediolateral nucleus of thoracolumbar region (C8 or T1 to L2 or L3), while parasympathetic preganglionic neurons are located in the cranial nerve nuclei(Edinger-Westphal, superior salivatory, inferior salivatory nucleus and dorsal motor nucleus of vagus) and sacral intermediolateral nucleus (S2 to S4). Postganglionic sympathetic neurons are aggregations of neurons found in paravertebral sympathetic trunks and prevertebral ganglia. Ciliary, submandibular, pterygopalatine, otic and numerous visceral ganglionic plexuses are aggregations of postganglionic parasympathetic neurons. The cranial nerves III(oculomotor), VII (facial) and IX (glossopharyngeal) affect the pupil and salivary gland secretion, whilst the vagus nerve (X) carries fibers to the heart, lungs, stomach, upper intestine and ureter. The sacral fibers form pelvic plexuses which innervate the distal colon, rectum, bladder and reproductive organs. The chemical transmitter at both pre and postganglionic synapses in the parasympathetic system as well as sympathetic preganglionic synapses is acetylcholine. At most postganglionic sympathetic endings, the chemical transmitter is norepinephrine, which is present in the presynaptic terminal as well as in the adrenal medulla. In sweat glands, however, postganglionic sympathetic fibers release acetylcholine. There are many receptors in the viscera whose afferent fibers travel centrally by way of the parasympathetic and sympathetic nerves. These afferent autonomic fibers which are concerned with the mediation of visceral sensation and the regulation of vasomotor and respiratory reflexes, for example the baroreceptors and chemoreceptors in the carotid sinus and aortic arch which are important in the control of heart rate, blood pressure and respiratory activity. Autonomic afferent fibers are usually carried to the CNS, mostly nucleus tractus solitarius by major autonomic nerves. The enteric nervous system (ENS) also forms a part of the ANS. This system exhibits a complexity that is found in no other group of neurons outside the CNS. The ENS is remarkably brainlike, both structurally and functionally. The complex intrinsic neuronal circuits of the ENS regulates gastrointestinal motility, mucosal secretion and absorption by intrinsic reflexes. Although the activity of the ENS can be modified from sympathetic and parasympathetic inputs, much of the control of the gastrointestinal function can be achieved independently. Preganglionic neurons of the ANS are maintained by a multitude of segmental and suprasegmental mechanisms. The hypothalamus is regarded as the principal site of central autonomic integration. Regulation of specific patterned autonomic responses is also achieved by neurons in the reticular formation of the ventrolateral medulla, the parabrachial nucleus and the periaqueductal gray of the midbrain.