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El sistema nervioso autónomo es un componente del sistema nervioso periférico que regula los procesos fisiológicos involuntarios que incluyen la frecuencia cardíaca, la presión arterial, la respiración, la digestión y la excitación sexual. Contiene tres divisiones anatómicamente distintas: simpática, parasimpática y entérica (Waxenbaum JA et al., 2024).

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SISTEMA NERVIOSO SIMPÁTICO Y PARASIMPÁTICO

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SISTEMA NERVIOSO SIMPÁTICO Y PARASIMPÁTICO

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El sistema nervioso simpático (SNS) y el sistema nervioso parasimpático (SNP) contienen fibras aferentes y eferentes que proporcionan información sensorial y salida motora, respectivamente, al sistema nervioso central (SNC). Generalmente, las vías motoras del SNS y del SNP constan de una serie de dos neuronas: una neurona preganglionar con un cuerpo celular en el SNC y una neurona posganglionar con un cuerpo celular en la periferia que inerva los tejidos diana (Waxenbaum JA et al., 2024).

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Abstract

The autonomic nervous system is a component of the peripheral nervous system that regulates involuntary physiologic processes including heart rate, blood pressure, respiration, digestion, and sexual arousal. It contains three anatomically distinct divisions: sympathetic, parasympathetic, and enteric. The sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS) contain both afferent and efferent fibers that provide sensory input and motor output, respectively, to the central nervous system (CNS). Generally, the SNS and PNS motor pathways consist of a two-neuron series: a preganglionic neuron with a cell body in the CNS and a postganglionic neuron with a cell body in the periphery that innervates target tissues. The enteric nervous system (ENS) is an extensive, web-like structure that is capable of function independently of the remainder of the nervous system. It contains over 100 million neurons of over 15 morphologies, greater than the sum of all other peripheral ganglia, and is chiefly responsible for the regulation of digestive processes. Activation of the SNS leads to a state of overall elevated activity and attention: the "fight or flight" response. In this process, blood pressure and heart rate increase, glycogenolysis ensues, gastrointestinal peristalsis ceases, etc. The SNS innervates nearly every living tissue in the body. The PNS promotes the "rest and digest" processes; heart rate and blood pressure lower, gastrointestinal peristalsis/digestion restarts, etc. The PNS innervates only the head, viscera, and external genitalia, notably vacant in much of the musculoskeletal system and skin, making it significantly smaller than the SNS. The ENS is composed of reflex pathways that control the digestive functions of muscle contraction/relaxation, secretion/absorption, and blood flow. Presynaptic neurons of both the SNS and PNS utilize acetylcholine (ACh) as their neurotransmitter. Postsynaptic sympathetic neurons generally produce norepinephrine (NE) as their effector transmitter to act upon target tissues, while postsynaptic parasympathetic neurons use ACh throughout. Enteric neurons have been known to use several major neurotransmitters such as ACh, nitrous oxide, and serotonin, to name a few.

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StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-.

Joshua A. Waxenbaum; Vamsi Reddy; Matthew Varacallo.
Author Information
Last Update: July 24, 2023.

Introduction

The autonomic nervous system is a component of the peripheral nervous system that regulates involuntary physiologic processes including heart rate, blood pressure, respiration, digestion, and sexual arousal. It contains three anatomically distinct divisions: sympathetic, parasympathetic, and enteric.

Structure and Function

Sympathetic Nervous System

Sympathetic neurons have cell bodies located in the intermediolateral columns, or lateral horns, of the spinal cord. The presynaptic fibers exit the spinal cord through anterior roots and enter the anterior rami of T1-L2 spinal nerves and onto the sympathetic trunks via white rami communicantes. From here, the fibers may ascend or descend the sympathetic trunk to a superior or inferior paravertebral ganglion, respectively, pass to adjacent anterior spinal nerve rami via gray rami communicantes, or cross through the trunk without synapsing and continue through an abdominopelvic splanchnic nerve to reach prevertebral ganglia. Because of the central location of the sympathetic ganglia, presynaptic fibers tend to be shorter than their postsynaptic counterparts.[2][9]

Paravertebral ganglia exist as nodules throughout the sympathetic trunk, adjacent to the spinal column, where pre- and postganglionic neurons synapse. While the numbers may vary by individual, generally, there are three cervical, 12 thoracic, four lumbar, and five sacral ganglia. Of these, only the cervical have names of superior, middle, and inferior cervical ganglia. The inferior cervical ganglion may fuse with the first thoracic ganglion to form the stellate ganglion.[2][9]

All nerves distal to the paravertebral ganglia are splanchnic nerves. These convey afferent and efferent fibers between the CNS and the viscera. Cardiopulmonary splanchnic nerves carry the postsynaptic fibers destined for the thoracic cavity.

The SNS is constantly active, even in non-stressful situations. In addition to the aforementioned tonic stimulation of blood vessels, the SNS is active during the normal respiratory cycle. Sympathetic activation complements the PNS by acting during inspiration to dilate the airways allowing for an appropriate inflow of air.[2][21]

The majority of postganglionic sympathetic neurons are noradrenergic, and also release one or more peptides such as neuropeptide Y or somatostatin. NE/neuropeptide Y neurons innervate blood vessels of the heart, thus regulating blood flow,[25] while NE/somatostatin neurons of the celiac and superior mesenteric ganglia supply the submucosal ganglia of the intestine and are involved in the control of gastrointestinal motility. The thinking is that these peptides serve to modulate the response of the postsynaptic neuron to the primary neurotransmitter.[1]

Parasympathetic Nervous System

Parasympathetic fibers exit the CNS via cranial nerves (CN) III, VII, IX, and X, as well as through the S2-4 nerve roots. There are four pairs of parasympathetic ganglia, and they are all located in the head. CN III, via the ciliary ganglion, innervates the iris and ciliary muscles of the eye. CN VII innervates the lacrimal, nasal, palatine, and pharyngeal glands via the pterygopalatine ganglion, as well as the sublingual and submandibular glands via the submandibular ganglion. CN IX innervates the parotid glands via the otic ganglion.[4]Every other presynaptic parasympathetic fiber synapses in a ganglion near or on the wall of the target tissue; this leads to the presynaptic fibers being significantly longer than the postsynaptic. The location of these ganglia gives the PNS its name: "para-" means adjacent to, hence, "parasympathetic."[2]

Dorsal nucleus: provides parasympathetic output to the viscera

Nucleus ambiguus: produces motor fibers and preganglionic neurons that innervate the heart

Nucleus solitarius: receives afferents of taste sensation and that from viscera, and lastly

Spinal trigeminal nucleus: receives information of touch, pain, and temperature of the outer ear, the mucosa of the larynx, and part of the dura

Additionally, the vagus nerve conducts sensory information from baroreceptors of the carotid sinus and the aortic arch to the medulla.[32]

Postganglionic parasympathetic neurons release ACh that acts on muscarinic and nicotinic receptors, each with various subunits: M1, M2, and M3, and N1 and N2, with "M" and "N" standing for muscarine and nicotine, respectively.[5] The postganglionic ACh receptors and those on the adrenal medulla are N-type, while the parasympathetic effectors and sweat glands are M-type.[2] As in sympathetic neurons, several peptides, such as vasoactive intestinal peptide (VIP), Neuropeptide Y (NPY), and calcitonin gene-related peptide (CGRP) are expressed in, and released from, parasympathetic neurons.[27][28][35][36] For more information, see the StatPearls article on cholinergic receptors, here.[37]

Enteric Nervous System (ENS)

The ENS is composed of two ganglionated plexuses: the myenteric (Auerbach) and the submucosal (Meissner). The myenteric plexus sits in between the longitudinal and circular smooth muscle of the GI tract, while the submucosal plexus is present within the submucosa. The ENS is self-contained, functioning through local reflex activity, but often receives input from, and provides feedback to, the SNS and PNS. The ENS may receive input from postganglionic sympathetic neurons or preganglionic parasympathetic neurons.[1][38]

The submucosal plexus governs the movement of water and electrolytes across the intestinal wall, while the myenteric plexus coordinates the contractility of the circular and longitudinal muscle cells of the gut to produce peristalsis.[39]

Motility is produced in the ENS through a reflex circuit involving the circular and longitudinal muscles. Nicotinic synapses between interneurons mediate the reflex circuits.[39] When the circuit activates by the presence of a bolus, excitatory neurons in the circular muscle and inhibitory neurons in the longitudinal muscle fire producing a narrow section of bowel proximal to the bolus; this is known as the propulsive segment. Simultaneously, excitatory neurons in the longitudinal muscle and inhibitory neurons in the circular muscle fire producing the "receiving segment" of the bowel in which the bolus will continue. This process repeats with each subsequent section of the bowel.[40]

The ENS maintains several similarities to the CNS. As in the CNS, enteric neurons can be bipolar, pseudounipolar, and multipolar, between which neuromodulation via excitatory and inhibitory communication.[1] Likewise, ENS neurons use over 30 neurotransmitters that are similar to those of the CNS, with cholinergic and nitrergic transmitters being the most common.[39]

While much of this discussion has focused on the efferent functions of the ANS, the afferent fibers are responsible for numerous reflex activities that regulate everything from heart rate to the immune system. Feedback from the ANS is usually processed at a subconscious level to produce reflex actions in the visceral or somatic portions of the body. The conscious sensation of the viscera is often interpreted as diffuse pain or cramps that may correlate with hunger, fullness, or nausea. These sensations most commonly result from sudden distention/contractions, chemical irritants, or pathological conditions such as ischemia.[41]

Embryology

Surgical Considerations

Horner syndrome is a mild, rare condition often presenting with unilateral ptosis, miotic, but a reactive pupil, and facial anhidrosis secondary to sympathetic nerve damage in the oculosympathetic pathway.[46] This damage may have a central cause such as infarction of the lateral medulla, or peripheral such as from damage secondary to thoracic surgery or from partial/total resection of the thyroid gland.[46][47] More centralized lesions tend to correlate with a constellation of symptoms that include Horner syndrome.[46] For more information, please see the associated StatPearls articles, here.[48][49]

Clinical Significance

Uremic neuropathy/chronic liver diseases

Toxin/drug-induced: alcohol, amiodarone, chemotherapy

Infections: Botulism, Chagas disease, HIV, leprosy, Lyme disease, tetanus

Autoimmune: Guillain-Barre, Lambert-Eaton myasthenic syndrome, rheumatoid arthritis, Sjogren, systemic lupus erythematosus

Neurological: multiple system atrophy/Shy-Drager syndrome, Parkinson disease, Lewy body dementia

Neoplasia: Brain tumors, paraneoplastic syndromes

Likewise, autonomic neuropathy can present in nearly any system. Orthostatic hypotension is the most common autonomic dysautonomia, but numerous other, less understood, findings may present[53]

The most prevalent symptoms of orthostatic hypotension are lightheadedness, tunnel vision, and discomfort in the head, neck, or chest. It may present concomitantly with supine hypertension due to increased peripheral resistance, which induces natriuresis, exacerbating orthostatic hypotension. There are numerous other, more benign stimuli that may either lower blood pressure (standing, food, Valsalva, dehydration, exercise, hyperventilation, etc.) or raise blood pressure (lying supine, water ingestion, coffee, head-down tilt, hypoventilation, etc.).[53]

Orthostatic hypotension evaluation is commonly done through orthostatic testing via repeated blood pressure and heart rate readings in supine and standing positions, but also through the use of the tilt-table test. However, the advantage of this latter test is minimal over the orthostatic test, with the main benefit being safety and convenience to the patient.[53]

References

Disclosure: Joshua Waxenbaum declares no relevant financial relationships with ineligible companies.

Disclosure: Vamsi Reddy declares no relevant financial relationships with ineligible companies.

Disclosure: Matthew Varacallo declares no relevant financial relationships with ineligible companies.

Copyright © 2024, StatPearls Publishing LLC.

This book is distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0) ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ), which permits others to distribute the work, provided that the article is not altered or used commercially. You are not required to obtain permission to distribute this article, provided that you credit the author and journal.

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