What is the Autonomic Nervous System? A Comprehensive Guide

The autonomic nervous system (ANS) is a vital component of the peripheral nervous system responsible for regulating involuntary physiological processes. These processes are crucial for survival and include heart rate, blood pressure, respiration, digestion, and even sexual arousal. Understanding the ANS is key to grasping how our bodies maintain homeostasis and respond to internal and external stimuli.

The ANS is anatomically divided into three distinct branches: the sympathetic nervous system (SNS), the parasympathetic nervous system (PNS), and the enteric nervous system (ENS). Each of these systems plays a unique role in maintaining overall bodily function.

Structure and Function of the Autonomic Nervous System

The SNS and PNS are comprised of both afferent and efferent nerve fibers. Afferent fibers carry sensory information from the body to the central nervous system (CNS), while efferent fibers transmit motor commands from the CNS to target tissues. Generally, the motor pathways of the SNS and PNS involve a two-neuron series: a preganglionic neuron located within the CNS and a postganglionic neuron situated in the periphery, which directly innervates target tissues.

The ENS, on the other hand, is an extensive, web-like network of neurons within the gastrointestinal tract. Uniquely, the ENS is capable of functioning independently from the rest of the nervous system. With over 100 million neurons exhibiting more than 15 distinct morphologies, the ENS surpasses the neuronal count of all other peripheral ganglia combined. Its primary function is the regulation of digestive processes.

The Sympathetic Nervous System: “Fight or Flight”

Activation of the SNS triggers a state of heightened activity and alertness, commonly known as the “fight or flight” response. During this response, the body prepares for immediate action. Blood pressure and heart rate increase, glycogenolysis (the breakdown of glycogen to glucose) occurs to provide energy, and gastrointestinal peristalsis (muscle contractions that move food through the digestive tract) ceases. The SNS innervates nearly every tissue in the body, enabling a widespread and coordinated response to perceived threats or stressful situations.

The Parasympathetic Nervous System: “Rest and Digest”

In contrast to the SNS, the PNS promotes “rest and digest” functions. When the PNS is active, heart rate and blood pressure decrease, while gastrointestinal peristalsis and digestion resume. The PNS primarily innervates the head, viscera (internal organs), and external genitalia. Notably, the PNS has limited innervation of the musculoskeletal system and skin, making it significantly smaller than the SNS.

The Enteric Nervous System: The “Brain” of the Gut

The ENS is composed of reflex pathways that govern the digestive functions of muscle contraction and relaxation, secretion and absorption, and regulation of blood flow within the gastrointestinal tract.

Image: A simplified diagram illustrating the basic organization of the sympathetic and parasympathetic nervous systems.

Neurotransmitters of the Autonomic Nervous System

Neurotransmitters are chemical messengers that transmit signals between neurons. Presynaptic neurons in both the SNS and PNS utilize acetylcholine (ACh) as their primary neurotransmitter. However, the postsynaptic neurotransmitters differ. Postsynaptic sympathetic neurons typically release norepinephrine (NE) as their effector transmitter to act upon target tissues, whereas postsynaptic parasympathetic neurons use ACh throughout. Enteric neurons utilize a variety of neurotransmitters, including ACh, nitric oxide, and serotonin.

Detailed Look at the Structure of Each System

Sympathetic Nervous System Anatomy

Sympathetic neurons originate in the intermediolateral columns, or lateral horns, of the spinal cord. Presynaptic fibers exit the spinal cord via anterior roots and enter the anterior rami of T1-L2 spinal nerves, connecting to the sympathetic trunks through white rami communicantes. From the sympathetic trunks, fibers may ascend or descend to superior or inferior paravertebral ganglia, respectively. They can also pass to adjacent anterior spinal nerve rami via gray rami communicantes or traverse the trunk without synapsing, continuing through abdominopelvic splanchnic nerves to reach prevertebral ganglia. The location of sympathetic ganglia closer to the spinal cord results in shorter presynaptic fibers compared to their postsynaptic counterparts.

Paravertebral ganglia are arranged as nodules along the sympathetic trunk, adjacent to the spinal column, where pre- and postganglionic neurons synapse. The number of ganglia varies among individuals, but typically includes three cervical, twelve thoracic, four lumbar, and five sacral ganglia. Among these, only the cervical ganglia are named superior, middle, and inferior cervical ganglia. The inferior cervical ganglion may fuse with the first thoracic ganglion to form the stellate ganglion.

All nerves distal to the paravertebral ganglia are splanchnic nerves, which carry afferent and efferent fibers between the CNS and the viscera. Cardiopulmonary splanchnic nerves carry postsynaptic fibers destined for the thoracic cavity. Nerves innervating the abdominal and pelvic viscera pass through the paravertebral ganglia without synapsing, becoming abdominopelvic splanchnic nerves, including the greater, lesser, least, and lumbar splanchnic nerves. These presynaptic nerves ultimately synapse in prevertebral ganglia closer to their target organs, forming part of the nervous plexuses surrounding the branches of the aorta, such as the celiac, aorticorenal, and superior and inferior mesenteric ganglia.

Image: Illustration of the sympathetic nerves pathways, highlighting the interconnected network.

The celiac ganglion receives input from the greater splanchnic nerve and innervates organs derived from the foregut: distal esophagus, stomach, proximal duodenum, pancreas, liver, biliary system, spleen, and adrenal glands. The superior mesenteric ganglion innervates derivatives of the midgut: distal duodenum, jejunum, ileum, cecum, appendix, ascending colon, and proximal transverse colon. Lastly, the inferior mesenteric ganglion provides sympathetic innervation to structures developed from the hindgut: distal transverse, descending, and sigmoid colon; rectum and upper anal canal; as well as the bladder, external genitalia, and gonads.

Parasympathetic Nervous System Anatomy

Parasympathetic fibers exit the CNS via cranial nerves (CN) III, VII, IX, and X, as well as through the S2-4 nerve roots. Four pairs of parasympathetic ganglia are located in the head. CN III, through 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, and the sublingual and submandibular glands via the submandibular ganglion. CN IX innervates the parotid glands via the otic ganglion. Other presynaptic parasympathetic fibers synapse in ganglia near or on the wall of the target tissue, resulting in significantly longer presynaptic fibers than postsynaptic fibers. The proximity of these ganglia to the target organs gives the PNS its name: “para-” meaning adjacent to, hence, “parasympathetic.”

The vagus nerve, CN X, constitutes approximately 75% of the PNS and provides parasympathetic input to most of the thoracic and abdominal viscera, with the sacral parasympathetic fibers innervating the descending and sigmoid colon and rectum. The vagus nerve has four cell bodies in the medulla oblongata:

• 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.
• 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.

Image: Illustration showing the parasympathetic nerves pathway, emphasizing cranial and sacral nerve distribution.

Enteric Nervous System Anatomy

The ENS is composed of two ganglionated plexuses: the myenteric (Auerbach) and the submucosal (Meissner). The myenteric plexus resides between the longitudinal and circular smooth muscle layers of the GI tract, while the submucosal plexus is located within the submucosa. The ENS functions autonomously through local reflex activity but also receives input from, and provides feedback to, the SNS and PNS. The ENS may receive input from postganglionic sympathetic neurons or preganglionic parasympathetic neurons.

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.

Embryological Development of the Autonomic Nervous System

The peripheral nervous system originates from neural crest cells, which are divided axially into cranial, vagal, truncal, and lumbosacral neural crest cells. Truncal neural crest cells contribute to the dorsal root of the spinal cord and the sympathetic ganglia. The parasympathetic innervation of the heart develops from the vagal neural crest. A significant portion of the parasympathetic nervous system, including all ganglia of the head, arises from glial cells rather than neural crest cells.

The ENS originates from the vagal neural crest, with cells migrating in a rostral-to-caudal pattern through the intestinal wall, forming a network of glia and neurons of various subtypes. Cells of the ENS complete their migration by four to seven weeks of development and express all varieties of ENS neurotransmitters by gestational week 24. However, mature gut motility is not fully realized until late gestation or shortly after birth.

Surgical Implications of the Autonomic Nervous System

Damage to the autonomic nervous system can have significant clinical consequences. Horner syndrome, for instance, is a relatively uncommon condition often resulting from sympathetic nerve damage in the oculosympathetic pathway, leading to unilateral ptosis (drooping eyelid), miosis (constricted pupil), and facial anhidrosis (lack of sweating). This damage may stem from central causes, such as infarction of the lateral medulla, or peripheral causes, such as injury during thoracic surgery or resection of the thyroid gland.

Hyperhidrosis, characterized by excessive sweating, primarily affects the face, palms, soles, and axilla. The cause of primary hyperhidrosis is not entirely understood, but it is linked to increased cholinergic stimulation. Treatment can be clinical, involving anticholinergic agents, or surgical, involving resection, ablation, or clipping of the thoracic sympathetic chain.

Clinical Significance: Disorders of the Autonomic Nervous System

The autonomic nervous system, due to its extensive reach throughout the body, can be affected by a wide variety of conditions, including:

• Inherited Disorders: Amyloidosis, Fabry disease, hereditary sensory autonomic neuropathy, porphyrias.
• Acquired Conditions:
  • Diabetes mellitus
  • Uremic neuropathy/chronic liver diseases
  • Vitamin B12 deficiency
  • 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

Autonomic neuropathy can manifest in nearly any system, with symptoms including:

• Cardiovascular: Fixed heart rate, postural hypotension, resting tachycardia.
• Gastrointestinal: Dysphagia, gastroparesis, constipation.
• Genitourinary: Bladder atony.
• Pupillary: Absent/delayed light reflexes, decreased pupil size.
• Sexual: Erectile dysfunction, retrograde ejaculation.
• Sudomotor: Anhidrosis, gustatory sweating.
• Vasomotor: Cold extremities, edema.

Orthostatic hypotension, the most common form of autonomic dysautonomia, presents with lightheadedness, tunnel vision, and discomfort in the head, neck, or chest. Evaluation commonly involves orthostatic testing via repeated blood pressure and heart rate readings in supine and standing positions, or a tilt-table test.

Most conditions related to the ENS are congenital and present during early childhood. Hirschsprung disease, a life-threatening disorder, results from a failure of embryologic ENS cells to colonize the distal bowel, leading to early constipation, vomiting, growth failure, and possible death.

Conclusion

The autonomic nervous system is a complex and critical system responsible for maintaining homeostasis and regulating a wide range of involuntary bodily functions. Understanding its structure, function, and clinical significance is essential for comprehending overall health and well-being. From the “fight or flight” response of the sympathetic nervous system to the “rest and digest” functions of the parasympathetic nervous system and the independent control of digestion by the enteric nervous system, the ANS plays a vital role in our daily lives. Further research into the autonomic nervous system holds great promise for improving our understanding and treatment of various conditions that affect this important system.

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