Autonomic Nervous System

The ANS is a component of the peripheral nervous system that uses both afferent (sensory) and efferent (effector) neurons, which control the functioning of the internal organs and involuntary processes via connections with the CNS. The ANS consists of the sympathetic and parasympathetic nervous systems. The efferent nerve fibers that terminate in the endocrine, vascular, and visceral structures coordinate the inner workings of the body in response to several afferent inputs. The sympathetic and parasympathetic neural circuits coordinate stress responses and relaxation responses, respectively. The enteric nervous system regulates visceral organ function. A balance between these systems results in homeostasis, whereas an imbalance leads to pathological conditions.

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Editorial responsibility: Stanley Oiseth, Lindsay Jones, Evelin Maza

Table of Contents

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Overview

Definition

The ANS is responsible for controlling functions that do not require conscious thought.

  • Controls unconscious, involuntary, and visceral body functions
  • Trajectory: CNS → ganglion → target tissue

Components

  • Afferent (sensory) neurons that originate in visceral receptors and provide inputs to the CNS
  • Efferent (effector or motor) neurons generally consist of a 2-neuron series:
    • Preganglionic (presynaptic) neuron with a cell body in the CNS
    • Postganglionic (postsynaptic) neuron with a cell body in the periphery that innervates target tissues
  • Presynaptic neurons of both the parasympathetic nervous system (PNS) and sympathetic nervous system (SNS) accomplish synaptic signal transmission using acetylcholine (ACh) as a neurotransmitter.
  • Postsynaptic sympathetic neurons generally accomplish synaptic signal transmission using norepinephrine (NE) as a neurotransmitter, although notable exceptions exist.
  • Postsynaptic parasympathetic neurons generally accomplish synaptic signal transmission using ACh as a neurotransmitter.
  • Enteric nervous system (ENS) neurons may have 3 or more neurons and accomplish synaptic signal transmission using several neurotransmitters:
    • ACh
    • NO
    • Serotonin (5-hydroxytryptamine (5HT))
Autonomic efferent pathway vs. Somatic efferent pathway

Autonomic efferent pathway vs. somatic efferent pathway

Image: “Comparison of Somatic and Visceral Reflexes” by Phil Schatz. License: CC BY 4.0

Divisions of the ANS

  • Sympathetic system:
    • Activation leads to a state of overall elevated function
    • Responsible for the “fight-or-flight” response
    • Connections to most tissues of the body
    • Sympathetic dominance → ↑ HR, ↑ blood pressure, cessation of GI peristalsis, glycogenolysis
  • Parasympathetic system:
    • Activation leads to a state of overall depressed function
    • Responsible for the “rest-and-digest” response
    • Parasympathetic dominance → ↓ HR, ↓ blood pressure, promotion of GI peristalsis, glycogenesis
  • ENS:
    • Involved in the regulation of digestive processes
    • Capable of functioning independently of the remainder of the nervous system
    • Consists of sympathetic and parasympathetic nerves
    • Exception to the 2-neuron innervation rule of the ANS
Schema showcasing the anatomical division of the nervous system

Schema showcasing the anatomical divisions of the nervous system

Image by Lecturio.

Sympathetic Nervous System

The SNS is involved with many of the functions associated with the “fight-or-flight” response. Although this response is at the extreme end of the sympathetic physiology spectrum, it serves as a model to understand that the SNS enables our tissues to respond appropriately to varying degrees of physiologic stress.

Sympathetic neurons

Cell bodies of preganglionic neurons are located in the spinal cord Spinal cord The spinal cord is the major conduction pathway connecting the brain to the body; it is part of the CNS. In cross section, the spinal cord is divided into an H-shaped area of gray matter (consisting of synapsing neuronal cell bodies) and a surrounding area of white matter (consisting of ascending and descending tracts of myelinated axons). Spinal Cord:

  • Emerge from the thoracolumbar regions
  • Located in the intermediolateral columns (lateral horns)
  • Presynaptic fibers exit the spinal cord Spinal cord The spinal cord is the major conduction pathway connecting the brain to the body; it is part of the CNS. In cross section, the spinal cord is divided into an H-shaped area of gray matter (consisting of synapsing neuronal cell bodies) and a surrounding area of white matter (consisting of ascending and descending tracts of myelinated axons). Spinal Cord through the anterior roots and enter the anterior rami of T1‒L2.
  • Anterior roots give off branches (white rami communicantes) to the sympathetic trunks. Sympathetic fibers may:
    • Travel (ascend or descend) in the sympathetic trunks to a paravertebral ganglion → synapse Synapse The junction between 2 neurons is called a synapse. The synapse allows a neuron to pass an electrical or chemical signal to another neuron or target effector cell. Synapses and Neurotransmission with postganglionic sympathetic fibers
    • Join the adjacent anterior spinal nerve rami via branches (gray rami communicantes) → synapse Synapse The junction between 2 neurons is called a synapse. The synapse allows a neuron to pass an electrical or chemical signal to another neuron or target effector cell. Synapses and Neurotransmission with postganglionic sympathetic fibers
    • Pass through the sympathetic trunk (without synapsing), becoming continuous with 1 of the splanchnic nerves to reach and ultimately synapse Synapse The junction between 2 neurons is called a synapse. The synapse allows a neuron to pass an electrical or chemical signal to another neuron or target effector cell. Synapses and Neurotransmission with the prevertebral ganglia
    • Pass through the celiac ganglion (without synapsing) and travel directly to their target organ (only applies to the adrenal gland) to synapse Synapse The junction between 2 neurons is called a synapse. The synapse allows a neuron to pass an electrical or chemical signal to another neuron or target effector cell. Synapses and Neurotransmission with chromaffin cells, which secrete epinephrine directly into the bloodstream (another exception to the 2-nerve rule of the ANS).
    • ACh is the neurotransmitter at all of the above synapses.

Paravertebral ganglia form a string of nodules known as the sympathetic trunk:

  • Located adjacent to the spinal column
  • Site of synapse Synapse The junction between 2 neurons is called a synapse. The synapse allows a neuron to pass an electrical or chemical signal to another neuron or target effector cell. Synapses and Neurotransmission between preganglionic and postganglionic neurons
  • Ganglia are generally organized as follows:
    • 3 cervical ganglia (superior, middle, and inferior)
    • 12 thoracic ganglia (inferior cervical ganglion and the 1st thoracic ganglion may fuse to form the “stellate” ganglion)
    • 4 lumbar ganglia
    • 5 sacral ganglia

Distal to the paravertebral ganglia, all nerves become splanchnic nerves.

Splanchnic nerves contain afferent and efferent fibers that convey information between the CNS and visceral structures:

  • Cardiopulmonary splanchnic nerves carry postsynaptic fibers that innervate the thoracic viscera.
  • Abdominopelvic splanchnic nerves carry postsynaptic fibers that innervate the abdominal and pelvic viscera.

The abdominal and pelvic viscera are innervated by abdominopelvic splanchnic nerves.

  • Recall, these nerves pass through the sympathetic trunks (without synapsing) to become splanchnic nerves.
  • Abdominopelvic splanchnic nerves include:
    • Greater splanchnic nerve
    • Lesser splanchnic nerve
    • Least splanchnic nerve
    • Lumbar splanchnic nerve
  • Presynaptic nerves ultimately synapse Synapse The junction between 2 neurons is called a synapse. The synapse allows a neuron to pass an electrical or chemical signal to another neuron or target effector cell. Synapses and Neurotransmission in the prevertebral ganglia.
  • Prevertebral ganglia lie close to their target organ and branches of the aorta.
  • Prevertebral ganglia include:
    • Celiac ganglion
    • Aorticorenal ganglion
    • Superior mesenteric ganglion
    • Inferior mesenteric ganglion
  • Splanchnic/ganglionic connections:
    • Greater splanchnic nerve → celiac ganglion
    • Lesser + least splanchnic nerves → aorticorenal ganglion
    • Least + lumbar splanchnic nerves → superior and inferior mesenteric ganglia
  • Ganglionic/visceral connections:
    • Celiac ganglion (innervates the foregut-derived organs) → distal esophagus Esophagus The esophagus is a muscular tube-shaped organ of around 25 centimeters in length that connects the pharynx to the stomach. The organ extends from approximately the 6th cervical vertebra to the 11th thoracic vertebra and can be divided grossly into 3 parts: the cervical part, the thoracic part, and the abdominal part. Esophagus, stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach, proximal duodenum, pancreas Pancreas The pancreas lies mostly posterior to the stomach and extends across the posterior abdominal wall from the duodenum on the right to the spleen on the left. This organ has both exocrine and endocrine tissue. Pancreas, liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver, biliary system, spleen Spleen The spleen is the largest lymphoid organ in the body, located in the LUQ of the abdomen, superior to the left kidney and posterior to the stomach at the level of the 9th-11th ribs just below the diaphragm. The spleen is highly vascular and acts as an important blood filter, cleansing the blood of pathogens and damaged erythrocytes. Spleen, adrenal glands Adrenal Glands The adrenal glands are a pair of retroperitoneal endocrine glands located above the kidneys. The outer parenchyma is called the adrenal cortex and has 3 distinct zones, each with its own secretory products. Beneath the cortex lies the adrenal medulla, which secretes catecholamines involved in the fight-or-flight response. Adrenal Glands
    • Aorticorenal ganglion → aorta, kidneys Kidneys The kidneys are a pair of bean-shaped organs located retroperitoneally against the posterior wall of the abdomen on either side of the spine. As part of the urinary tract, the kidneys are responsible for blood filtration and excretion of water-soluble waste in the urine. Kidneys
    • Superior mesenteric ganglion (innervates the midgut-derived organs) → distal duodenum, jejunum, ileum, cecum, appendix, ascending colon Colon The large intestines constitute the last portion of the digestive system. The large intestine consists of the cecum, appendix, colon (with ascending, transverse, descending, and sigmoid segments), rectum, and anal canal. The primary function of the colon is to remove water and compact the stool prior to expulsion from the body via the rectum and anal canal. Colon, Cecum, and Appendix, proximal transverse colon Colon The large intestines constitute the last portion of the digestive system. The large intestine consists of the cecum, appendix, colon (with ascending, transverse, descending, and sigmoid segments), rectum, and anal canal. The primary function of the colon is to remove water and compact the stool prior to expulsion from the body via the rectum and anal canal. Colon, Cecum, and Appendix
    • Inferior mesenteric ganglion (innervates the hindgut-derived organs) → distal transverse colon Colon The large intestines constitute the last portion of the digestive system. The large intestine consists of the cecum, appendix, colon (with ascending, transverse, descending, and sigmoid segments), rectum, and anal canal. The primary function of the colon is to remove water and compact the stool prior to expulsion from the body via the rectum and anal canal. Colon, Cecum, and Appendix, descending colon Colon The large intestines constitute the last portion of the digestive system. The large intestine consists of the cecum, appendix, colon (with ascending, transverse, descending, and sigmoid segments), rectum, and anal canal. The primary function of the colon is to remove water and compact the stool prior to expulsion from the body via the rectum and anal canal. Colon, Cecum, and Appendix, sigmoid colon Colon The large intestines constitute the last portion of the digestive system. The large intestine consists of the cecum, appendix, colon (with ascending, transverse, descending, and sigmoid segments), rectum, and anal canal. The primary function of the colon is to remove water and compact the stool prior to expulsion from the body via the rectum and anal canal. Colon, Cecum, and Appendix, rectum Rectum The rectum and anal canal are the most terminal parts of the lower GI tract/large intestine that form a functional unit and control defecation. Fecal continence is maintained by several important anatomic structures including rectal folds, anal valves, the sling-like puborectalis muscle, and internal and external anal sphincters. Rectum and Anal Canal, anal canal, bladder, external genitalia, gonads

Postganglionic sympathetic neurons then travel to their target tissues where they stimulate sympathetic activity specific to the target organ.

  • NE is released as a neurotransmitter at adrenergic receptors in most organs.
  • Adrenergic receptors include:
    • Alpha-1 and 2 receptors
    • Beta-1, 2, and 3 receptors
  • Peptides may be released at the synapse Synapse The junction between 2 neurons is called a synapse. The synapse allows a neuron to pass an electrical or chemical signal to another neuron or target effector cell. Synapses and Neurotransmission in addition to NE:
    • Neuropeptide Y is released at the cardiac vessels to regulate coronary blood flow.
    • Somatostatin is released at the intestinal mucosa to regulate GI motility.
  • ACh is released as a neurotransmitter at the sympathetic cholinergic receptors in:
    • Sweat glands
    • Piloerector muscles of the skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Structure and Function of the Skin (stimulation causes “goosebumps”)
    • Precapillary resistance vessels located in skeletal muscle beds
  • Cholinergic receptors in the SNS are of the muscarinic subtype.
  • Peptides may be released in addition to ACh at the synapse Synapse The junction between 2 neurons is called a synapse. The synapse allows a neuron to pass an electrical or chemical signal to another neuron or target effector cell. Synapses and Neurotransmission:
    • Vasoactive intestinal peptide (VIP) is released in the skeletal muscle beds and acts as a potent vasodilator.
    • Calcitonin gene-related peptide (CGRP) is released in the intracranial and extracranial vessels and acts as a potent vasodilator.
Pathways of the sympathetic nervous system

Pathways of the sympathetic nervous system
T: thoracic
L: lumbar

Image by Lecturio.

Sympathetic functions

At its extreme, the SNS triggers the “fight-or-flight” response in reaction to physiologic stress.

  • Primarily a vascular response
  • Most vessels respond to sympathetic stimulation by vasoconstriction to:
    • Abdominal visceral organs to redirect blood flow to vital “fight-or-flight” organs 
    • Pelvic visceral organs to redirect blood flow to vital “fight-or-flight” organs
  • Notable exceptions:
    • Coronary vessels dilate to enhance myocardial perfusion and enhance cardiac performance.
    • Vessels of the skeletal muscle beds dilate to enhance muscular perfusion and enhance muscular performance.
    • Vessels of the external genitalia dilate to allow arousal and erection.
  • The vascular response of a given target tissue depends on:
    • The relative proportion of alpha to beta receptors
    • The presence of peptides released with neurotransmission Neurotransmission The junction between 2 neurons is called a synapse. The synapse allows a neuron to pass an electrical or chemical signal to another neuron or target effector cell. The plasma membranes of the 2 neurons are placed very close together, and the space between the 2 neurons is called the synaptic cleft. The molecules that mediate the interaction are called neurotransmitters. Synapses and Neurotransmission at the synapse Synapse The junction between 2 neurons is called a synapse. The synapse allows a neuron to pass an electrical or chemical signal to another neuron or target effector cell. Synapses and Neurotransmission

In times of normal physiologic stress, the SNS is still constitutively active (but in balance with the PNS):

  • There is tonic stimulation of blood vessels but not to the extent of extreme vasoconstriction:
    • SNS and PNS allow coordinated activities at the level of the alveolar-capillary interface in the lung to allow optimal O2 exchange.
    • SNS and PNS allow coordinated activities at the level of the intestinal epithelium Epithelium The epithelium is a complex of specialized cellular organizations arranged into sheets and lining cavities and covering the surfaces of the body. The cells exhibit polarity, having an apical and a basal pole. Structures important for the epithelial integrity and function involve the basement membrane, the semipermeable sheet on which the cells rest, and interdigitations, as well as cellular junctions. Surface Epithelium-capillary interface to allow optimal absorption.
  • At times of rest, SNS-mediated vascular tonicity is at a minimum.

The SNS plays a role in immune regulation.

  • SNS innervates the organs of immunity:
    • Spleen
    • Thymus
    • Lymph nodes
  • Immune system cells have adrenergic receptors.
  • Adrenergic receptor stimulation may modify the immune and/or inflammatory response.
The dynamics of sympathetic outflow

The dynamics of sympathetic outflow

Image by Lecturio.

Parasympathetic Nervous System

The PNS is involved with many of the functions associated with “rest and digest.” Although the “rest-and-digest” response is at the extreme end of the parasympathetic physiology spectrum, it serves as a model to understand that the PNS enables our tissues to recover appropriately from and/or balance varying degrees of physiologic stress.

Parasympathetic neurons

  • Parasympathetic fibers emerge from the craniosacral regions of the nervous system.
  • Cell bodies of the preganglionic neurons are located in the cranial nerve (CN) nuclei (CN III, CN VII, CN IX, and CN X) and in the terminal portion of the spinal cord Spinal cord The spinal cord is the major conduction pathway connecting the brain to the body; it is part of the CNS. In cross section, the spinal cord is divided into an H-shaped area of gray matter (consisting of synapsing neuronal cell bodies) and a surrounding area of white matter (consisting of ascending and descending tracts of myelinated axons). Spinal Cord:
    • CN X (vagus nerve, comprises the majority of the PNS) has 4 cell bodies located in the medulla oblongata:
      • Dorsal nucleus → efferent parasympathetic output to the viscera
      • Nucleus ambiguus → preganglionic and motor neurons to the heart
      • Nucleus solitarius → receives afferent parasympathetic input from the viscera and tongue Tongue The tongue, on the other hand, is a complex muscular structure that permits tasting and facilitates the process of mastication and communication. The blood supply of the tongue originates from the external carotid artery, and the innervation is through cranial nerves. Oral Cavity: Lips and Tongue (taste sensation)
      • Spinal trigeminal nucleus → receives afferent sensory input from the ear, laryngeal mucosa, and meninges Meninges The brain and the spinal cord are enveloped by 3 overlapping layers of connective tissue called the meninges. The layers are, from the most external layer to the most internal layer, the dura mater, arachnoid mater, and pia mater. Between these layers are 3 potential spaces called the epidural, subdural, and subarachnoid spaces. Meninges (touch, pain Pain Pain has accompanied humans since they first existed, first lamented as the curse of existence and later understood as an adaptive mechanism that ensures survival. Pain is the most common symptomatic complaint and the main reason why people seek medical care. Physiology of Pain, and temperature)
  • Some preganglionic neurons synapse Synapse The junction between 2 neurons is called a synapse. The synapse allows a neuron to pass an electrical or chemical signal to another neuron or target effector cell. Synapses and Neurotransmission inside the cranial vault in 1 of the 4 pairs of intracranial sympathetic ganglia:
    • CN III → ciliary ganglion → innervates the:
      • Iris
      • Pupillary ciliary muscles
    • CN VII → pterygopalatine and submandibular ganglia → innervate the:
      • Lacrimal glands
      • Nasal glands
      • Palatine glands
      • Pharyngeal glands
      • Sublingual glands
      • Submandibular glands
    • CN IX → otic ganglion → innervates the parotid glands
  • Some preganglionic neurons outside the cranial vault synapse Synapse The junction between 2 neurons is called a synapse. The synapse allows a neuron to pass an electrical or chemical signal to another neuron or target effector cell. Synapses and Neurotransmission in the terminal or intramural ganglia of the target organ(s):
    • CN X (vagus nerve) → terminal ganglia of the:
      • Thoracic viscera: heart, lungs Lungs Lungs are the main organs of the respiratory system. Lungs are paired viscera located in the thoracic cavity and are composed of spongy tissue. The primary function of the lungs is to oxygenate blood and eliminate CO2. Lungs, distal esophagus Esophagus The esophagus is a muscular tube-shaped organ of around 25 centimeters in length that connects the pharynx to the stomach. The organ extends from approximately the 6th cervical vertebra to the 11th thoracic vertebra and can be divided grossly into 3 parts: the cervical part, the thoracic part, and the abdominal part. Esophagus, stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach, proximal duodenum, pancreas Pancreas The pancreas lies mostly posterior to the stomach and extends across the posterior abdominal wall from the duodenum on the right to the spleen on the left. This organ has both exocrine and endocrine tissue. Pancreas, liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver, biliary system, spleen Spleen The spleen is the largest lymphoid organ in the body, located in the LUQ of the abdomen, superior to the left kidney and posterior to the stomach at the level of the 9th-11th ribs just below the diaphragm. The spleen is highly vascular and acts as an important blood filter, cleansing the blood of pathogens and damaged erythrocytes. Spleen
      • Midgut viscera: distal duodenum, jejunum, ileum, cecum, appendix, ascending colon Colon The large intestines constitute the last portion of the digestive system. The large intestine consists of the cecum, appendix, colon (with ascending, transverse, descending, and sigmoid segments), rectum, and anal canal. The primary function of the colon is to remove water and compact the stool prior to expulsion from the body via the rectum and anal canal. Colon, Cecum, and Appendix, proximal transverse colon Colon The large intestines constitute the last portion of the digestive system. The large intestine consists of the cecum, appendix, colon (with ascending, transverse, descending, and sigmoid segments), rectum, and anal canal. The primary function of the colon is to remove water and compact the stool prior to expulsion from the body via the rectum and anal canal. Colon, Cecum, and Appendix
      • Hindgut viscera: distal transverse colon Colon The large intestines constitute the last portion of the digestive system. The large intestine consists of the cecum, appendix, colon (with ascending, transverse, descending, and sigmoid segments), rectum, and anal canal. The primary function of the colon is to remove water and compact the stool prior to expulsion from the body via the rectum and anal canal. Colon, Cecum, and Appendix, bladder, external genitalia, gonads, kidneys Kidneys The kidneys are a pair of bean-shaped organs located retroperitoneally against the posterior wall of the abdomen on either side of the spine. As part of the urinary tract, the kidneys are responsible for blood filtration and excretion of water-soluble waste in the urine. Kidneys, upper ureters
    • Pelvic splanchnic nerves (S2–S4) → terminal ganglia of the:
      • Remaining hindgut viscera: descending colon Colon The large intestines constitute the last portion of the digestive system. The large intestine consists of the cecum, appendix, colon (with ascending, transverse, descending, and sigmoid segments), rectum, and anal canal. The primary function of the colon is to remove water and compact the stool prior to expulsion from the body via the rectum and anal canal. Colon, Cecum, and Appendix, sigmoid colon Colon The large intestines constitute the last portion of the digestive system. The large intestine consists of the cecum, appendix, colon (with ascending, transverse, descending, and sigmoid segments), rectum, and anal canal. The primary function of the colon is to remove water and compact the stool prior to expulsion from the body via the rectum and anal canal. Colon, Cecum, and Appendix, rectum Rectum The rectum and anal canal are the most terminal parts of the lower GI tract/large intestine that form a functional unit and control defecation. Fecal continence is maintained by several important anatomic structures including rectal folds, anal valves, the sling-like puborectalis muscle, and internal and external anal sphincters. Rectum and Anal Canal, anal canal
      • Pelvic viscera: lower ureters, bladder, urethra, uterus, cervix, external genitalia
  • Afferent parasympathetic fibers comprise visceral afferent fibers that convey input to the vagus nerve and pelvic splanchnic nerves:
    • From the carotid and aortic baroreceptors
    • From the heart, lungs Lungs Lungs are the main organs of the respiratory system. Lungs are paired viscera located in the thoracic cavity and are composed of spongy tissue. The primary function of the lungs is to oxygenate blood and eliminate CO2. Lungs, and alimentary tract
    • From the pelvic organs and external genitalia
  • The majority of parasympathetic nerves are sensory and innervate most major organs.
  • Preganglionic and postganglionic PNS release ACh as a neurotransmitter onto the cholinergic receptors:
    • Muscarinic cholinergic receptors:
      • M-1, M-2, M-3
      • All cholinergic receptors at the preganglionic/postganglionic synapse Synapse The junction between 2 neurons is called a synapse. The synapse allows a neuron to pass an electrical or chemical signal to another neuron or target effector cell. Synapses and Neurotransmission are of the muscarinic subtype.
      • Cholinergic receptors at the sweat glands are of the muscarinic subtype.
    • Nicotinic cholinergic receptors: Cholinergic receptors at most other target organs are of the nicotinic subtype.
    • Peptides may be released in addition to ACh at the synapse Synapse The junction between 2 neurons is called a synapse. The synapse allows a neuron to pass an electrical or chemical signal to another neuron or target effector cell. Synapses and Neurotransmission:
      • Neuropeptide Y
      • VIP
      • CGRP

Parasympathetic functions

At its extreme, the PNS triggers the “rest-and-digest” response in reaction to a need for physiologic recovery and balances the actions of the SNS.

  • Decreases contractility in the atrial and ventricular myocardium
  • Reduces cardiac conduction velocity to slow tachycardic rhythms and prevent arrhythmias
  • Promotes salivation
  • Promotes/increases GI peristalsis and secretory activity
  • Contracts smooth muscle in the airways during inspiration to maintain patency

The PNS plays a role in immune regulation:

  • Parasympathetic ganglia inflammatory cytokine receptors:
    • Activates the hypothalamic–pituitary–adrenal axis → cortisol release
    • Indirect activates SNS-mediated immune/inflammatory functions
Pathways of the sympathetic nervous system

Pathways of the sympathetic and parasympathetic nervous systems
T: thoracic
L: lumbar

Image by Lecturio.

Sympathetic vs Parasympathetic Nervous System

Table: Sympathetic and parasympathetic nervous system functions and receptors
Target Sympathetic effects and receptors Parasympathetic effects and receptors
Brain α1: cerebral blood flow regulation M1: ↑ memory and attention
Eye α1: contraction (mydriasis) of the iris dilator muscle
β2: focusing on distant objects
M3: miosis and accommodation
Bladder α1: constriction of the bladder sphincter
control of micturition and urine flow
β2: bladder relaxation
M3: relaxation of vesical sphincter muscle;
contraction of the detrusor muscle
Prostate Prostate The prostate is a gland in the male reproductive system. The gland surrounds the bladder neck and a portion of the urethra. The prostate is an exocrine gland that produces a weakly acidic secretion, which accounts for roughly 20% of the seminal fluid. Prostate and other Male Reproductive Glands and reproductive organs α1: cause ejaculation by prostate contraction M1: erection
Kidney α1: ↓ renin secretion
β2: ↑ renin secretion
None
Veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins and arterioles α1: contraction of smooth muscles of the peripheral blood vessels
β2: promote dilation of arterioles and veins; consequently, a decrease in total peripheral resistance, blood pressure, and afterload
Most vessels do not possess parasympathetic innervation.
Platelets Platelets Platelets are small cell fragments involved in hemostasis. Thrombopoiesis takes place primarily in the bone marrow through a series of cell differentiation and is influenced by several cytokines. Platelets are formed after fragmentation of the megakaryocyte cytoplasm. Platelets α2: ↑ platelet aggregation None
Heart β1: ↑ HR (positive chronotropic);
↑ conduction velocity (positive dromotropic);
↑ contractibility (positive inotropic)
M2: ↓ HR (negative chronotropic);
↓ conduction velocity (negative dromotropic);
↓ contractibility (negative inotropic)
Bronchioles β2: bronchiolar smooth muscle relaxation M3: bronchoconstriction
Liver β2: ↑ glycogenolysis M3: gluconeogenesis Gluconeogenesis Gluconeogenesis is the process of making glucose from noncarbohydrate precursors. This metabolic pathway is more than just a reversal of glycolysis. Gluconeogenesis provides the body with glucose not obtained from food, such as during a fasting period. The production of glucose is critical for organs and cells that cannot use fat for fuel. Gluconeogenesis
Adipose tissue Adipose tissue Adipose tissue is a specialized type of connective tissue that has both structural and highly complex metabolic functions, including energy storage, glucose homeostasis, and a multitude of endocrine capabilities. There are three types of adipose tissue, white adipose tissue, brown adipose tissue, and beige or "brite" adipose tissue, which is a transitional form. Adipose Tissue α2: ↓ lipolysis
β1, β2: ↑ lipolysis
None
Skeletal muscle β3: thermogenesis M3: contraction of the detrusor muscle

Enteric Nervous System

The enteric portion of the ANS is associated with functions of digestion and regulation of the GI secretions and the smooth muscle.

Intramural nervous system

  • Located in the wall of the GI tract (plexus entericus)
  • > 100 million neurons of > 15 morphologies that form a web-like structure innervating the abdominal viscera
  • Involved in the regulation of digestive processes and is capable of functioning independently of the remainder of the nervous system
    • Fully self-contained and capable of independent function via local reflex activity
    • Connected to, communicates with, and receives feedback from the SNS and PNS.
  • Consists of 2 ganglion-rich plexuses:
    • Submucous plexus (Meissner plexus): found in the submucosa → regulates fluid and electrolyte movement across the intestinal mucosa
    • Myenteric plexus (Auerbach plexus): found in the muscularis propria between the longitudinal and circular layers of the smooth muscle in walls of the GI tract → coordinates smooth muscle contractions involved in peristalsis
    • Sensory neurons detect chemical changes in the GI tract.
    • Enteric motor neurons:
      • Regulation of smooth muscle contractions via interneuronal connections that communicate in an excitatory/inhibitory manner via nicotinic receptor activation
      • Control of the secretions of the GI organs
  • > 30 neurotransmitters/peptides involved in the ENS signaling pathways

Sympathetic innervation

  • Prevertebral ganglia:
    • Celiac ganglia
    • Superior mesenteric ganglia
    • Inferior mesenteric ganglia
  • Nerves:
    • Splanchnic nerve(s)
    • Hypogastric nerve(s)
    • Colonic nerves(s)
  • Effects:
    • ↓ GI peristalsis and secretion
    • Constriction of GI sphincters

Parasympathetic innervation

  • Nerves:
    • Vagus
    • Pelvic splanchnic nerves (S2–S4)
    • Afferent sensory nerves → conscious sensation of visceral function (e.g., hunger, distention, nausea)
  • Effects:
    • ↑ GI peristalsis and secretion
    • Constriction of GI sphincters

Clinical Relevance

  • Horner syndrome Horner syndrome Horner syndrome is a condition resulting from an interruption of the sympathetic innervation of the eyes. The syndrome is usually idiopathic but can be directly caused by head and neck trauma, cerebrovascular disease, or a tumor of the CNS. Horner Syndrome: a condition that damages the sympathetic nerves of 1 side of the face, affecting the sympathetic output of the superior cervical ganglion. Horner syndrome Horner syndrome Horner syndrome is a condition resulting from an interruption of the sympathetic innervation of the eyes. The syndrome is usually idiopathic but can be directly caused by head and neck trauma, cerebrovascular disease, or a tumor of the CNS. Horner Syndrome results from an injury, a disease, or a hereditary mutation Mutation Genetic mutations are errors in DNA that can cause protein misfolding and dysfunction. There are various types of mutations, including chromosomal, point, frameshift, and expansion mutations. Types of Mutations. A commonly examined scenario is where Horner syndrome Horner syndrome Horner syndrome is a condition resulting from an interruption of the sympathetic innervation of the eyes. The syndrome is usually idiopathic but can be directly caused by head and neck trauma, cerebrovascular disease, or a tumor of the CNS. Horner Syndrome can present in a Pancoast lung tumor and its metastatic spread can invade the sympathetic chain.
  • Multiple sclerosis Multiple Sclerosis Multiple sclerosis (MS) is a chronic inflammatory autoimmune disease that leads to demyelination of the nerves in the CNS. Young women are more predominantly affected by this most common demyelinating condition. Multiple Sclerosis: The name of this disease describes the presence of multiple indurations (scleroses) in the CNS. Multiple sclerosis Multiple Sclerosis Multiple sclerosis (MS) is a chronic inflammatory autoimmune disease that leads to demyelination of the nerves in the CNS. Young women are more predominantly affected by this most common demyelinating condition. Multiple Sclerosis is a chronic, immune-mediated, progressive inflammatory CNS disease that damages the myelin sheath and nerve cells to varying degrees, leading to physical disabilities.
  • ANS dysfunction (dysautonomia): leads to nonfunctional organs of the ANS. Some causes of ANS dysfunction include autonomic neuropathy, HIV/AIDS, multiple sclerosis, paraneoplastic syndromes Paraneoplastic syndromes Paraneoplastic syndromes are a heterogeneous group of disorders caused by an abnormal immune response to a neoplasm. The substances produced are not due to the direct effect of the tumor, such as metastasis, mass effect, or invasion. Antibodies, hormones, cytokines, and other substances are generated and affect multiple organ systems. Paraneoplastic Syndromes, and Parkinson disease. Affected individuals present with features of unopposed efferent neuronal functions that include anhidrosis, anxiety, constipation Constipation Constipation is common and may be due to a variety of causes. Constipation is generally defined as bowel movement frequency < 3 times per week. Patients who are constipated often strain to pass hard stools. The condition is classified as primary (also known as idiopathic or functional constipation) or secondary, and as acute or chronic. Constipation, orthostatic hypotension Hypotension Hypotension is defined as low blood pressure, specifically < 90/60 mm Hg, and is most commonly a physiologic response. Hypotension may be mild, serious, or life threatening, depending on the cause. Hypotension, tachycardia, vertigo Vertigo Vertigo is defined as the perceived sensation of rotational motion while remaining still. A very common complaint in primary care and the ER, vertigo is more frequently experienced by women and its prevalence increases with age. Vertigo is classified into peripheral or central based on its etiology. Vertigo, bowel incontinence, difficulty in swallowing, exercise intolerance, and chronic fatigue.

Resources

  1. Waxenbaum, J. A., Reddy, V., & Varacallo, M. (2021). Anatomy, Autonomic Nervous System. In StatPearls. Treasure Island (FL): StatPearls Publishing. Retrieved October 10, 2021, from https://www.ncbi.nlm.nih.gov/books/NBK539845/
  2. Chawla, J. (2016). Autonomic Nervous System Anatomy. Retrieved October 10, 2021, from https://emedicine.medscape.com/article/1922943-overview

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