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The key beta-1 selective drug is dobutamine. Lastly, beta-2 selective drugs are bronchodilators, such as albuterol and salmeterol. Adrenergic drugs can also be non-selective and hence bind to a combination of adrenergic receptors. Norepinephrine binds to the alpha-1, alpha-2, and beta-1 receptors.

An alpha and beta adrenergic agonist used to treat nasal and sinus congestion, as well as allergic rhinitis. An alpha and beta-adrenergic agonist used for the management of respiratory conditions related to bronchial asthma, such as wheezing, shortness of breath, and tightness of chest.

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The AHS rests, and the HPA axis returns to normal function. This is the “rest and digest” response. This system is primarily regulated by the parasympathetic nervous system or PNS. With this system, the body relaxes, any leftover adrenaline is metabolized, and the body recovers from the effects of stress.

The main overall end effect of the SNS is to prepare the body for physical activity, a whole-body reaction affecting many organ systems throughout the body to redirect oxygen-rich blood to areas of the body needed during intense physical demand Actions

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The sympathetic nervous system (SNS) is one of the two divisions of the autonomic nervous system (ANS), along with the parasympathetic nervous system (PNS), These systems primarily work unconsciously in opposite ways to regulate many functions and parts of the body.

The SNS governs the "fight or flight" response while the PNS controls the "rest and digest" response.

Fibers from the SNS innervate tissues in almost every organ system and provide physiological regulation over diverse body processes including pupil diameter, gut motility (movement), and urinary output.

The main overall end effect of the SNS is to prepare the body for physical activity, a whole-body reaction affecting many organ systems throughout the body to redirect oxygen-rich blood to areas of the body needed during intense physical demand

The actions of the sympathetic nervous system occur in concert with other neural or hormonal responses to stress eg increases in corticotropin and cortisol secretion. Messages travel through the SNS in a bidirectional flow. Efferent messages can trigger changes in different parts of the body simultaneously. eg, the sympathetic nervous system can accelerate heart rate, widen bronchial passages, decrease motility (movement) of the large intestine, constrict blood vessels, cause pupil dilation, activate goose bumps, start sweating and raise blood pressure.

The effects in which SNS acts in direct contrast to the PNS function include the following:

Eye, sympathetic activation causes the radial muscle of the iris to contract, which leads to mydriasis, allowing more light to enter. The ciliary muscle relaxes, allowing for far vision to improve.

Heart, sympathetic activation causes an increased heart rate, the force of contraction, and rate of conduction, allowing for increased cardiac output to supply the body with oxygenated blood.

Lungs, bronchodilation and decreased pulmonary secretions occur to allow more airflow through the lungs.

Stomach and intestines, decreased motility and sphincter contraction, as well as contraction of the gallbladder, occur to slow down digestion to divert energy to other parts of the body.

Exocrine and endocrine pancreas decreases both enzyme and insulin secretion.

Urinary bladder, there is relaxation of the detrusor muscle and contraction of the urethral sphincter to help stop urine output during sympathetic activation.

Kidney increases renin secretion to increase intravascular volume.

Salivary glands work through small volume potassium and water secretion.

In humans, chronic stress results in long-term stimulation of the fight-or-flight response, which leads to constant production and secretion of catecholamines (e.g., epinephrine) and hormones such as cortisol.

Long-term stress-induced secretion of these substances is associated with a variety of physiological consequences eg hyperglycemia (high blood glucose levels), which can lead to type 2 diabetes mellitus, and hypertension (high blood pressure), which can lead to cardiovascular disease.

The sympathetic nerves run parallel to the spinal cord on both sides of the vertebral column as described below:

Preganglionic neurons: Neurons of the intermediolateral column of the spinal cord, found within the levels T1-T12 and L1-L3

Preganglionic fibers: The axons of the preganglionic neurons that leave the spinal cord through the anterior rami of spinal nerves and continue their path as white rami communicantes

Sympathetic ganglia: Sympathetic trunk (paravertebral ganglia); Prevertebral (splanchnic) ganglia; The neuronal bodies of the sympathetic ganglia synapse with the white rami communicantes

Postganglionic fibers The axons of the ganglionic neurons that leave the ganglia in the form of gray rami communicantes which join the rami of the spinal nerves.

Spinal nerves C2-C8 carry sympathetic innervation to head, neck, upper limbs and thorax

Spinal nerves T1-L2 carry sympathetic innervation for the trunk wall, as well as participate in comprising the splanchnic nerves for innervation of the abdominopelvic viscera

Spinal nerves L3-Co carry sympathetic innervation to the cutaneous structures of the lower limbs.

Central sympathetic overactivity has been identified in a plethora of clinical conditions. Aside from a role in the development of hypertension, sympathetic overactivity has been implicated in the initiation and progression of numerous pathophysiological processes independent of increases in BP (e.g. vascular hypertrophy, atherosclerosis, glomerulosclerosis, and insulin resistance).

In this condition, the actions of the stress hormones (eg cortisol, epinephrine and norepinephrine) are overly exerted on various organs. Long-term stress-induced secretion of these substances is associated with a variety of physiological consequences eg

It affects both physical and mental health and can trigger somatic symptoms like heart palpitations, diarrhea, sweating and insomnia as well as panic attacks and chronic stress.

Some of the most common causes of this condition include:

Stress and Overstimulation: Provoked by a stimulus or something that prompts focus and vigilance, it can be a stressful event or a demanding task at work. The more the stimuli, the higher activity of the sympathetic nervous system.

Not enough sleep

Poor diet (see the Gut Brain Axis)

Many people have an overactive sympathetic nervous system. Assess your clients lifestyles and be aware of the health implications. Educate them when and if appropriate. Techniques to stimulate the reduce sympathetic nervous system activity and increase vagal tone include:

Exercise training (widely applied non-pharmacological option in the case of the overactive sympathetic nervous system) The World Health Organization recommends that adults aged 18-64 years old should do at least either 150-300 minutes of moderate-intensity aerobic activity or 75-150 minutes of vigorous-intensity aerobic physical activity every week. It has been proven that regular exercise training effectively calms the overactive sympathetic nervous system and visibly reduces its symptoms like hypertension, signs of heart failure and diabetes as well as anxiety

Diet: Eg The sympathetic nervous system responds to changes in caloric intake; caloric restriction decreases and carbohydrate administration increases sympathetic nervous system activity. Insulin may be a major link between changes in dietary intake and changes in central sympathetic outflow. Coffee and caffeine-containing teas can be very stimulating for the nervous system. Choosing less caffeine, changing the time you consume it, or having a caffeine-free tea is a better choice;

Acupuncture stimulation is found to decrease the sympathetic outflow of certain neurotransmitters and stress hormones that typically flood our brains in response to chronic stress. In other words, acupuncture promotes the body's healing response by down-regulating the sympathetic "fight or flight" nervous activity. This allows the parasympathetic "rest and digest" response to take the lead and establish a natural healing mode of repair and regeneration.

1. Alshak MN. Neuroanatomy, sympathetic nervous system.2019 Available from: https://www.ncbi.nlm.nih.gov/books/NBK542195/(accessed 7.2.2021)

2. Britannica SNS Available from:https://www.britannica.com/science/sympathetic-nervous-system (accessed 7.2.2021)

3. Fisher JP, Young CN, Fadel PJ. Central sympathetic overactivity: maladies and mechanisms. Autonomic Neuroscience. 2009 Jun 15;148(1-2):5-15.Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2679852/ (accessed 7.2.2021)

4. Healthnutrition news SNS Available from:https://www.healthnutritionnews.org/health/overactive-sympathetic-nervous-system (accessed 7.2.2021)

5. Review of Pertinent Anatomy and Physiology Sandy Fritz MS, NCTMB, ... Glenn M. Hymel EdD, LMT, in Clinical Massage in the Healthcare Setting, 2008Available from: https://www.sciencedirect.com/topics/neuroscience/parasympathetic-nervous-system(accessed 30.1.2021)

6. Lu WA, Kuo CD. The effect of Tai Chi Chuan on the autonomic nervous modulation in older persons. Medicine and science in sports and exercise. 2003 Dec 1;35(12):1972-6.Available from: https://pubmed.ncbi.nlm.nih.gov/14652490/(accessed 31.1.2021)Jump up↑ Stromberg SE, Russell ME, Carlson CR. Diaphragmatic breathing and its effectiveness for the management of motion sickness. Aerospace medicine and human performance. 2015 May 1;86(5):452-7.Available from: https://pubmed.ncbi.nlm.nih.gov/25945662/(accessed 31.1.2021)

7. Young JB, Landsberg L. Diet-induced changes in sympathetic nervous system activity: possible implications for obesity and hypertension. Journal of chronic diseases. 1982 Jan 1;35(12):879-86 Available from:.https://pubmed.ncbi.nlm.nih.gov/6816809/ (accessed 7.2.2021)

8. Review of Pertinent Anatomy and Physiology Sandy Fritz MS, NCTMB, ... Glenn M. Hymel EdD, LMT, in Clinical Massage in the Healthcare Setting, 2008Available from: https://www.sciencedirect.com/topics/neuroscience/parasympathetic-nervous-system(accessed 30.1.2021)

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The sympathetic nervous system (SNS) is one of the two divisions of the autonomic nervous system (ANS), along with the parasympathetic nervous system (PNS), These systems primarily work unconsciously in opposite ways to regulate many functions and parts of the body. The SNS governs the "fight or flight" response while the PNS controls the "rest and digest" response.

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Fight/Flight - Breathing becomes faster, increased heart rate and blood pressure. More energy to react to a stressful situation.

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Author: Adrian Rad BSc (Hons) * Reviewer: Elizabeth O. Johnson, PhD
Last reviewed: December 14, 2021
Reading time: 18 minutes

Sympathetic nervous system (diagram)

The autonomic system is made up of two divisions, the sympathetic and parasympathetic systems. They usually work antagonistically in the organs, but in a well integrated manner. It is the balance of the actions of both divisions that maintains a stable internal environment in the body.

While the sympathetic system is also important at rest, it is essential for preparing us for emergencies, in other words, for "fight-or-flight" responses. If you have ever been scared or anxious, attacked or threatened, you have basically experienced activation of your sympathetic system. To prepare yourself for an emergency in a "fight-or-flight" response, the sympathetic system activates numerous complex pathways and components to achieve faster breathing, increased heart rate and blood pressure, dilation of pupils, changes in blood flow, so blood leaves the skin, stomach and intestines and goes to the brain, heart and muscles where it's needed, increased sweating and "goose-bumps" as the hair on your skin stands on end; all those things you feel during a fight-or-flight response.

This article will discuss the anatomy and function of the sympathetic nervous system.

Definition

Thoracolumbar division of the autonomic nervous system which is in charge to initiate bodily stress response ("flight or fight")

Preganglionic neurons

Neurons of the intermediolateral column of the spinal cord, found within the levels T1-T12 and L1-L3

Preganglionic fibers

The axons of the preganglionic neurons that leave the spinal cord through the anterior rami of spinal nerves and continue their path as white rami communicantes

Sympathetic ganglia

- Sympathetic trunk (paravertebral ganglia)
- Prevertebral (splanchnic) ganglia
The neuronal bodies of the sympathetic ganglia synapse with the white rami communicantes

Postganglionic fibers

The axons of the ganglionic neurons that leave the ganglia in the form of gray rami communicantes which join the rami of the spinal nerves.
- Spinal nerves C2-C8 carry sympathetic innervation to head, neck, upper limbs and thorax
- Spinal nerves T1-L2 carry sympathetic innervation for the trunk wall, as well as participate in comprising the splanchnic nerves for innervation of the abdominopelvic viscera
- Spinal nerves L3-Co carry sympathetic innervation to the cutaneous structures of the lower limbs

Function

Stress response of the body: increases heart rate, miosis of the eye, vasoconstriction, bronchodilation, energy release from liver, adrenaline release from suprarenal gland

Visceral nervous system

The human nervous system is the most complex product of evolution. It enables the body to react to continuous internal and external or environmental changes. It also controls and integrates various internal activities of the body. The nervous system can be divided structurally or functionally, as follows:

﻿

Nervous system breakdown (diagram)

Structurally, the nervous system is divided into the central nervous system (CNS) and peripheral nervous system (PNS). The CNS consists of the brain and the spinal cord, while the PNS consists of all the neural tissues outside the CNS. Namely, these structures are the cranial nerves, spinal nerves, and peripheral ganglia.

The peripheral part of the nervous system can also be broken down functionally. By this classification, the nervous system consists of the somatic nervous system (SNS) and autonomic nervous system (ANS). The somatic nervous system is a voluntary part of the nervous system. It consists of the sensory and motor nerves. Somatic sensory nerves carry the sensory information from the skin, joints and muscles to the CNS, while the somatic motor nerves innervate skeletal muscles and enable us to move them upon our will.

The autonomic nervous system, however, contains only the motor nerves. These nerves control the involuntary functions of the body viscera, namely the smooth and cardiac muscles, as well as the glands. The ANS is further subdivided into the sympathetic nervous system (SANS) and parasympathetic nervous system (PANS). These two subdivisions cause opposite effects on the bodily organs, but their synchronized activity is essential for maintaining the homeostasis of the body.

The anatomical distinction between the sympathetic and parasympathetic divisions is given by the location of the presynaptic cell bodies and the types of nerves conducting presynaptic nerve fibers.

Autonomic nervous system starter pack is waiting for you in our study unit:

General sympathetic pathway

The general sympathetic pathway can be simplified into the following components:

preganglionic

ganglionic

postganglionic

The preganglionic components consist of preganglionic neurons located inside the spinal cord and their fibers (axons), which are called preganglionic fibers. The axons synapse with the postganglionic neuron inside sympathetic ganglia. These ganglia are actually a collection of cell bodies of postganglionic neurons, usually situated outside the CNS. Postganglionic components consist of postganglionic neurons and their fibers. The axons leave the ganglia and project onto visceral effectors, where they release the neurotransmitter norepinephrine. Both preganglionic and postganglionic neurons aremultipolar.

Preganglionic components

The cell bodies of the preganglionic neurons of the SNS are found only in the intermediolateral cell columns (ICLs) of the spinal cord, one on the left side and on the right. ICLs are part of the lateral horns of the gray matter of the thoracic (T1-12) and upper lumbar (L1-2 or 3) spinal cord segments, hence the alternative name "thoracolumbar" for the sympathetic division. This region consists of the visceral motor region of the spinal gray matter. You can think of the ICLs as longitudinal tubes passing through the respective lateral horns of the spinal cord. The preganglionic SNS cell bodies are organised somatotopically, meaning the arrangement of the cell bodies is a close representation to that of the body. Basically, the T1-6 cell bodies that are located superiorly innervate the head, upper limb and thoracic viscera. T7-11 located in the middle innervate the body wall and abdominal viscera, while T11-L2(3) located inferiorly innervate the lower limb and pelvic viscera.

The preganglionic fibers leave the ICLs and thus, the spinal cord through the anterior roots. They travel very briefly through the anterior rami of spinal nerves T1-L2(3), before leaving them and passing to the sympathetic trunks (more details later) through the white rami communicantes (white because nerve fibers are covered with white myelin).

Ganglionic components

Types of ganglia

The ganglionic compartment is actually composed of the cell bodies of the postganglionic neurons. It consists of two types, paravertebral and prevertebral ganglia.

Paravertebral ganglia ("para" = alongside, beside) occur on either side of the vertebral column and are independently linked on either side, forming two sympathetic trunks (chains). The paravertebral ganglia are the site where preganglionic fibers synapse with postganglionic neurons. The trunks extend the entire length of the column, from the base of the cranium to the coccyx. They converge anteriorly to the coccyx, forming the ganglion impar (ganglion of Walther). Each trunk is attached to the anterior rami of the T1-L2(3) spinal nerves.

Prevertebral ganglia (splanchnic ganglia) are located in the abdominal cavity around the origin of the major branches of the abdominal aorta. The prevertebral ganglia form aggregations around the abdominal prevertebral plexus and are referred to as the celiac, aorticorenal and superior and inferior mesenteric ganglia. Various nerve plexuses branch from these ganglia.

Course of fibers

In general, after passing briefly through the anterior rami, preganglionic fibers enter the sympathetic trunk via white rami communicantes. Inside the trunk, preganglionic fibers can follow one of four courses:

1. Ascend and synapse in a higher paravertebral ganglion

Within the sympathetic trunk, preganglionic fibers usually from T1-5 spinal cord levels can ascend to other vertebral levels and synapse inside ganglia located at a more superior level. The ganglia might not necessarily be associated with inputs directly from the spinal cord (other nerves than T1-L2/3 can participate in the synapse).

2. Descend and synapse in a lower paravertebral ganglion

These are similar to the ascending preganglionic, but in contrast, they descend to ganglia located at a more inferior level. This pathway usually involves fibers from T5-L2(3). The ascending and descending preganglionic fibers gives the sympathetic trunk the appearance of a chain with connections between the ganglia.

3. Synapse directly in a paravertebral ganglion at the same level

After synapsing inside the ganglion, postganglionic fibers leave through a gray ramus communicans (grey due to absence of myelin) and re-enters the same anterior ramus, which it initially travelled through.

The fibers are subsequently distributed to effector structures with peripheral branches of the anterior and posterior rami of the same spinal nerve. The fibers can also combine with fibers from other levels to form splanchnic nerves, which then pass onto the thoracic viscera (more details later).

4. Travel without synapsing all the way to the prevertebral ganglia

Preganglionic fibers can also pass through the sympathetic trunk without synapsing. These fibers are usually derived from the spinal cord levels T5 to L2(3). Once they pass through the sympathetic trunk, they combine with fibers from other levels to form and exit the trunk as a splanchnic nerve. Splanchnic nerves synapse on a prevertebral ganglia, and the postsynaptic fibers then pass onto the abdomen and pelvic viscera via a visceral motor nerve plexus.

Postganglionic components

Gray ramus communicans (cranial view)

The postganglionic compartment consists of postganglionic fibers travelling to effectors. The number of postganglionic fibers are greater than preganglionic ones. Approximately one preganglionic fiber synapses with at least thirty postganglionic fibers. After synapsing, postganglionic fibers leave the ganglia through gray rami communicantes and travel through the anterior and posterior rami of the spinal nerves. These rami carry the fibers all the way to the periphery and visceral components.

Ascending sympathetic fibers through the sympathetic trunk join peripheral nerves from C2-8 spinal nerves. These project onto effectors in the head, neck, upper limbs and thoracic cavity. For example, a cephalic arterial nervous branch leaves the superior cervical ganglion and projects onto the peri-arterial plexus on the carotid arteries. From here they project onto the dilator muscle of iris.

Descending sympathetic fibers through the sympathetic trunk join peripheral nerves from L3 to coccyx spinal nerves. These project onto the skin in the lower limbs, where they stimulate vasomotion, sudomotion and pilomotion.

Sympathetic fibers that enter and leave the trunk at the same level join peripheral nerves from T1-L2(3) spinal nerves. These project onto the body wall via cutaneous branches, but also via visceral motor nerves to sweat glands, smooth muscle and arrector pili muscles. Postganglionic fibers can also combine to form splanchnic nerves. These nerve types convey visceral efferent and afferent fibers to and from the viscera. Postganglionic fibers projecting onto thoracic viscera (e.g., heart, lungs, esophagus) pass through cardiopulmonary splanchnic nerves.

Splanchnic nerves

Sympathetic fibers which pass through the trunk without synapsing also combine with other fibers to form splanchnic nerves, of which there are five: greater, lesser, least, lumbar and sacral splanchnic nerves. Collectively these are called abdominopelvic splanchnic nerves. In this case, the synapsing happens in prevertebral ganglia rather than paravertebral ganglia. Postganglionic fibers from these prevertebral ganglia follow the main branches of the aorta and subsequently project onto all the organs (except adrenal glands) in the abdominal and pelvic cavities.

The adrenal glands are an exception. For every single human body organ, the postganglionic fibers synapse and release norepinephrine for regulation. However, for these glands, the nerves project directly onto the medullary cells without synapsing. The cells themselves play the role of the postganglionic neurons by releasing neurotransmitters, such as epinephrine (adrenaline), directly into the bloodstream. This results in a widespread sympathetic response.

Function

The reach of the sympathetic system is extremely broad within the human body. It is a component of virtually all spinal nerves and peri-arterial plexuses, and sympathetic fibers innervate all the blood vessels, sweat glands, arrector pili and viscera. The only structures the sympathetic system does not reach are avascular structures, like nails and cartilage.

Eyes

Mydriasis (dilation of the pupil)

Skin

Goosebumps, vasoconstriction, sweating

Lacrimal and salivary glands

Decreases secretion

Heart

Increases heart rate and strength of contraction

Blood vessels

Contracts smooth muscle (vasoconstriction)

Lungs

Bronchodilation, decreases secretion of bronchial glands

Digestive system

Inhibits peristalsis, constricts blood vessels and redirects blood to skeletal muscles, contracts anal sphincters

Liver and gallbladder

Stimulates breakdown of glycogen to glucose - energy release

Urinary system

Decreases urine production, contracts internal bladder sphincter

Genital system

Ejaculation

Suprarenal gland

Stimulates release of epinephrine (adrenaline) into blood

The sympathetic and parasympathetic divisions of the nervous system work in very close association, with contrasting, yet tightly coordinated effects. The sympathetic system is involved in energy-expending (catabolism), enabling the body to use energy appropriately to respond to stressful situations and emergencies, as in the "fight or flight" response. Activation of the sympathetic system results in pupil dilation, piloerection, vasoconstriction of cutaneous blood vessels, sweating, release of adrenaline, bronchodilation, increased cardiac contraction and reduced digestion.

During normal conditions, blood vessels are tonically maintained in a resting state of moderate vasoconstriction. If sympathetic signals are increased, vasoconstriction increases and vice-versa. However, in coronary vessels, skeletal muscles and vessels of the external genitalia, sympathetic stimulation results in vasodilation.

Feeling ready to test yourself on the sympathetic nervous system anatomy? Try out our quiz below:

References

K. L. Moore, A. F. Dalley, A. M. R. Agur: Clinically Oriented Anatomy, 7th edition, Lippincott Williams & Wilkins

Richard L. Drake, A. Wayne Vogl, Adam. W.M. Mitchell: Gray's Anatomy for Students, 2nd Edition, Churchill Livingstone Elsevier

Susan Standring: Gray's Anatomy: The Anatomical Basis of Clinical Practice, 41st edition, Elsevier

Bandyk, D.F., et al., Surgical sympathectomy for reflex sympathetic dystrophy syndromes. J Vasc Surg, 2002. 35(2): p. 269-77.

Vinik, A.I. and D. Ziegler, Diabetic Cardiovascular Autonomic Neuropathy. Circulation, 2007. 115(3): p. 387-397.

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Sympathetic nervous system (diagram) - Paul Kim

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# data[].attributes.attachment_link.description
Author: Adrian Rad BSc (Hons) * Reviewer: Elizabeth O. Johnson, PhD Last reviewed: December 14, 2021 Reading time: 18 minutes The autonomic system is made up of two divisions, the sympathetic and parasympathetic systems. They usually work antagonistically in the organs, but in a well integrated manner.

# data[].attributes.body
Rest and digest- controls smooth cardiac muscles slows down the heart rate, relaxing sphincter muscles in GI tract and urinary tract.

# data[].attributes.attachment_link.readable_text
Author: Adrian Rad BSc (Hons) * Reviewer: Elizabeth O. Johnson, PhD
Last reviewed: December 14, 2021
Reading time: 18 minutes

Sympathetic nervous system (diagram)

The autonomic system is made up of two divisions, the sympathetic and parasympathetic systems. They usually work antagonistically in the organs, but in a well integrated manner. It is the balance of the actions of both divisions that maintains a stable internal environment in the body.

While the sympathetic system is also important at rest, it is essential for preparing us for emergencies, in other words, for "fight-or-flight" responses. If you have ever been scared or anxious, attacked or threatened, you have basically experienced activation of your sympathetic system. To prepare yourself for an emergency in a "fight-or-flight" response, the sympathetic system activates numerous complex pathways and components to achieve faster breathing, increased heart rate and blood pressure, dilation of pupils, changes in blood flow, so blood leaves the skin, stomach and intestines and goes to the brain, heart and muscles where it's needed, increased sweating and "goose-bumps" as the hair on your skin stands on end; all those things you feel during a fight-or-flight response.

This article will discuss the anatomy and function of the sympathetic nervous system.

Definition

Thoracolumbar division of the autonomic nervous system which is in charge to initiate bodily stress response ("flight or fight")

Preganglionic neurons

Neurons of the intermediolateral column of the spinal cord, found within the levels T1-T12 and L1-L3

Preganglionic fibers

The axons of the preganglionic neurons that leave the spinal cord through the anterior rami of spinal nerves and continue their path as white rami communicantes

Sympathetic ganglia

- Sympathetic trunk (paravertebral ganglia)
- Prevertebral (splanchnic) ganglia
The neuronal bodies of the sympathetic ganglia synapse with the white rami communicantes

Postganglionic fibers

The axons of the ganglionic neurons that leave the ganglia in the form of gray rami communicantes which join the rami of the spinal nerves.
- Spinal nerves C2-C8 carry sympathetic innervation to head, neck, upper limbs and thorax
- Spinal nerves T1-L2 carry sympathetic innervation for the trunk wall, as well as participate in comprising the splanchnic nerves for innervation of the abdominopelvic viscera
- Spinal nerves L3-Co carry sympathetic innervation to the cutaneous structures of the lower limbs

Function

Stress response of the body: increases heart rate, miosis of the eye, vasoconstriction, bronchodilation, energy release from liver, adrenaline release from suprarenal gland

Visceral nervous system

The human nervous system is the most complex product of evolution. It enables the body to react to continuous internal and external or environmental changes. It also controls and integrates various internal activities of the body. The nervous system can be divided structurally or functionally, as follows:

﻿

Nervous system breakdown (diagram)

Structurally, the nervous system is divided into the central nervous system (CNS) and peripheral nervous system (PNS). The CNS consists of the brain and the spinal cord, while the PNS consists of all the neural tissues outside the CNS. Namely, these structures are the cranial nerves, spinal nerves, and peripheral ganglia.

The peripheral part of the nervous system can also be broken down functionally. By this classification, the nervous system consists of the somatic nervous system (SNS) and autonomic nervous system (ANS). The somatic nervous system is a voluntary part of the nervous system. It consists of the sensory and motor nerves. Somatic sensory nerves carry the sensory information from the skin, joints and muscles to the CNS, while the somatic motor nerves innervate skeletal muscles and enable us to move them upon our will.

The autonomic nervous system, however, contains only the motor nerves. These nerves control the involuntary functions of the body viscera, namely the smooth and cardiac muscles, as well as the glands. The ANS is further subdivided into the sympathetic nervous system (SANS) and parasympathetic nervous system (PANS). These two subdivisions cause opposite effects on the bodily organs, but their synchronized activity is essential for maintaining the homeostasis of the body.

The anatomical distinction between the sympathetic and parasympathetic divisions is given by the location of the presynaptic cell bodies and the types of nerves conducting presynaptic nerve fibers.

Autonomic nervous system starter pack is waiting for you in our study unit:

General sympathetic pathway

The general sympathetic pathway can be simplified into the following components:

preganglionic

ganglionic

postganglionic

The preganglionic components consist of preganglionic neurons located inside the spinal cord and their fibers (axons), which are called preganglionic fibers. The axons synapse with the postganglionic neuron inside sympathetic ganglia. These ganglia are actually a collection of cell bodies of postganglionic neurons, usually situated outside the CNS. Postganglionic components consist of postganglionic neurons and their fibers. The axons leave the ganglia and project onto visceral effectors, where they release the neurotransmitter norepinephrine. Both preganglionic and postganglionic neurons aremultipolar.

Preganglionic components

The cell bodies of the preganglionic neurons of the SNS are found only in the intermediolateral cell columns (ICLs) of the spinal cord, one on the left side and on the right. ICLs are part of the lateral horns of the gray matter of the thoracic (T1-12) and upper lumbar (L1-2 or 3) spinal cord segments, hence the alternative name "thoracolumbar" for the sympathetic division. This region consists of the visceral motor region of the spinal gray matter. You can think of the ICLs as longitudinal tubes passing through the respective lateral horns of the spinal cord. The preganglionic SNS cell bodies are organised somatotopically, meaning the arrangement of the cell bodies is a close representation to that of the body. Basically, the T1-6 cell bodies that are located superiorly innervate the head, upper limb and thoracic viscera. T7-11 located in the middle innervate the body wall and abdominal viscera, while T11-L2(3) located inferiorly innervate the lower limb and pelvic viscera.

The preganglionic fibers leave the ICLs and thus, the spinal cord through the anterior roots. They travel very briefly through the anterior rami of spinal nerves T1-L2(3), before leaving them and passing to the sympathetic trunks (more details later) through the white rami communicantes (white because nerve fibers are covered with white myelin).

Ganglionic components

Types of ganglia

The ganglionic compartment is actually composed of the cell bodies of the postganglionic neurons. It consists of two types, paravertebral and prevertebral ganglia.

Paravertebral ganglia ("para" = alongside, beside) occur on either side of the vertebral column and are independently linked on either side, forming two sympathetic trunks (chains). The paravertebral ganglia are the site where preganglionic fibers synapse with postganglionic neurons. The trunks extend the entire length of the column, from the base of the cranium to the coccyx. They converge anteriorly to the coccyx, forming the ganglion impar (ganglion of Walther). Each trunk is attached to the anterior rami of the T1-L2(3) spinal nerves.

Prevertebral ganglia (splanchnic ganglia) are located in the abdominal cavity around the origin of the major branches of the abdominal aorta. The prevertebral ganglia form aggregations around the abdominal prevertebral plexus and are referred to as the celiac, aorticorenal and superior and inferior mesenteric ganglia. Various nerve plexuses branch from these ganglia.

Course of fibers

In general, after passing briefly through the anterior rami, preganglionic fibers enter the sympathetic trunk via white rami communicantes. Inside the trunk, preganglionic fibers can follow one of four courses:

1. Ascend and synapse in a higher paravertebral ganglion

Within the sympathetic trunk, preganglionic fibers usually from T1-5 spinal cord levels can ascend to other vertebral levels and synapse inside ganglia located at a more superior level. The ganglia might not necessarily be associated with inputs directly from the spinal cord (other nerves than T1-L2/3 can participate in the synapse).

2. Descend and synapse in a lower paravertebral ganglion

These are similar to the ascending preganglionic, but in contrast, they descend to ganglia located at a more inferior level. This pathway usually involves fibers from T5-L2(3). The ascending and descending preganglionic fibers gives the sympathetic trunk the appearance of a chain with connections between the ganglia.

3. Synapse directly in a paravertebral ganglion at the same level

After synapsing inside the ganglion, postganglionic fibers leave through a gray ramus communicans (grey due to absence of myelin) and re-enters the same anterior ramus, which it initially travelled through.

The fibers are subsequently distributed to effector structures with peripheral branches of the anterior and posterior rami of the same spinal nerve. The fibers can also combine with fibers from other levels to form splanchnic nerves, which then pass onto the thoracic viscera (more details later).

4. Travel without synapsing all the way to the prevertebral ganglia

Preganglionic fibers can also pass through the sympathetic trunk without synapsing. These fibers are usually derived from the spinal cord levels T5 to L2(3). Once they pass through the sympathetic trunk, they combine with fibers from other levels to form and exit the trunk as a splanchnic nerve. Splanchnic nerves synapse on a prevertebral ganglia, and the postsynaptic fibers then pass onto the abdomen and pelvic viscera via a visceral motor nerve plexus.

Postganglionic components

Gray ramus communicans (cranial view)

The postganglionic compartment consists of postganglionic fibers travelling to effectors. The number of postganglionic fibers are greater than preganglionic ones. Approximately one preganglionic fiber synapses with at least thirty postganglionic fibers. After synapsing, postganglionic fibers leave the ganglia through gray rami communicantes and travel through the anterior and posterior rami of the spinal nerves. These rami carry the fibers all the way to the periphery and visceral components.

Ascending sympathetic fibers through the sympathetic trunk join peripheral nerves from C2-8 spinal nerves. These project onto effectors in the head, neck, upper limbs and thoracic cavity. For example, a cephalic arterial nervous branch leaves the superior cervical ganglion and projects onto the peri-arterial plexus on the carotid arteries. From here they project onto the dilator muscle of iris.

Descending sympathetic fibers through the sympathetic trunk join peripheral nerves from L3 to coccyx spinal nerves. These project onto the skin in the lower limbs, where they stimulate vasomotion, sudomotion and pilomotion.

Sympathetic fibers that enter and leave the trunk at the same level join peripheral nerves from T1-L2(3) spinal nerves. These project onto the body wall via cutaneous branches, but also via visceral motor nerves to sweat glands, smooth muscle and arrector pili muscles. Postganglionic fibers can also combine to form splanchnic nerves. These nerve types convey visceral efferent and afferent fibers to and from the viscera. Postganglionic fibers projecting onto thoracic viscera (e.g., heart, lungs, esophagus) pass through cardiopulmonary splanchnic nerves.

Splanchnic nerves

Sympathetic fibers which pass through the trunk without synapsing also combine with other fibers to form splanchnic nerves, of which there are five: greater, lesser, least, lumbar and sacral splanchnic nerves. Collectively these are called abdominopelvic splanchnic nerves. In this case, the synapsing happens in prevertebral ganglia rather than paravertebral ganglia. Postganglionic fibers from these prevertebral ganglia follow the main branches of the aorta and subsequently project onto all the organs (except adrenal glands) in the abdominal and pelvic cavities.

The adrenal glands are an exception. For every single human body organ, the postganglionic fibers synapse and release norepinephrine for regulation. However, for these glands, the nerves project directly onto the medullary cells without synapsing. The cells themselves play the role of the postganglionic neurons by releasing neurotransmitters, such as epinephrine (adrenaline), directly into the bloodstream. This results in a widespread sympathetic response.

Function

The reach of the sympathetic system is extremely broad within the human body. It is a component of virtually all spinal nerves and peri-arterial plexuses, and sympathetic fibers innervate all the blood vessels, sweat glands, arrector pili and viscera. The only structures the sympathetic system does not reach are avascular structures, like nails and cartilage.

Eyes

Mydriasis (dilation of the pupil)

Skin

Goosebumps, vasoconstriction, sweating

Lacrimal and salivary glands

Decreases secretion

Heart

Increases heart rate and strength of contraction

Blood vessels

Contracts smooth muscle (vasoconstriction)

Lungs

Bronchodilation, decreases secretion of bronchial glands

Digestive system

Inhibits peristalsis, constricts blood vessels and redirects blood to skeletal muscles, contracts anal sphincters

Liver and gallbladder

Stimulates breakdown of glycogen to glucose - energy release

Urinary system

Decreases urine production, contracts internal bladder sphincter

Genital system

Ejaculation

Suprarenal gland

Stimulates release of epinephrine (adrenaline) into blood

The sympathetic and parasympathetic divisions of the nervous system work in very close association, with contrasting, yet tightly coordinated effects. The sympathetic system is involved in energy-expending (catabolism), enabling the body to use energy appropriately to respond to stressful situations and emergencies, as in the "fight or flight" response. Activation of the sympathetic system results in pupil dilation, piloerection, vasoconstriction of cutaneous blood vessels, sweating, release of adrenaline, bronchodilation, increased cardiac contraction and reduced digestion.

During normal conditions, blood vessels are tonically maintained in a resting state of moderate vasoconstriction. If sympathetic signals are increased, vasoconstriction increases and vice-versa. However, in coronary vessels, skeletal muscles and vessels of the external genitalia, sympathetic stimulation results in vasodilation.

Feeling ready to test yourself on the sympathetic nervous system anatomy? Try out our quiz below:

References

K. L. Moore, A. F. Dalley, A. M. R. Agur: Clinically Oriented Anatomy, 7th edition, Lippincott Williams & Wilkins

Richard L. Drake, A. Wayne Vogl, Adam. W.M. Mitchell: Gray's Anatomy for Students, 2nd Edition, Churchill Livingstone Elsevier

Susan Standring: Gray's Anatomy: The Anatomical Basis of Clinical Practice, 41st edition, Elsevier

Bandyk, D.F., et al., Surgical sympathectomy for reflex sympathetic dystrophy syndromes. J Vasc Surg, 2002. 35(2): p. 269-77.

Vinik, A.I. and D. Ziegler, Diabetic Cardiovascular Autonomic Neuropathy. Circulation, 2007. 115(3): p. 387-397.

Illustrators:

Sympathetic nervous system (diagram) - Paul Kim

Sympathetic nervous system: want to learn more about it?

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Author: Adrian Rad BSc (Hons) * Reviewer: Elizabeth O. Johnson, PhD Last reviewed: December 14, 2021 Reading time: 18 minutes The autonomic system is made up of two divisions, the sympathetic and parasympathetic systems. They usually work antagonistically in the organs, but in a well integrated manner.

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Neurotransmitter in the parasympathetic systems

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Neurotransmitter in the parasympathetic systems

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The parasympathetic nervous system uses chiefly acetylcholine (ACh) as its neurotransmitter, although peptides (such as cholecystokinin) can be used. The ACh acts on two types of receptors, the muscarinic and nicotinic cholinergic receptors.

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PSNSSympathetic
Location | Key areas affected include the lungs, heart, bladder, and stomach. | Key areas affected include the lungs, heart, smooth muscle, and exocrine and endocrine glands, like the sweat glands and saliva.
Actions | Constricts pupils; causes salivation; slows down the heart rate; tightens the bronchi in the lungs; enacts digestion; releases bile; makes the bladder contract | Dilates pupils; keeps you from salivating; speeds up the heart; widens the bronchi; inhibits digestion; keeps the bladder from contracting
Speed | Slower than the sympathetic division | Faster than the PSNS

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Both the sympathetic and parasympathetic nerves release neurotransmitters, primarily norepinephrine and epinephrine for the sympathetic nervous system, and acetylcholine for the parasympathetic nervous system.

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Neurotransmitters are synthesized by neurons and are stored in vesicles, which typically are located in the axon ’s terminal end, also known as the presynaptic terminal. The presynaptic terminal is separated from the neuron or muscle or gland cell onto which it impinges by a gap called the synaptic cleft.