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Sympathetic nervous system

Sympathetic nervous system anatomy
Sympathetic nervous system (diagram)

The sympathetic nervous system is part of the autonomic nervous system, along with its counterpart, parasympathetic nervous system.

The origin of the sympathetic nervous system is found within the thoracic and lumbar segments of the spinal cord also known as the thoracolumbar division (T1 to L2,3).

The sympathetic pathway can be divided into three following components:

  • The preganglionic neurons,
  • The sympathetic ganglia,
  • The postganglionic neurons.

While the sympathetic nervous system is also important at rest, it is essential for preparing the body for emergency response in endangering situations, also known as the “fight-or-flight” response. The sympathetic system activates numerous complex pathways to enable an adequate response to a threat or trauma. Some of these physical effects include faster breathing, increased heart rate and blood pressure, dilation of pupils, redirection of blood flow to important organs (e.g. brain and muscles), and increased sweating.

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

Key facts about 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
Contents
  1. Autonomic (visceral) nervous system
  2. General sympathetic pathway
  3. Preganglionic neurons
  4. Sympathetic ganglia
    1. Types of ganglia
    2. Course of fibers
  5. Postganglionic neurons
    1. Splanchnic nerves
  6. Function
  7. Clinical considerations
    1. Complex regional pain syndrome
    2. Sympathectomy
    3. Diabetic cardiovascular autonomic neuropathy
  8. Sources
+ Show all

Autonomic (visceral) nervous system

The nervous system can be divided into central and peripheral nervous systems. The peripheral nervous system is further divided into the somatic and autonomic nervous systems.


Nervous system breakdown (diagram)

The autonomic nervous system (ANS) is a functional division of the nervous system that controls involuntary actions of muscles, glands and internal organs (e.g. bowel movements). Together with endocrine glands, the ANS affects important body functions without the direct involvement of the cerebral cortex. In contrast, the somatic nervous system mediates voluntary responses of the body (e.g. skeletal muscle function) and it’s under the direct control of the cerebral cortex.

The ANS can be divided according to its location (central and peripheral parts) and function. Functionally, the ANS is divided into sympathetic (SNS) and parasympathetic (PSNS) nervous 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.

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.

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General sympathetic pathway

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

  • Preganglionic neurons;
  • Sympathetic ganglia;
  • Postganglionic neurons.

The preganglionic neurons located inside the thoracic and lumbar segments (T1-L2,3) of the spinal cord and their fibers (axons), which are called preganglionic fibers. These preganglionic fibers synapse with the postganglionic neurons inside the sympathetic ganglia, which are typically found near the vertebral column. These ganglia are actually a collection of cell bodies of postganglionic neurons. The postganglionic neurons give off long axons (postganglionic fibers) that leave the ganglia and project onto visceral effectors, where they release the neurotransmitter norepinephrine. Both preganglionic and postganglionic neurons are multipolar.

Preganglionic neurons

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 (L2 or L3) 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).

Sympathetic ganglia

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 at 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 neurons

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.

Functions of the sympathetic nervous system
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.

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