Sympathetic nervous system


The sympathetic nervous system (SNS) is one of the two main divisions of the autonomic nervous system, the other being the parasympathetic nervous system. 



The sympathetic nervous system stimulates the body’s fight-flight-or-freeze response. 



It is constantly active at a basic level to maintain homeostasis homeodynamics.


The  SNS  is described as being antagonistic to the parasympathetic nervous system which stimulates the body to “feed and breed” and to “rest-and-digest”.

Two types of neurons involved in the transmission of any signal through the sympathetic system: pre-ganglionic and post-ganglionic. 


The sympathetic nervous system innervates tissues in almost every organ system, providing at least some regulation of functions as diverse as pupil diameter, gut motility, and urinary system output and function.

The SNS mediates the neuronal and hormonal stress response: fight-or-flight response. 


The sympathetic nervous system primes the body for action, particularly in survival threatening situations.

 Moments before waking sympathetic outflow  increases in preparation for action.

Sympathetic NS  causes vasoconstriction of most blood vessels, including many of those in the skin, the digestive tract, and the kidneys. 

Sympathetic vasoconstriction is a result of activation of alpha-1 adrenergic receptors by norepinephrine released by post-ganglionic sympathetic neurons. 

Alpha-1 adrenergic receptors exist throughout the blood vessels  of the body but are inhibited and counterbalanced by beta-2 adrenergic receptors.

Beta-2 adrenergic receptors are stimulated by epinephrine release from the adrenal glands in the skeletal muscles, the heart, the lungs, and the brain during a sympathoadrenal response. 

The sympatho-adrenal response of the body, as the preganglionic sympathetic fibers , end in the adrenal medulla.

Messages travel through the sympathetic nervous system in a bi-directional flow. 

Efferent messages affect different parts of the body simultaneously: can accelerate heart rate; widen bronchial passages; decrease motility of the large intestine; constrict blood vessels; increase peristalsis in the esophagus; cause pupillary dilation, piloerection and sweating, and raise blood pressure. 


The shunting of blood away from the organs not necessary to immediate survival, and increased blood flow to those organs involved in intense physical activity is the net affect of sympathetic activity.

The afferent fibers of the autonomic nervous system, transmits sensory information from the internal organs back to the central nervous system.

The autonomic sensory information is conducted by general visceral afferent fibers, and not divided into parasympathetic and sympathetic fibers 


These general visceral afferent sensations are mostly unconscious visceral motor reflex sensations from organs and glands being transmitted to the CNS. 

With increased sympathetic tone, cerebral and coronary arteries dilate rather than constrict: there is a proportional increase in the presence of β2 adrenergic receptors rather than α1 receptors. 


Activity essential for changing posture.

β2 receptors promote vessel dilation instead of constriction like α1 receptors. 

Movement from supine to sitting or standing position requires adjustment in blood flow and blood pressure coordinated by sympathetic nerves in conjunction with parasympathetic modulation of the heart rate.

Accounts for adjustment to maintain blood flow to the brain. without which syncope would result.

Sympathetic neural activity measurements obtained by microneurography, which measures the electrical activity of sympathetic nerves in intact, conscious individuals.

Influences: via cardiac sympathetic nerves, vascular sympathetic nerves, adrenal medulla via circulating epinephrine and nor epinephrine, and stimulation of renal juxtaglomerular cells that activates the renin-angiotensin-aldosterone axis.

Most cardiovascular innervation is noradrenergic in nature.

Norepinephrine is the primary neurotransmitter.

Epinephrine and other transmitters perform secondary functions, except for the sympathetic sudomotor innervation.

The sympathetic sudomotor innervation is cholinergic and is the action of sympathetic active vasodilator nerves in skin.

Heart sympathetic innervation includes the sinoatrial node (SA), which allows sympathetic nerves to increase heart rate by increasing the slope of diastolic depolarization.

The sympathetic  response that acts primarily on the cardiovascular system is mediated directly via impulses transmitted through the sympathetic nervous system and indirectly via catecholamines secreted from the adrenal medulla.

Innervates the myocardium increasing myocardial contractility and stroke volume.

Innervation of the peripheral blood vessels causes vasoconstriction mainly via norepinephrine at postsynaptic alpha-adrenergic receptors.

Neuropeptide Y also plays a role in peripheral vasoconstriction and is a co-transmitter with norepinephrine.

Primary role is cardiovascular maintenance of blood pressure and blood low regulation for seconds to minutes via the arterial baroreflex.

Arterial baroreflex detects changes in blood pressure via bar receptors the sensory afferent nerve endings located in the carotid sinus and the aortic arch.

Baroreceptors respond to vessel wall stretching.

Generally, increased stretching results from a short-term increases in blood pressure leading to an increased in afferent input into central autonomic nuclei.

Preganglionic neurons originate in the thoracolumbar division of the spinal cord specifically at T1 to L2~L3.



The preganglionic neurons travel to a ganglion, often one of the paravertebral ganglia, where they synapse with a postganglionic neuron. 


Presynaptic nerve axons terminate in either the paravertebral ganglia or prevertebral ganglia. 

The axon enters the paravertebral ganglion at the level of its originating spinal nerve. 


In the sympathetic nervous system and other components of the peripheral nervous system, synapses are made at ganglia. 

The cell that sends its fibers to the ganglion are called a preganglionic cell.

Cels  whose fiber leaves the ganglion is called a postganglionic cell. 

Preganglionic cells of the sympathetic nervous system are located between the first thoracic segment and third lumbar segments of the spinal cord. 

Postganglionic cells have their cell bodies in the ganglia and send their axons to target organs or glands.

The ganglia include sympathetic trunks and also the cervical ganglia (superior, middle and inferior), which send sympathetic nerve fibers to the head and thorax organs, and the celiac and mesenteric ganglia, which send sympathetic fibers to the gut.

From the paravertebral ganglion it synapses in this ganglion, ascends to a more superior or descend to a more inferior paravertebral ganglion and synapses there, or it can descend to a prevertebral ganglion and synapse there with the postsynaptic cell.



The  long postganglionic neurons extend from there across most of the body.



At the sympathetic NS synapses within the ganglia, preganglionic neurons release acetylcholine, a neurotransmitter that activates nicotinic acetylcholine receptors on postganglionic neurons. 


The preganglionic sympathetic fibers secrete acetylcholine, which activates the great secretion of adrenaline and to a lesser degree norepinephrine from it. 


The postsynaptic cell then innervates the targeted end effector such as a gland, or smooth muscle.

Paravertebral and prevertebral ganglia are relatively close to the spinal cord.

Presynaptic neurons are generally much shorter than their postsynaptic counterparts, which must extend throughout the body to reach their destinations.


Dopamine is the immediate metabolic precursor to norepinephrine.

The sympathetic nervous system extends from the thoracic to lumbar vertebrae and has connections with the thoracic, abdominal, and pelvic plexuses.

Sympathetic nerves arise from near the middle of the spinal cord in the intermediolateral nucleus of the lateral grey column, beginning at the first thoracic vertebra of the vertebral column and are thought to extend to the second or third lumbar vertebra. 

Then sympathetic nervous system is said to have a thoracolumbar outflow. 


Axons of these nerves leave the spinal cord through the anterior root. 

They pass near the spinal ganglion, where they enter the anterior rami of the spinal nerves. 

They separate out through white rami connectors  to either the paravertebral or prevertebral ganglia extending alongside the spinal column.

To reach target organs and glands, the axons must travel long distances.

Many axons relay their message to a second cell through synaptic transmission. 


Then postganglionic neurons release norepinephrine, which activates adrenergic receptors that are present on the peripheral target tissues. 


The activation of such target tissue receptors causes the sympathetic system effects. 


Sympathetic system action on various organs:

Eye Dilates

Heart Increases rate and force of contraction

Lungs Dilates bronchioles via circulating adrenaline

Blood vessels dilate in skeletal muscle.

Digestive system Constricts in gastrointestinal organs

Inhibits peristalsis

Sweat glands Activates sweat secretion

Kidney Increases renin secretion

Penis   Inhibits tumescence

Ductus deferens promotes emission prior to ejaculation

There are three exceptions to postganglionic neurons release of norepinephrine: 

Postganglionic neurons of sweat glands release acetylcholine for the activation of muscarinic receptors, except for areas of thick skin, the palms and the plantar surfaces of the feet, where norepinephrine is released and acts on adrenergic receptors.


The  sympathetic innervation of the adrenal medulla varies from above: In presynaptic neurons pass through paravertebral ganglia, on through prevertebral ganglia and then synapse directly with suprarenal tissue. 

The adrenal medulla consists of cells that have pseudo-neuron like qualities in that when activated by the presynaptic neuron, they will release their neurotransmitter directly into the bloodstream.

The chromaffin cells of the adrenal medulla are analogous to post-ganglionic neurons.

The adrenal medulla develops in tandem with the sympathetic nervous system and acts as a modified sympathetic ganglion. 

Within the adrenal medulla gland, pre-ganglionic neurons synapse with chromaffin cells, triggering the release of a small proportion of norepinephrine, and more substantially, epinephrine. 

Postganglionic sympathetic nerves terminating in the kidney release dopamine, which acts on dopamine D1 receptors of blood vessels to control how much blood the kidney filters. 


The sympathetic nervous system increases its activity in heart failure , increasing  heart muscular contraction force, stroke volume, as well as peripheral vasoconstriction to maintain blood pressure. 

Sympathetic  effects in heart failure accelerates disease progression, eventually increasing mortality.



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