Sympathetic Nervous System

The sympathetic nervous system is a network of neurons that helps your body to tackle emergencies and increases the chances of survival. It’s a division of the autonomic nervous system that controls the body’s unconscious or involuntary processes. The neuronal system is responsible for the well-known “fight or flight” response in emergencies. It affects mainly those body parts actively involved in managing stressful scenarios and emergencies, such as the heart, muscles, eyes, GIT, etc. 

It works unconsciously and increases our heart rate, blood pressure, and muscular blood flow. It alters our GIT motility and urinary output. The overall working physiology of the sympathetic nervous system involves multiple organs and organ systems. It is based on diverting the oxygen-rich blood flow to the parts of the body actively involved in coping with emergencies and preparing the body to either face danger or escape from danger. However, it shouldn’t be assumed that it becomes inactive at the resting phase. Instead, it performs many vital functions at the basal level to maintain the body’s normal homeostasis.

Sympathetic Nervous System

The sympathetic nervous system (SNS) is one of the three divisions of the autonomic nervous system; other divisions are the parasympathetic nervous system (PSNS) and the enteric nervous system. The sympathetic nervous and parasympathetic systems control the body’s most involuntary processes by innervating inner body organs, glands, and smooth muscles. In contrast, the enteric nervous system is limited to the gastrointestinal tract. The sympathetic nervous system works opposite to the parasympathetic nervous system, which is directed to the “rest and digest” phase of the body. Synapses densely interconnect the neuronal network of SNS. Different neurotransmitters, such as epinephrine, norepinephrine, and acetylcholine, perform the function of neuronal transmission at the synapses.

Read more about the Peripheral Nervous System

Anatomy of the Sympathetic Nervous System

The sympathetic nervous system comprises many pathways and neuronal networks that affect multiple body organs. It has preganglionic and postganglionic fibers with sympathetic ganglia.

Sympathetic Fibers

Preganglionic fibers of SNS leave the spinal cord from thoracic and lumber segments ranging from T1 to L2, with cell bodies distributed in the gray matter of the spinal cord. The preganglionic fibers of SNS are shorter than the postganglionic fibers as the sympathetic ganglia are usually far off from the target tissues. The neurotransmitter linking presynaptic and postsynaptic fibers is usually acetylcholine which activates the nicotinic receptors. These postganglionic fibers innervate the target tissue by releasing neurotransmitters epinephrine or norepinephrine, except for the sympathetic innervation of sweat glands and arrector pili muscles, which use acetylcholine as the postganglionic neurotransmitter for innervation. Epinephrine and norepinephrine activate adrenergic receptors in the target tissues. These adrenergic receptors are of four types; alpha 1, alpha 2, beta 1, and beta 2. These receptors are distributed differently in various body parts, and the response depends upon the type of receptors activated. 

Sympathetic Ganglia

Sympathetic ganglia, where preganglionic and postganglionic sympathetic fibers synapse, include paravertebral and prevertebral ganglia. 

Paravertebral ganglia are located on the sides of the vertebral column in the form of a sympathetic chain extending from the base of the skull downwards. The sympathetic chain allows the preganglionic fibers to ascend superior to T1 and descend below L2/L3. The superior end of the sympathetic chain continues upward and forms a plexus around the internal carotid artery while its lower parts converge and form ganglion impar. The ganglia in the sympathetic chain are labeled as cervical, thoracic, lumbar, and sacral. There are 4 cervical and 12 thoracic ganglia. The last cervical and 1st thoracic ganglia fuse to form stellate cervicothoracic ganglia. The lumber region has 4, while the sacral region has 4 or 5 ganglia in the sympathetic chain. These ganglia provide the location for the synapse of preganglionic fibers coming from the spinal cord, with the postganglionic fibers innervating the effectors.

Prevertebral ganglia also provide a location where preganglionic fibers synapse with the postganglionic fibers. The postganglionic nerves from the prevertebral ganglia mostly innervate the pelvic viscera, such as the renal system, bladder, and enteric nervous system. The preganglionic nerve fibers of the prevertebral ganglia leave the spinal cord, pass through the sympathetic chain without synapsing, and reach the prevertebral ganglia located near the target organ and synapse with postganglionic neurons. These postganglionic neurons, in turn, innervate the target organ. These prevertebral ganglia include celiac, aortic renal, superior, and inferior mesenteric ganglia.

Effects of the Sympathetic Nervous System on the Body

Sympathetic activation produces different effects on different body parts. 

• It increases cardiac output by increasing heart rate, the force of contraction, and the conduction rate, allowing more oxygen and nutrients to the body tissues.
• In the lungs, it causes bronchodilation and decreases the secretions to allow more ventilation and gaseous exchange.
• In the eyes, it causes the constriction of the radial muscle of the iris, leading to mydriasis, which allows more and more light to enter the eyes. It also promotes ciliary muscle relaxation allowing far vision to improve.
• It decreases the secretion of enzymes and insulin from the pancreas.
• It increases renin secretion from the kidneys, increasing fluid retention and blood pressure by the renin-angiotensin system.
• It causes relaxation of the detrusor muscle of the bladder and constriction of the urethral sphincter. As a result, urinary output decreases.
• It decreases the GIT motility and blood flow and diverts blood flow to muscles.
• It increases the secretion of epinephrine and norepinephrine from the adrenal medulla, which augments the effects of the sympathetic nervous system.

The Physiological Role of the SNS

Fight or Flight Response

The sympathetic nervous system is well known for the “fight or flight” response. It is a normal physiological response of the body to any emergency due to sympathetic activation. When our body senses any emergency or threat, the sympathetic system becomes active subconsciously and produces many changes to maximize survival chances. It raises blood pressure, cardiac output, and nutrient delivery to the body tissues to counter emergencies. It increases the breathing rate to increase oxygen supply and dilates pupils so that eyes can visualize the danger enabling a prompt decision to fight or flight. Moreover, it causes constriction of blood vessels in the skin and GIT; and diverts this blood toward muscles that require an increased oxygen supply to escape danger.

Autonomic Processes

In addition to this “fight or flight” response, the sympathetic system also plays an essential role with the parasympathetic nervous system to regulate the body’s normal physiological processes, such as breathing rate, heart rate, GIT motility, urination, sweating, vascular tone, etc.

The most pronounced role here is the regulation of body temperature and cardiovascular effects of SNS.

Body temperature regulation is controlled by the hypothalamus and is regulated by a combined action of SNS and PSNS; however, the leading role is played by SNS. These systems work in collaboration to maintain the constant internal body temperature despite fluctuations in the external environment. When cold outside, SNS activates nonshivering thermogenesis to produce heat (shivering thermogenesis is a somatomotor response). When it is hot out, SNS promotes sweating to increase heat loss so that internal body temperature stays the same.

SNS plays a vital role in decreasing momentary fluctuations in blood pressure. When we tend to rise from a sitting posture, blood pressure drops due to the blood pooling in lower body parts. SNS becomes active due to baroreceptors, increases the heart rate and myocardial contractility, and causes vasoconstriction. These changes bring blood pressure to normal.

Difference Between the Sympathetic and Parasympathetic Systems

The sympathetic and parasympathetic nervous systems are the two divisions of the autonomic nervous system. Both have different structural and functional characteristics. Contrary to SNS working in emergencies, PSNS dominates during the “rest and digest” phase of the body and brings the body processes towards the resting stage. These two systems produce opposite effects on the body parts. PSNS also has preganglionic, ganglia, and postganglionic fibers like the sympathetic system.

In contrast to SNS, PSNS has longer preganglionic fibers and shorter postganglionic fibers. Parasympathetic ganglia are present near or in the walls of the target organs, so preganglionic fibers travel a considerable distance to reach the ganglia. In contrast, postganglionic fibers just leave the ganglia and innervate the target tissues. Parasympathetic fibers leave the CNS via cranial nerves III, VII, IX, X, and S2-S4 spinal nerves. The common neurotransmitter in PSNS is acetylcholine. When PSNS dominates during the rest phase, heart rate, blood pressure, and breathing rate decrease; pupils constrict to limit light entering the eyes; and digestion and GIT motility increase.

Abnormalities of SNS

Different conditions altering the SNS are as follows:

Orthostatic Hypotension

Orthostatic hypotension is a common problem of SNS. Usually, SNS prevents postural hypotension by making compensatory measures. However, when it fails to do so, blood pressure drops upon changing the posture, and the person feels dizzy or may become unconscious. The causes behind this SNS failure are diverse.

Pheochromocytoma

Pheochromocytoma is an adrenal medulla tumor that secretes large quantities of epinephrine and norepinephrine. These hormones mimic the actions of SNS activation and produce similar effects, such as increased heart rate, blood pressure, sweating, tremors, etc.

Horner Syndrome

Horner syndrome is characterized by ptosis (drooping of the upper eyelid), miosis (constricted pupils), anhidrosis( absence of sweating on the face), and sinking the eyeball into the bony orbital. Horner syndrome arises due to damage to the sympathetic supply to the eye region. The causes of sympathetic damage may include carotid artery dissection, neck tumor, brain lesion, spinal cord injury, etc.

Overactive Sympathetic Nervous System

Sometimes SNS becomes abnormally active. When it happens, your heart rate shoots up, and hypertension develops. It damages the vascular walls and increases the risk of vascular hypertrophy, atherosclerosis, kidney problems, and insulin resistance. It increases the risk of metabolic syndrome and diabetes and can cause panic attacks. Some conditions which can cause overactive SNS include chronic stress, poor sleeping habits, chronic pain, etc.

Diabetes Mellitus

Diabetic autonomic neuropathy is a common cause of sympathetic neuropathy. When sympathetic innervation becomes faulty, the myocardium is badly affected. Due to sympathetic abnormality, myocardial blood flow through coronary arteries drops, which reduces myocardial contractility and leads to ischaemic heart disease. This neuronal myopathy leads to the failure of multiple systems, such as cardiovascular, gastrointestinal, and genitourinary systems, and plays a crucial role in mortality in diabetes patients.

Drugs Affecting the Sympathetic Nervous System

Drugs can be used either to increase the sympathetic effect or to block the sympathetic nervous system. Different drugs affecting the sympathetic nervous system are as under:

Sympathomimetics

The drugs which augment the effects of the sympathetic nervous system are called sympathomimetic drugs. Our body has a wide distribution of adrenergic receptors. Sympathomimetic drugs work by activating these receptors and have different therapeutic effects. These drugs can be direct-acting sympathomimetics or indirect sympathomimetics. Direct-acting sympathomimetics directly act on adrenergic receptors and activate them, while indirectly acting drugs increase the number of endogenous catecholamines present at the synaptic cleft by displacing the stored catecholamines in vesicles, reuptake inhibition, or inhibiting degradation.

Sympathomimetics has several uses, such as:

• They are used to treat hypotension due to hypovolemia or shock.
• They are used to improve breathing in asthma and COPD.
• They help with eye problems such as glaucoma.
• They are also helpful in treating nasal congestion and urinary incontinence etc.

Adrenergic Blockers

Adrenergic blockers are the class of drugs used to inhibit the actions of the sympathetic nervous system. These drugs can be alpha 1 blockers, alpha 2 blockers, beta 1 blockers, or beta 2 blockers. They can be selective or nonselective blockers. They produce their effects by blocking the activation of adrenergic receptors and antagonizing the effects of the sympathetic nervous system.

They are used differently in different clinical conditions. They commonly treat tachycardia, hypertension, arrhythmias, heart failure, migraines, anxiety, etc.

How to Prevent SNS Abnormalities

Sympathetic regulation is crucial to maintain the normal functioning and homeostasis of the body. You can prevent SNS abnormalities by adopting a healthy lifestyle. However, genetic and congeniality-acquired abnormalities can’t be avoided.

Following are a few tips to help prevent SNS abnormalities:

Adopt a Balanced Diet

Vitamin deficiencies, especially vitamin B12, affect the normal functioning of the CNS. Moreover, overuse of vitamin B6 can also cause CNS toxicity. The best approach is to adopt a balanced diet to stay healthy.

Avoid Smoking and Alcohol

Smoking affects all body parts and alters the neuronal response to the messengers. Similarly, alcohol also affects your health badly. You should avoid smoking and alcoholism to maintain your physical and mental health.

Exercise Regularly

Adopt the habit of regular exercise. Exercise positively affects all body parts and improves nutrient distribution to all body tissues. Exercise also helps to prevent chronic conditions such as CVS issues and diabetes, which affect your neuronal health badly.

Manage Chronic Conditions

The sympathetic nervous system is prone to injury by chronic conditions such as diabetes. If you have such issues, manage them appropriately as recommended by the physician. Mismanagement and ignorance can cause neuropathy and can affect your SNS badly.

Summary

The sympathetic nervous system (SNS) is the central division of the autonomic nervous system concerned with emergencies. It works opposite to the parasympathetic nervous system (PSNS), which is concerned with the “rest and digest” phase of the body. It has preganglionic fibers, sympathetic ganglia, and postganglionic fibers. Preganglionic neurons have their cell bodies located in the CNS, and their axons leave the CNS through cranial nerves III, VII, IX, X, and spinal nerves S2-S4. Preganglionic fibers of SNS are usually smaller than the postganglionic fibers as they don’t have to travel a large distance to reach the sympathetic ganglia. Sympathetic ganglia involve the paravertebral ganglia in the sympathetic chain extending downwards from the skull at the sides of the vertebral column and prevertebral ganglia present near the target organs. These ganglia are the site where preganglionic and postganglionic fibers synapse. 

SNS uses acetylcholine, epinephrine, and norepinephrine as neurotransmitters and activates adrenergic receptors, including alpha 1, alpha 2, beta 1, and beta 2. The sympathetic system (SNS) activates the “fight or flight” response in emergencies. This response increases heart rate, blood pressure, and breathing rate and dilates pupils. It also shifts blood flow from GIT and skin towards muscles to increase the chances of survival in times of danger. It also regulates body temperature by triggering nonshivering thermogenesis in cold and sweating in hot environments. Moreover, it regulates blood pressure and inhibits transient changes in blood pressure. Abnormalities of SNS may arise due to congenital defects, traumas, diabetes, or others. Overactive SNS causes hypertension, tachycardia, and sweating and may lead to chronic heart issues. SNS depression decreases blood pressure and heart rate.

Different drugs can alter the SNS activity. Drugs that increase the activity of SNS are sympathomimetics, while which decrease the activity are called adrenergic blockers. Sympathomimetics are used in hypotension, asthma, COPD, and urinary incontinence, while adrenergic blockers treat hypertension, cardiac arrhythmias, migraines, and anxiety. A balanced diet, regular exercise, a healthy lifestyle, and avoiding smoking and alcoholism help to prevent SNS abnormalities.

References

Schmidt, A; Thews, G (1989). “Autonomic Nervous System”. In Janig, W (ed.). Human Physiology (2 ed.). New York, NY: Springer-Verlag. pp. 333–370

Furness, John (9 October 2007). “Enteric nervous system”. Scholarpedia. 2 (10): 4064. Bibcode:2007SchpJ…2.4064F. doi:10.4249/scholarpedia.4064.

Willis, William D. (2004). “The Autonomic Nervous System and its central control”. In Berne, Robert M. (ed.). Physiology (5. ed.). St. Louis, Mo.: Mosby. ISBN 0323022251.

Pocock, Gillian (2006). Human Physiology (3rd ed.). Oxford University Press. pp. 63–64. ISBN 978-0-19-856878-0.

Image source: The sympathetic nervous system