Sympathetic Nervous System Functions


What happens when the body goes under a stressful situation? Usually, our heart rate rises, we breathe faster, our eyes widen and our muscles become alert and super responsive. How does the body bring about this action? The answer is the sympathetic nervous system.

Sympathetic nervous system, component of the nervous system that produces regional modifications (such as perspiration in reaction to an increase in temperature) and cardiovascular system reflex modifications.

In this article, the structural components that are involved in the network of sympathetic nervous system are explained, along with the functional role of the system. Some complications that involve the sympathetic nervous system are also described.


  • Under stressful conditions, the whole sympathetic nervous system is stimulated, resulting in an immediate broad reaction known as the fight-or-flight reaction.
  • This pattern is mediated by the adrenal gland releasing significant amounts of adrenaline, a rise in heart rate, an increase in cardiac output, skeletal muscle vasodilation, and cutaneous and gastrointestinal vasoconstriction.
  • The launch of large amounts of epinephrine from the adrenal gland, a rise in heart rate, an increase in cardiac output, skeletal muscle vasodilation, cutaneous and gastrointestinal vasoconstriction, eyelid dilation, bronchial elongation, and piloerection are all characteristics of this reaction.
  • The total impact is to warn the person for impending danger.
  • The sympathetic nervous system’s functions occur in tandem with the other neurological or hormonal stress response, such as rises in corticotropin and cortisol release.
  • Chronic stress in people causes long-term activation of the fight-or-flight response, which leads to persistent production and release of catecholamines (epinephrine, for example) and hormones like cortisol.
  • Extended stress-induced release of these chemicals is linked to a number of physiological outcomes, notably hyperglycaemia (high blood glucose levels), which can progress to type 2 diabetes mellitus, and hypertension (high blood pressure), that can lead to heart disease.


The sympathetic nervous system is supported by brain structures, spinal cord, and peripheral nervous system. Internal organs in the abdomen and chest have sensors that gather sensory information and transfer it to the relevant parts of the brain via the spinal cord and cranial nervous system. The hypothalamus, a brain region vital for homeostatic regulation, collects information from the body and adjusts autonomic nervous system behaviour as a reaction.

This brain component also receives input from higher-level brain structures such as the amygdala. In moments of stress, the amygdala, often known as the emotional brain, sends a signal to the hypothalamus. (Lanese, 2019)

The warning is subsequently relayed to the sympathetic nervous system, and the message is passed on to the adrenal glands, which create epinephrine, also referred to as adrenaline. This hormone is responsible for the copious perspiration, rapid heartbeat, and quick breaths that we connect with stress.

If the threat remains, the brain then sends a fresh signal via the grapevine nerve system, telling the adrenal glands to generate the stress hormone called cortisol in order to keep the response to stress going.

As explained below, the sympathetic nerves travel parallel to the spinal cord on the both sides of the vertebral column:

  • Preganglionic neurons are nerve cells present in the intermediolateral column of the spinal cord at regions T1-T12 and L1-L3.
  • Preganglionic fibres are the axons of preganglionic nerve cells that exit the spinal cord via the anterior rami of spinal nerves and make their way as white rami communicantes.
  • Sympathetic ganglia (sympathetic ganglia): Sympathetic trunk (paravertebral ganglia); prevertebral (splanchnic) ganglia; sympathetic ganglia neuron structures synapse to white rami communicantes
  • Postganglionic fibres are a type of postganglionic fibre. The axons of ganglionic neurons which exit the ganglia as grey rami communicantes and link to the rami of the spinal nerves.
  • Sympathetic innervation is carried by spinal nerves C2-C8 to the forehead, neck, upper body, and chest. 


When confronted with a dangerous animal, an incoming automobile, or even a looming appointment, our bodies initiate a physical stress reaction that enables us to battle or leave the scene. The sympathetic nervous system drives this “fight-or-flight” reaction, a typically balanced system of brain regions, nerves, and chemicals that, if knocked off balance, might result in catastrophic consequences.

The sympathetic nervous system is a component of the autonomic nervous system, which is often referred to as the involuntary nervous system. The autonomic nerve system governs vital bodily activities such as heartbeat, blood pressure, dilated pupils, and temperature of the body, perspiration, and digestive processes without conscious control.

According to studies, several kinds of neurons manage these many physical responses by guiding the secretion of glands, , function of cardiac muscle, and function of skeletal muscles.. Animals can use the platform to manage immediate internal changes and respond without pausing to worry about it.

The sympathetic nervous system is in charge of the body’s fast involuntary reaction to harmful or traumatic experiences. A surge of hormones increases the body’s awareness and heartbeat, directing extra blood to the muscle cells. Breathing becomes more rapid, supplying more oxygen to the brain, and sugar is injected into the blood circulation for an immediate extra boost.

Once the problem has faded away or the threat is gone, the sympathetic nervous system does not de-stress the organism. The parasympathetic nervous system is another element of the autonomic nervous system that tries to relax the body down. This mechanism promotes the body to “rest and digest” in order to counteract the fight-or-flight reaction. As the body heals, blood pressure, respiration rate, and hormonal circulation resume to the normal state because the body regains equilibrium, or homeostasis.

To sustain this foundation and regular physiological function, the sympathetic and parasympathetic nerve systems collaborate.

The sympathetic nervous system’s exiting instructions leave the spinal cord in the thoracolumbar area, or the middle to lower spine. Sympathetic nerve cells emerge from the spinal cord and form two columns on either sides of it. These nerve cells subsequently incorporate a second group of neurons into the circuit, signalling them with the assistance of the chemical messenger acetylcholine. 

The second group of nerve cells extends to smooth muscles that perform involuntary muscular contractions, cardiac muscles, and glands throughout the body, having taken up the reins. To maintain the functioning of those systems in control, the parasympathetic nervous system frequently interacts with the same systems as the sympathetic nervous system.

The sympathetic and parasympathetic nerve systems are located on opposite sides of a swaying scale; one system will remain activated and serves to balance the effect of the other. When the opposite forces are in a balanced state, the body reaches homeostasis, and activities resume as they do in routine.  

The sympathetic nervous system (SNS), along with the parasympathetic nervous system (PNS), is one of the two parts controlled by the autonomic nervous system (ANS). These networks primarily work silently in opposite ways to govern numerous processes and regions of the body.

The sympathetic nervous system (SNS) is in charge of the “fight or flight” reaction, whereas the parasympathetic nervous system (PNS) is in charge of the “rest and digest” reaction.

Sympathetic nervous system (SNS) fibres innervate practically each organ system as well as provide physiological regulation across a variety of body functions such as pupil diameter, gut motion (movement), and urine output. The sympathetic nervous sysem (SNS)’s main overall end result is to ready the body for action, which is a whole-body response impacting multiple organ systems all throughout body to reroute oxygen-rich blood to regions of need.

The sympathetic nervous system (SNS)’s functions happen in tandem with other neurological or hormonal reactions to stress, such as elevations in corticotropin and cortisol secretion. Information move in both directions through the sympathetic nervous system (SNS). 

Different messages can cause changes in different regions of the body at the same time. For example, the sympathetic nervous system can increase heart rate, enlarge bronchial passageways, reduce large intestine motion (movement), constrict blood vessels, induce dilated pupils, trigger goose bumps, initiate sweat, and increase blood pressure.

Below are some of the repercussions of the sympathetic nervous system (SNS) acting in direct opposition to the parasympathetic nervous system (PNS) function:

Sympathetic activation of the eye leads the radial muscle of the iris to tighten, causing mydriasis and enabling more light to enter. The ciliary muscle softens, allowing better eyesight (Figure 1).

Sympathetic activation of the heart raises the pulse rate, strength of contraction, and rate of conductivity.

Sympathetic stimulation of the heart induces a rise in heart rate, force of contraction, and conduction rate, leading to higher cardiac output to provide the body with oxygenated blood.

To allow increased airflow via the lungs, bronchodilation, and less pulmonary secretions happen.

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

Reduced movement and sphincter contraction, and also gallbladder contraction, happen to halt digestion and transfer energy to many other areas of the body. Both the exocrine and endocrine pancreas reduces enzyme and insulin release.

In the urinary bladder, the detrusor muscle relaxes and the urethral sphincter contracts to try and stop urine output in sympathetic activation.

Renin secretion is increased by the kidney to raise intravascular volume.

Salivary glands function by secreting modest amounts of potassium and water.

Chronic stress in humans causes long-term activation of the fight-or-flight reaction, which leads to persistent production and release of catecholamines (epinephrine, for example) and hormones like cortisol.

Lengthy stress-induced release of these chemicals has been linked to a number of physiological repercussions, including hyperglycemia (high blood glucose levels), that can progress to type 2 diabetes mellitus, and hypertension (high blood pressure), that can cause a heart attack.

Complications involving sympathetic nervous system:

In a variety of disorders, the sympathetic nervous system grows overactive. Heart disease such as coronary heart disease, heart failure, and hypertension are examples of these. A surge in sympathetic signalling boosts blood pressure and increases tone in smooth muscles, potentially leading to hypertension.

Sympathetic malfunction has been linked to renal disease, type II diabetes, obesity, metabolic disease, and sometimes even Parkinson’s disease, in addition to cardiovascular illness. Everybody considers about Parkinson’s disease in light of its motor symptoms, although autonomic symptoms arise far earlier. Variations in sympathetic nerve activity can be seen in the skin, pupils, and, most notably, the heart.

Certain patients report feeling more weary or fatigued, but cardiac problems lead to these general complaints. Parkinson’s disease destroys sympathetic nerve cells, which help to maintain adrenaline and norepinephrine content in the brain — chemicals that signal the heart when to pump harder, including when you shift to start standing up or workout.

Destruction to these nerve cells can contribute to a lack of blood flow in Parkinson’s patients, causing them to feel disoriented when standing, significantly increasing their risk of falling.

According to an article, sympathetic dysregulation also underpins mental health issues such as depression, despair, and chronic stress. The body’s physical stress reaction can be beneficial in brief spurts and provide an energetic boost of focused attention.

The stress impulses flying through the body, on the other hand, cause problems if left unchecked. Aside from sustaining a mental state of chronic stress, the increased adrenaline and cortisol harm blood vessels, raise blood pressure, and encourage fat accumulation.

A variety of clinical disorders have been linked to central sympathetic overstimulation. And apart from its factor in the process of hypertension, sympathetic overactivity has been linked to the pathogenesis and maintenance of a variety of pathophysiological mechanisms that are unrelated to changes in blood pressure (e.g., glomerulosclerosis, atherosclerosis, insulin resistance and vascular hypertrophy).

The effects of stress hormones (– for example, epinephrine, cortisol and norepinephrine) on numerous organs are too pushed in this situation. Lengthy stress-induced release of these chemicals has been linked to a number of physiological repercussions, including hyperglycaemia (a condition that is marked with high blood glucose levels), that can progress to type 2 diabetes mellitus.

Hypertension (excessive blood pressure), which can contribute to heart disease and renal diseases (Figure 2).

Obesity, metabolic diseases, gastrointestinal disease, stress, and depression are some other possibilities.

It has an impact on both physical and mental health and wellbeing, and can cause somatic symptoms such as heart palpitations, diarrhoea, perspiration, and sleeplessness, and also panic attacks and chronic stress.

The following are among the most likely reasons of this situation:

  • Tension and Overstimulation: Anxiety and overstimulation are caused by a signal or something which causes focus and alertness, such as a traumatic event or a challenging assignment at job. More and more stimuli there are, the more active the sympathetic nervous system is.
  • Inadequate sleep
  • Dietary deficiencies
  • Chronic Ache

The sympathetic nervous system is hyperactive in many persons. Examine your clients’ behaviours and be mindful of the health consequences. If and when necessary, enlighten them. Methods for increasing vagal tone and decreasing sympathetic nervous system activity involve:

Physical conditioning (massively implied non-pharmacological choice in the condition of the overactive sympathetic nervous system) The World Health Organization suggests that persons between the ages of 18 and 64 engage in at least two and a half hours to five hours of moderate-intensity aerobic activity or 75-150 minutes of vigorous-intensity aerobic physical activity per week. 

Daily exercise training was shown to successfully soothe the overactive sympathetic nervous system and lessen symptoms such as hypertension, indicators of heart problems, anxiety and diabetes. (Sympathetic Nervous System)

The sympathetic nervous system reacts to variations in calorie consumption; calorie reduction lowers sympathetic nervous system activity, while carbohydrate delivery enhances sympathetic nervous system activity. Insulin may play a significant role in the relationship between food changes and adjustments in central sympathetic outflow. Caffeine-containing drinks and cappuccino can be quite stimulating to the nervous system.

Choosing minimal caffeine, consuming it at a different time, or drinking caffeine-free teas is a better alternative.

Acupuncture has been shown to reduce the sympathetic outflow of specific neurotransmitters and anxiety chemicals that normally fill our brains in reaction to prolonged stress. Acupuncture, in other terms, increases the body’s healing process by suppressing the sympathetic nervous system’s “fight or flight” response. 

This permits the parasympathetic “rest and digest” reaction to lead the way and develop a healthy repair and renewal phase.


The sympathetic nervous system (SNS), together with the parasympathetic nervous system, is among the two parts controlled by the autonomic nervous system (ANS), these networks primarily work in opposite directions involuntarily to govern numerous functions and regions of the body. The sympathetic nervous system (SNS) controls the “fight or flight” reaction, whereas the parasympathetic nervous system (PNS) governs the “rest and digest” reaction. The SNS’s largest contributing end result is to equip the body for action, which is a whole-body response impacting multiple organ systems all throughout body to reroute oxygen-rich blood to parts of the body that require it under strong physical activity.


  • Lanese, N. (2019, May 10). Fight or Flight: The Sympathetic Nervous System. Retrieved from Live Science:
  • Sympathetic Nervous System. (n.d.). Retrieved from Physiopedia: