Sympathetic Nervous System: Difference between revisions

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[1]

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[1].

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[2].

Connections Sympathetic Nervous System.jpg

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[3][4].

Overactive Sympathetic Nervous System[edit | edit source]

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)[5].

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)
  • Chronic Pain[6]
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Physiotherapy Implications[edit | edit source]

Many people have an overactive sympathetic nervous system[7]. 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[5]
  • Relaxation Breathing[8]
  • 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. [9] 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[10].
  • Yoga[11]Massage[12]Tai Chi;
  1. 1.01.1 Alshak MN. Neuroanatomy, sympathetic nervous system.2019 Available from: 7.2.2021)
  2. 2.02.1 Britannica SNS Available from: (accessed 7.2.2021)
  3. Ken Hub Sympathetic NS Available from: (accessed 7.2.2021)
  4. Academic kids SNS Available from: (accessed 7.2.2021)
  5. 5.05.1 Fisher JP, Young CN, Fadel PJ. Central sympathetic overactivity: maladies and mechanisms. Autonomic Neuroscience. 2009 Jun 15;148(1-2):5-15.Available from: (accessed 7.2.2021)
  6. Healthnutrition news SNS Available from: (accessed 7.2.2021)
  7. Review of Pertinent Anatomy and Physiology Sandy Fritz MS, NCTMB, … Glenn M. Hymel EdD, LMT, in Clinical Massage in the Healthcare Setting, 2008Available from: 30.1.2021)
  8. 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: 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: 31.1.2021)

  9. 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:. (accessed 7.2.2021)
  10. Haker E, Egekvist H, Bjerring P. Effect of sensory stimulation (acupuncture) on sympathetic and parasympathetic activities in healthy subjects. Journal of the autonomic nervous system. 2000 Feb 14;79(1):52-9.Available from: 7.2.2021)
  11. Streeter CC, Gerbarg PL, Saper RB, Ciraulo DA, Brown RP. Effects of yoga on the autonomic nervous system, gamma-aminobutyric-acid, and allostasis in epilepsy, depression, and post-traumatic stress disorder. Medical hypotheses. 2012 May 1;78(5):571-9.Available from: 31.1.2021)
  12. Review of Pertinent Anatomy and Physiology Sandy Fritz MS, NCTMB, … Glenn M. Hymel EdD, LMT, in Clinical Massage in the Healthcare Setting, 2008Available from: 30.1.2021)

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