Sympathetic nervous system

The autonomic nervous system functions to regulate the body's unconscious actions. The sympathetic nervous system's primary process is to stimulate the body's fight or flight response. It is, however, constantly active at a basic level to maintain homeostasis.^[4] The sympathetic nervous system is described as being antagonistic to the parasympathetic nervous system. The latter stimulates the body to "feed and breed" and to (then) "rest-and-digest".


The SNS has a major role in various physiological processes such as blood glucose levels, body temperature, cardiac output, and immune system function. The formation of sympathetic neurons being observed at embryonic stage of life and its development during aging shows its significance in health; its dysfunction has shown to be linked to various health disorders.^[5]

Disorders[edit]

The dysfunction of the sympathetic nervous system is linked to many health disorders, such as heart failure, gastrointestinal problems, immune dysfunction as well as, metabolic disorders like, hypertension and diabetes. Highlighting the importance of the sympathetic nervous system for health.


The sympathetic stimulation of metabolic tissues is required for the maintenance of metabolic regulation and feedback loops. The dysregulation of this system leads to an increased risk of neuropathy within metabolic tissues and therefore can worsen or precipitate metabolic disorders. An example of this includes the retraction of sympathetic neurons due to leptin resistance, which is linked to obesity.^[22] Another example, although more research is required, is the observed link that diabetes results in the impairment of synaptic transmission due to the inhibition of acetylcholine receptors as a result of high blood glucose levels. The loss of sympathetic neurons is also associated with the reduction of insulin secretion and impaired glucose tolerance, further exacerbating the disorder.^[23]


The sympathetic nervous system holds a major role in long-term regulation of hypertension, whereby the central nervous system stimulates sympathetic nerve activity in specific target organs or tissues via neurohumoral signals. In terms of hypertension, the overactivation of the sympathetic system results in vasoconstriction and increased heart rate resulting in increased blood pressure. In turn, increasing the potential of the development of cardiovascular disease.^[24]


In heart failure, the sympathetic nervous system increases its activity, leading to increased force of muscular contractions that in turn increases the stroke volume, as well as peripheral vasoconstriction to maintain blood pressure. However, these effects accelerate disease progression, eventually increasing mortality in heart failure.^[25]


Sympathicotonia is a stimulated^[26] condition of the sympathetic nervous system, marked by vascular spasm,^[27] elevated blood pressure,^[27] and goose bumps.^[27]


Heightened sympathetic nervous system activity is also linked to various mental health disorders such as, anxiety disorders and post-traumatic stress disorder (PTSD). It is suggested that the overactivation of the SNS results in the increased severity of PTSD symptoms. In accordance with disorders like hypertension and cardiovascular disease mentioned above, PTSD is also linked with the increased risk of developing mentioned diseases, further correlating the link between these disorders and the SNS.^[28]


The sympathetic nervous system is sensitive to stress, studies suggest that the chronic dysfunction of the sympathetic system results in migraines, due to the vascular changes associated with tension headaches. Individuals with migraine attacks are exhibited to have symptoms that are associated with sympathetic dysfunction, which include reduced levels of plasma norepinephrine levels, sensitivity of the peripheral adrenergic receptors.^[29]


Insomnia is a sleeping disorder, that makes falling or staying asleep difficult, this disruption in sleep results in sleep deprivation and various symptoms, with the severity depending on whether the insomnia is acute or chronic. The most favoured hypothesis for the cause of insomnia is the hyperarousal hypothesis, which is known as a collective over-activation of various systems in the body, this over-activation includes the hyperactivity of the SNS. Whereby during sleep cycle disruption sympathetic baroreflex function and neural cardiovascular responses become impaired.^[30][31]


However more research is still required, as methods used in measuring SNS biological measures are not so reliable due to the sensitivity of the SNS, many factors easily effect its activity, like stress, environment, timing of day, and disease. These factors can impact results significantly and for more accurate results extremely invasive methods are required, such as microneurography. The difficultly of measuring the SNS activity does not only apply to insomnia, but also with various disorders previously discussed. However, overtime with advancements in technology and techniques in research studies the disruption of the SNS and its impact on the human body will be explored further. ^[32][33]