The Autonomic Nervous System – the stressed part, the relaxed part and stopping fear through ‘shock’

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The autonomic nervous system is perhaps a system known all too well by medical students (having spent hours learning the cranial nerves) and known too little about by the outside world. Part of it, the parasympathetic system, is what keeps things ticking over in the background, while its counterpart, the sympathetic nervous system, is the opposite, ‘helping’ us in our hour of need.

The autonomic nervous system’s role can be predicted from its etymology (autonomy meaning self-government). It is the part of the peripheral nervous system that supplies the viscera (internal organs). As mentioned in the caption, there are 2 key divisions that are usually made – the sympathetic nervous system and the parasympathetic system.

The sympathetic system’s most prominent role comes in the body’s fight or flight response and in general, its role can be described as a stimulatory one. It is the system that urges the body to respond, when one is faced with an imminent threat to life. In response to a critical situation, there are key processes that occur.

The first of these is to increase the heart rate and this is moderated by neurotransmitters – the key one being norepinephrine. The heart is innervated (has a nerve supply) by fibres of the sympathetic system (which originate in the spinal cord), and it is at the ends of these fibres (known as post ganglionic fibres) where norepinephrine is released. While it is correct that the heart is myogenic and maintains its own rhythm by the sino-atrial node, intervention from the sympathetic nervous system is needed to increase heart rate.

The effects go beyond increasing heart rate – during the fight or flight response, pupils dilate, the digestive actions are inhibited and the skin pales. These responses all make sense when danger is imminent – having eyes open means one is less likely to fall asleep, and the body needs to direct blood supply to the muscles (to contract) rather than to the skin. However, as ever, the brain isn’t perfect and these response can occur on a much milder scale, such as panic before an exam, which many students can probably attest to.

The parasympathetic system is the more relaxed system, and if there was a rhyme to remember it by, it works best at rest. It deals with the nitty-gritty, such as digestion, defecation, salivation and relaxing the heart. As with the sympathetic nervous system, a lot of the origins of the nerve supply comes from the brainstem and the spinal cord.

 To go into further detail about its physiology and anatomy, the parasympathetic nervous system is mainly mediated by pre-ganglionic neurons (note its antithesis to the sympathetic nervous system’s post-ganglionic neurons). These, as part of the cranial nerves, innervate various areas of the body. The main one is undoubtedly the vagus nerve. This is the 10th cranial nerve (meaning it comes directly from the brain). This innervates the viscera of the thorax and abdomen – pretty much every organ you’d think of, such as the heart, liver, lungs, stomach, pancreas and the small intestine, with all being beneficiaries of the vagus nerve’s supply.

 Like the sympathetic system, using norepinephrine to stimulate the vagus nerve’s (and in general the parasympathetic nervous system’s) modus operandi, is done through using acetylcholine. These have specific receptors called cholinergic receptors. Its role manifests as relaxing functions of the body (barring digestion, where the receptors promote secretion of gastric juices). For example, by causing internal sphincter muscles to relax (and external sphincter muscles to contract as they are an antagonistic pair), defecation can occur.

Recent research into the role of such a varied system has been intriguing and one highlight is the long-term inhibition of fear by stimulating the vagus nerve transcutaneously (i.e. applied across the skin), known as tVNS. Quoting researchers from the universities of Greifswald and Potsdam in Germany:  ‘We found that administration of tVNS during extinction training facilitated inhibition of fear potentiated startle responses and cognitive risk assessments, resulting in facilitated formation, consolidation and long-term recall of extinction memory, and prevention of the return of fear.’ They go on to suggest that such stimulation in an ‘exposure based environment’ could be used clinically to treat anxiety.

The autonomic nervous system, a ‘machine’ of 2 vastly different sides, is one with a lot yet still to be discovered. It is one of the body’s key tools in achieving the optimal conditions of homeostasis, and its role is one we sometimes forget, despite its importance.


References

  1. Schmidt, A; Thews, G (1989). “Autonomic Nervous System”. In Janig, W (ed.). Human Physiology (2 ed.). New York, NY: Springer-Verlag. pp. 333–370.
  2. Gordan, R ; Gwathmey J K, Xie, L: Autonomic and endocrine control of cardiovascular function. World Journal of Cardiology. 2015 Apr 26; 7(4): 204–214
  3. Walker, H. Kenneth (1990). “Cranial Nerve XI: The Spinal Accessory Nerve”. NCBI Bookshelf. PMID 21250228. Retrieved 30 May 2019.
  4. Szeka, C, Richter J, Wendt J, Weymar M, O.Hamm, A Promoting long-term inhibition of human fear responses by non-invasive transcutaneous vagus nerve stimulation during extinction training Scientific Reports volume 10, Article number: 1529 (2020)
  5. Laurie Kelly McCorry, Physiology of the Autonomic Nervous System Am J Pharm Educ. 2007 Aug 15; 71(4): 78.

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