• Question from:Marianne

I was chatting to a friend recently who had been reading a book which described acts of cannibalism and he said it made him feel faint. He asked me what was going on in his body to produce this response and I realized I had no idea. Can you help?

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  • University of Bristol
  • Posts: 2,455

The reason for these sorts of emotional reactions (similar to fainting at the sight of blood) are almost always vagally mediated which cause bradycardia and nausea. See
http://en.wikipedia.org/wiki/Vasovagal_response

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  • South London
  • Posts: 528

David,

That certainly covers the 'how' of the question but to me, doesn't cover the 'why'. Has anyone done any research in why some people are more prone to vasovagal syncopes than others? i.e. why do people have different 'squeamishness levels'?? Is it learnt behaviour?

In other words, can people with low response levels (that are very squeamish) become inured through exposure? (I'm guessing people can, otherwise we might be short a good number of doctors and nurses!) Thus, do we have any idea as to why response rates vary and how much exposure / 'getting your hands dirty' actually makes a difference?

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  • University of Bristol
  • Posts: 2,455

Good question Dave and predictably the answer is no! Other than rare causes of autonomic neuropathy (inherited or acquired as a complication of diseases such as diabetes) the cause of the huge variation is largely unknown. The simple answer is it must be down to genetic variation in neurotransmitters and cognate receptors in the brain stem which is where the vagus relay is located. On top of that it is indeed clear that these sorts of responses can be attenuated by exposure (similar to the psychological techniques of desensitization) - that would almost certainly be mediated at a cortical rather than brainstem level.

  • University of Bristol
  • Posts: 349

Agree with David re. brainstem variations, and/or inputs into this region. In very general terms behavioural and neuroendocrine responses to acute stress (the sight of blood is an acute psychological or emotional stressor for some) are influenced by many factors, including childhood exposure (and its impact on stress in adults, which can involve epigenetic changes e.g., in the expression of hormones driving pituitary adrenocorticotropic hormone, and hence adrenal cortisol secretion), stress intensity and duration (e.g., acute, prolonged acute, etc. which may be sensitised or desensitised on repeated or chronic exposure), type of stressor (physical, emotional, etc), age, gender, immune system status, levels and activity of hormones/neurotransmitters and their receptors, and when the stress is experienced (day or night).

You may find interesting http://www.cnsspectrums.com/aspx/articl … icleid=527 , which offers some evolutionary perspective(s) on fainting as a response to acute stress (and also gives references for some genetic predisposition rather than learned behaviour in stress-induced fainting).

http://www.ncbi.nlm.nih.gov/pubmed/17272328 from 2007 argues that the genetic basis for vasovagal syncope is not that convincing, and describes how a number of neurochemicals such as serotonin may be involved (but then receptors and some neurotransmitters vary at the DNA level, e.g., as in single nucleotide polymorphisms, and these may underly changes in neurotransmitter/receptor function, and likely influence responses in a number of conditions as David suggests).

Last edited by Steve Lolait (18th Jul 2012 08:53:50)