The perception of pain in the teenage brain | Natural mechanisms of limiting pain perception

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What is more painful? The agonizing 8 hours of childbirth or the excruciating 8 minutes of being kicked in the testicles? The answer seems obvious. However, in a randomized social poll of 157 teenagers the preference of these two experiences was not made so clear.

For us to appreciate the ability to perceive pain we must understand the basis of which this reactionary expression came about in living organisms.  ‘When the air hits your brain’, an interesting novel by a retired neurosurgeon Frank Vertosick Jr., MD, offers an adventurous insight into the development of pain in scientific terms. As animals are constantly at odds with changing environment, peripatetic metazoans require complex responses which can be customized in milliseconds, resulting in the evolution of the nervous system which serves as “an organ system denoted purely to cognition”1.

Within diagnosis, the experience of pain is separated into two categories: acute and chronic. Acute pain is limited in its duration and usually caused by a well-defined pathology whereas chronic pain is long term, lasting greater than 3 months (for example, fibromyalgia pain and chronic headaches arising from migraines), in some cases the pathophysiology may have completely resolved however the patient retains an emotional and unpleasant sensory experience, continuing to give descriptions denoting to tissue damage when in fact there is none, it is this kind of psychogenic pain that multidisciplinary teams of psychiatrists, physiologists and anaesthesiologists deal with in the pain clinic.

It may be hard to believe but our response to physiological pain does have its benefits; the reaction offers protection from ongoing injury and further damage. The mechanism of a reflex, which does not involve the conscious part of the brain, prevents you from keeping your hand in boiling water (or listening to 2010 Justin Bieber’s “Baby” as an equivalent).

We shall use the infamous ‘kick in the balls’ to follow the journey of nociception to the brain. After just being awkwardly injured, several processes occur from the moment of noxious stimulation but most importantly immune cells (such as neutrophils and mast cells) begin to produce Prostaglandin (PGD2) as part of the inflammatory response. Nociceptors are sensory receptors at the ends of or C fibres (primary afferent fibres) found in the skin along with muscles, joints, and the meninges. The Að Mechanosensitive nociceptor responds to the prostaglandin produced and turns the stimuli into a nerve signal.

The signal then travels along the primary afferent fibre, which we will call the 1st Order Nerve, to the Dorsal Horn, the 1st Order Nerve synapses with a 2nd Order Nerve, transmitting the pain signal. The 2nd Order Nerve then crosses over to the opposite side of the spinal cord. Within the white matter of the spinal cord there are different tracts for several neuronal pathways- the white matter is essentially the neuron highway to the brain. In this case the nerve signal enters the Spinothalamic Tract and travels all the way up to the Thalamus (the relay station of the brain). The 2nd order neurone terminates and synapses with the 3rd Order Neurone that transmits the nerve signal from the Thalamus to a region of the Somatosensory Cortex, corresponding to the area of the noxious stimuli (in this case the scrotum); hence, pain is perceived.

The body has two ingenious ways of limiting pain perception- the gating mechanism and opioids. You will find that the first mechanism is a simple idea that touch sensory neurons can override and block certain pain sensory neurons at the spinal cord level, this is because touch sensation reduces c fibre transmission through the Dorsal Horn. Through the conjoining of both these sensory fibres into the spinal nerve, touch sensation has dominion. Hence why you tend to rub on areas that are stimulating pain.

The descending inhibition pathway is a cascade that runs opposite to the ascending pain pathway, and ultimately leads to interception in dorsal horn of the spinal cord – the critically acclaimed Substantia Gelatinosa region.

The descending neurone is specialised with its ability to produce Serotonin and Noradrenaline, these chemicals inhibit the transmission of the pain signal across the synaptic cleft of the two ascending neurons in two ways.  SR/NAD stimulates a small interneuron within the region, an opioid neurone that releases enkephalin, this leads us to the body’s second way of limiting pain perception – Opioids.

The body produces three endogenous opioids which are involved heavily in mitigating pain perception, these are Enkephalin, Endorphins and Dynorphins2 (no, they are not power rangers).  Each opioid has its own receptor (or as I like to remember them, Frat Houses) which it is most likely to bind onto and cause agonistic (stimulatory) effects. Enkephalin binds onto the Delta Opioid Peptide Receptor (DOPP), Dynorphin binds onto the Kappa Opioid Peptide Receptor (KOPP) and Endorphin binds onto the Mu Opioid Peptide Receptor (MOPP). These chemicals and their receptors are distributed throughout the body and in the Central Nervous System, however they are all present in extremely high concentrations in the dorsal horn.

Now that we know what Enkephalin is and where it binds on to, we can go back to the famous Substantia Gelatinosa. The opioid neurone produces enkephalin which binds onto its DOPP receptors on the 1st and 2nd Order Neurons of the Ascending Pain Pathway and exhibits agonistic effects.3

Ultimately, the pain nerve signal is prevented from reaching the Thalamus due to the agonistic effects which power your flight response to acute injury – the less pain you feel the more likely you are to withstand a predatory attack.

Many would agree that pain is intricately subjective; to me it is a phenomenon, to gynaecologists it is the reason why women, although few, can stand 8 hours of active labour without experiencing any pain.4 Loesser’s onion skin model depicts how multiple factors (personality, environment, genetics & beliefs etc.) augment the pain that you perceive – giving leeway to the Shaolin monks of China who are able to balance their abdomen, and various other parts of their body, on sharp spheres by ‘channelling their Chi’.

The almost equal split of 77:80 in the social poll I mentioned in the introduction proves that defining pain has its difficulties. However, it is important to consider its metaphysical side -for a progressive but compassionate society to exist we must understand that “pain is experienced by people and families, not simply by nerve endings”.5


References

  1. Vertosick, F.T. (2008). When the air hits your brain: tales from neurology. New York; London: W.W. Norton.
  2. Sprouse-Blum AS, Smith G, Sugai D, Parsa FD. Understanding endorphins and their importance in pain management. Hawaii Medical Journal. 2010 Mar;69(3):70-71.
  3. Waldron, K. (2007). The chemistry of everything. Upper Saddle River, Nj: Pearson Prentice Hall.
  4. Kitching, C. (2019). Gran “feels no pain” even when she gave birth due to her mutated genes. [online] mirror. Available at: https://www.mirror.co.uk/news/uk-news/gran-who-felt-no-pain-14196838 [Accessed 30 Jun. 2020].
  5. Watson, M.S. (2009). Oxford handbook of palliative care : [the essential and holistic guide to palliative care] : [include all the latest guidelines and management options] : [provides advice on the nursing aspects of palliative care] : places an increased emphasis on patients with non-malignant diseases. Oxford: Oxford University Press.

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