What is pain?

Not many are fans of pain. In fact, the majority dislike it so much that it is oftentaken for granted. We focus on all of the bad aspects of it because pain is a part of life we try to avoid. Yet, it is a necessary part of life that teaches us when we might hurt ourselves. It offers protection. It is something we need to make sense of our surroundings. Without it, we would die young and be disfigured in the process, like those who are born without the ability to feel pain1 . Congenital insensitivity to pain is a genetic condition that doesn’t allow transmission of signals from dangerous (noxious) stimuli, so they never produce a model of pain in their brains to learn from.

How do We Generate Pain? It starts with Nociception.

Pain is a sensory experience that causes modification of the nervous system, of which the foundation is neuroplasticity. The acute physiological sensation of hurt that we all know and love is the result of our peripheral nervous system functioning well, or adaptive plasticity. It is giving us valuable information that will protect us through receptors called nociceptors 2.

These are located throughout the body and organs, in the skin and tissues, and are activated by dangerous forms of pressure, temperature or chemicals 3. They are part of the nociceptive system that is a sensory system in its own right from primary afferent (nociceptor) to the dorsal horn and then onto multiple brain areas 4. There are 2 main types of pain afferents (nociceptors) and they may be specialised for what they respond to or they are activated by several classes of stimuli or polymodal 5.

  • A fibres made of A-delta/ A-Beta fibres have myelinated axons that transmit faster sharper noxious sensations such as at the time of injury.
  • C fibres are unmyelinated small fibres that slowly transmits input more diffuse in nature.

Nociception is Protective

Nociceptors offer protection in 2 ways. They are high threshold receptors which means they require a strong enough stimulus to activate them and usually transmit noxious information much slower than things like soft touch. The most common are c-fibres. This is advantageous because we don’t want to feel discomfort all of the time. We also have to control the feeling of discomfort by rubbing because that information travels faster to the brain.

Additionally, the protective function of nociceptors is further enhanced by a phenomenon called peripheral sensitisation. Repeated dangerous stimuli or even one that is strong enough lowers the threshold required to transmit “painful” information so that it is amplified. This type of adaptive plasticity in the periphery returns to baseline where the threshold to activate is once again high, shortly after the stimulus is gone or in the absence of on-going tissue injury (Woolf).

Pain is an experience and output of the brain

Dangerous information enters the spinal cord through the dorsal horn and transmits to the brain through ascending pathways. Pain perception occurs in the brain downstream of nociceptive input and is separate from nociception. The pain experience is controlled by the interaction of ascending and descending pathways (Apkarian) so that only the salient input is perceived by the brain. Pain can occur in the absence of nociception and nociception can occur without pain. An example of the latter is withdrawing from touching a hot object that occurs through a spinal reflex without the need of descending control from the brain.

Pain, it’s More Than a Feeling.

Pain isn’t just about tissues being injured. It is a whole nervous system experience that has become hyper-alert following conditions where the actual or potential risk of further damage is high. Pain has many influences on it and influences many things in turn. How many times have you noticed that things hurt when you are more stressed, angry, or just having one of those bad days? Moreover, the loss of a loved one can make you feel like you are in pain that literally takes control of your life and ability to function. This is because pain is an experience and cannot be separated from the person’s mental state, life stress, cultural background, environment, past experiences, genetics and all other nuances that make you who you are. Pain is unique, just like you.

Other highly influential factors include anxiety or fears about pain, attention to pain, and the social context in which the pain is experienced. How many of you have felt like an injury will prevent you from continuing in your career? These factors can be so critical that they can actually cause the brain to trigger or abolish the experience of pain independent of what is going on in your tissues. This last remark is more apparent in persistent (chronic pain) states, where pain outlasts the normal tissue healing process, is disproportionate to the injury or is there without any obvious sign of tissue damage.

References 

1. Drissi I, Woods WA, Woods CG. Understanding the genetic basis of congenital insensitivity to pain. Br. Med Bull. 2020; 133(1):65-78. doi: 10.1093/bmb/ldaa003.

2. Latremoliere A and Woolf C. Central sensitisation: a generator of pain hypersensitivity by central neural plasticity. J Pain.  2009; 10(9): 895-926. doi: 10.1016/j.jpain.2009.06.012.

3. Tracey WD. Nociception. Curr Biol. 2017; 27(4): R129-R133. doi: 10.1016/j.cub.2017.01.037.

4. Apkarian VA, Bushnell CM, Treede RD et al. Human brain mechanisms of pain perceptions and regulation in health and disease.  European Journal of Pain. 2005; 9:463-484. doi: 10.1016/j.ejpain.2004.11.001.

5. Dubin AE, Patapoutian A. Nociceptors: the sensors of the pain pathway.  J Cliln Invest. 2010; 120(11): 3760-3772. 

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