The Physiology of Pain Explained

Understanding how pain works, can help you gain greater control of the pain you experience


Pain is one of a series of sensory systems in the human body, including the visual system (sight), the auditory system (hearing), the olfactory system (smell), the gustatory system (taste), and the somatosensory system (touch). While many consider pain to be a part of the somatosensory system/touch system, actually pain has its own dedicated pathways in the body and spinal cord, as well as its own unique areas in the brain. This makes sense when you think of what the purpose of the pain system is.  

The pain system is designed as an early warning system. First, it is important to recognize that the pain system responds to stimuli that are approaching tissue damaging range. While pain responds when tissue damage occurs, the pain system actually responds significantly before that tissue damage happens. This is logical, when you consider the purpose of the system.  Pain is designed to give you a warning signal. It is designed to allow you to get away from the stimulus that might hurt you, BEFORE tissue damage can occur. But this is not the only function of the pain system. It is also designed as a protective system – so that if you do have tissue damage or injury you will protect the damaged extremity/body part, to give it time to rest and heal.  

But when it comes to chronic pain, the first purpose can sometimes be the most important – in that patients can have significant pain, even when there is nothing “wrong,” where there are no identifiable injuries, no evidence of tissue damage.  Similarly, a person can continue to have pain even after that damage has long healed – because of adaptive systems that are unique to the pain system. Finally, these pathways can malfunction, and someone can feel pain even in the complete absence of tissue damaging stimuli, or stimuli that are approaching tissue damaging range. In other words, a person can have pain even without any exposure to a potentially damaging stimulus, or may respond to stimuli that would not normally trigger a pain/nociceptive response. 

Nociceptors

Nociceptors are sensory receptors that encode pain information, or “noxious” stimulus receptors. Nociceptors are unique among sensory receptors, in that they can respond to a variety of stimuli, including pressure (mechanoreceptors), chemicals (chemoreceptors) and temperature (thermoreceptors). While there are many kinds of nociceptors, there are two general types – A fiber receptors and C fiber receptors. It is important to recognize these two major classes of nociceptors, because their functional purpose is quite different, and can have significant implications for persons who are trying to increase their activity levels when they have pain. For individuals who are hurting, the difference in these two types of pain can have significant implications for persons trying to deal with their pain. 

C fibers

C fibers are primarily touch receptors – they respond to increasing amounts of pressure/pulling that can ultimately result in tissue damage. What is important about these receptors is that they begin firing long before tissue damage actually occurs. This is part of the pain system’s early warning function. If you put your finger on your arm, and press harder and harder and harder, when you first start to feel pain from that excess pressure, this is C fiber pain. That initial pain response is called a pain threshold.

C fibers are typically quite slow in their transmission speeds – they carry information at a rate of about 1 meter per second. The classic example of this is when you drop a weight on your foot, and it takes a second or two for you to actually feel the pain from the weight hitting your foot. This is C fiber pain. C fiber pain has very different characteristics than other types of pain.

C fiber pain is not focused – it tends to be very diffuse, dull, and aching in nature. While it does act as an early warning system, anyone who has engaged in high level athletics knows very well that you can push through this type of pain relatively easily, particularly with practice. Again, just because you have dull and aching pain does not mean that you have actual tissue damage – it means that the stimuli are approaching tissue damaging range, and you need to be careful.  However, it should be noted that chronic C fiber pain can also be quite debilitating, particularly because of its diffuse nature. Finding where you might be injured can be difficult, if you only experience this type of pain. But in general, mild to moderate levels of C fiber pain can be worked through without further damage to the body, without risk of serious injury.

A fibers

A fiber pain is strikingly different. A type nociceptors primarily respond to chemicals that are normally found inside cells – what this means is that they do not respond until cells are damaged, or once tissue damage occurs. They are extremely fast in their transmission speeds, some sending their signals at up to 100 meters/second. Most importantly, the character of A fiber pain is quite different, yielding often severe, very focused, sharp, intense pain. A fiber pain is not primarily designed as an early warning system, as C fiber pain tends to be. A fiber pain is often much more intense and highly localized, and is designed to encourage the organism not to use the damaged body part, to allow for adequate healing. A fiber pain usually registers as much more unpleasant than C fiber pain, and is extremely difficult to work through. 

What these differences mean

Being aware of these two types of pain can be very useful, particularly if you have pain and you are trying to increase your levels of activity. Dull, aching pain means you can proceed with caution. Sharp, lancing pain means you need to stop what you are doing, and try to find another kind of activity if possible. These differences can be used functionally, when trying to increase levels of activity when one is experiencing chronic pain. If the pain is sharp and focused, stop what you are doing, and reassess the situation. If the pain is dull and aching, proceed with caution but you can proceed. 

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