The complexity of the nervous system allows for pain to be experienced. In fact, the function of pain is to warn us of dangers, informing us that something is wrong. Our brain then tells us to remove the stimulus that causes pain by avoidance or by looking at approaches that help relieve it.
A lot about pain is still not well understood however certain theories were developed to explain how this is experienced. One of the most popular theories is the Gate Control Theory proposed by Ronald Melzack and Patrick Wall in the 1960’s. The theory explains that pain is experienced through gated channels. Another theory is about the concept of nociception in that different pain signals are felt because the stimulus that causes pain travels to different types of nerve fibres. But despite what theory you want to use to explain pain, it is important to remember that pain is a subjective experience. Our interpretations of pain differ from one another which may be influenced by experience, genetics, illness, culture and many others.
What causes pain?
The receptors for painful stimuli are called nociceptors. These receptors from nerves are located almost everywhere in the body which explains why pain can be felt from various areas as long as there's noxious stimuli (the stimuli that causes pain). The following are types of noxious stimuli:
Mechanical – this is the pain stimulus caused by mechanical trauma or pressure such as pain experienced from swelling, abscess or tumour growth.
Thermal – stimulus coming from heat (e.g. from burns)
Chemical – stimulus that may excite neurotransmitter response such as in the presence of toxic substances or infection resulting to inflammation. Infections can sometimes lead to a cascade of events initiating the release of certain chemical mediators like prostaglandins, substance P and bradykinins causing inflammation and pain. When these mediators bathe the nerve cells, it will be picked up as painful stimuli thus leading to the experience of pain.
The Perception of Pain
Different pain theories are used to explain the perception of pain. Pain can be attributed to stimulation of the nociceptors of certain nerve fibres called the A delta and C fibres.
A delta fibres are myelinated fibres that respond to mechanical and thermal stimuli. Since they are myelinated (the insulating layer that surrounds the axon of nerves), the speed of pain transmission is rapid or fast and is associated with sharp or cutaneous types of pain like the ones experienced when the skin is damaged (eg, punctures and cuts). The C fibers are non-myelinated which makes pain transmission slower and this has been associated to the dull and aching types of pain. C fibers can respond to all mechanical, thermal, and chemical stimuli.
The Nerve Cell
There’s another fibre from the delta fibres group called B delta that responds to touch and pressure. Touch and pressure sometimes do not cause pain unless more pressure is applied which then result to noxious stimuli from mechanical pressure. This then activates the A delta fibres.
The perception of pain comes in different stages. The stages describe how the stimulus is picked up, transmitted, interpreted and actioned.
Transduction Stage: This is the stage when a noxious stimulus is transformed to electrical nerve impulses by nociceptors.
Transmission Stage: This is the stage when the nerve impulses brought by noxious stimuli travel through nerve fibres (A delta or C fibres) towards the dorsal root of the spinal cord which then reaches the brainstem and thalamus to be interpreted or perceived.
Perception Stage: This is the awareness of pain. The nerve impulses coming from nerve tracts are processed by the thalamus of the brain. The thalamus is the area for processing somatosensory information (Steeds, 2016). Depends on the type of pain and intensity, the thalamus may also stimulate other parts of the brain such as the reticular activating system (RAS), the limbic system and the somatosensory system.
When the RAS is activated by pain stimulus, this may result to autonomic nervous system and motor responses which explains why vital signs such as the heart rate and blood pressure are increased when a person is in pain. Muscle spasms can also occur from pain because of the activation of the RAS.
Limbic system activation can result to emotional and behavioural responses. Sometimes, pain or even the thought of past pain experiences, can trigger emotional responses like anxiety and fear, depression, deprivation of sleep and poor appetite. This is associated to limbic system activation.
When the somatosensory cortex is activated from pain stimulus, interpretation of the pain sensation occurs which allows for recognising where the pain is located, its intensity and quality.
Modulation Stage: If you’ve noticed that the intensity of pain changed to either increased or decreased when you were hurt, this is because of the excitatory and inhibitory neurotransmitters. When excitatory neurotransmitters are released such as prostaglandin, bradykinin or substance P, this can result to an increase in the level of pain. If the pain level goes down, the inhibitory neurotransmitters may be in action such as endorphins and enkephalins. Endorphins are sometimes called natural pain killers. What makes the modulation excitatory or inhibitory is of multifactorial reasons and depends on the situation (e.g. being happy and excited releases endorphins which makes a person forget about the mild pain from a small wound).
The Gate Control Theory
When mild pain is relieved by massage or just by simply rubbing it after you’ve just accidentally bumped your elbow on a hard surface, the stimulus coming from the massage and rubbing motion overrides the pain stimulus. This explanation can be attributed to the gate control theory because in this theory, pain travels through gating mechanisms which is believed to be in the dorsal horn of the spine (Cherry, 2020). When the gates are overridden by stronger stimulus, the weaker stimulus can’t enter. This theory developed by Melzack and Wall has been one of the most influential of the pain theories. It has been the basis of some of the devices used to treat pain such as the Transcutaneous Electrical Nerve Stimulation device or TENS.
The Gate Control Theory of Pain/ Ronald Melzack
/ Video from Barzckar, E. (2015)
How is pain assessed?
Pain is a subjective experience and whatever a person says about their pain must be considered as true. Each of us will have our own definition of pain. That is why in practice, in order to have a better understanding of the patient’s pain, clinicians use tools such as the numerical rating scale and Wong-Baker face rating scale to assess it.
Numerical scales can be in 0-10 wherein 0 refers to no pain and 10 being the most painful experience ever felt. In some cases, this may be reduced to a scale of 0-3 to make it easy for patients to rate with 0 as no pain, 1 as mild, 2 as moderate and 3 as severe pain. The Wong-Baker face rating scale is used for those who can’t rate using numbers such as those who may be unfamiliar to what numbers are, or for children (Giddens and Langford, 2004).
There are many types of pain assessment tools and the ones mentioned above are just an example. In some cases, more details of the pain experience may be needed by identifying not just the severity of the pain, but also the location, quality, precipitating and aggravating factors, and the observed behaviours (Giddens and Langford, 2004). Clinicians will have to use the tool that is best for the situation.
Pain is even more complex than you think
Understanding pain is quite broad and there are a lot of theories that have been developed to explain why it happens. We also have to understand that pain comes in many forms and types. Pain can sometimes be acute (short-term) or chronic (long-term). Pain thresholds also change over time which commonly occurs for those who suffer from chronic pain because of central sensitization (pain threshold changes because of presently high reactivity of the nociceptors). There are also those types of pain that are associated to malignancies which triggers both the A delta and C fibres such as cancer pain. Furthermore, there are also those types that result from nerve lesions or damage such as neuropathic pain. Phantom limb pain again is of another type which is more complex. This is sometimes experienced by amputees when they still feel pain on an extremity that is no longer there. This may be due to the reorganisation of the somatosensory centre. Therefore, understanding pain experience is taking into account the overall narrative of the person’s pain and considering whatever they say about their pain as it is, regardless of whether it is a description that’s straightforward or vague.
Disclaimer:
The content in this blog is for informational purposes only and should not be taken as medical advice. It is always best to consult your doctor for medical questions.
If you are a healthcare provider, the content here should not be used to make any diagnosis, give advice or prescribe treatment as this blog is for informational and educational purposes only. Healthcare is an everchanging field and each patient is unique. It is your responsibility as a healthcare provider to always refer to current care standards and practices.
Reference List:
Briggs, E. (2010) Understanding the experience and physiology of pain, Nursing Standard, 25 (3), pp. 35–39. DOI:10.7748/ns2010.09.25.3.35.c7989.
Cherry, K., (2020). Gate Control Theory for Pain Signals to the Brain. [online] Verywell Mind. Available at: <https://www.verywellmind.com/what-is-gate-control-theory-2795208>
Giddens, J.F. and Langford, R.W. (2004). Mosby’s Nursing PDQ. Pain Assessment Factors. Singapore: Elsevier (Singapore) PTE. LTD.
Steeds, C. E. (2016) The anatomy and physiology of pain, Surgery (Oxford), 34 (2), pp. 55–59. DOI:10.1016/j.mpsur.2015.11.005.
Wood, S. (2008) Anatomy and physiology of pain | Nursing Times. [online] Nursing Times. Available at: <https://www.nursingtimes.net/clinical-archive/pain-management/anatomy-and-physiology-of-pain-18-09-2008/>
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