The Science of Pain

Pain serves a vital purpose for living creatures with a nervous system, including humans. The unpleasant feelings of pain convince us to move away from situations that are causing damage to our bodies. Without this built-in alarm system, we would have to try hard to avoid biting our own tongues, we might not notice our hand is on a burning stove, and we wouldn’t go to the doctor to mend broken bones. As unpleasant as pain is, it is extremely important to our survival.

Pain is a defining factor in inflammatory bowel disease, irritable bowel syndrome, functional dyspepsia, GERD, and diverticular disease, and is common in many other GI diseases and disorders.

What is Pain?

According to the International Association for the Study of Pain, it is “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.”

When something injures – or otherwise threatens – your body, the nerve fibres send a message to the brain letting it know that something isn’t right. Most pain experience takes place in the brain, which processes the information from these signals. However, pain isn’t just a simple message from nerve-to-brain. The brain often mediates this message based on your experiences, other stimuli, various environmental factors, and more.1

For example, people tend to feel more pain when they believe that the injury is major and less pain when they think the injury is minor, regardless of the actual degree of injury. The brain can even send a message back to the nerves to silence them if it finds the situation to be safe. Conversely, the brain can instruct the nerves to continue to fire when it is uncalled for, causing pain that is needlessly strong or pain that lasts much longer than necessary. Your emotional state of mind can also affect the types of messages exchanged between the brain and nerves. Anxiety causes the brain to ask for more signals from the nerves, which increases pain length and severity, whereas feeling safe makes it ask for fewer signals, thereby reducing the amount of pain you feel.

The impact of the brain on pain is clear when you consider that placebo pain relief pills often work remarkably well, and that sometimes people don’t feel pain until they have other stimuli that cause them to acknowledge the injury, such as seeing a wound.

Before we understood that pain is simply neurons signalling something is wrong, many people believed that pain was a punishment from God. The word pain is actually derived from the Latin word poena, meaning ‘punishment, penalty’ and the Greek word ποινή (poine), which translates to ‘price paid, penalty, punishment’. These same root words show up in the English words penalty, pain, punish, etc.11


What are the Different Types of Pain?

Although we typically refer to anything that hurts as pain, there are actually several types of pain, with unique stimuli and treatments, which our bodies process and acknowledge differently.

Nociceptive Pain

Most of the pain you experience is nociceptive pain, which results from stimulation of a type of peripheral nerve fibre, called nociceptors. This is what you feel when you stub your toe, pull a hot dish from the oven with your bare hands, scrape your knee, break your arm, and most other things that cause tissue damage. We can break down nociceptive pain even further by looking at either the type of stimulus causing the pain, or where in the body someone is feeling this pain.

There are three main types of stimuli that cause nociceptive pain:

  1. mechanical stimuli, which causes injuries such as cuts, bruises, sprains, and breaks;
  2. thermal stimuli, typically from extreme temperatures, which results in injuries such as burns from heat and frostbite; and
  3. chemical stimuli, when a substance harms the body, such as with chemical burns and getting soap in your eye.


The way we feel nociceptive pain also depends on where in the body the pain originates:

  1. superficial somatic pain is from injuries to the skin. It is typically very easy to locate the source of the pain, which is very specific and often a sharp feeling. This type of pain is what you feel from cuts, bruises, and burns;
  2. somatic pain occurs when the wound is deeper, such as injuries to the tendons and ligaments, broken bones, sprains, and pulled muscles; and
  3. visceral pain originates in organs, and is often general and hard to pinpoint. Individuals with digestive diseases and disorders are typically most familiar with visceral pain.

Neuropathic Pain

That odd feeling of hitting your ‘funny bone’ is a common example of neuropathic pain, which occurs when the injury affects the nervous system itself. This pain is different from nociceptive pain in that it doesn’t feel aching, throbbing, or stabbing. Neuropathic pain typically presents as the sensation of pins and needles, numbness, itching, burning, coldness, or electric shock feelings. This type of pain is common in people with damage to the central nervous system, such as those with multiple sclerosis or spinal cord injuries, and in those with ailments that affect the peripheral nervous system such as nutritional deficiencies, some types of cancer, HIV, herpes, diabetes, and those exposed to certain toxins. In addition, individuals with celiac disease are more likely to experience this type of pain.

Phantom pain is another type of neuropathic pain, which occurs in amputees when they perceive pain in a missing limb.

Psychogenic Pain

This type of pain occurs when emotional or mental issues cause, increase, or prolong pain. Some examples include stomach aches, headaches, muscle pain, and back pain that are caused by strong emotions such as grief, anxiety, and depression. Psychogenic pain can also occur when mental or emotional distress increases the severity of other pains. As mentioned earlier, most pain occurs in the brain, thus mental illness and emotions can have a huge impact on how we interpret pain signals. This is common in those who have irritable bowel syndrome, for instance, who typically experience more pain and discomfort in their bowels when under stress or experiencing mental health symptoms.

Treating Pain

The first line of defense against pain is to treat the cause. Since pain is typically a reaction to something harming your body, the simplest way to reduce pain is to fix or eliminate the injurious stimulus. However, this isn’t always possible. There are many ailments that can chronically affect your body, which are persistent, incurable, or even untreatable. In some individuals, there is not even an identifiable trigger for the pain, which makes it very difficult to treat. Because of these problems, many people experience chronic pain – pain in one location that has lasted for at least three months.

Right now, one in five Canadians are experiencing chronic pain, which more commonly affects females and seniors.2 This chronic pain can have a huge influence on an individual’s quality of life. Studies show that pain can affect quality of life more than any other diseases or disorders where pain is not present. Those who are in chronic pain have an increased chance of experiencing depression, anxiety, and anger,3 and are twice as likely to commit suicide as those without chronic pain.4 Many individuals develop impairments in attention, memory, problem solving, and mental flexibility, often losing the ability to work.5 Children who experience chronic pain miss the opportunity to live a normal life, as they often find it hard to participate in most childhood activities, such as social events and school.6 Many elderly people experience chronic pain; it affects 25-65% of seniors living in communities and up to 80% of seniors in long-term care facilities, which has a huge impact on their quality of life and, unfortunately, medical professionals often ignore this suffering.7

For individuals in chronic pain, and those in the process of healing from ailments that cause short-term (acute) pain, medications that treat pain (analgesics) can help reduce the discomfort and make the pain more manageable or, ideally, eliminate as much pain as possible.

Chronic pain affects 9% of males and 12% of females aged 12-44 in Canada.12

Pain Medications8,9,10


Non-steroidal anti-inflammatory drugs (NSAIDs) prevent you from producing compounds called prostaglandins, which are responsible for causing inflammation. In addition, NSAIDs interfere with your body’s ability to send pain signals because prostaglandins are necessary to send signals of pain to the brain. This reduction in prostaglandins means that you experience less pain both from the reduction in inflammation in the injured area as well as from the fact that fewer pain messages are reaching your brain.

Examples include ibuprofen, Aspirin® (acetylsalicylic acid), and naproxen, which are primarily available over-the-counter (OTC), but some higher-dose formulations are only accessible by prescription.

Acetaminophen is widely used to relieve many types of pain because it increases the body’s pain threshold and modifies how your brain perceives pain to reduce pain severity.

It is sold under the brand name Tylenol® as well as in many generic formulations. Acetaminophen is often an active pain relief drug in many OTC medications, including medicines for treating cold, flu, arthritis, menstrual cramps, back pain, and more. If you have a liver disease, be sure to ask your physician whether it’s safe for you to take acetaminophen because it’s processed in the liver and relatively small amounts can worsen liver damage.

Opioids are narcotic agents, which are either natural alkaloids from the opium poppy plant (opiates) or synthetic drugs made to work by mimicking opiates. These medications bind to opioid receptors in the brain and modify pain messages. Side effects include nausea, vomiting, constipation, drowsiness, itching, and respiratory distress. Opioids are also highly addictive and it is easy to develop a tolerance to these medicines, so increasing dosages are often required during long-term use. These drugs eliminate pain by preventing pain messages from getting to the brain, and by working in the brain to alter the sensation of pain. Typically, when we feel pain, our bodies produce chemicals known as endorphins to counteract the pain messages. Endorphins interact with opioid receptors in the brain, reducing pain and making us feel more relaxed. Opioid medicines mimic this natural pain relief by using this same pathway, but they are much more potent than endorphins.

Opioids are primarily available through prescription, and include morphine, codeine, oxycodone, and fentanyl.

Adjuvant Analgesics

The following medicines aren’t typical analgesics; they have a primary indication for specific ailments and pain relief is a secondary effect. They often only work in very specific situations, but they are an important part of the pain-killing arsenal. These are the most commonly used:

  • muscle-relaxants cause a sedative action in the central nervous system, reducing pain from tense muscles;
  • corticosteroids are very powerful anti-inflammatory drugs;
  • anti-anxiety medicines relax muscles, reduce anxiety, and help people cope with pain;
  • antidepressants reduce pain transmission in the spinal cord; and
  • anticonvulsive medications stabilize nerve cells.

Alternative Therapies

There are a variety of non-medication techniques you can use to help reduce chronic pain. Adequate sleep, stress management, relaxation, and hypnotherapy offer varying degrees of relief.

Marijuana and Pain

While not a common analgesic, there is research showing medical marijuana to have pain-relieving properties. A recent meta-analysis13 compared studies conducted from 1948 to 2015 on the effects of marijuana and found “high-quality evidence” that marijuana can help reduce chronic pain. It isn’t clear exactly how marijuana relieves pain, but one study shows that it might modify the way we perceive pain.14


Since pain serves a vital purpose for us, we must give it the respect it deserves and respond appropriately. If you have any type of pain, but particularly gastrointestinal pain, be sure to discuss this with your physician. It is important that the treatments and medications you are using are appropriate for your condition, considering that certain medications might adversely affect your existing GI condition.

First published in the Inside Tract® newsletter issue 194 – 2015
Image Credit: © atamanenko
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