What is pain?
The pain process
Everyone has felt pain but not everyone feels it in the same way. Pain is a complex sensation that differs greatly from person to person, even when the injury or cause seems the same. It can be an instant sensation, like when you cut yourself. Or it can be a dull ache, like from a tension headache. It can also be an inexplicable, nagging tingle on the skin. But, for everyone who feels it, pain means something is not right, and pain that goes untreated for a long period can alter one’s mood and well-being.
So what is pain?
The International Association for the Study of Pain defines it as an unpleasant sensory and emotional experience associated with actual or potential tissue damage.
This is a simplified explanation of how we feel pain: Pain starts when sensors or receptors trigger an electrical signal that travels to the spinal cord. The spinal cord then relays the signal to the brain. The spinal cord acts as a relay center, where the pain signal can be blocked, enhanced or modified before gets to the brain.
In reality, pain is a very complicated process. It involves the interplay of various chemicals (called neurotransmitters) in the body, which together act to produce the sensation of pain.
Neurotransmitters transmit nerve signals from one cell to another. Receptors on the cells receive the signals and act as gates that allow pain messages to be passed to neighboring cells or to the spinal cord and brain.
Receptors are important because they also play a role in blocking pain. For instance, some pain medications lock on to certain receptors, which stop the pain process and subsequently relieve pain.
Why do we feel pain?
We feel pain for different reasons. Pain can be broadly divided into three classes.
Nociceptive pain provides an ‘early warning’, detecting and protecting the body from damaging or toxic elements. This is the kind of pain we feel when we touch something too hot, cold or sharp. Nociceptive pain activates a withdrawal reflex to protect the body from further damage (for example, we instantly pull our hand away when we touch something burning hot.
Inflammatory pain is associated with tissue damage. Inflammatory pain has a protective effect – pain and tenderness after an ankle sprain, for example, discourages us from moving the joint and helps promote recovery. Inflammatory pain is caused by the activation of the immune system in response to tissue injury or infection. Although this kind of pain helps in recovery, it still needs to be reduced in people with ongoing inflammation (eg, in rheumatoid arthritis) or in very severe injuries.
Pathologic pain is pain that is not protective. It is a result of abnormal and amplified functioning of the nervous system (the brain, spinal cord and nerves). It may be due to nerve damage (neuropathic pain). Sometimes there may be no obvious cause, damage or inflammation (dysfunctional pain). Examples of dysfunctional pain include pain from fibromyalgia and irritable bowel syndrome, and some headaches.
Each class of pain has a different mechanism, so treatment can differ according to the type of pain and mechanism.
Acute versus chronic pain
Pain is also classified based on how long the pain lasts.
Acute pain is short-term pain. It is often caused by disease, inflammation or injury. Acute pain is felt immediately, like after an accident or surgery. The cause of the pain is usually obvious. Acute pain is usually treatable, but sometimes it can go away without treatment.
Chronic pain is long-term pain. Acute pain that is not treated effectively can become chronic pain. Chronic pain can be much harder to treat than acute pain. Like acute pain, chronic pain can be made worse by environmental and psychological factors. Chronic pain can also affect quality of life. Chronic pain conditions include low back pain, rheumatoid arthritis, fibromyalgia and pain due to cancer.