Pain is a part of everyday life. Typically, pain results when we are exposed to situations that are likely to lead to injury or tissue damage. In this respect, pain is a useful occurrence and a vital mechanism by which we protect ourselves from possible further damage and aid the healing process. A reflex flinching from a hot kettle or nursing a sore, bruised hand are examples of this function.
We each learn our own association with the word "pain" through our experiences of injury in early life. As such, pain will always be subjective. By definition it is unpleasant and it therefore always has an emotional association.
Pain is in the brain
When we hurt ourselves, we usually feel pain. If we touch a hot object we feel a burning pain in our hand, but it is our brain that is creating the perception of pain. The brain cannot detect pain directly, but relies upon a sophisticated system of nerves and receptors in the skin, muscles, joints, and organs, and their connections to the spinal cord and brain. When we touch the hot object the brain is able to detect that damage is occurring, locate where it is occurring, and tell the body how to respond. To us it feels as if our hand is burning, but this feeling is created within our brain from the information it receives.
But how does the brain receive these pain messages?
Activation of pain messages
Most events that can cause us harm, such as a blow or extreme heat, are detected by specialized nerves—sensory neurons—called nociceptors [say: NO-see-sep-tors}. These send messages from the body, where the injury occurs, to the brain.
Nociceptors are able to respond to different types of painful events, including heat, cold, and pressure, such as pinching. The tips of the nociceptors in the skin contain special structures called receptors. There are particular receptors for each of these different painful events. They detect that damage may be occurring and generate the nociceptive message. That message is then sent to the spinal cord and on to the brain so that we are able to not only feel pain, but know what sort of pain it is.
Transmission of pain messages
There are other sensory neurons that detect non-painful touch. In this way sensory nerves in the skin can tell the difference between, for example, a light brushing sensation and a pinch. The threshold which a painful, potentially harmful stimulus needs to reach before a nociceptive message is sent to the brain is normally high enough so that our normal behaviour isn’t affected, but also low enough to stop any actual tissue damage occurring. This threshold is not fixed and can be shifted up or down, either in a helpful, protective way or in a way that is not helpful and can lead to chronic pain.
Nociceptive messages start their journey by travelling along nerves made up of sensory neurons. Sensory neurons are nerve cells that have two long connections called processes. One process stretches all the way to the skin, joints, muscles, and organs, while the other is connected to the spinal cord. Nociceptive messages are sent along these processes and into the spinal cord.
Sorting out pain from other sensations
The spinal cord cannot feel pain directly. Signals of all sorts, be they nociceptive or those generated from touch, warmth, or cooling, all arrive at the spinal cord from the skin. There are also messages sent from our muscles and joints to the spinal cord when we move. In this way, the spinal cord receives many different types of messages at the same time.
Imagine a large busy railway station with trains arriving from every direction but with nothing on them to tell where they have come from. If all the trains coming from a particular town always go to one particular platform in the railway station, then it is possible to tell where a train has come from by where it stops. In much the same way, the spinal cord has different "platforms" called laminae where separate "trains" of messages arrive. The nociceptive messages from the skin go to a different lamina than do touch messages from the skin, which go to different laminae from movement messages from the muscles and joints. In this way, the spinal cord knows where on the body the message has come from.
Regulating the intensity of pain messages
Each of these platforms/laminae of the spinal cord consist of many neurons. There are several sorts of neurons in the spinal cord. Some transfer messages on to the brain, but most connect with other cells locally, both within laminae and also between them, like bridges connecting platforms in the railway station. These cells, called interneurons, can increase or decrease the strength of the messages being sent to the brain, acting like a volume control, turning the volume up or down, depending which neurons are activated.
This is why when we bang our knee, rubbing it afterwards helps reduce the pain. Knocking our knee sends a nociceptive message to the spinal cord and on to the brain and we feel a sharp pain. Rubbing the knee activates touch receptors, and they send their own message to the spinal cord, which also goes to the brain, and we feel a rubbing sensation. However, the rubbing message also turns on some of the interneurons in the spinal cord, which turn down the volume of the nociceptive message being sent to the brain, and some of the pain is relieved.
The brain interprets nociceptive messages and translates them into pain
Once the nociceptive message has entered the spinal cord it continues up to the thalamus. The thalamus is a region deep inside the brain that is like a switchboard where messages are passed to and from many other parts of the brain. When we touch a hot object, messages are sent to the thalamus and then distributed to different parts of the brain. Each of these has a different function. One part will decide that the messages are coming from the hand. Another will decide that the sensation "heat" is being felt.
The thalamus also connects to parts of the brain responsible for memory, emotion, attention, and motivation. The nociceptive message triggers our memory of pain and anxiety about the pain to come. All of these actions occur at the same time and the brain decides what the best course of action is. In the case of us putting our hand on a hot object, the brain decides that it is hot and painful, sends a signal to the part of the brain that controls movement, and we move our hand away.
The brain can change pain
The traffic of messages is not all one way. In addition to messages going from skin to spinal cord to brain, messages are also continuously being sent from the brain down nerves to the spinal cord. These nerves also pass on their messages to the neurons of the spinal cord and are able to change the volume of the signal being produced. The flow of messages up and down the spinal cord is happening all the time. When a nociceptive message reaches the spinal cord, local and descending nerves can adjust the volume.
There are several regions of the brain that contain neurons that send these messages down to the spinal cord. Some of these regions are closely linked to other regions of the brain that are involved with emotions such as fear and anxiety, and with attention. The pain perception that our brain generates in response to nociceptive messages is therefore affected by the emotional effect the pain sensation created. At the same time, the emotional response itself is able to control the nociceptive message reaching the spinal cord through these descending nerves. How much pain we are in is therefore controlled in part by our emotional and attentional state.
Pain and emotion
The emotional component of pain is very strong. Pain is something to avoid and the possibility or anticipation of pain produces fear and anxiety. This connection between pain and the fear of pain is so strong that the emotional context in which we feel pain can moderate its intensity. In other words, fear of pain can make things more painful than if the fear was not present. And, a lack of fear tends to reduce pain. To illustrate this, here are some examples:
A toddler falls off a slide and immediately gets up seemingly fine. Her father rushes over and notices a small cut that is bleeding. When the child notices the blood, she starts crying in pain even though there was no pain before.
A ten-year-old boy feels a sharp pain in his toe and looks down. He realises that he’s stubbed his toe against the leg of a table. Understanding the source of his pain, and knowing that it is temporary and not serious, it begins to subside.
A ten-year-old boy feels a sharp pain in his toe and looks down, but there is no table leg or anything else around that he might have stubbed his toe on. The cause of his pain is unknown and so, perhaps, the pain increases as his anxiety over the incident increases: if I didn’t stub it, why is it so painful? Maybe I have some terrible condition.
Mind and body
If the above examples illustrate how complex and sometimes bizarre pain can be, they also indicate that it is probably best to think of pain in both emotional and physical terms at the same time. Especially when trying to assess and relieve pain, it is probably best not to think of the actual physical injury alone. The fear, anxiety, and worry that a child experiences during a painful episode are all part of being in pain. In a practical sense, the emotional and physical responses to pain are one and the same because the physical part of pain is always felt within an emotional and intellectual context.
Through years of research we have learned to view pain from both the mind and body, and have come to understand many aspects of pain. But we should remember there is still much about pain we do not yet fully understand.