Pain, like everything else that you experience, sight, sound, smell, taste, thought, and emotion, is the result of activity in your brain. To the classic philosophical question “If a tree falls in the forest and there is no one there to hear it, does it make a sound?” a neuroscientist would unhesitatingly answer “No”. Sound is what a person (or animal) experiences when they are exposed to a certain kind of energy (sound waves). When a tree falls in the forest, it generates sound waves, but sound is produced in the mind of the hearer. Similarly, pain is what you experience when a particular pattern of electrical activity is produced in a particular area of your brain. Pain is your mind’s interpretation of what different sensors are telling you about the status of your body. In general, pain indicates an assessment that your body has been injured, or is about to be injured (from something like touching a hot stove), and that you should probably stop doing whatever it is that you are doing, in order to avoid further injury.
Pain may seem like all one thing, but scientists actually recognize at least 4 distinct kinds of pain with different origins and different potential targets for control: Acute, Persistent, Neuropathic, and Emotional or Psychogenic Pain. Acute pain is the simplest and easiest to understand. There is one type of nerve cell (neuron) which specializes in responding to extreme stimulation of the type commonly associated with tissue injury. These pain neurons or nociceptors are usually pretty quiet. In the noisy meeting house of the mind, nociceptors may occasionally burp or clear their throats but they rarely raise their voices above the tumult of general conversations. However, if nociceptors are stretched, crushed, torn, burned, or otherwise injured or distorted, they begin screaming loudly and continuously and drown out their more polite neighbors. If the electrical signals produced by nociceptors are converted into sound, they sound like machine guns when the nerve cells are damaged, but they remain pretty quiet the rest of the time. Nociceptors do not respond to the kinds of moderate warm and cold or light touch or pressure stimuli that other neurons concern themselves with, and they only occasionally fire off a few random rounds to remind everyone that they are still there if nothing very extreme is happening to them. It is the prolonged rapid firing of these nerve impulses which occurs when you prick your finger on a thorn, stub your toe, or pass a kidney stone that you experience as acute pain. When the distortion of the nerve is relieved and damage to the nociceptor has been repaired, (usually in a matter of minutes, or at most a few hours), the nerve cell becomes quite again. However, if the damage to the surrounding tissue is more than very minor, or if the injury happens to be from something like a bee sting, inflammatory chemicals produced by the damaged cells and the immune system’s repair crew or the toxins in the bee sting, will cause the pain neurons to continue to fire, (albeit at a somewhat slower rate). This ongoing chemically-induced firing of the pain neurons causes the sensation of pain to persist after the nerve has repaired the immediate damage to itself. If the inflammation is caused by something like arthritis, there may never have been any significant nerve injury to begin with, only the ongoing chemical signal irritating the pain neurons and producing a persistent sensation of pain. Alternatively, the pain neurons may be damaged in a way that does not allow them to repair themselves properly or causes other neurons to begin acting like pain neurons. Also some rare genetic diseases can cause pain sensations in the absence of nerve damage. This kind of failed repair situation causes neuropathic pain. The so-called “phantom limb pain” in amputees, diabetic nerve pain, and the burning sensation caused by the genetic condition called erythromyalgia all represent versions of this neuropathic pain. The final type of pain, emotional pain is caused by a kind of overflow of information from the cross-connections between the emotional centers of your brain and the parts of your brain involved in processing and interpreting sensory information. People tend to feel depressed when they are in pain, but the signals go both ways. People who are very depressed also tend to feel a lot of aches and pains and people who are anxious or depressed tend to feel pain more intensely.
The different ways that pain can be generated offer different targets for pain control. Ice can slow down the nerve firing and may be sufficient to prevent or treat acute pain from ear piercing or removing a thorn from your finger. Novocain or other local nerve blockers can prevent pain signals (as well as touch and other sensations) from being carried to the brain and are often sufficient for minor surgeries such as a tooth pulling which will heal quickly with relatively little ongoing inflammation. Alcohol intoxication also slows and partially blocks nerve conduction. When no better pain treatments were available, having a cowboy swallow a couple of shots of whisky before digging out a bullet was better than nothing. Non-steroidal anti-inflammatory drugs (NSAIDs) such as aspirin, ibuprofen, and Aleve and corticosteroids such as prednisone can reduce persistent pain due to inflammation. Drugs like Tylenol do not actually block inflammation, but it blocks one of the key pathways by which inflammatory molecules cause your pain neurons to fire (Cox inhibitors) and it also reduce pain by blocking the inflammatory signals stimulating your nociceptors. Medications which block inflammation or inflammation-mediated signaling can be useful for controlling persistent pain, but there is a limit to how much of these drugs you can take, and for how long, before your kidneys fail or your immune system collapses. Opioid drugs like codeine, Percocet, Vicodin, Oxycodone, and morphine act at the level of the brain where the pain signal is being analyzed and interpreted. When opioid drugs are taken, pain signals get sent up to the brain, but the relevant committees keep tabling any discussion on the subject of whether there is any pain occurring out in your body. With opioid drugs, pain is not completely numbed, you are kind of aware that something is wrong with your body, but it just doesn’t feel like something that you have to worry about. Opioids work quite well on both acute and persistent pain but there are problems with their use. In addition to being addictive and requiring more and more of the drug over time to keep having the desired effect, opioids have side effects like constipation at low doses and suppression of breathing at higher doses. Much of the need to keep patients in a hospital after a surgery is due to the need to manage the potentially deadly side effects of the pain medications necessary to make surgery tolerable. Opioids also do not work on neuropathic pain. Although some of the same chemicals involved in inflammatory pain are also involved in neuropathic pain, anti-inflammatory medications also have little or no effect on neuropathic pain. In fact, one of the ways that doctors use to determine that a patient’s pain is neuropathic in origin is that NSAID pain medications and opioids are generally ineffective. Neuropathic pain is usually treated with antiepileptic drugs which quiet down nerve firing in a way that your brain no longer registers the sensation as pain. Getting the drug levels for antiepileptic medications just right to block pain sensation without messing up everything else however can be tricky and it may take several months of trial and error to get it right. Furthermore, even when antiepileptic medications do work, they often do not completely relieve all of the pain. The pain of a broken heart is not likely to be helped by any of these other treatments, but anti-depressants can help. Anti-depressants may also “take the edge off” of the pain if someone is going through a stressful situation at the same time that they are suffering from a painful condition. And let’s face it, dealing with a chronically painful condition can be pretty stressful and depressing. Reducing the portion of the pain that is caused by the emotional overflow may reduce the dosages of more conventional pain medications needed for pain relief. In addition, antidepressants have also been found to partially block the cellular machinery necessary for neurons to fire in a way that you experience as pain. So antidepressants can help with pain control in two different ways
There is one more aspect of pain perception and pain control which I have not yet talked about. Up until now I have only talked about the pain signal getting to your brain. In order to be experienced as pain however, the signal also has to get form the pain analysis centers to your consciousness. This conscious attention to the pain signal represents the final common pathway for all pain perception. There is a lot going on in your body at any one time and your conscious mind cannot pay attention to all of it. Usually pain gets a pretty high priority for attention, but there are exceptions. Pain is usually a signal telling you that you have been injured, or are about to be, and that you should stop doing whatever it is you are doing and attend to your condition. However, if you are fleeing from a lion or escaping a burning building, you wouldn’t want a little thing like a broken ankle to slow you down. If you experience a relatively serious injury, you will often not feel the pain for several minutes or even for as long as half an hour after the injury. This blocking out of pain after a serious injury can create problems for first responders dealing with accidents involving many casualties, as some of the wounded may start wandering around, unaware of the extent of their injuries until they collapse. Pain may also be blocked from your consciousness if you need to perform a task which will almost certainly result in your being injured. Parents have been known to be able to lift extraordinarily heavy objects in order to rescue their children from danger, not because they have suddenly become possessed of super strength, but because they have shut out the pain of the torn ligaments and bone fractures which would normally have caused them to stop lifting. There are many other less dramatic examples of pain perception being influenced by other thought processes. If you scald your mouth with a cup of hot coffee and then slam the car door on your finger, you will suddenly no longer feel the burn in your mouth because the greater source of pain will seize all of your conscious attention. Pain that is perceived as resulting from something beneficial will also generally be perceived as less intense than pain coming from something that only causes harm. Childbirth that results in a healthy and much wanted child is generally perceived as less painful than if the child has serious health problems or will be given up for adoption. A physical therapy regime which the patient believes will rapidly restore their independence will generally result in fewer pain complaints than if the patient expects only to be going back to a nursing home. Likewise, many people report feeling better while waiting in the doctor’s office for their appointments. This seems to be because they have handed off the problem to their doctors. When you first make a doctor’s appointment, you are often worried about your pain and whether or not it indicates that something is seriously wrong. You are focusing a lot of your attention on the pain. When you no longer feel that you have to give the matter much attention, because your doctor will be working on the problem for you, the pain got pushed to the back of your mind. Placebos work primarily in this manner. Part of your brain convinces the rest of your brain that any pain messages that it receives have been sent in error and should simply be ignored. Deliberate selective inattention to pain represents the final strategy that can be employed for pain management. Using biofeedback techniques or visualization exercises, some people are able to isolate their pain sensations from their consciousness or think about them as so unimportant that they are beneath notice. These kinds of strategies usually require quite a bit of practice to master and some people will have greater skills in this than others, just like any other skill. There are also limits to what this sort of mental control can be accomplished. A doctor probably would not want to perform surgery on a patient who was trying to use mental techniques alone to block out the pain of his knife, but a patient who was trying to minimize the need to take a lot of pain medications for a chronic condition might find these techniques useful.