BIOLOGICAL ASPECTS OF PAIN
Richard W. Hanson, Ph.D.
We believe that it is important for persons with chronic pain to have an accurate and realistic understanding of some of the biological aspects of pain. The following, non-technical information is intended to give you a greater appreciation for the biological complexities regarding the way our bodies experience pain.
Role of the Nervous System.
In most cases, painful sensations arise from injured or damaged body tissues. Sensitive nerve endings pick up the painful sensations and then carry these pain messages along nerves to the spinal cord. From the spinal cord they are transmitted upward and ultimately registered by various parts of the brain. In other words, the nervous system plays an essential role in the experience of pain.
The most important body structure involved in the experience of pain is the brain. At least five parts of the brain are involved in the experience of pain. These may be referred to as the sensory, emotional, memory, evaluative, and action centers. The sensory centers give us information as to the location, quality, and intensity of pain sensations. The emotional centers serve to motivate us to act in ways to minimize the pain or seek relief. The memory centers enable us to compare our immediate experience of pain with previous experiences (memories) of pain. The memory centers also work together with the evaluative centers. Based upon our past experiences with pain, we automatically make judgments (evaluations) about the cause or meaning of the pain, as well as decisions regarding what to do about the pain. Finally, the action centers become activated as we seek to do something about the pain. These action centers include both automatic reflex actions, as well as actions based upon our conscious evaluation of the pain and decisions regarding what to do about it.
Relationship between Physical Injury and the Experience of Pain.
Normally, what the mind/brain tells us is a very accurate representation of what is happening in the injured part of our bodies. If you accidentally stub your toe, your brain immediately tells you that your toe is hurt. In other words, our nervous system enables us to quickly respond to pain as a signal warning us of injury. This all assumes of course that your nervous system is fully intact and operational. If you have some kind of nerve damage, it may be possible to stub your toe and not feel any pain sensations in the brain. In other words, it is possible to have an injury and no pain. This is one situation in which the relationship between injury and pain do not coincide.
Injury but No Pain
There are several situations in which it is possible to be injured and not experience pain. First of all, as just mentioned, it is possible to have an injury and not experience pain because of nerve damage affecting the part of the body that is injured. Second, there are some tissues in our bodies that are relatively insensitive to injury. You may be surprised to know that most of the brain tissue itself is insensitive to pain.
Third, there is evidence that people with intact nervous systems can have injuries but no pain. For example, recent studies have been done on large groups of normal people (those who are not reporting any pain problems) using sophisticated imaging tests such as the MRI. A certain percentage of these so-called "normal" persons have been found to have herniated discs in their spine. Despite clear medical evidence of injury, these persons do not complain of back pain. Furthermore, autopsy studies of persons who have died for one reason or another have found evidence of severe spinal degeneration, even though these persons did not complain of back pain prior to their deaths. In other words, some people show clear medical evidence of injury, but for some reason that is not clearly understood, do not report pain.
Fourth, it is possible for persons to have injuries and not experience pain because their minds are involved in something else that is considered more important at the moment. For example, some athletes sustain injuries during sporting events, but do not know they are injured until after the game is over.
Finally, if an injured person is unconscious there can be no experience of pain. One must be awake and aware in order to experience pain.
Pain but No Injury
It is also possible for one to experience pain in the absence of injury. In other words, the brain tells us that there is pain in a particular part of the body, even though that part of the body is not actually injured. The most obvious and dramatic example of this is phantom limb pain. Some persons who have had a limb amputated experience pain in a body part that is no longer there. For example, a person may have the lower part of his or her leg amputated; however, the brain says that pain is coming from the foot.
Another situation in which pain may occur in the absence of injury, is when there is damage to the brain or other nerve structures. For example, some people develop what is referred to as a "thalamic pain syndrome" following a stroke. The brain experiences pain sensations on one side of the body, even though that side of the body is not actually injured.
Much more troublesome, and also more common, are situations in which the brain experiences pain in the absence of any known nerve or tissue injury. This occurs most often following an actual injury which has since healed. In other words, the person experienced acute pain following a painful physical injury. However, the brain continues to experience pain even though the original injury has long since fully healed. This is a very puzzling situation and is not fully understood. This situation is also responsible for many misunderstandings between patients and doctors. Some times people with honest reports of pain show absolutely no evidence of any physical problems on the medical tests. Does this mean that they are just imagining their pain? The answer to that question depends on what you mean by imagining. People with phantom limb pain are in one sense imagining their pain. However, does this make their pain any less real? The important point is that so-called imagined pain is every bit as real as pain experienced by persons with verified physical injuries.
Active Vs. Passive Brain
A large part of the apparent puzzle of pain results from a failure to understand the active role played by the brain in pain perception. Many incorrectly think that the brain plays a relatively passive role in the experience of pain. According to this view, the brain is simply a passive receiver which picks up pain signals that are generated and transmitted directly from the site of the injury. In other words, the brain is thought to work like something like a telephone or radio receiver which accurately reproduces whatever messages are sent to it. Thus, it is assumed that the greater the physical injury, the more pain messages are transmitted to and received by the brain.
In reality, the brain is not simply a passive receiver, nor is the spinal cord a passive conveyor of pain messages originating in some injured part of the body. Rather, both of these central nervous system structures play an active role in modifying the pain messages which are ultimately registered in the brain. Furthermore, the brain serves as both a receiver and an active transmitter. It can transmit signals which block the experience of pain. It can also significantly magnify the experience of pain out of proportion to the actual injury or it can even generate signals which lead a person to experience pain in a part of the body that is not actually injured.
Role of the Spinal Gate
A number of years ago, pain researchers discovered the existence of a complex gate mechanism within the spinal cord which determines the amount of pain signals that actually reach the brain. If the spinal gate is wide open, all available pain messages reach the brain, whereas, if the gate is completely closed, no pain messages reach the brain. In reality, the gate is neither completely open nor completely shut. Rather, it works like a control valve which determines the amount (intensity) of pain signals which actually get through.
This gate mechanism is affected by a number of factors, the most important of which concerns the pattern of nerve impulses which reach the spinal cord from the rest of the body, and nerve impulses generated by the brain itself. Sometimes the pattern of nerve impulses reaching the spinal gate can be altered by other forms of physical stimulation such as rubbing and massaging a painful area, applying electrical stimulation (e.g., TENS), applying extremes of temperature (e.g., either heat or cold), or sticking needles into the body (e.g., acupuncture, trigger point injections). It is also important to realize that certain mental activities, taking place in the brain, can also result in nerve stimulation which can close the spinal gate.
Role of the Endorphins
The endorphins refer to a class of chemicals, which are produced in the brain, and serve an important role in the experience of pain. These chemicals act to reduce pain in a way that is very similar to morphine and other opiates. In fact, the very word "endorphin" means an endogenous or internally produced, morphine-like substance. These endorphins act on specific receptor sites on special nerve cells deep within the brain. These receptor sites function like a lock which, when turned, generate nerve impulses that block pain. The endorphins act as a key which turns the lock. Morphine and all the other opiates have a similar chemical structure and can also serve as keys and turn the lock.
What this means is that the more endorphins that are produced by the brain, the less pain one feels. Of course if the locks (receptor sites) are already filled by the opioids which you are taking, there may not be any need for endorphin.
Following are some situations and conditions which are thought to stimulate endorphin production.
1) Emergency situations requiring the person to perform physical actions despite pain and injury.
2) Intense physical activity. Moderate physical activity over an extended period of time can also increase the supply of endorphin.
3) Positive beliefs and expectations (e.g., the "placebo effect").
4) Positive emotional states such as happiness, joy, laughter, and love.
What this means is that psychological factors, including positive mental attitudes and positive emotional states can have a beneficial effect on your brain chemistry, which in turn can reduce your experience of pain.
The Role of Stress
The biology of pain is not complete without also considering the role of stress. Even though the relationships between pain and stress are discussed in greater detail below, it is important to understand that stress in and of itself activates structures in the brain which are also involved in the perception of pain. These brain structures involved in stress also trigger powerful shifts in hormonal activities, alter the body's immune system, and activate a part of the nervous system known as the autonomic nervous system. The autonomic nervous system has connections to all internal organ systems in the body.
Although stress can be either physical or psychological in nature, the effects on the brain, secretion of hormones, and autonomic nervous system are essentially the same. Whenever there is an acute injury, there is obviously physical stress. However, when pain becomes chronic, psychological stressors often mount up and can directly contribute to the amount of pain that is experienced.