How do we Feel Pain

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Pain is a complex, highly individual phenomenon that involves both physical and mental elements.

Physical pain may top the list of challenging human experiences. As the body’s primary alarm system, pain is a vital survival mechanism. But even the thought of pain fills most of us with dread. And although pain is a nearly universal phenomenon, no two people perceive and suffer pain in quite the same way. An injury that causes one person to writhe in agony may cause only moderate discomfort in another. Even the same person may react very differently to the same kind of pain at different times. Moreover, a person may suffer a serious injury and feel no pain at all for a time or, contrarily, feel serious pain without any apparent sign of illness or injury. The explanation lies in pain’s complex nature as both a physiological (biological) and psychological (mental) phenomenon.

At its most basic, pain is an unpleasant physical sensation ranging from discomfort and distress to intolerable agony resulting from the stimulation of specialized nerve endings in the body. In general, the greater the stimulation—that is, the more severe an injury or the more serious an illness—the more intense our pain will be. But the intensity of our pain and our tolerance for it also depend on our psychological makeup—our emotional state, ability to cope with difficulties, general outlook on life, and previous experiences. External factors, such as where we are when we experience pain, play a role as well. Even the culture we live in may affect our response.

In addition to warning us of injury and disease, pain helps us locate the source of the problem. It also helps prevent additional injury. For example, pain causes us to drop a hot plate before a minor burn becomes a major burn. Pain also plays a critical role in the healing process by limiting our use of an injured area, such as a sprained ankle, to encourage rest and recuperation.

Pain means suffering

But these benefits are usually not our primary focus when we consider pain. After all, pain causes both physical and emotional suffering. Short-term pain may hinder recuperation by interfering with sleep and appetite and may cause serious medical complications. Long-term pain may generate feelings of frustration, anxiety, helplessness, and depression. Sometimes, pain serves only to torment us, worsening an already difficult situation, such as terminal cancer. Pain may create spiritual anxiety by reminding us of our mortality.

As a result, much of the art and science of medicine is directed at the relief of pain. Pain is probably the most common and compelling reason people seek medical attention. Almost half of all Americans seek treatment for pain each year.

The four components of pain

Pain is the result of a chain reaction that involves all parts of the nervous system, including the brain. Pain has four components: stimulus, perception, suffering, and pain behavior. To explain how this complicated communication network functions, let’s follow what happens when we touch a hot pan.

The stimulus is the trigger that sets the pain process in motion—in this case, the heat of the pan. The perception, or recognition, of that stimulus is the work of specialized touch receptors called nociceptors. Nociceptors appear as widely branching, bushy clusters of free-standing nerve endings that overlap each other extensively. They exist by the hundreds of millions in the skin, bones, muscles, joints, and blood vessels. Internal organs have relatively few.

The name nociceptor comes from a Latin word meaning destructive or physically harmful. Like ordinary touch receptors, nociceptors register information about our surroundings, such as cold, heat, and pressure. Unlike ordinary touch receptors, which respond to low-level stimuli, nociceptors respond only to damaging or potentially damaging stimuli. These include trauma, such as cuts in the skin and muscles; abnormal stretching; dangerously high levels of heat or cold; and high levels of chemicals released by the body when tissue is inflamed. For example, the nociceptors in your knee would ignore a gentle tapping. But they would fire away if you banged your knee on the corner of a desk.

Both nociceptors and ordinary touch receptors are distributed unevenly over the body. That is why some parts of the body, such as the lips, fingertips, and soles of the feet feel pain more readily than other areas, such as the back of the shoulders.

The pain message

When your fingers touch a hot pan, the nociceptors in the skin register the extreme heat and translate the heat energy into electrical signals called impulses. These signals create the sensations we identify as pain or suffering, the third component in the pain process. (We do not actually “feel” the pain, however, until the pain message is recognized by the brain.)

In general, the intensity of our pain depends on the strength and duration of the stimulus as well as the number of nociceptors stimulated. For example, the hotter the pan we have touched and the longer we have touched it, the more numerous and longer-lasting the pain impulses transmitted by the nociceptors will be. Moreover, the larger the area affected—in this case, the larger the area of skin burned—the greater number of nociceptors broadcasting pain signals. But not all stimuli are equal. For example, a pressure stimulus must be four to five times more powerful than a heat stimulus to produce the same level of pain.

Pain’s relay race

From the nociceptors, pain impulses jump to nearby sensory neurons. We have more than 15 billion of these nerve cells that keep the body informed about conditions around us. The sensory neurons, which form pathways through the body to the central nervous system (the spinal cord and brain), function like runners in a relay race. Their branched receiving ends, called dendrites, collect information from the nociceptors, while their tubelike transmitting ends, called axons, pass the information along to neighboring dendrites. Axons may split into branches at their end. A single neuron may have enough axon branches to make contact with as many as 1,000 other neurons. The structures we commonly call nerves are actually bundles of axons stacked next to each other like logs.

Sensory neurons are separated from each other by microscopic spaces called synapses. Communication across synapses is controlled by chemicals called neurotransmitters, which are stored in tiny sacs in axons. The arrival of electrical impulses at a neuron’s dendrites changes the chemical environment of the neuron, causing it to fire (become activated). As the impulses travel along that neuron’s axon, they cause the sacs to burst open, releasing neurotransmitters that pass across synapses to change the chemical environment at the tip of nearby dendrites. This causes these neurons to fire electrically and pass the impulses along their axons. Then the process starts all over again.

Neurotransmitters and pain

Neurotransmitters also affect the intensity of the pain we feel. For example, some neurotransmitters, such as bradykinin and prostaglandin, which are released by injured or inflamed tissue, increase the sensitivity of nociceptors. As a result, we feel much more pain holding a burned finger several inches from a flame than we would holding an uninjured finger there.

Several other neurotransmitters activate the fight-or-flight response, an automatic revving up of body function we experience when we are frightened or face physical danger. Our heart rate shoots up, our breathing quickens, our blood pressure rises, and the sugar level in our blood increases. These changes provide the energy to “fight” or “take flight.”

In contrast, other neurotransmitters dampen pain. For instance, pain-suppressing chemicals called endogenous opioids reduce the sensitivity of sensory nerves, partially blocking the pain route. Some research suggests that people who are particularly sensitive to pain may produce relatively lower levels of endogenous opioids or have fewer brain receptors for these chemicals.

Pain superhighways

Pain impulses travel mainly along two pathways in the nerve communication system. Sensory nerves called A-delta fibers form a fast route, carrying impulses to the spinal cord and brain at a speed of about 40 miles per hour. These nerves are covered with a sheath of myelin, a white, fatty substance that speeds the transmission of impulses. A-delta fibers produce sensations of sharp, immediate pain, such as you would experience when you first touch a hot pan.

In contrast, sensory nerves known as C fibers form a slow route, along which impulses travel at a speed of about 3 miles per hour. C fibers, which do not have a myelin sheath, relay sensations of dull and aching pain that follow an initial shock.

Pain processors

Special groups of nerves in the spinal cord act as a processing center for the barrage of pain impulses flowing from the sensory nerves. The spinal nerves give some messages priority treatment and speed them on to the brain. The spinal nerves also may lower the intensity of some impulses, combine impulses, or cancel some messages altogether. Even before the brain recognizes the pain message from our burned fingers, the processing station in the spinal cord may order a reflex action, an automatic and involuntary response to a stimulus. In the case of the hot pan, spinal nerves would “read” the pain impulses and instantaneously signal motor nerves (nerves that control our muscles) in the fingers to jerk back.

Pain impulses that make it through the spinal cord proceed to two areas of the brain. The first is the thalamus, a structure at the base of the brain that serves as the main relay center for sensory information. The thalamus recognizes the impulses as a pain message and routes them to appropriate areas of the cerebral cortex, an area of the brain that controls thought and learning. The cerebral cortex identifies the source of the pain and plays a major role in governing the steps we take to deal with the pain.

Pain’s psychological connection

Nociceptors, nerves, and neurotransmitters provide only a partial explanation of why and how we feel pain. Psychological factors play an equal role. These factors include our emotional state, memories, our mental health, how distracted we are at the time we experience the pain, and cultural influences.

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In fact, psychological factors may actually play a dominant role in determining our pain threshold and pain tolerance. Pain threshold is the point at which a sensation—such as warmth or pressure—becomes painful. Pain tolerance is the intensity of pain a person can endure before being compelled to run away or take other action to escape the pain. Pain threshold and pain tolerance are nearly as individual as fingerprints.

The limbic system

Pain’s emotional components result from input mainly from the limbic system, a group of brain structures, including the thalamus, that regulate emotion. The limbic system determines how unpleasant pain is and how strongly we want it to end. In the limbic system, feelings of anger, anxiety, stress, loneliness, or sadness intensify pain impulses.

In contrast, feelings of happiness, relaxation, and peacefulness dampen pain impulses. As a result, when the impulses are processed by the cerebral cortex, a conscious experience—that is, the recognition and identification of pain—becomes an emotional experience as well. The limbic system also deals with memory. Unhappy or frightening memories connected with a sensation can increase feelings of pain as well.

As a result, after touching a hot pan, you would probably feel more pain if you had previously suffered a serious burn. Your pain threshold also might be lower if you had a bad cold and were feeling tired or if you were cooking dinner for your spouse’s family for the first time. Your pain probably would be greater if you had been fired recently from your job or had just put your dog to sleep after 15 years of companionship.

Mindset and pain

Mental attitude also plays a central role in pain. Think about getting an injection. If you dread shots, an injection will almost certainly hurt more than it would if you thought of a shot as a minor annoyance. Anxiety, such as that triggered by not knowing the source of pain, may intensify suffering. In contrast, many people believe that mental preparation for a painful experience, such as childbirth, can reduce pain levels.

Closely related to mental attitude are cultural beliefs. For example, your pain threshold might be higher if you had been brought up in a culture that believed that enduring pain without complaint was a sign of personal or moral strength.

Pain’s psychological components may play their greatest role in determining the fourth component of pain—pain behavior. Pain behavior is the action, both physical and mental, we take in reaction to pain. It may be as simple as a brief yell or muttered curse or as complex as depression and a withdrawal from life.

The gate control theory of pain

At the same time pain messages travel through the nervous system, the nervous system takes steps to limit their effect. According to the gate control theory of pain, proposed in 1965 by psychologist Ronald Melzack and neuroanatomist Patrick D. Wall, then of the Massachusetts Institute of Technology in Cambridge, the nervous system has built-in mechanisms, described as gates, to suppress pain. When the gates are open, pain messages travel freely through the nervous system. When they are closed, however, at least part of the pain message is blocked.

The gates may close for both physiological and psychological reasons. For example, the brain may release pain-suppressing endogenous opioids that desensitize sensory neurons. The brain also closes the gates by releasing neurotransmitters that, in turn, stimulate spinal neurons to release pain-suppressing chemicals.

Competition from other sensations may lessen pain impulses. It works like this: You hit your knee on a desk and you feel pain. You rub your knee. This causes sensory neurons in the knee that transmit messages of touch and pressure to release neurotransmitters that reduce the sensitivity of neurons in the spinal cord. Your knee starts to feel better. Other stimuli such as ice and heat, electricity, and acupuncture also may suppress the relay of pain messages.

Blocking pain

One powerful psychological mechanism for closing the pain gates is a state of high excitement or arousal. For example, a football player in the last few moments of a hotly contested game might not notice that he has dislocated his shoulder. After the game, however, he feels the pain and recognizes that he has been injured. Studies of soldiers wounded in battle during World War II (1939-1945) reported that some of the injured were unaware they had suffered serious wounds until after the fighting had died down.

Another psychological blocking mechanism is a positive state of mind. Studies of wounded soldiers in World War II also revealed that the battle survivors required less pain medication than civilians who had suffered similar injuries off the battlefield. The soldiers’ feelings of relief at knowing they would be shipped home or at least sent out of harm’s way apparently subdued their pain response.

A third blocking mechanism is distraction. Your pain from burned fingers might be less bothersome, for example, if you were fantasizing about spending the inheritance check you had just picked up. Or your pain might seem minor if you burned yourself while your attention was focused on an uproarious joke or an engrossing news report. On the other hand, the ache in your fingers might worsen once you were in bed at night and you had no distractions.

Acute pain

We experience two main types of pain: acute and chronic. Acute pain is sharp and may be severe—the pain of burning your fingers, suffering a heart attack, giving birth, or recovering from surgery. Acute pain sounds a warning that something is amiss and acts as a protective mechanism, making a person want to fight or to escape the cause of the pain.

Acute pain can last from several seconds to several hours. Severe untreated acute pain can cause serious medical problems, including increased blood pressure and heart rate. Generally, acute pain subsides as healing occurs.

Chronic pain

Chronic pain is a long-lasting pain that may be a symptom of an injury or disease or may have no apparent physical cause. An estimated one-fifth of Americans suffer from chronic pain. For many, the pain is so severe that they are unable to work, attend school, or socialize. In some cases, chronic pain escalates from being a symptom of an injury or disease to becoming a disease itself.

The most common type of chronic pain is lower-back pain. Other common types include arthritis, cancer, nerve damage, spinal cord injury, and burns. Chronic pain most often affects middle-aged and older people.

A misunderstood ailment

Despite intense research, chronic pain is still poorly understood by the medical community. In some cases, the pain begins with an injury or disease but continues even after the injured tissue seems to have healed or the disease seems to have been cured. In many cases, low-level inflammation or muscle spasms may play a role. In some cases, however, chronic pain seems unrelated to any physical ailment at all.

Many pain specialists believe chronic pain occurs because of a breakdown in the normal system of checks and balances that keep the nervous system functioning properly. For example, for reasons that remain unclear, nerves that have been injured may continue to transmit pain messages even after they apparently have healed. In some cases, severe and unrelenting pain signals may so damage pain-processing centers in the spinal cord that the centers become incapable of dampening or suppressing pain signals as they normally do.

People with chronic pain are deprived of the peace of mind that comes with knowing their pain will end. And because peace of mind is important to the healing process, chronic pain can convert a person into an invalid. Physical deterioration, such as a loss of muscle strength, often accompanies chronic pain. Some people may need assistance with walking, climbing stairs, dressing, or other activities they used to take for granted. The never-ending pain signals also may depress hunger centers in the brain, leading to loss of appetite and subsequent poor nutrition.

Chronic pain’s psychological cost

Chronic pain often produces serious psychological damage and alters the sufferer’s personality. Although some people show amazing inner strength when faced with chronic pain, others with chronic pain often feel depressed, bitter, anxious, or helpless. They may find their need for physical assistance humiliating. They may become angry because doctors cannot ease their pain. They may turn on family members who, they believe, are not supportive or understanding. They even may feel angry at themselves because they cannot get better on their own.

People with chronic pain may withdraw from life by giving up pleasurable activities, such as sports, hobbies, or socializing with friends. They may refrain from going out in public or visiting with family members. In their isolation, they may believe they are the only people in the world with this problem. They may worry that their pain is a sign of mental instability.

Chronic pain also may bring on feelings of guilt because sufferers are not interacting with their family as they would like or not performing well at work. This guilt, in turn, may lead to feelings of low self-esteem. People who suffer chronic pain in the absence of any apparent disease or injury become especially demoralized.

All these negative thoughts and feelings may help create a vicious cycle of worsening pain and increasing unhappiness called chronic pain syndrome. Although there is no cure for chronic pain syndrome, people with this condition can take steps to help close the pain gates and reduce their psychological suffering. With personal effort and help from pain experts, they might be able to develop their communications skills so that they can obtain the help they need from family, friends, and medical personnel. They also might sharpen their problem-solving abilities so they can continue to enjoy life despite their pain.

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