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Anaesthesia => Nervous System => Pain
Pain
INTRODUCTION
Pain, unpleasant sensory and emotional experience caused by real or potential injury or damage to the body or described in terms of such damage. Scientists believe that pain evolved in the animal kingdom as a valuable three-part warning system. First, it warns of injury. Second, pain protects against further injury by causing a reflexive withdrawal from the source of injury. Finally, pain leads to a period of reduced activity, enabling injuries to heal more efficiently.
Pain is difficult to measure in humans because it has an emotional, or psychological component as well as a physical component. Some people express extreme discomfort from relatively small injuries, while others show little or no pain even after suffering severe injury. Sometimes pain is present even though no injury is apparent at all, or pain lingers long after an injury appears to have healed.
PHYSIOLOGY OF PAIN
The signals that warn the body of tissue damage are transmitted through the nervous system. In this system, the basic unit is the nerve cell or neuron. A nerve cell is composed of three parts: a central cell body, a single major branching fiber called an axon, and a series of smaller branching fibers known as dendrites. Each nerve cell meets other nerve cells at certain points on the axons and dendrites, forming a dense network of interconnected nerve fibers that transmit sensory information about touch, pressure, or warmth, as well as pain.
Sensory information is transmitted from the different parts of the body to the brain via the spinal cord, which is a complex set of nerves that extends from the brain down along the back, protected by the bones of the spine. About as wide as a finger, the spinal cord is like a cable packed with many bundles of wires. The bundles are nerve pathways for transmitting information. But the spinal cord is more than just a message transmitter, it is also an extension of the brain. It contains neurons that process incoming sensory information, and generates messages to be sent back down to cells in other parts of the body.
Information being transmitted between and within the brain and spinal cord travels through the nervous system using both chemical and electrical mechanisms. A message-carrying impulse travels from one end of a nerve cell to another by means of an electric signal. When the electric signal reaches the terminal end of a nerve cell, a gap called a synapse prevents the electric signal from crossing to the next cell. The electric signal triggers the cell to release chemicals called neurotransmitters, which float across the synapse to the neighboring nerve cell. These neurotransmitters fit into specialized receptors found on the adjacent nerve cell, much as a key fits into a lock, generating an electric impulse in the neighboring cell. This new impulse travels to the end of the long cell, in turn triggering the release of neurotransmitters to carry the message across the next synapse. Not all neurotransmitters initiate a message in a neighboring nerve cell. Some specialize in preventing neighboring cells from generating an electrical signal, while others function as helpers, fascilitating the message's journey to the brain.
While most of the sensory nerves in the skin and other body tissues have special structures covering their nerve endings, those nerves that signal injury have free nerve endings. These simple nerve endings specialize in detecting noxious stimuli-a catchall term for injury-causing stimuli such as intense heat, extreme pressure, or sharp pricks or cuts. The nerve endings that detect pain are called nociceptors, and the process of transmitting pain signals when harmful stimulation occurs is called nociception. Several million nociceptors are interlaced through the tissues and organs of the body.
An injury triggers pain signals in two types of nociceptors, one with large, insulated axons known as A-delta fibers and one with small, uninsulated axons known as C fibers. The large A-delta fibers conduct signals quickly, and the smaller C fibers transmit information slowly. The difference in the functions of these two fibers becomes obvious to a person who stubs a toe. At first the injured person is aware of a sharp, flashing pain at the point of injury. Generated by the A-delta fibers, this short-lived pain intrudes upon the thoughts and perceptions occurring in the brain. Just as this first pain subsides, a second pain begins that is vague, throbbing, and persistent. This sensation is derived from the C fibers.
Pain information from the A-delta and C fibers travels through the spinal cord to the brain. When it receives the pain message, the spinal cord generates impulses that travel back down to muscles, which lead to a reflexive contraction that pulls the body away from the source of injury. Other reflexes may affect skin temperature, blood flow, sweating, and other changes.
While this reflex action is underway, the pain message continues up the spinal cord to relay centers in the brain. The sensory information is routed to many other parts of the brain, including the cortex, where thinking processes occur.
PSYCHOLOGY OF PAIN
When messages from pain-generating nerve endings finally reach higher centers in the brain, they are processed much like other forms of perception-that is, the sensory information is integrated with memories, expectations, emotions, and thoughts in order to form a complete perceptual experience. While it seems convenient to think of pain as a simple message that sounds an alarm in the brain, contemporary understanding stresses that pain is much more complicated. The emotional aspects of an injury may be more significant than the extent of the physical damage in determining the perceived intensity of pain.
Each person perceives pain a little differently, and as a result, each person also responds to painful stimulation differently. Pain research specialists have observed a wide variety of subtle variations in pain response. For instance, children are quicker to cry after a relatively minor injury than are adults. Learned cultural behaviors often dominate the way individuals express pain. Older children and young adults are often taught that crying, sometimes viewed as a sign of weakness, is inappropriate behavior, while younger children have no such understanding. Some people are more willing to express pain than others, but this does not mean they hurt more.
Broad cultural differences in pain responsiveness have also been documented. In some aboriginal societies, extreme tissue injury is often incurred willingly by people undergoing important rituals, and typically, pain is not expressed. Male Australian aborigines, for instance, traditionally celebrated passage into manhood with a ritual that involved circumcision, extensive scarring of the chest, and extraction of the two upper front teeth. The initiate was expected to show no reaction to the injury. It may be that the person undergoing the rite managed to suppress expressions of suffering, but it may also be that the individual was able to perceive less pain by making use of natural pain control mechanisms.
PAIN CONTROL
The body has many mechanisms that amplify or reduce pain. When cells are damaged, they release chemicals, such as bradykinins and prostaglandins. These chemicals intensify pain sensation both by making nociceptor nerve endings more sensitive and by causing inflammation around the damaged cells. Without these chemicals, nociceptors would cease transmitting pain information as soon as the source of injury was removed. Some scientists suspect that bradykinins activate nociceptors in the first place.
Other mechanisms reduce pain sensation by blocking, or inhibiting, the transmission of the pain message to the brain. To alter the pain sensation, the brain and spinal cord release specialized neurotransmitters called endorphins and enkephalins. These chemicals interfere with pain impulse transmission by occupying the nerve cell receptors required to send the impulse across the synapse. By making the pain impulse travel less efficiently, endorphins and enkephalins can significantly lessen the perception of pain. In extreme circumstances, they can even make severe injuries nearly painless. If an athlete is injured during the height of competition, or a soldier injured during combat, they may not realize they have been injured until after the stressful situation has ended. This happens because the brain produces abnormally high levels of endorphins or enkephalins in periods of intense stress or excitement.
In addition to the body's own mechanisms, humans have devised many different ways to manipulate the body's ability to control pain. Drugs that relieve pain, known as analgesics, usually interfere with pain impulse transmission in the nervous system. Narcotic analgesics, such as codeine, have chemical structures that are similar to the pain-blocking neurotransmitter endorphin. Other drugs that relieve pain alter the way damaged nerves transmit information. Nonsteroidal anti-inflammatory drugs, such as aspirin and ibuprofen, are analgesics that reduce pain by inhibiting the synthesis of prostaglandins, the body chemicals that intensify pain and cause inflammation.
Another way humans control pain is by injection of drugs that temporarily deaden the nerves that transmit pain signals. These drugs bring about anesthesia, a loss of sensation that renders the body completely or partially insensitive to pain, or even touch. Local anesthetics, such as procaine, deaden nerves in a particular area of the body but interfere little with other body functions. General anesthesia renders people unconscious so they do not feel pain at all. People who undergo general anesthesia also have no memory of events that occurred while they were unconscious.
Many people learn to control their pain with strategies that do not rely on drugs or surgery. Some people control the normally involuntary components of pain message transmission using a behavior modification technique called biofeedback. Acupuncture is widely used for pain relief. Many scientists now believe that this ancient medical procedure may trigger the release of endorphins and enkephalins, the body's own pain-inhibiting neurotransmitters. Others suspect that the pain-relieving attributes of acupuncture are due, in part, to a patient's expectation of relief. Although it is not completely understood, physicians and pain specialists have found that when a person suffering from pain expects that a particular procedure-in this case acupuncture-will make their pain subside, it actually does.
In cases where no treatment effectively relieves pain, doctors may recommend a surgical procedure in which pain-transmitting nerves in the brain or spinal cord are severed. Only a small fraction of pain sufferers need such surgical treatment. Another pain-relieving procedure involves placing electrical stimulators on the skin, nerves, spinal cord, or brain to reduce pain sensation.
Some injuries take a long time to heal, and even then, pain does not always completely subside. People suffering from this condition, known as chronic pain, may continue to experience debilitating pain for years, without having any apparent tissue damage. This may be the result of permanent damage to the nervous system. There is new evidence that the nerves in the spinal cord and brain can alter their connections after severe pain-that is, even after healing, the nervous system never returns to normal. Pain that subsides and then returns periodically, such as headaches or low back pain, also falls under the category of chronic pain. In their search for pain relief, many chronic pain sufferers become dependent on strong painkilling medicines, and they often fall into an endless cycle of pain, depression, and inactivity.
The complexity of human pain often requires a combination of pain therapies to achieve relief. Pain management specialists are usually medical doctors with specialized training in neurology, psychiatry, or surgery who have restricted their practice to the analysis and treatment of pain. Psychologists are usually important members of a pain management team. Many people are turning to alternative healthcare practitioners, such as those that specialize in acupuncture or chiropractic, for pain relief. Often, pain management specialists and practitioners of alternative pain therapies join forces in multidisciplinary pain clinics.
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