Pain

Pain is an unpleasant sensation which may be associated with actual or potential tissue damage and which may have physical and emotional components. According to the International Association for the Study of Pain (IASP), one should distinguish between pain and nociception. The term "pain" is a subjective experience that typically accompanies nociception, but can also arise without any stimulus, and thus includes the emotional response. Nociception, on the other hand, is a neurophysiological term and denotes specific activity in nerve pathways. It is the transmission mechanism for physiological pain, and does not describe psychological pain. These pathways transmit the nominally "painful" signals, though they are not always perceived as painful. Although pain can be associated with tissue damage or inflammation, this is often not the case.

Despite its unpleasantness, pain is a critical component of the body's defense system. It is part of a rapid warning relay instructing the motor neurons of the central nervous system to minimize detected physical harm.

Types of pain
Pain can be classified as acute or chronic.


 * Acute pain is defined as short-term pain or pain with an easily identifiable cause. Acute pain is the body's warning of present damage to tissue or disease. It is often fast and sharp followed by aching pain. Acute pain is centralized in one area before becoming somewhat spread out.  This type of pain responds well to medications.


 * Chronic pain was originally defined as pain that has lasted 6 months or longer. It is now defined as pain that persists longer than the normal course of time associated with a particular type of injury. This constant or intermittent pain has often outlived its purpose, as it does not help the body to prevent injury. It is often more difficult to treat than acute pain. Expert care is generally necessary to treat any pain that has become chronic. An anterior cingulectomy, neurosurgery that disconnects the anterior cingulate gyrus, can be used in extreme cases to treat chronic pain. Post-surgery the patient will still feel the sensation of pain, but not the accompanying emotion.

The experience of physiological pain can be grouped according to the source and related nociceptors (pain detecting neurons).


 * Cutaneous pain is caused by injury to the skin or superficial tissues. Cutaneous nociceptors terminate just below the skin, and due to the high concentration of nerve endings, produce a well-defined, localised pain of short duration. Examples of injuries that produce cutaneous pain include paper cuts, minor (first degree) burns and lacerations.


 * Somatic pain originates from ligaments, tendons, bones, blood vessels, and even nerves themselves. It is detected with somatic nociceptors. The scarcity of pain receptors in these areas produces a dull, poorly-localised pain of longer duration than cutaneous pain; examples include sprains and broken bones.


 * Visceral pain originates from body's viscera, or organs. Visceral nociceptors are located within body organs and internal cavities. The even greater scarcity of nociceptors in these areas produces pain that is usually more aching and of a longer duration than somatic pain. Visceral pain is extremely difficult to localise, and several injuries to visceral tissue exhibit "referred" pain, where the sensation is localised to an area completely unrelated to the site of injury. Myocardial ischaemia (the loss of blood flow to a part of the heart muscle tissue) is possibly the best known example of referred pain; the sensation can occur in the upper chest as a restricted feeling, or as an ache in the left shoulder, arm or even hand. Referred pain can be explained by the findings that pain receptors in the viscera also excite spinal cord neurons that are excited by cutaneous tissue. Since the brain normally associates firing of these spinal cord neurons with stimulation of somatic tissues in skin or muscle, pain signals arising from the viscera are interpreted by the brain as originating from the skin.  The theory that visceral and somatic pain receptors converge and form synapses on the same spinal cord pain-transmitting neurons is called "Ruch's Hypothesis".


 * Phantom limb pain is the sensation of pain from a limb that has been lost or from which a person or no longer receives physical signals. It is an experience almost universally reported by amputees and quadriplegics.


 * Neuropathic pain, or "neuralgia", can occur as a result of injury or disease to the nerve tissue itself. This can disrupt the ability of the sensory nerves to transmit correct information to the thalamus, and hence the brain interprets painful stimuli even though there is no obvious or known physiologic cause for the pain.

Pain receptors
All pain receptors are free nerve endings. There are mechanical, thermal and chemical pain receptors. They are found in skin and on internal surfaces such as periosteum and joint surfaces. Deep internal surfaces are only weakly supplied with pain receptors and will propagate sensations of chronic, aching pain if tissue damage in these areas is experienced.

Pain receptors do not adapt to stimulus. In some conditions, excitation of pain fibres becomes greater as the pain stimulus continues, leading to a condition called hyperalgesia.

Nociceptors are the free nerve endings of neurons that have their cell bodies outside the spinal column in the dorsal root ganglion and are named based upon their appearance at their sensory ends. These sensory endings look like the branches of small bushes.

Two main types of nociceptor, A&delta; and C fibres, mediate fast and slow pain respectively. Thinly myelinated type A&delta; fibres, which transmit signals at rates of between 6 to 30 metres per second mediate fast pain. This type of pain is felt within a tenth of a second of application of the pain stimulus. It can be described as sharp, acute, pricking pain and includes mechanical and thermal pain. Slow pain, mediated by slower, unmyelinated ("bare") type C pain fibres that send signals at rates of between 0.5 to 2 metres per second, is an aching, throbbing, burning pain. Chemical pain is an example of slow pain.

Transmission of pain signals in the central nervous system
The perception of pain occurs when the nociceptors are stimulated and transmit signals through sensory neurons in the spinal cord. These neurons release glutamate, a major exicitory neurotransmitter that relays signals from one neuron to another. The signals are sent to the thalamus, in which pain perception occurs. From the thalamus, the signal travels to the somatosensory cortex in the cerebrum, at which point the individual becomes fully aware of the pain.

There are 2 pathways for transmission of pain in the CNS. These are the neospinothalamic tract (for fast pain) and the paleospinothalamic tract (for slow pain).
 * Fast pain travels via type A&delta; fibres to terminate on lamina I (lamina marginalis) of the dorsal horn. Second order neurons of the neospinothalamic tract then take off and give rise to long fibres which cross the midline through the anterior commisure and pass upwards in the contralateral anterolateral columns. These fibres then terminate on the Ventrobasal Complex (VBC) of the thalamus. From here, third order neurons communicate with the somatosensory cortex. Fast pain can be localised easily if A&delta; fibres are stimulated together with tactile receptors.
 * Slow pain is transmitted via slower type C fibres to laminae II and III of the dorsa horns, together known as the substantia gelatinosa. Second order neurons take off and terminate in lamina V, also in the dorsal horn. Third order neurons then join fibres from the fast pathway, crossing to the opposite side via the anterior commisure, and travelling upwards through the anterolateral pathway. These neurons terminate widely in the brain stem, with one tenth of fibres stopping in the thalamus, and the rest stopping in the medulla, pons and mesencephalon. Slow pain is poorly localized.

Analgesia
The gate control theory of pain, proposed by Patrick Wall and Ron Melzack, postulates that pain is "gated" by non-painful stimuli such as vibration. Thus, rubbing a bumped knee seems to relieve pain by preventing its transmission to the brain. Pain is also "gated" by signals that descend from the brain to the spinal cord to suppress (and in other cases enhance) incoming pain information.

The analgesia system is mediated by 3 major components : the periaquaductal grey matter (in the midbrain), the nucleus raphe magnus (in the medulla), and the pain inhibitory neurons within the dorsal horns of the spinal cord, which act to inhibit pain-transmitting neurons also located in the spinal dorsal horn.

The body has several different types of opioid receptors that are activated in response to the binding of the body's endogenous endorphins. These receptors, which exist in a variety of areas in the body, inhibit firing of neurons that would otherwise be stimulated to do so by nociceptors.

Survival benefit
Despite its unpleasantness, pain is an important part of the existence of humans and other animals; in fact, it is vital to survival. Pain encourages an organism to disengage from the noxious stimulus associated with the pain. Preliminary pain can serve to indicate that an injury is imminent, such as the ache from a soon-to-be-broken bone. Pain may also promote the healing process, since most organisms will protect an injured region in order to avoid further pain. People born with congenital insensitivity to pain usually have short life spans, and suffer numerous ailments such as broken bones, bed sores, and chronic infection.

The study of pain has in recent years diverged into many different fields from pharmacology to psychology and neurobiology. It was even proposed that fruit flies may be used as an animal model for pharmacological pain research. Pain is also of interest in the search for the neural correlates of consciousness, as pain has many subjective psychological aspects besides the physiological nociception.

Interestingly, the brain itself is devoid of nociceptive tissue, and hence cannot experience pain. Thus, a headache is not due to stimulation of pain fibers in the brain itself. Rather, the membrane surrounding the brain and spinal cord, called the dura mater, is innervated with pain receptors, and stimulation of these dural nociceptors (pain receptors) is thought to be involved to some extent in producing headache pain. Some evolutionary biologists have speculated that this lack of nociceptive tissue in the brain might be because any injury of sufficient magnitude to cause pain in the brain has a sufficiently high probability of being fatal that development of nociceptive tissue therein would have little to no survival benefit.

Since pain is defined as a signal of present or impending tissue damage effected by a harmful stimulus, the ability to experience pain or irritation is observable in most multicellular organisms. Even some plants have the ability to retract from a noxious stimulus. Whether this sensation of pain is equivalent to the human experience is debatable.

Chronic pain, in which the pain becomes pathological rather that beneficial, is an exception to the idea that pain is helpful to survival. Furthermore, it is not clear what the survival benefit of sometimes extreme forms of pain (e.g. toothache) might be; and the intensity of some forms of pain (for example as a result of injury to fingernails or toenails) seem to be out of all proportion to any survival benefits.

Children and pain
Children have been proven to be markedly more sensitive to pain, but this fact is commonly dismissed as a fear reaction or a lack of coping abilities. Research has been carried out on how children can cope with pain due to increased sensitivity and it has been established that strategies that remove pain can help prevent long-term increases in sensitivity as the nervous system is still developing.

Pain and alternative medicine
A recent survey by NCCAM found pain was the most common reason that people use complementary and alternative medicine (CAM). Among American adults who used CAM in 2002, 16.8% used CAM to treat back pain; 6.6% for neck pain; 4.9% for arthritis; 4.9% for joint pain; 3.1% for headache; and 2.4% used CAM to treat recurring pain. (Some survey respondents may have used CAM to treat more than one of these pain conditions.)

One such alternative, traditional Chinese medicine, views pain as a qi "blockage" equivalent to electrical resistance, or as "stagnation of blood" – theorized as dehydration inhibiting metabolism. Traditional Chinese treatments such as acupuncture are more effective for nontraumatic pain than traumatic pain.