| Pain is both a sensory and emotional experience, generally associated tissue
damage, or inflammation. Pain is ultimately a perception, and not an objective bodily state.
Despite its unpleasantness, pain is a critical component of the body's defense system.
It is part of a rapid warning and defence relay instructing the motor neurons
of the central nervous system to minimize detected physical harm.
The gate control theory of pain offers
insight into how cognitive and emotional factors might dramatically influence painful sensations. Developed by Ronald Melzack and Pat Wall, it
focuses on different pain states at the brain, rather than at the perceived site of injury.
Nociception
Nociception is the physiological sense for perception of physiological
pain. Nociception does not describe psychological pain.
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.
The interpretation of pain occurs when the nociceptors are stimulated and
subsequently transmit signals through sensory neurons in the spinal cord, which releases glutamate, a major exicitory neurotransmitter that
relays signals from one neuron to another and ultimately to the thalamus, in which
pain perception occurs. From the thalumus, the signal travels to the cerebrum, at
which point the individual becomes fully aware of the pain.
Interestingly, the brain itself is devoid of nociceptive tissue, and hence cannot experience pain (thus a headache is not pain in the brain itself). Some evolutionary biologists have speculated
that this lack of nociceptive tissue might be due to the fact that 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.
If pain is defined as a signal of present or impending tissue damage effected by a harmful stimulus then the ability to
experience pain or irritation is observable in most multi-cellular
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.
Interpretation of pain
The unpleasantness of pain encourages an organism to use any means at its disposal to disengage from the noxious stimuli that
it assumes cause the pain. It may, of course, have incorrectly determined the cause. 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 as most organisms will protect an injured region from further damage in order to avoid further
pain.
Despite its unpleasantness, pain remains an important part of human existence.
The study of pain has in recent years diverged into many different fields from pharmacology to psychology and neurobiology.
Pain has also provided an interested take for the search for the neural correlates of consciousness, as pain has many subjective
psychological aspects besides the physiological nociception.
Types of pain
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 is medically defined as pain that has lasted 6
months or longer. 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. When opioids are used for prolonged periods drug tolerance, chemical
dependency and even psychological addiction may
occur. While drug tolerance and chemical dependency are common among opioid users,
psychological addiction is rare.
The experience of physiological pain can be grouped into four categories according to the source and related nociceptors (pain
detecting nerves).
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. Example 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, and are 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 sprained ankle and broken bones.
Visceral pain originates from body organs. Visceral nociceptors are located within body organs and internal cavities.
The even greater scarcity of nociceptors in these areas produces a pain 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.
Phantom limb pain is the sensation of pain from a limb that
one no longer has or no longer gets physical signals from - an experience almost universally reported by amputees and quadriplegics.
Finally neuropathic pain ("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 documented physiologic cause for the
pain.
Classification of Pain
Pain can be functionally classified into fast pain and slow pain.
- Fast pain - Fast pain is felt within 0.1s of application of the pain stimulus. It can be described as sharp, acute, pricking
pain and includes mechanical and thermal pain. It is mediated by type A &Delta fibres at rates of between 6-30m/s.
- Slow pain - Slow pain is an aching, throbbing, burning, chronic pain. Chemical pain is an example of slow pain. It is
mediated by slowed type C pain fibres at rates of between 0.5-2m/s
Pain Receptors
All pain receptors are free nerve endings. There are mechanical, thermal and chemical pain receptors. They are found in skin,
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
hyperalgesia.
Transmission of Pain Signals in the Central Nervous System
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).
- Pathway for Fast Pain - Fast pain travels via type A &Delta fibres to terminate on lamina I (lamina marginalis) of the
dorsal horns. 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 somatic sensory cortex.
Fast pain can be localised easily if A &delta fibres are stimulated together with tactile receptors.
- Pathway for Slow Pain - Slow pain is transmitted via slower type C fibres to lamina II and III of the dorsa horns, together
known as the substantia gelatinosa. Second order neurons take off and terminatein lamina V, also in the dorsal horn. Third order
neurons then take off and 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 .
The Analgesia System of the Central Nervous System
The analgesia system is mediated by 3 major components : the periaquaductal grey, the nuclear raphe magnus, and the pain
inhibitory complex of the dorsal horns of the spinal cord.
Referred Pain
Referred pain is a phenomenon which arises when visceral pain fibres and pain fibres from the skin synapse on the same second
order pain fibres. Thus pain arising from the viscera appear to come from the skin.
Pain and alternative medicine
A recent survey (http://nccam.nih.gov/news/2004/052704.htm) by NCCAM found pain was the most common reason to 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 relatively more effective for nontraumatic pain than with traumatic pain.
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