PAIN SENSATION According to The International Association for the Study of Pain (IASP): Definition: Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage. 1) warning signal against tissue damage . Pain is one of the most prominent symptoms of tissue damage. 2) Initiate protective reflexes which causes the subject to get rid of the painful stimulus, or at least, to minimize tissue injury or damage
If persistent, physiological pain may progress to a pathological condition itself, often referred to as maladaptive pain, in which case pain is dissociated from the original noxious stimulation or the healing process and thus does not represent anymore a symptom of disease but rather abnormal sensory processing due to damage to tissues (inflammatory pain) or the nervous system (neuropathic pain), or to abnormal function of the nervous system itself (functional pain) . pain resulting from activation of pain receptors may be referred to as adaptive or physiological pain, because it minimizes tissue damage and promotes healing.
Pain is classified into nociceptive , neuropathic and psychogenic ; all can be either acute or chronic.
Pain is defined as chronic if persists more than 7 weeks.
1. Nociceptive is pain caused by tissue damage (inflammation) which stimulate pain receptors (nociceptors). 2. Neuropathic: (pain due to injury of nerve pathway) site of injury: Central Central pain (thalamic infarct). Mixed Plexus avulsion, Post herpetic neuralgia. Peripheral Neuroma, nerve compression, phantom, neuralgias. character: burning, tingling, numbness, pressing, squeezing, itching, constant +/- intermittent shooting, lancinating, electric. 3. Psychogenic: (difficult to differentiate whether secondary to or actual cause of pain), anxiety, depression (30% of depressives complain of pain on initial presentation).
Types of Pain Receptors
Free nerve endings which are morphologically similar but functionally specific . They are classified according to their sensitivity into:
Polymodal Pain Receptors (most pain receptors)
These respond to a combination of mechanical, thermal, and chemical noxious stimuli.
Mechanical Pain Receptors
respond to strong mechanical forces, such as cutting, crushing, pricking, or even firm pressure on tissues.
Thermal Pain Receptors
respond to excessive changes in temperature (above 45 o C and below 10 o C).
Chemical Pain Receptors
respond to noxious chemical stimuli.
Pain Receptors (Nociceptors)
Distribution of Pain Receptors
Pain receptors are found in most tissues of the body but varies in their density.
They are abundant and widely spread in the skin and some internal tissues such as: - the periosteum of the bone,
- arterial walls,
- joint surfaces,
- the dura of the falx and tentorium in the cranial cavity,
- the skeletal muscle,
- the parietal layer of serous membranes.
Many of the other deep tissues and viscera are poorly supplied with pain receptors. So,
- for pain to occur, painful stimulus must by intense and widespread.
- The deep & visceral pain are poorly localized.
On the other hand, the brain itself and also the parenchymal tissues of the liver , kidneys , and lungs have no pain receptors. They are called “pain insensitive structures”
N.B.: Serious diseases in these structures don’t produce pain till they extend to a pin sensitive structure like arterial wall or serous covering.
Pain threshold is the lowest intensity of stimulus that can cause pain when the stimulus is applied for sufficient period of time.
Pain threshold can be measured in many ways.
One of the accurate methods to quantify the threshold is heating the skin with measured amounts of radiant heat from a calibrated electric lamp.
It has been shown that the majority of subjects begin to perceive pain when the skin temperature reaches 45 o C, and almost everyone perceives pain before the temperature reaches 47oC.
So, it seems that the great majority of people do not show significant differences in their sensitivity to painful stimuli. However, they differ widely in their reaction to pain.
Stimulation of Pain Receptors:
noxious stimuli are strong enough -----> tissue damage ------> release of chemical agents from destructed cells into the surrounding interstitial spaces which are called “pain producing compounds” (PPCs) ------> stimulate pain receptors in the affected tissues.
PGE 2 IL-1 Both threshold of pain receptors facilitating their stimulation
The PPCs may be classified into:
1- Direct stimulators Substances which when reach specific threshold directly stimulate pain receptors ----> pain, as: - K + ions. - H + ions - Serotonin. - Histamine - Bradykinin 2- Sensitizers Substances which lower the threshold for stimulation of pain receptors by direct stimulators ----> facilitate pain production. They include: a) Substances released by the injured tissues as: PGE2 & IL-1 b) Substances released by pain receptors through antidromic impulses as: substance P N.B.: Substance P also stimulate mast cells to release histamine which is a direct stimulator.
The surface membrane of pain receptors contain several molecular receptors which can be activated by various PPCs.
Pain receptors do not adapt to continuing noxious stimuli.
Non adaptation to pain serves a protective function to keep the individual trying to remove the damaging stimulus or to get away from it.
THE CHARACTER (QUALITY) of pain 1) Pricking or Cutting Pain 2) Burning Pain 3) Aching Pain 4) Throbbing Pain 5) Colicky Pain
A sharp and localized pain. It is of cutaneous origin and is caused by pricking or cutting the skin by a sharp object.
A less well localized pain. It is usually of cutaneous origin and is caused by burns or inflammations of the skin .
A dull-aching nature. It is more diffuse and felt coming from deeper tissues, e.g. rheumatic pains.
is characterized by fluctuation of its intensity with arterial pulsations. It results from localized inflammation in deep tissues, as in abscess formation.
Pain results from spasm of plain muscles in the walls of hollow viscera.
Visceral Somatic sympathetically innervated organs can be transferred to body surface cutaneous, deep tissues site vague distribution and Quality deep, ache, dragging, squeezing acute: colic, paroxysmal, +/- N/V, sweating, BP and heart rate changes constant, localised aching, throbbing, gnawing character Acute nociceptive pain:
Slow (delayed) pathophysiological pain Fast (Immediate) physiological pain Shortly after application if tissue damage occurs Longer duration Burning Poorly-localized C-fibers Thalamus Substance-P * Associated with arousal, autonomic & emotional reactions Abolished by local anaethesia & morphine onset: during application of the stimulus Duration: short duration. Nature: pricking Localization: well-localized Afferent: A-delta fibers Higher center: CC Neurotransmitter: glutamate Significance: * determine site & severity. * Initiate withdrawal reflexes. Abolished by deep pressure and not abolished by morphine.