Pain is not simply a perception, it is a complex phenomenon or syndrome, only one component of which is the sensation actually reported as pain.
Pain has four major aspects present to varying extent in any one case:
Nociception6 is a consequence of tissue injury (trauma, inflammation) causing the release of chemical mediators which activate nociceptors, defined as receptors that are capable of distinguishing between noxious and innocuous stimuli in the tissue. That said, it is widely assumed that there is no specific single histological structure that is a nociceptive receptor, but that free unmyelinated terminals in skin, muscle, joints and viscera are activated by noxious stimuli and transmit information by thin myelinated (A-delta) and nonmyelinated (C) fibres to the spinal cord and brain. Thus nociception is not, for example, due to overstimulation of touch or other receptors. A number of receptors, identified by anatomical, electrophysiological and pharmacological means, have been associated with nociceptors, and include acetylcholine, prostaglandin E, adrenergic, 5-hydroxytryptamine, glutamine, brady-kinin, opioid and adenosine. The ligands for these receptors may be released in the periphery from neurones or be of non-neuronal origin.
Pain perception is a result of nociceptive input plus a pattern of impulses of different frequency and intensity from other peripheral receptors, e.g. heat,
5 Twycross R G 1984 Journal of the Royal College of Physicians of London 18: 32.
6 Latin: noxa: injury.
and mechanoreceptors whose threshold of response is reduced by the chemical mediators. These are processed in the brain whence modulating inhibitory impulses pass down to regulate the continuing afferent input. But pain can occur without nociception (some neuralgias7) and nociception does not invariably cause pain; pain is a psychological state, though most pain has an immediately antecedent physical cause.
Suffering is a consequence of pain and of lack of understanding by patients of the meaning of the pain; it comprises anxiety and fear (particularly in acute pain) and depression (particularly in chronic pain), which will be affected by patients' personalities, and their beliefs about the significance of the pain, e.g. whether merely a postponed holiday, or death, or a future of disability with loss of independence. Depression makes a major contribution to suffering; it is treatable, as are the other affective concomitants of pain.
Pain behaviour comprises consequences of the other three aspects (above); it includes behaviour that is interpreted by others as signifying pain in the victim, e.g. such immediate and obvious aspects as facial expression, restlessness, seeking isolation (or company), medicine-taking, as well as, in chronic pain, the development of querulousness, depression, despair and social withdrawal.
It is thus useful to distinguish between acute pain (an event whose end can be predicted) and chronic pain (a situation whose end is commonly unpredictable, or will only end with life itself).
The clinician's task is to determine the significance of these items for each patient and to direct therapy accordingly. Analgesics may, but not necessarily will, be the mainstay of therapy; adjuvant (nonanalgesic) drugs may be needed, as well as nondrug therapy (radiation, surgery).
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