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Analgesia
Guide
Pain, in the sense of physical pain, is a typical sensory
experience that may be described as the unpleasant
awareness of a noxious stimulus or bodily harm. For
scientific and clinical purposes, pain is defined by the
International Association for the Study of Pain (IASP) as “an
unpleasant sensory and emotional experience associated
with actual or potential tissue damage, or described in
terms of such damage”. Pain is part of the body’s defense
system, triggering a reflex reaction to retract from a painful
stimulus, and helps adjust behavior to increase avoidance
of that particular harmful situation in the future. Given its
significance, physical pain is also linked to various cultural,
religious, philosophical, or social issues.
The word “pain” does not equate with nociception, which
is a preconscious neural activity that is normally
necessary, but not sufficient, for pain. The term
nociception was coined by Charles Scott Sherrington to
make clear the difference between the physiological
nature of nervous activity signaling tissue damage and
the psychological response of pain to this physiological
event. In animal models, we have to speak of
“nociceptive transmission” instead of “pain transmission”
since pain per se cannot be communicated.
Nociception is the afferent activity produced in the
peripheral and central nervous system by stimuli that
have the potential to damage tissue. This activity is
initiated by nociceptors that can detect mechanical,
thermal or chemical changes, above a certain
threshold. All nociceptors are free nerve endings of fastconducting
myelinated A delta fibers or slow conducting
unmyelinated C fibers, respectively responsible for fast,
localized, sharp pain and slow, poorly localized, dull
pain. Once stimulated, the nociceptors transmit signals
that travel along the spinal cord and within the brain.
Brain areas that are particularly studied in relation with
pain include the somatosensory cortex which mostly
accounts for the sensory discriminative dimension of
pain, and the limbic system, of which the thalamus and
the anterior cingulated cortex are said to be especially
involved in the affective dimension. Nociception, even
in the absence of pain, may trigger withdrawal reflexes
and a variety of autonomic responses. The control of
nociceptive transmission is complex and involves
numerous peripheral and central mechanisms.
In this pathological manifestation, pain is a major
symptom in many medical conditions, significantly
interfering with a person’s quality of life and general
functioning. Diagnosis is based on characterizing pain in
various ways, according to duration, intensity, type (dull,
burning or stabbing), source, or location in body.
Among the most frequent technical terms for referring to
abnormal perturbations in pain experiences, there are:
allodynia (pain due to a stimulus which does not normally
provoke pain) hyperalgesia (an increase response to a
stimulus which is normally painful) and hypoalgesia
(diminished pain in response to a normally painful
stimulus). As an example, allodynia is a clinical feature of
many painful conditions, such as neuropathies, posttherapeutic
neuralgia, fibromyalgia, and migraine.
Usually pain stops without treatment or responds to
simple measures such as resting or taking an analgesic,
and it is then called “acute” pain. However, it may
become intractable and develop into a condition called
chronic pain, in which pain is no longer considered a
symptom but an illness by itself.
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