Page 25 Guide to Pain Management in Low-Resource Settings

Chapter 3

Physiology of Pain

Nilesh B. Patel

Pain is not only an unpleasant sensation, but a complex may show no behavior indicating pain. Nociception can
sensory modality essential for survival. Th ere are rare lead to pain, which can come and go, and a person can
cases of people with no pain sensation. An often-cited have pain sensation without obvious nociceptive activi-
case is that of F.C., who did not exhibit a normal pain ty. Th ese aspects are covered in the IASP defi nition: “An
response to tissue damage. She repeatedly bit the tip of unpleasant sensory and emotional experience associ-
her tongue, burned herself, did not turn over in bed or ated with actual or potential tissue damage, or described
shift her weight while standing, and showed a lack of in terms of such damage.”
autonomic response to painful stimuli. She died at the
age of 29. Physiology of pain
Th e nervous system mechanism for detection of
stimuli that have the potential to cause tissue damage is Nociceptors and the transduction
of painful stimuli
very important for triggering behavioral processes that
protect against current or further tissue damage. Th is is Th e nervous system for nociception that alerts the
done by refl ex reaction and also by preemptive actions brain to noxious sensory stimuli is separate from the
against stimuli that can lead to tissue damage such as nervous system that informs the brain of innocuous
strong mechanical forces, temperature extremes, oxy- sensory stimuli.
gen deprivation, and exposure to certain chemicals. Nociceptors are unspecialized, free, unmyelin-
Th is chapter will cover the neuronal recep- ated nerve endings that convert (transduce) a variety of
tors that respond to various painful stimuli, substances stimuli into nerve impulses, which the brain interprets
that stimulate nociceptors, the nerve pathways, and the to produce the sensation of pain. Th e nerve cell bodies
modulation of the perception of pain. Th e term nocicep- are located in the dorsal root ganglia, or for the trigemi-
tion (Latin nocere, “to hurt”) refers to the sensory pro- nal nerve in the trigeminal ganglia, and they send one
cess that is triggered, and pain refers to the perception nerve fi ber branch to the periphery and another into the
of a feeling or sensation which the person calls pain, spinal cord or brainstem.
and describes variably as irritating, sore, stinging, ach- Th e classifi cation of the nociceptor is based on
ing, throbbing, or unbearable. Th ese two aspects, noci- the classifi cation of the nerve fi ber of which it is the ter-
ception and pain, are separate and, as will be described minal end. Th ere are two types of nerve fi bers: (1) small-
when discussing the modulation of pain, a person with diameter, unmyelinated nerves that conduct the nerve
tissue damage that should produce painful sensations impulse slowly (2 m/sec = 7.2 km/h), termed C fi bers,
Guide to Pain Management in Low-Resource Settings, edited by Andreas Kopf and Nilesh B. Patel. IASP, Seattle, © 2010. No responsibility is assumed by IASP 13
for any injury and/or damage to persons or property as a matter of product liability, negligence, or from any use of any methods, products, instruction, or
ideas contained in the material herein. Because of the rapid advances in the medical sciences, the publisher recommends that there should be independent
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recommendation by the editors, authors, or IASP in favor of other medical products or procedures that are not covered in the text.