167
CHAPTER
SECTION III CENTRAL &
PERIPHERAL NEUROPHYSIOLOGY
10
Pain & Temperature
OBJECTIVES
After studying this chapter, you should be able to:
■
Name the types of peripheral nerve fibers and receptor types that mediate
warmth, cold, and nociception.
■
Explain the difference between pain and nociception.
■
Explain the differences between fast and slow pain and acute and chronic pain.
■
Explain hyperalgesia and allodynia.
■
Describe and explain referred pain.
INTRODUCTION
One of the most common reasons an individual seeks the
advice of a physician is because he or she is in pain. Pain was
called by Sherrington, “the physical adjunct of an imperative
protective reflex.” Painful stimuli generally initiate potent
withdrawal and avoidance responses. Pain differs from other
sensations in that it sounds a warning that something is
wrong, preempts other signals, and is associated with an
unpleasant affect. It turns out to be immensely complex
because when pain is prolonged and tissue is damaged, cen-
tral nociceptor pathways are sensitized and reorganized.
NOCICEPTORS &
THERMORECEPTORS
Pain and temperature sensations arise from unmyelinated
dendrites of sensory neurons located around hair follicles
throughout the glabrous and hairy skin as well as deep tissue.
Impulses from
nociceptors
(pain) are transmitted via two fi-
ber types. One system comprises thinly myelinated A
δ
fibers
(2–5
μ
m in diameter) which conduct at rates of 12–30 m/s.
The other is unmyelinated C fibers (0.4–1.2
μ
m in diameter)
which conduct at low rates of 0.5–2 m/s.
Thermoreceptors
also span these two fiber types. Cold receptors are on dendritic
endings of A
δ
fibers and C fibers, whereas warmth (heat) re-
ceptors are on C fibers.
Mechanical nociceptors
respond to strong pressure (eg,
from a sharp object).
Thermal nociceptors
are activated by
skin temperatures above 45 °C or by severe cold.
Chemically
sensitive nociceptors
respond to various agents like bradyki-
nin, histamine, high acidity, and environmental irritants
. Poly-
modal nociceptors
respond to combinations of these stimuli.