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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.