Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 10

Pain & Temperature 171

CHAPTER SUMMARY

■

Pain impulses are transmitted via lightly myelinated A

δ

and un-

myelinated C fibers. Cold receptors are on dendritic endings of

A

δ

fibers and C fibers, whereas heat receptors are on C fibers.

■

Pain is an unpleasant sensory and emotional experience associat-

ed with actual or potential tissue damage, or described in terms of

such damage, whereas nociception is the unconscious activity in-

duced by a harmful stimulus applied to sense receptors.

■

Fast pain is mediated by A

δ

fibers and causes sharp, localized

sensation. Slow pain is mediated by C fibers and causes a dull,

intense, diffuse, and unpleasant feeling.

■

Acute pain has a sudden onset, recedes during the healing pro-

cess, and serves as an important protective mechanism. Chronic

pain is persistent and caused by nerve damage; it is often refrac-

tory to NSAIDs and opiates.

■

Hyperalgesia is an exaggerated response to a noxious stimulus; al-

lodynia is a sensation of pain in response to an innocuous stimulus.

■

Referred pain is pain that originates in a visceral organ but is

sensed at a somatic site. It may be due to convergence of somatic

and visceral nociceptive afferent fibers on the same second-

order neurons in the spinal dorsal horn that project to the thal-

amus and then to the somatosensory cortex.

FIGURE 10–2
Pain innervation of the viscera.
Pain afferents from structures between the pain lines reach the CNS via sympathetic path-
ways, whereas, they traverse parasympathetic pathways from structures above the thoracic pain line and below the pelvic pain line.
(After White
JC. Reproduced with permission from Ruch TC: In
Physiology and Biophysics,
19th ed. Ruch TC, Patton HD [editors]. Saunders, 1965.)

PARASYMPATHETIC

SYMPATHETIC

PARASYMPATHETIC

Glossopharyngeal nerve

Superior laryngeal nerve

Upper thoracic vagal rami

Brachial plexus

THORACIC

PAIN LINE

PELVIC

PAIN LINE

Vagus

Ap

ica

l

ple

ura

P

ar

ie

ta

lp

le

ur

a

(Intercostal

nerves)

Vi

sc

er

al

pl

eu

ra

(in

se

ns

itiv

e)

Central

diaphragm

(phrenic nerve)

Peripheral

diaphragm

(intercostal nerves)

Lower

splanch-

nic nerves

(T10–L1)

Ureter

(T11–L1)

Somatic nerves (T11–L1)

Par

ietalperitoneum

Fundus (T11–L1)

Cervix and

upper vagina

(S2–4)

Trigone

Prostate

Urethra

(pelvic nerves, S2–4)

Testicle

(sacral nerves, S2–4)

(genitofemoral nerves, L1–2)

(spermatic plexus, T10)

Parasympathetic

rami (S2–4)

Colon (T11–L1)

Splanchnic

nerves

(T7–9)

lleum

Small intestine

splanchnic nerves

(T9–11)

Duodenum and jejunum

(splanchnic nerves)

R splanchnic

nerves (T7–9)

FIGURE 10–3
Diagram of the way in which convergence of
somatic and visceral nociceptive fibers in lamina VII of the dorsal
horn may cause referred pain.
When a visceral stimulus is pro-
longed, somatic fiber facilitation occurs. This leads to activation of spi-
nothalamic tract neurons, and of course the brain cannot determine
whether the stimulus came from the viscera or from the somatic area.

To

brain

Somatic

structure

Spinothalamic tract

Viscus