C H A P T E R 2 6
Opioids, opioid antagonists and antimigraine agents
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According to the
gate control theory
, the transmission
of these impulses can be modulated or adjusted all along
these tracts. All along the spinal cord, the interneurons
can act as “gates” by blocking the ascending trans
mission of pain impulses. It is thought that the gates can
be closed by stimulation of the larger A fibres and by
descending impulses coming down the spinal cord from
higher levels in such areas as the cerebral cortex, the
limbic system and the reticular activating system.
The inhibitory influence of the higher brain centres
on the transmission of pain impulses helps to explain
much of the mystery associated with pain. Several
factors, including learned experiences, cultural expecta-
tions, individual tolerance and the placebo effect, can
activate the descending inhibitory nerves from the upper
central nervous system. These other factors need to be
considered and incorporated into pain management
strategies, which usually involve the use of drugs. For
example, the placebo effect, stress reduction, acupunc-
ture and back rubs (which stimulate the A fibres) can
all play important roles in the effective management of
pain.
Pain receptors
Opioid receptors
are receptor sites that respond to
naturally occurring peptides, the endorphins and the
encephalins. These receptor sites are found in the CNS,
on nerves in the periphery and on cells in the gastro-
intestinal (GI) tract. In the brainstem, opioid receptors
help to control blood pressure, pupil diameter, GI secre-
tions and the chemoreceptor trigger zone (CTZ) that
regulates nausea and vomiting, cough and respiration.
In the spinal cord and thalamus, these receptors help to
integrate and relate incoming information about pain.
The endorphins and encephalins normally modulate
the pain information coming into the brain. Endor-
phins are released during stress to block the sensation
of pain. Professional athletes may be injured during an
important game and have no sensation of pain or injury
because their stress reaction is highly activated, and
the endorphins are blocking pain transmission into the
brain. In the hypothalamus, stimulation of the opioid
receptors may interrelate the endocrine and neural
responses to pain. In the limbic system, the receptors
Response
from brain
Cross-section
of medulla
Spinal cord
Spinothalamic
tracts
Spinal
nerve
Reflex
withdrawal
tract
Reticular
formation
Pain signals
to brain
Hypothalamus,
limbic system Thalamus
Cerebral
cortex
Pain receptors in skin
C fibre—
unmyelinated,
slow conducting
A-delta fibre—
small, myelinated
acute pain
FIGURE 26.1
Neural pathways of pain.
