443
CHAPTER TWELVE
Nervous System III
intensity of pain and locate its source, and it is also respon-
sible for emotional and motor responses to pain.
RECONNECT
To Chapter 10, Postsynaptic Potentials, pages 371–372.
Still other parts of the brain, including areas of gray mat-
ter in the midbrain, pons, and medulla oblongata, regulate
the fl
ow of pain impulses from the spinal cord (see chap-
ter 11, pp. 407–408). Impulses from special neurons in these
areas descend in the lateral funiculus to various levels within
the spinal cord. The impulses stimulate the ends of certain
nerve F
bers to release biochemicals that can block pain sig-
nals by inhibiting presynaptic nerve F
bers in the posterior
horn of the spinal cord.
Among the inhibiting substances released in the pos-
terior horn are neuropeptides called
enkephalins
and the
monoamine
serotonin
(see chapter 10, p. 374). Enkephalins
can suppress both acute and chronic pain impulses; thus,
they can relieve strong pain sensations, much as morphine
and other opiate drugs do. In fact, enkephalins were dis-
covered because they bind to the same receptors on neuron
membranes as does morphine. Serotonin stimulates other
neurons to release enkephalins.
Pain impulses that originate from tissues of the head
reach the brain on sensory F bers of the F fth, seventh, ninth,
and tenth cranial nerves. All other pain impulses travel on
sensory F bers of spinal nerves, and they pass into the spinal
cord by way of the dorsal roots of these spinal nerves.
Upon reaching the spinal cord, pain impulses enter the
gray matter of the posterior horn, where they are processed.
The fast-conducting F bers synapse with long nerve F bers
that cross over to the opposite side of the spinal cord in the
anterior and lateral spinothalamic tracts (anterolateral sys-
tem). The impulses carried on the slow-conducting F
bers
pass through one or more interneurons before reaching the
long F bers that cross over and ascend to the brain.
In the brain, most of the pain fibers terminate in the
reticular formation (see chapter 11, p. 409), and from there
are conducted on F bers of still other neurons to the thala-
mus, hypothalamus, and cerebral cortex. ±ibers of the spi-
nothalamic tracts transmit pain and temperature information
directly to the thalamus.
Regulation of Pain Impulses
Awareness of pain occurs when pain impulses reach the
level of the thalamus—that is, even before they reach the
cerebral cortex. However, the cerebral cortex must judge the
Spinal
cord
Dorsal root
ganglion
Paravertebral
ganglion
Sensory
pathway
to brain
Sensory
nerve fiber
Skin
Heart
Pain
receptor
FIGURE 12.3
Pain originating in the heart may feel as if it is
coming from the skin because sensory impulses from the heart
and the skin follow common nerve pathways to the brain.
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