375
CHAPTER TEN
Nervous System I
O
piate drugs, such as morphine, heroin,
codeine, and opium, are potent painkill-
ers derived from the poppy plant. These
drugs alter pain perception, making it easier to
tolerate, and elevate mood.
The human body produces opiates, called
endorphins (for “endogenous morphine”), that
are peptides. Like the poppy-derived opiates that
they structurally resemble, endorphins inF
uence
mood and perception of pain.
The discovery of endorphins began in 1971
in research laboratories at Stanford University
and the Johns Hopkins School of Medicine,
where researchers exposed pieces of brain tissue
from experimental mammals to morphine. The
morphine was radioactively labeled (some of the
atoms were radioactive isotopes) so researchers
could follow its destination in the brain.
The morphine bound receptors on neurons
that transmit pain. Why, the investigators won-
dered, would an animal’s brain cells have recep-
tors for a plant chemical? One explanation was
that a mammal’s body could manufacture opiates.
The opiate receptors, then, would normally bind
the body’s opiates (the endorphins) but would
also bind the chemically similar compounds from
poppies. Researchers have since identified sev-
eral types of endorphins in the human brain and
associated their release with situations involving
pain relief, such as acupuncture and analgesia to
mother and child during childbirth. Endorphin
release is also associated with “runner’s high.” PET
scans reveal endorphins binding opiate receptors
after conditioned athletes run for two hours.
Endorphins explain why some people
addicted to opiate drugs such as heroin experi-
ence withdrawal pain when they stop taking the
drug. Initially, the body interprets the frequent
binding of heroin to its endorphin receptors as
an excess of endorphins. To bring the level down,
the body slows its own production of endorphins.
Then, when the person stops taking the heroin,
the body becomes short of opiates (heroin and
endorphins). The result is pain.
Opiate drugs can be powerfully addicting
when abused—that is, taken repeatedly by a per-
son who is not in pain. These same drugs, how-
ever, are extremely useful in dulling severe pain,
particularly in terminal illnesses.
10.4
CLINICAL APPLICATION
Opiates in the Human Body
may reach threshold if it receives additional stimulation from
a second input neuron. Thus, an output impulse triggered
from this neuron refl
ects summation of impulses from two
sources
(f g. 10.21
a
)
. Such an output impulse may travel to
a particular effector and evoke a response.
synapse with each other and perform a common function,
even though their cell bodies may be in different parts of the
CNS. Each neuronal pool receives input from neurons (which
may be part of other pools), and each pool generates output.
Neuronal pools may have excitatory or inhibitory effects on
other pools or on peripheral effectors.
As a result of incoming impulses and neurotransmit-
ter release, a particular neuron of a neuronal pool may be
excited by some presynaptic neurons and inhibited by oth-
ers. If the net effect is excitatory, threshold may be reached,
and an outgoing impulse triggered. If the net effect is excit-
atory but subthreshold, an impulse will not be triggered, but
because the neuron is close to threshold, it will be much
more responsive to any further excitatory stimulation. This
condition is called
facilitation
(fah-sil
ı˘-ta
˘
shun).
Convergence
Any single neuron in a neuronal pool may receive impulses
from two or more other neurons. Axons originating from dif-
ferent parts of the nervous system leading to the same neu-
ron exhibit
convergence
(kon-ver
jens).
Incoming impulses often represent information from
various sensory receptors that detect changes. Convergence
allows the nervous system to collect, process, and respond
to information.
Convergence makes it possible for a neuron to sum
impulses from different sources. For example, if a neuron
receives subthreshold stimulation from one input neuron, it
1
2
3
(a)
(b)
4
5
6
FIGURE 10.21
Impulse processing in neuronal pools. (
a
) Axons of
neurons 1 and 2 converge to the cell body of neuron 3. (
b
) The axon of
neuron 4 diverges to the cell bodies of neurons 5 and 6.
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