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The sensation of pain cannot accurately be described as
“located” at the point of an injury, or, for that matter,
in any one place in the nerves or brain. Rather, pain
signals—and pain relief—are delivered through a highly
(5) complex interacting circuitry.
When a cell is injured, a rush of prostaglandin’s
sensitizes nerve endings at the injury. Prostaglandins are
chemicals produced in and released from virtually all
mammalian cells when they are injured: these are the only
(10) pain signals that do not originate in the nervous system.
Aspirin and other similar drugs (such as indomethacin and
ibuprofen) keep prostaglandins from being made by inter-
fering with an enzyme known as prostaglandin synthetase,
or cyclooxygenase. The drugs’ effectiveness against pain is
(15) proportional to their success in blocking this enzyme at the
site of injury.
From nerve endings at the injury, pain signais move to
nerves feeding into the spinal cord. The long, tubular
membranes of nerve cells carry electrical impulses. When
(20) electrical impulses get to the spinal cord, a pain-signaling
chemical known as substance P is released there.
Substance P then excites nearby neurons to send impulses
to the brain. Local anesthetics such as novocaine and
xylocaine work by blocking the electrical transmission
(25)along nerves in a particular area. They inhibit the flow of
sodium ions through the membranes, making the nerves
electrically quiescent; thus no pain signals are sent to the
spinal cord or to the brain.
Recent discoveries in the study of pain have involved
(30) the brain itself—the supervising organ that notices pain
signals and that sends messages down to the spinal cord
to regulate incoming pain traffic. Endorphins—the brain’s
own morphine—are a class of small peptides that help to
block pain signals within the brain itself. The presence
(35) of endorphins may also help to explain differences in
response to pain signals, since individuals seem to differ
in their ability to produce endorphins. It now appears that
a number of techniques for blocking chronic pain—such
as acupuncture and electrical stimulation of the central
(40) brain stem—involve the release of endorphins in the brain
and spinal cord.
1. The passage is primarily concerned with
(A) analyzing ways that enzymes and other chemicals
influence how the body feels pain
(B) describing the presence of endorphins in the brain
and discussing ways the body blocks pain within the
brain itself.
(C) describing how pain signals are conveyed in the
body and discussing ways in which the pain signals
can be blocked
(D) demonstrating that pain can be influenced by
acupuncture and electrical stimulation of the central
brain stem.
(E) differentiating the kinds of pain that occur at
different points in the body’s nervous system.
2. According to the passage, which of the following is one
of the first things to occur when cells are injured?
(A) The flow of electrical impulses through nerve cells
at the site of the injury is broken.
(B) The production of substance P traveling through
nerve cells to the brain increases.
(C) Endorphins begin to speed up the response of nerve
cells at the site of the injury.
(D) A flood of prostaglandins sensitizes nerve endings at
the site of the injury.
(E) Nerve cells connected to the spinal cord become
electrically quiescent.
3. Of the following, which is most likely attributable to the
effect of endorphins as described in the passage?
(A) After an injection of novocaine, a patient has no
feeling in the area where the injection was given.
(B) After taking ibuprofen, a person with a headache
gets quick relief.
(C) After receiving a local anesthetic, an injured person
reports relief in the anestherized area.
(D) After being given aspirin, a child with a badly
scraped elbow feels better.
(E) After acupuncture, a patient with chronic back pain
reports that the pain is much less severe.
4. It can be inferred from the passage that if the
prostaglandin synthetase is only partially blocked, which
of the following is likely to be true?
(A) Some endorphins will be produced, and some pain
signals will be intensified.
(B) Some substance P is likely to be produced, so some
pain signals will reach the brain.
(C) Some sodium ions will be blocked, so some pain
signals will not reach the brain.
(D) Some prostaglandins will be produced, but
production of substance P will be prevented.
(E) Some peptides in the brain will receive pain signals
and begin to regulate incoming pain traffic.