The sensation of pain cannot accurately1 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 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 pain signals that do not originate in the nervous system. Aspirin2 and other similar drugs keep prostaglandins from being made by inter- fering with an enzyme3 known as prostaglandin synthetase, or cyclooxygenase. The drugs' effectiveness against pain is 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 spinal4 cord. The long, tubular membranes5 of nerve cells carry electrical impulses. When 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 along nerves in a particular area. They inhibit7 the flow of sodium8 ions through the membranes, making the nerves electrically quiescent9; thus no pain signals are sent to the spinal cord or to the brain.
1. The passage is primarily concerned with
analyzing10 ways that enzymes11 and other chemicals influence how the body feels pain
describing the presence of endorphins in the brain and discussing ways the body blocks pain within the brain itself.
describing how pain signals are conveyed in the body and discussing ways in which the pain signals can be blocked
demonstrating that pain can be influenced by acupuncture12 and electrical stimulation13 of the central brain stem.
differentiating14 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?
The flow of electrical impulses through nerve cells at the site of the injury is broken.
The production of substance P traveling through nerve cells to the brain increases.
Endorphins begin to speed up the response of nerve cells at the site of the injury.
A flood of prostaglandins sensitizes nerve endings at the site of the injury.
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?
After an injection of novocaine, a patient has no feeling in the area where the injection was given.
After taking ibuprofen, a person with a headache gets quick relief.
After receiving a local anesthetic6, an injured person reports relief in the anestherized area.
After being given aspirin, a child with a badly scraped elbow feels better.
After acupuncture, a patient with chronic15 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 partially16 blocked, which of the following is likely to be true?
Some endorphins will be produced, and some pain signals will be intensified17.
Some substance P is likely to be produced, so some pain signals will reach the brain.
Some sodium ions will be blocked, so some pain signals will not reach the brain.
Some prostaglandins will be produced, but production of substance P will be prevented.
Some peptides in the brain will receive pain signals and begin to regulate incoming pain traffic
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