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