Pain mechanisms exist to provide a useful
warning of possible injury or to caution
against repeating an action that has led to
injury. Certain diseases, such as arthritis
and extensive cancer, may set off these
same mechanisms, causing chronic pain
that has no apparent function.
Reflex action
The nerve pathways that warn of noxious
stimuli (through the sensation of pain) may
also initiate automatic, reflex actions that
help prevent harm.
B r a in r e g i o n
c o n c e r n e d w ith
Perception of pain
When an injury occurs, signals pass along nerve
pathways concerned with pain, first to the
spinal cord and then to the thalamus in the
brain; there the pain is perceived.
Initiation ofpain signals
Pain signals are set off by stimulation of special
nerve endings - by pressure, heat, or the
release ofchemicals, including prostaglandins,
by cells that have been damaged.
Signal transmission to brain
Within the brain and spinal cord, pain signals
pass between nerve cells by means of chemicals
that cross the gaps between the cells.
Receptors in the fingertip detect heat.
Signals are sent along a sensory nerve
to the spinal cord.
The signals arriving in the spinal cord pass
instantaneously to a motor nerve that
connects to a muscle in the arm. The signals
received via the motor nerve cause the muscle
in the arm to contract, moving the arm away
from the source ofdanger (the flame).
A referred pain is one felt in a
site other than an injured or
diseased part. Sensorynerves
from certain bodyareas
converge before they enter the
brain, causing confusion about
the source ofpain signals.
Tooth to ear region
A toothache may be felt in the ear,
because the same sensory nerve
supplies both parts.
Diaphragm to right shoulder
Inflammation ofthe diaphragm,
often due to pneumonia, maybe
felt as a pain in the right shoulder.
Heart to left arm
Angina, a pain caused byreduced
blood supplyto the heart muscle, is
often felt in the left shoulder or arm.
Hip to knee
Disordersthat affectthe hip, such
as arthritis, may be felt as pain in
the knee rather than in the hip.
previous page 581 BMA A Z Family Medical Encyclopedia   2004 read online next page 583 BMA A Z Family Medical Encyclopedia   2004 read online Home Toggle text on/off