How Pain is Perceived
Pain results when nocireceptors (nerve endings) are stimulated by mechanical, thermal, or chemical stimuli that have the potential to cause tissue damage. A pain impulse is transmitted from the nociceptors to the spinal cord along either A-delta or C-delta fibers. Because they are covered with a sheath of insulating myelin, A-delta fibers transmit impulses quicker than the smaller C-delta fibers. A-delta fiber impulses are perceived as sharp, localized pain, such as a nose hit by a punch. C-delta fiber impulses are perceived as a diffused, dull, aching pain, such as the residual pain of a bruised nose.
Nociceptive fibers enter the spinal cord through the dorsal horn, synapses in the spinal cord, and then descend as the spinothalamis tract, which has two divisions. One the neospinothalamic tract ascends to the thalamus and projects to the somatosensory cortex where it transmits information about the quality, intensity, and location of the pain stimulus. The other, the paleospinothalamic tract occurs at many synapses and transmits impulses through the reticular system to terminate in the thalamus, with projections to the limbic and subcortical areas. The brain then processes the impulses for a physical response.