Pain receptors are relatively unspecialized
nerve fiber endings that respond to
a variety of stimuli signaling possible or
real damage to tissues. These free nerve
endings also respond to other stimuli,
such as mechanical movement of the tissue
and temperature changes. Pain
fibers respond to small peptides, such as
substance P and bradykinins, which are
released by injured cells. This type of
response is termed slow pain. Fast pain responses (for example, a pin prick,
cold or hot stimuli) are a more direct
response of the nerve endings to
mechanical or thermal stimuli.
Pain is a distress call from the body signaling
some noxious stimulus or internal disorder.
Although there is no cortical pain center,discrete
areas have been located in the brain
stem where pain messages from the periphery
terminate. These areas contain two
kinds of small peptides, endorphins and
enkephalins, that have morphinelike or opiumlike
activity. When released, they bind
with specific opiate receptors in the midbrain.
They are the body’s own analgesics.
Just as pain is a sign of danger,
sensory pleasure is a sign of a stimulus
useful to the subject. Pleasure depends
on the internal state of an animal and
is judged with reference to homeostasis
and some physiological set point.
Evidence suggests that pleasure and
pain states are produced by release of
small neuropeptides, called endogenous
opioids, within the central nervous