Pain can be classified according to its source,
whether external or internal to the body’s
nervous system. The type of pain that most
readily comes to mind is called somatic pain,
which is experienced when pain receptors in
tissues such as skin, muscles, bones, and joints
detect a painful stimulus. This type of pain is
usually sharp and well localized, meaning that
it is easy to determine where in the body it
originates. On the other hand, visceral pain
originates in the viscera, or internal organs,
when pain receptors are stimulated, and is
experienced as a vague dull ache, difficult to
pinpoint. Visceral pain may even be referred to
a different part of the body so that, for instance,
pain caused by a heart attack maybe felt in the
left arm. Neuropathic pain differs from somatic
and visceral pain in that it originates in the
nervous system itself, occurring when nerves
are injured through degeneration, pressure,
inflammation, or infection. Injured nerves
become electrically unstable, firing off random
and unprovoked signals that the brain perceives
as pain, often described as lancinating, shooting,
or burning pain. Like visceral pain, neuropathic
pain may be referred to another part of the body
as in the case of phantom pain, when a patient
feels pain in a limb that is no longer physically
capable of sensing pain or any other stimulus as
a result of paralysis or amputation.