|
Somatic
vs. Visceral Nociceptive Processing
The previous
discussion emphasized the neural mechanisms involved in transmitting
nociceptive information arising from somatic structures
from the periphery to the cerebral cortex. The neural processing
of visceral information has several different features
compared to processing of somatic information (Cervero
and Foreman, 1990; Foreman,
1997).
First, input
arising from somatic structures causes conscious sensation, but
visceral structures are innervated by neurons whose activation
does not result in conscious sensation but does produce reflexes.
Second, receptors
located in somatic structures can lead to a wide range of sensory
experience, including touch, tickle, pressure, or pain. In contrast,
visceral receptors either lead to no sensation or to discomfort
or pain.
Third, in
somatic structures the type of stimulus, such as a wisp of cotton
or a sharp needle, will evoke responses in different types of
receptors that ultimately lead to different sensations. On the
other hand, the range of visceral sensations is quite narrow:
degree of fullness (such as in the bladder) or pain that can arise
from the heart or gallbladder. Indeed, pain is the only sensation
that is elicited from the heart and gallbladder.
Fourth, the
location of somatic stimuli can be determined very precisely,
but visceral sensation is poorly localized and often referred
to nearby somatic structures.
Finally, there
are far fewer visceral sensory than somatic sensory fibers in
the dorsal roots. For example, in the cat only 2% of sensory fibers
originate from viscera in the thoracic and lumbar dorsal roots
(Cervero and Foreman, 1990;
Foreman, 1997).
Most of the
remainder of this chapter will relate the functional and anatomical
significance of these general considerations of visceral sensory
processing to the specific topic of cardiac pain.
|