Point 11
Intro
VISCEROSENSORY information reaches
the medulla, primarily (but not exclusively) via C.N. X
(vagus; the wanderer). We are generally not aware of the
viscerosensory information conveyed by the vagus. Most of
these messages are related to the status of the viscera (for
example, information from receptors in the walls of the
viscera, including the entire digestive system to the middle
of the transverse colon and in the respiratory system from
the larynx to the pulmonary air sacs of the lung).
The
cells of origin of vagal fibers that convey messages from
the viscera lie in the INFERIOR GANGLIA X.
The peripheral processes of these neurons pass to
viscera, while the central processes pass into the medulla
to comprise FASCICULUS SOLITARIUS and synapse in the
adjacent NUCLEUS SOLITARIUS. As I have mentioned
above, the solitary complex is functionally divided into
CAUDAL and ROSTRAL portions, and the
viscerosensory information that we are talking about now
synapses within the CAUDAL
NUCLEUS SOLITARIUS. Neurons in the CAUDAL part
of nucleus solitarius possess axons that convey information
about the status of the viscera to many areas of the brain
that are involved in the reflex control of the viscera. For
instance, information is sent from the caudal nucleus
solitarius to the dorsal motor nucleus X (preganglionic
parasympathetic; increases peristalsis) and to the lateral
cell column of the spinal cord (preganglionic sympathetic;
decreases peristalsis). Messages are also sent to
respiratory centers in the brain stem (we will not cover
these areas in this course, but you will learn them in
Physiology).
VISCEROSENSORY information regarding blood pressure is conveyed via both C.N. IX and X. For example, information from the carotid sinus travels over C.N. IX (cell bodies are in INFERIOR GANGLION IX). The carotid sinus is a region near the bifurcation of the internal and external carotids. In this area the wall of the artery is thinner and contains a large number of branching, vinelike endings of C.N. IX. This area serves as a pressure receptor (baroreceptor; baros=weight). An increase in arterial pressure increases the rate of impulses in the fibers of C.N. IX that innervate the carotid sinus, above the baseline ("normal") number of impulses, and this information passes into caudal nucleus solitarius. This results in more impulses being sent from excitatory neurons in nucleus solitarius to the DORSAL MOTOR X (C.N. X). This leads to an increase in the number of impulses sent from dorsal motor X to the heart (of course not directly). This will SLOW the heart rate. Cells in nucleus solitarius also project to the preganglionic sympathetic neurons in the upper thoracic spinal cord. An increase in blood pressure in the carotid sinus will lead to an increase in firing of the fibers of C.N. IX that reach caudal nucleus solitarius. This will result in an increase in firing of inhibitory neurons in caudal nucleus solitarius that project to preganglionic sympathetic neurons in the thoracic cord. This increase in the amount of inhibition reaching the preganglionic sympathetic neurons will lead eventually to reflex-lowering of the blood pressure. While similar connections and functions are associated with the baroreceptors in the arch of the aorta, C.N. X instead of C.N. IX is involved. You should be able to figure out what happens following a decrease in blood pressure in the carotid sinus.