NUCLEUS & TRACTUS SOLITARIUS (VISCEROSENSORY)
The cell group in the brain stem that receives VISCEROsensory information is called NUCLEUS SOLITARIUS. This sad, little, lonely nucleus lies lateral to the dorsal motor nucleus X (pregang. parasym. of C.N. X), and next to (and sometimes surrounds, but this is hard to see in our sections) the solitary fasciculus or tract (somewhat similar arrangement as spinal nuc. and tract V eh?). Only a portion of the entire rostrocaudal axis of the solitary complex can be seen in our series of 10 brain stem sections, and this is on level #3. In order to show the rostral and caudal divisions of the solitary complex (which differ functionally), I have taken some artistic license in the drawing below by schematically illustrating the solitary complex as a column extending above (rostral part) and below (caudal part) level #3. I hope this is not confusing.
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 GANGLION of 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.Ns. 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.
For our PROBLEM SOLVING EXERCISES, WE WILL EQUATE A UNILATERAL LESION OF CAUDAL NUCLEUS SOLITARIUS WITH AN INCREASE IN HEART RATE (just like DORSAL MOTOR X). Loss of excitation of the dorsal motor nucleus X means dorsal motor X is firing LESS and loss of inhibition to the sympathetic neurons in the cord, means that they are firing more. This results in sympathetics dominating = heart rate UP!
READ ON ONLY IF YOU ARE INTERESTED. NOT ON EXAM!
There are chemoreceptors in the carotid and aortic bodies that affect respiration. The afferent information from the carotid and aortic bodies travel in C.N.s IX (carotid body; cell bodies in INFERIOR GANGLION IX) and X (aortic body; cell bodies in INFERIOR GANGLION X). The receptors in the carotid and aortic bodies respond to a decrease in arterial oxygen tension (P02) and an increase in arterial carbon dioxide (PCO2). For instance, an increase in PCO2 will result in an increase in the number of impulses traveling over C.N.s IX and X to the caudal nucleus solitarius. Neurons in nucleus solitarius project to the phrenic nucleus, which consists of a group of neurons in the ventral horn of the spinal cord from C3-C5. Axons arising from the phrenic nucleus comprise the phrenic nerve that innervates the diaphragm. Cells in nucleus solitarius also project to neurons in the spinal cord that innervate the intercostal muscles. Therefore, an increase in PCO2 will result in an increase in the depth and rate of breathing, while a decrease in PCO2 will have the opposite effect.
This is NOT a course in respiratory or cardiovascular physiology! However, it is extremely important for you to remember that the brain stem, especially the medulla, is an important region for the control of respiration and cardiovascular functions. BILATERAL lesions of the caudal nucleus solitarius will result in major respiratory and cardiovascular problems that result in death.
AN INTERESTING CLINICAL OBSERVATION - KNOW THIS COLD
Clinical case reports mention that lesions of the medulla that involve the area slightly ventral and lateral to nucleus and tractus solitarius result in HICCUP. One (of several) explanations for this finding is that such a lesion irritates descending information from nucleus solitarius to the phrenic nucleus. The phrenic nucleus consists of a functionally related group of cell bodies in the ventral horn from C3-C5. Axons arising from the phrenic nucleus comprise the phrenic nerve, which innervates the diaphragm. The hiccups result from spasmodic lowering of the diaphragm that causes a short, sharp inspiratory cough.
I want you to remember that brain stem lesions involving the area ventral and lateral to nucleus and tractus solitarius (I have cleverly designated it the HICCUP area, but this has not been carefully studied) result in HICCUP. KEEP THIS IN MIND WHEN THE DOING PROBLEM SOLVING EXERCISES.
The ROSTRAL portion of the solitary complex is a component of the TASTE PATHWAY. The axons within the rostral tractus solitarius are the central processes of cells within THREE cranial nerve ganglia, the GENICULATE GANGLION OF C.N. VII, the INFERIOR GANGLION of C.N. IX and the INFERIOR GANGLION of C.N. X. These central processes comprise the rostral tractus solitarius and terminate within the ROSTRAL or GUSTATORY portion of the nucleus solitarius. The peripheral processes of these neurons innervate the TASTE BUDS of the tongue in the following distribution:
C.N. VII=anterior twothirds
C.N. IX=posterior onethird
C.N. X=taste buds on epiglottis
Like other ascending sensory pathways, taste information heads for the thalamus (the great GATEWAY to the cerebral cortex!), and in particular to the ventral posteromedial nucleus (VPM; the nucleus of the HEAD!; after all, the tongue is in the head; remember the trigeminal-VPM relationship). The third neuron in the pathway i.e., the thalamic VPM neuron, then sends its axon to the ventral lateral portion of the postcentral gyrus, areas 3, 1, and 2. UNLIKE OTHER ASCENDING SENSORY PATHWAYS, THE SOLITARIOTHALAMIC TRACT (STT) IS UNCROSSED, repeat, UNCROSSED, repeat, UNCROSSED.
A LESION OF THE ROSTRAL NUCLEUS AND TRACTUS SOLITARIUS WILL RESULT IN THE LOSS OF TASTE FROM THE IPSILATERAL ONE-HALF OF THE TONGUE. SO WILL A LESION OF THE SOLITARIOTHALAMIC TRACT
You should also keep in mind that the interruption of the solitariothalamic tract will not result in major problems in respiratory and cardiovascular control, since most of the pathways over which the nucleus solitarius controls these functions pass caudally in the brain stem. Lets reserve a loss of taste from the ipsilateral side of the tongue for lesions of the solitariothalamic tract. Lesions of the rostral nucleus solitarius will also result in loss of taste from the ipsilateral side of the tongue. In contrast, a lesion of the CAUDAL portion of nucleus solitarius will result in an INCREASE IN HEART RATE.
LESION of the ROSTRAL SOLITARIUS=LOSS of IPSILATERAL TASTE
LESION of the CAUDAL SOLITARIUS=INCREASE HEART RATE