DORSAL MOTOR NUCLEUS OF THE VAGUS (C.N. X)
This nucleus lies slightly dorsal and lateral to the hypoglossal nucleus. Axons arising from cells within the dorsal motor X give rise to PREGANGLIONIC PARASYMPATHETIC fibers that course ventral and lateral from the nucleus to exit the brain stem dorsal and lateral to the inferior olive. These axons comprise the visceromotor component of the vagus nerve (C.N. X). Preganglionic parasympathetic fibers from the vagus wander all over the place, but eventually terminate in ganglia that contain postganglionic neurons. These terminal ganglia (NOT the sympathetic chain!) are located close to, or within the structures innervated by the short postganglionic fibers (do you remember Auerbachs plexus [myenteric] and Meissners plexus [submucosal] within the gut?).
Preganglionic parasympathetic visceromotor fibers from the vagus activate postganglionic neurons in ganglia associated with the pharynx, larynx and esophagus. Short postganglionic parasympathetic fibers in turn innervate GLANDS and SMOOTH MUSCLE in these structures.
Cardiac branches of the vagus carrying visceromotor fibers synapse on ganglia within the cardiac plexuses (superficial and deep). While some investigators have shown that stimulation of the right and left vagi have different effects upon the heart, I will leave those differences for the cardiovascular section of your physiology course. For now I want you to know that vagal stimulation slows heart rate. For our PROBLEM SOLVING exercises a unilateral lesion of either dorsal motor X (right or left) will result in an INCREASE IN HEART RATE (TACHYCARDIA). This increase in heart rate is the result of losing input from the dorsal motor nucleus, which itself slows the heart (the dorsal motor X is sometimes called the cardioinhibitory center). The sympathetic portion of the autonomic nervous system is left in control. The preganglionic sympathetic fibers arise from the lateral cell column of spinal cord segments T1-T5 and synapse in the three cervical sympathetic ganglia. Postganglionic sympathetic fibers arising from these ganglia pass through the cardiac plexuses and innervate sinoatrial and atrioventricular nodal tissue, conducting tissue and ventricular myocardium. Stimulation of the sympathetics increases nodal firing rate, conduction rate and ventricular force. The brain stem input to the lateral cell column will be discussed later under Point #11 (nucleus solitarius).
LESION OF DORSAL MOTOR X = INCREASE IN HEART RATE
READ ON ONLY IF YOU ARE INTERESTED. I WILL NOT, REPEAT, WILL NOT ASK YOU ANY QUESTIONS ABOUT WHAT FOLLOWS.
The visceromotor fibers from the vagus that reach the stomach and gut end in postganglionic neurons that lie near (gastric plexus) or in (myenteric and submucosal plexuses) the organs. Stimulation of the dorsal motor nucleus results in increased peristalsis and secretion of gastric and intestinal juices, and relaxation of sphincters. You might remember from Gross Anatomy that the right and left vagus nerves exchange fibers on the outer surface of the esophagus. They then enter the abdomen as anterior (or ventral) and posterior (or dorsal) vagal trunks. There is considerable mixing of the right and left vagi as they innervate the stomach and intestine.
Afferent sources (inputs) to the dorsal motor nucleus include the hypothalamus, olfactory system, autonomic centers in the reticular formation, and especially the nucleus solitarius. Most of these afferent sources you have never heard of. So, right now remember that the dorsal motor nucleus X plays an important role in various visceral reflexes. Thus, information about the internal milieu reaches the dorsal motor nucleus of X via visceral afferent pathways that we will soon talk about. The dorsal motor nucleus X receives these afferent messages and then sends information to the appropriate organ(s) (via the terminal ganglia).