Point
8
Intro
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 #10 (nucleus solitarius).