Sinoatrial Node - Innervation

Innervation

The SA node is richly innervated by parasympathetic nervous system fibers (CN X: Vagus Nerve) and by sympathetic nervous system fibers (T1-4, Spinal Nerves). This unique anatomical arrangement makes the SA node susceptible to distinctly paired and opposed autonomic influences.

• Stimulation of the vagus nerves (the parasympathetic fibers) causes a decrease in the SA node rate (thereby decreasing the heart rate). Parasympathetic fibers cannot change the force of contraction, however,because they only innervate the SA node and AV node (which control heart rate only)***(NOTE: This statement is not entirely accurate. The following study: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2278718/ demonstrates how the parasympathetic nervous system, through the action of vagus nerve, exerts a negative inotropic effect upon the heart)
• Stimulation via sympathetic fibers causes an increase in the SA node rate (thereby increasing the heart rate and force of contraction). Sympathetic fibers can increase the force of contraction because in addition to innervating the SA and AV nodes, they innervate the atria and ventricles themselves.