Heart Parasympathetic Innervation

The heart parasympathetic innervation is conducted by specialized nerve fibers that arise from the left branch of the vagus nerve (cranial nerve X), which is the longest of the twelve pair of cranial nerves. It belongs to the parasympathetic division of the peripheral nervous system. Innervation from these type of nerve fibers expands of the walls of arteries, dilating its diameter, and slow down the heart rate, and it happens when we are relaxed, free from stress. Its function is the opposite to that of the sympathetic innervation, which accelerates the heart beat when it is induced by adrenaline.

As it descends down the neck, the left branch of the vagus nerve runs down into the thorax between the left common carotid artery and the left subclavian artery. Having wound around the aortic arch, it gives off pulmonary branches that travel to the lungs to form the pulmonary plexus as well as the thoracic cardiac branches, which join the sympathetic nerve fibers that come from the sympathetic paravertebral ganglia to form the cardiac plexus. Having made their way into the heart, the parasympathetic fibers innervate the sinoatrial node, contributing to reducing the heart rate.