The heart sympathetic innervation is carried out by nerve fibers which arise from neurons located in the medulla oblongata. Most of these fibers nerve cells are found in the rostral ventrolateral aspect of medulla. When the activity of these neurons increases, there is cardiac stimulation and systemic vasoconstriction. Here, sympathetic nerve cells have spontaneous action potential activity, resulting in tonic stimulation of the heart muscle and vasculature (coronary arteries). Thus, acute denervation of the heart and systemic blood vessels generally results in cardiac slowing and systemic vasodilation.
During the sympathetic innervation of the heart, the axons from the sympathetic neurons extend out of the medulla and run down the spinal cord to establish synapse within the intermediolateral cell column of spinal cord. Then they exit at specific thoracolumbar levels (T1-T2). Then these preganglionic fibers synapse within sympathetic paravertebral ganglia (cervical, stellate, and thoracolumbar sympathetic chain), which lie on each side of spinal column. They also establish synapse within prevertebral ganglia, such as the celiac, mesenteric, and inferior mesenteric ganglia.
Some sympathetic nerve fibers also extend to the adrenal glands where they make synapse there with local nerve cells. Meanwhile, postganglionic sympathetic fibers runs towards target organs, where they innervate arteries and veins. As they travel, they send small nerve branches towards the adventitia layer of blood vessels. Meanwhile, varicosities, which are tiny enlargements within the sympathetic nerve fibers, are the sites of neurotransmitter release.
Below, diagram/drawing of the heart sympathetic innervation, which is marked with the black straight and dotted lines and arrows arising form spinal cord and the paravertebral ganglionic chains that run parallel to the spinal column. The red line and arrow is the parasympathetic innervation carried out by the vagus nerve.
