Cardiac plexus

Cardiac plexus refers to a meshwork of neurons and ganglia which are responsible for innervating the cardiac tissue. It comprises of two sub-parts: superficial and deep. The superficial cardiac plexus lies between the arch of aorta and the pulmonary trunk, whereas the deep cardiac plexus is sandwiched deeply, between the arch of aorta and the tracheal bifurcation.

Both superficial and deep cardiac plexuses receive neuronal contributions from sympathetic and parasympathetic nervous systems:

Parasympathetic innervation comes from the cardiac branches of the vagus nerve. They contain preganglionic parasympathetic neurons which originate from the dorsal motor nucleus of vagus in the brainstem. They synapse with cell bodies of post-ganglionic neurons within the cardiac ganglia, located inside the cardiac plexus or within the atrial walls. Thereupon, post-ganglionic neurons project to the sinoatrial and atrioventricular nodes and to the atrial and ventricular musculature.

The sympathetic innervation arises from the cervical and thoracic cardiac nerves. These include:

• Superior cervical cardiac nerve (a visceral branch of the superior cervical ganglion),

• Middle cervical cardiac nerve (a visceral branch of the middle cervical ganglion),

• Inferior cervical cardiac nerve (a visceral branch of the inferior cervical ganglion or cervicothoracic ganglion; a.k.a. stellate ganglion),

• Thoracic cardiac nerves (branches of the upper four or five thoracic sympathetic ganglia).

The cervical and thoracic cardiac nerves contain post-ganglionic sympathetic neurons, which arise from the above-mentioned cervical and thoracic sympathetic ganglia. Whereas, their preganglionic sympathetic neurons originate from the lateral gray horns of the upper thoracic spinal segments.

Small cardiac branches then arise from the cardiac plexuses to distribute sympathetic and parasympathetic neurons to the tissues of the heart, including the nodal tissue of the conducting system, coronary vessels and cardiac musculature.

Functionally, the parasympathetic innervation decreases heart rate, reduces force of cardiac contraction and constricts the coronary arteries. While, the sympathetic innervation increases the heart rate and force of contraction.

Please note that some additional neurons situated within the cardiac plexus include the visceral afferent neurons. They channelize through the vagal cardiac branches and the sympathetic cervical and thoracic cardiac nerves to reach back to the central nervous system. The visceral afferents accompanying the parasympathetic neurons in the vagus nerve form an integral part of cardiac reflexes, by transmitting sensory signals regarding change in blood pressure and blood chemistry. Whereas, the visceral afferents accompanying the sympathetic neurons carry ischemic pain sensation from the heart to the sympathetic trunk.