Innervation of the Heart
The innervation of the heart refers to the network of nerves that are responsible for the functioning of the heart. The heart is innervated by sympathetic and parasympathetic fibres from the autonomic branch of the peripheral nervous system. The network of nerves supplying the heart receives contributions from the right and left vagus nerves, as well as contributions from the sympathetic trunk. These are responsible for influencing heart rate, cardiac output, and contraction forces of the heart.
The Cardiac Plexus
The cardiac plexus is a network of nerves including both the sympathetic and parasympathetic systems. It is split into two parts. The superficial part is located below the arch of the aorta, and between the arch and the pulmonary trunk. The deep part lies between the arch of the aorta and the bifurcation of the trachea. Small mixed fibres (containing both sympathetic and parasympathetic fibres) branch off of the cardiac plexus and supply:
- the conduction system of the heart
- the coronary vasculature
- the myocardium (muscle) of the atria and ventricles
The parasympathetic portions of the cardiac plexus receive contributions from the vagus nerve only. The preganglionic fibres, branching from the right and left vagus nerves, reach the heart. They enter the cardiac plexus by synapsing with ganglia within this plexus and walls of the atria.
Parasympathetic innervation is responsible for:
- reducing the heart rate
- reducing the force of contraction of the heart
- vasoconstriction (narrowing) of the coronary arteries
The sympathetic part of the cardiac plexus is composed of fibres from the sympathetic trunk, arising from the upper segments of the thoracic spinal cord. Fibres from the sympathetic trunk reach the cardiac plexus via cardiac nerves. The preganglionic fibres branch from the upper thoracic spinal cord and synapse in the lower cervical and upper thoracic ganglia. Postganglionic fibres extend from the ganglia to the cardiac plexus.
Sympathetic nerves are responsible for:
- increasing heart rate
- increasing the force of contraction of the myocardium
- the ‘fight or flight’ response, causing our heart to beat faster.
Afferent fibres also form part of the cardiac plexus. They return to the central nervous system via both the sympathetic cardiac branches and the cardiac nerves from the right and left vagus nerves.
The afferents passing through the vagal cardiac nerves return to the corresponding vagus nerve. These afferents provide feedback on blood pressure and blood chemistry.
In the sympathetic branch, the visceral afferents return to the upper thoracic and lower cervical ganglia. The fibres entering the upper cervical region are typically redirected down towards the upper thoracic portions of the sympathetic trunk, where they reenter the upper thoracic regions of the thoracic spinal cord, joining afferents from the thoracic ganglia. Sympathetic afferents relay pain sensation from the heart.
Cardiac Plexus Injury
Injury to the cardiac plexus, or in fact, to any of the contributaries to the cardiac plexus can impair the function of the heart. Damage to the vagus nerves, providing the parasympathetic innervation of the heart, will affect the ability to decrease the heart rate, leading to tachycardia. Similarly, damage to the sympathetic fibres contributing to the cardiac plexus can reduce the ability to increase heart rate, causing bradycardia.
The pain conducting sympathetic afferents transmit pain sensation. This pain sensation can be detected at a cellular level in incidences where tissue damage to the heart occurs. These incidences include cardiac ischemia and myocardial infarction. The pain can often be felt in cutaneous regions. This is because some of the dermatomes responsible for cutaneous innervation are supplied by the same spinal cord levels as the visceral afferents from the heart. This is the mechanism behind pain being felt in the right shoulder region that is a characteristic symptom of a myocardial infarction.