Control of heart contractions is mediated through a combination of autonomic innervation via the cardiac plexus as well as the conduction pathway through the heart muscle itself. To understand heart innervation, we must study the autonomic contribution to the cardiac plexus and how it influences the conduction system of the heart.
The cardiac plexus is divisible into a superficial cardiac plexus, between the aortic arch and the pulmonary artery, and a deep cardiac plexus, between the aortic arch and the tracheal bifurcation. The plexuses receive a combination of both sympathetic and parasympathetic fibers in the manner discussed below (Figure 4-4).
Autonomic innervation and conducting system of the heart.
Preganglionic sympathetic fibers originate bilaterally in the lateral horns of the gray matter of the spinal cord between the T1 and the T5 spinal cord levels and enter the sympathetic chain via the white rami communicantes. After entering the sympathetic chain, fibers travel to the cardiac plexus via two possible routes:
- Preganglionic sympathetic fibers synapse in the superior parts of the thoracic sympathetic chain and send postganglionic sympathetic fibers directly from the sympathetic ganglia to the cardiac plexuses via thoracic cardiac nerves.
- The preganglionic sympathetic fibers ascend through the sympathetic chain and synapse in either the superior, middle, or inferior cervical ganglia before sending off postganglionic sympathetic fibers via cervical cardiac nerves to the cardiac plexuses.
Both sympathetic and parasympathetic fibers carry visceral sensory fibers from the heart to the spinal cord and brain, respectively. However, the visceral sensory fibers within the cardiac branches from the cervical and superior five thoracic sympathetic ganglia are sensitive to ischemia
(tissue damage due to lack of oxygen
). These sensory fibers mediate the visceral pain associated with angina pectoris and myocardial infarctions. Such myocardial ischemic pain is often referred to regions of the T1–T4 dermatomes simply because the visceral sensory fibers enter the spinal cord at the same levels of the segments for the superior four thoracic spinal nerves. The brain cannot differentiate between sensory input from the spinal nerves and that from the visceral nerves and thus refers ischemic pain to the same dermatome.
Preganglionic parasympathetic (vagal) fibers in the left and right vagus nerves originate in the medulla oblongata and descend through the neck and into the thorax to the cardiac plexuses. The synapse of vagal preganglionic and postganglionic parasympathetic fibers occurs either in the cardiac plexus or in the walls of the heart near the SA node of the right atrium. Therefore, the cardiac plexus serves as a conduit not only for parasympathetic preganglionic and postganglionic and visceral sensory fibers but also for sympathetic postganglionic fibers.
In summary, mixed nerves from the cardiac plexus supply the heart with sympathetic fibers, which increase the heart rate and the force of contraction and cause dilation of the coronary arteries, and parasympathetic fibers, which decrease the heart rate, reduce the force of contraction, and constrict the coronary arteries.
Conducting System of the Heart
The autonomic branches from the cardiac plexus help to regulate the rate and force of heart contractions, through influencing the SA node and the AV node, as follows:
- SA node. The rhythm of the heart is normally controlled by the SA node, a group of automatically depolarizing specialized cardiac muscle cells located at the superior end of the crista terminalis, where the right atrium meets the superior vena cava. The SA node is considered the “pacemaker of the heart” and initiates the heart beat, which can be altered by autonomic nervous stimulation (sympathetic stimulation speeds it up, whereas vagal stimulation slows it down). The wave of depolarization sweeps down the walls of the atria, stimulating them to contract, and eventually reaches the AV node.
- AV node. The AV node is located in the interatrial septum just superior to the opening of the coronary sinus. This node receives impulses from the SA node and passes them to the AV bundle (of His).
- AV bundle (of His). The AV bundle begins at the AV node and descends through the fibrous skeleton of the heart before dividing into the left and right bundles (of His), corresponding to the left and right ventricles, respectively. This divergent pathway ensures that ventricular contraction begins in the region of the apex. Conduction ends near the aortic and pulmonic valves. Impulses also pass from the left and right bundle branches to the papillary muscles in the corresponding ventricles. In the right ventricle, the moderator band (septomarginal trabeculum) contains the right bundle branch.