Innervation of Heart
|NERVES OF HEART|
The heart is supplied by sympathetic and parasympathetic nerves that arise primarily in the cervical region because initially the heart develops in the neck. Later the heart migrates downward into the thorax, along with its nerves.
The cervical and upper thoracic sympathetic trunk ganglia contribute cardiac branches, all of which pass through the cardiac plexus, usually without forming synapses. These ganglia are ultimately distributed to the various layers of the heart wall through the coronary plexuses. Three pairs of sympathetic cardiac nerves are derived from the cervical ganglia of the sympathetic trunks, and others arise from the upper thoracic ganglia.
The superior cervical sympathetic cardiac nerve originates by several rootlets from the corresponding ganglion. It often unites with the superior vagal cardiac nerve(s), and this conjoined nerve then descends behind the carotid sheath, communicating en route through slender rami with the pharyngeal, laryngeal, carotid, and thyroid nerves. On the right side, the conjoined nerve passes posterolateral to the subclavian and brachiocephalic arteries and aortic arch; on the left it curves downward across the left side of the aortic arch. The middle cervical sympathetic cardiac nerve is often the largest of the cervical cardiac nerves. It is formed by filaments from the middle and vertebral ganglia of the sympathetic trunk. This cardiac nerve usually runs independent of the cardiac plexus but may unite with other cardiac nerves, and it is interconnected with tracheal, esophageal, and thyroid branches of the sympathetic trunks.
The inferior cervical sympathetic cardiac nerves consist of filaments arising from the stellate (cervicothoracic) ganglion and ansa subclavia. These cardiac nerves often combine with each other or with other cardiac nerves before reaching the cardiac plexus, and inconstant communications exist between these nerves and the phrenic nerves.
The thoracic sympathetic cardiac nerves are four or five slender branches on each side that arise from the corresponding upper thoracic sympathetic trunk ganglia. These cardiac nerves run forward and medially to the cardiac plexus. Some enter the plexus directly, whereas others are united for variable distances with filaments destined for the lungs, aorta, trachea, and esophagus.
The vagal (parasympathetic) cardiac branches vary in size, number, and arrangement but can be grouped as superior and inferior cervical and thoracic vagal cardiac nerves. The superior cervical vagal cardiac nerve forms from two or three filaments that leave the vagus in the upper part of the neck and usually unites with the corresponding sympathetic cardiac nerve. This conjoined nerve then descends to the cardiac plexus (see earlier). The inferior cervical vagal cardiac nerve(s), one to three in number, arise in the lower third of the neck and often join or communicate with the cardiac branches from the middle cervical sympathetic ganglia and the vertebral and/or stellate sympathetic ganglia. If they remain separate, these cardiac nerves lie posterolateral to the brachiocephalic artery and aortic arch on the right side and lateral to the left common carotid artery and aortic arch on the left side.
The thoracic vagal cardiac nerves are a series of filaments arising from the vagus nerve of each side, at or below the level of the thoracic inlet, and also from both recurrent laryngeal nerves, with the left contributing more filaments than the right. These often unite with other cardiac nerves in their passage to the cardiac plexus.
All the vagal and the sympathetic cardiac nerves converge on the cardiac plexus, and filaments from the right and left sides of the plexus surround and accompany the coronary arteries and their branches. The cardiac plexus lies between the concavity of the aortic arch and the tracheal bifurcation and is sometimes described as consisting of superficial and deep parts, although their depths vary minimally, and they are intimately interconnected. However, a superficial tenuous preaortic plexus exists over the ascending aorta.
A proportion of the vagal fibers relay in several ganglia present in the cardiac plexus. The largest, the ganglion of Wrisberg, lies below the aortic arch between the division of the pulmonary trunk and the tracheal bifurcation. Other, smaller collections of parasympathetic cells—the intrinsic cardiac ganglia—are located mainly in the atrial subendocardial tissue, along the AV sulcus and near the roots of the great vessels. Relatively few cardiac ganglia are found over the ventricles, but enough exist to question the view that the ventricular innervation is entirely or predominantly sympathetic.
The cardiac sympathetic and parasympathetic nerves carry both afferent and efferent fibers. The afferents transmit impulses to the central nervous system from discrete cardiac receptor endings and terminal networks plentiful in these reflexogenous zones, such as the endocardium around openings of the caval and pulmonary veins, over the interatrial septum, and in the AV valves. The efferents carry impulses that are modified reflexively by afferent impulses from the heart and great vessels. Efferent fibers are under the overall control of the higher centers in the brain, the hypo- thalamus, and the brainstem.
The more important pathways are illustrated in Plates 1-15 and 1-16. Afferents from the heart and the great vessels are shown traveling to the cord via the sympathetic cardiac nerves, whereas others are carried upward to nuclei in the medulla oblongata by the vagus nerves. The efferents pursue similar routes but travel in a centrifugal direction. The cell bodies of the afferent neurons are situated in the dorsal root ganglia of the upper four or five thoracic nerves and in the inferior vagal ganglia.
The preganglionic parasympathetic fibers are the axons of cells in the dorsal vagal nuclei, and these fibers relay in cardiac plexus or intrinsic cardiac ganglia. The preganglionic sympathetic fibers are the axons of cells located in the lateral gray columns of the upper four or five thoracic segments. These fibers enter the corresponding spinal nerves and leave them in white rami communicantes. which pass to adjacent ganglia in the sympathetic trunks. Some fibers relay in these ganglia, however, and the postganglionic fibers (the axons of ganglionic cells) are conveyed to the heart in the thoracic sympathetic cardiac nerves. Others ascend in the sympathetic trunks to form synapses with cells in the superior, middle, and vertebral ganglia, and the postganglionic fibers reach the heart via cardiac branches of these ganglia. Therefore the parasympathetic relays occur in ganglia near or in the heart, whereas the sympathetic relays are located in ganglia at some distance from the heart. Consequently, the parasympathetic postganglionic fibers are relatively short and circumscribed in their distribution.
Afferent and efferent fibers probably run in all the sympathetic and the parasympathetic cardiac nerves, although afferents may not be present in the superior cervical sympathetic cardiac nerves. Many afferent vagal fibers from the heart and great vessels are involved in reflexes depressing cardiac activity, and in some animals these fibers are aggregated in a separate “depressor nerve” and in humans may run in cardiac branches of the laryngeal nerves.
Despite their insignificant size, the thoracic sympathetic cardiac nerves carry many efferent accelerator and afferent fibers to and from the heart and great vessels. Other cardiac pain afferents run in the middle and inferior cervical sympathetic cardiac nerves, but after entering the corresponding cervical ganglia, they descend within the sympathetic trunks to the thoracic region before passing through rami communicantes into the upper four or five thoracic nerves, then to the spinal cord. Because many cardiac pain fibers run through the preaortic plexus, some advocate excision of this plexus as a simpler, safer alternative to upper thoracic sympathetic ganglionectomy for relief of angina pectoris.
|SCHEMA OF INNERVATION|
Afferent fibers from the pericardium are carried mainly in the phrenic nerves, although afferents from the visceral serous pericardium are conveyed in the coronary plexuses.