The nerves which supply the heart are from the cardiac plexuses which lie on the bifurcation of the
trachea. They consist of sympathetic, parasympa-thetic, and sensory fibres. Sympathetic fibres come from the cervical and upper thoracic ganglia of the sympathetic trunk. Sensory fibres transmit-ting pain run with the sympathetics. Parasym-pathetic fibres arise in the cells of the cardiac plexuses and in ganglion cells scattered along the vessels of the heart. The preganglionic fibres come from the vagus. The vagus also transmits sensory fibres. Stimulation of the sympathetic fibres di-lates the coronary arteries and causes an increase in the rate and strength of cardiac contraction. Va-gal stimulation slows or even stops the heart. The nerve fibres reach the heart along the great arteries (forming coronary plexuses on the coronary ves-sels) and veins. Many nerves accompany the con-ducting tissue of the heart [Fig. 4.45].
Superficial cardiac plexus
The superficial cardiac plexus is the left extremity of a complicated plexus of nerve cells and fibres which extends from the bifurcation of the trachea
Sino-atrial node Superior vena cava
Inferior vena cava
Fibrous trigone Right auricle
Left auricle
Left atrioventricular valve
Branch of L. crus to ant. papillary M., cut Branch to post.
papillary M.
L. crus between membranous and muscular parts of septum R. crus entering septomarginal trabecula R. crus entering subendocardial
net at ant. papillary M.
Atrioventricular sinus and atrioventricular node Atrioventricular bundle and
cut edge of septal cusp
Post. papillary muscle, R. ventricle
Fig. 4.44 The conducting system of the sheep’s heart as seen in an injected specimen.
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Conducting system of the heart
to the concavity of the arch of the aorta. It is con-tinuous on the surface of the principal bronchi with the pulmonary plexuses. Together these plexuses send nerve fibres to the heart and lungs.
The superficial cardiac plexus receives nerve fibres from the superior cervical cardiac branch of the left sympathetic trunk and the inferior cervical cardiac branch of the left vagus. (The deeper part receives the remaining cardiac branch-es from the sympathetic trunk and the vagus on
both sides.) The cardiac branches of the vagus and sympathetic trunks descend obliquely from the neck as a result of the caudal displacement of the heart during development. Branches from the superficial part of the cardiac plexus pass to the heart predominantly on the pulmonary trunk, but it is virtually impossible to demonstrate the car-diac plexus satisfactorily by dissection.
Using instructions given in Dissection 4.15 trace the branches of the left vagus to the heart.
Right sympathetic trunk Left sympathetic trunk
Right vagus nerve Left vagus nerve
Right recurrent laryngeal nerve
Left recurrent laryngeal nerve Left recurrent laryngeal nerve
Thoracic cardiac branch of vagus nerve
Thoracic cardiac branch of vagus nerve Thoracic (sympathetic)
cardiac branches
Third thoracic sympathetic ganglion
Left common carotid artery Left subclavian artery
Thoracic (sympathetic) cardiac branches Trachea
Oesophagus
Deep cardiac plexus
Arch of aorta Superficial cardiac plexus
Ligamentum arteriosum Left pulmonary artery Superior vena cava
Brachiocephalic trunk
Fig. 4.45 The cardiac plexus.
This figure was published in Gray’s Anatomy: The Anatomical Basis of Clinical Practice, 40th Edition, Standring S. Copyright © Elsevier (2008).
DISSECTION 4.15 Branches of the left vagus Objective
I. To identify the ligamentum arteriosum, left recurrent laryngeal nerve, and cardiac nerves arising from the left vagus.
Instructions
1. Find the left vagus on the aortic arch. Follow its re-current laryngeal branch beneath the concavity of
the arch. The nerve lies postero-inferior to the liga-mentum arteriosum which unites the root of the left pulmonary artery to the aortic arch.
2. Identify the cardiac nerves on the aortic arch. They turn medially under the arch to join a plexus of nerve cells and fibres—the superficial cardiac plexus. This is the left extremity of the extensive cardiac plexus which sends branches to the heart and great vessels.
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The cavity of the thorax
Deep cardiac plexus
This interlacing plexus of parasympathetic (vagus) and sympathetic nerve fibres lies on the lowest part of the trachea, posterior to the arch of the aorta. It contains scattered groups of ganglion cells which are mainly parasympathetic.
The deep cardiac plexus receives numerous car-diac nerves from the sympathetic trunks and the vagus. (1) Cervical sympathetic branches from the right superior cardiac nerve, right and left mid-dle cardiac nerves, and right and left inferior car-diac nerves (the left superior cervical carcar-diac nerve goes to the superficial cardiac plexus). (2) Thorac-ic sympathetThorac-ic cardiac branches from the second to fourth thoracic sympathetic ganglia. (3) Cervi-cal cardiac branches of the vagi through their su-perior cervical, middle cervical, and right inferior cervical cardiac branches (the inferior cervical car-diac branch of the left vagus goes to the superficial cardiac plexus). (4) Thoracic cardiac branch of the right vagus. (5) Cardiac branches of both recurrent laryngeal nerves.
The deep and superficial cardiac plexuses form a single mass. Together they send efferent (motor) fibres: (1) directly to the atria and great vessels, and to the rest of the heart through the coronary plexuses; (2) to the lungs through the anterior parts of the lung roots (anterior part of the pulmonary plexus).
The cardiac plexuses are pathways through which the central nervous system controls the action of the heart and monitors blood pressure and respiration. In addition to efferent fibres, the plexuses transmit afferent (sensory) fibres from the great arteries, veins, and lungs through the vagus and upper thoracic ganglia of the sympa-thetic trunk. These fibres are responsible for car-rying the pain of ischaemic disease of the heart (angina pectoris).
Bronchi
The right and left principal bronchi are branches of the trachea [Figs. 4.16, 4.46, 4.47]. Each bron-chus passes inferolaterally into the hilus of the corresponding lung, in line with the inferior lobar bronchus. The extrapulmonary parts of the prin-cipal bronchi contain U-shaped cartilaginous bars similar to those in the trachea. As such, they are flattened posteriorly. The intrapulmonary parts of the bronchial tree are supported by irregular carti-lage plates and tend to be cylindrical in shape.
Right principal bronchus
The right principal bronchus is approximately 2.5 cm long. This bronchus is wider and more vertical than the left. % As such foreign bodies which en-ter the trachea tend to fall into the right bronchus, and lodge in one or other of its branches, most usually in the right inferior lobar bronchus [Figs.
4.46, 4.48]. When this happens, the air distal to the block is rapidly absorbed and that part of the lung collapses and becomes solid.
The right principal bronchus begins anterior to the right margin of the oesophagus and has the azygos vein posterior and superior to it. More later-ally, it is posterior to the ascending aorta, superior
Thyroid cartilage
Trachea
Common carotid A.
Left subclavian A.
Arch of aorta
Left principal bronchus
Left pulmonary A.
L. superior lobar bronchus R. sup.
lobar bronchus
Desc. aorta Oesophagus R. stem bronchus R. pulmonary A.
Fig. 4.46 The larynx, trachea, and bronchi. The thyroid gland is shown by the broken line.
Left pulmonary artery Right
pulmonary artery Right upper lobe bronchus
Carina
Descending aorta Fig. 4.47 Axial CT image through the upper thorax, in the lung window, showing the carina and the right upper lobe bronchus.