Other cardiac veins. Two additional groups of cardiac veins are also involved in the venous drainage of the heart.
The anterior veins of the right ventricle (anterior cardiac veins) are small veins that arise on the anterior surface of the right ventricle (Fig. 3.82A). They cross the coronary sulcus and enter the anterior wall of the right atrium. They drain the anterior portion of the right ventricle. The right marginal vein may be part of this group if it does not enter the small cardiac vein.
A group of smallest cardiac veins (venae cordis minimae or veins of Thebesius) have also been described. Draining directly into the cardiac chambers, they are numerous in the right atrium and right ventricle, are occasionally associated with the left atrium, and are rarely associated with the left ventricle.
The lymphatic vessels of the heart follow the coronary arteries and drain mainly into: brachiocephalic nodes, anterior to the brachiocephalic veins; and tracheobronchial nodes, at the inferior end of the trachea.
The musculature of the atria and ventricles is capable of contracting spontaneously. The cardiac conduction system initiates and coordinates contraction. The conduction system consists of nodes and networks of specialized cardiac muscle cells organized into four basic components: the sinu-atrial node, the atrioventricular node, the atrioventricular bundle with its right and left bundle branches, and the subendocardial plexus of conduction cells (the Purkinje fibers).
The unique distribution pattern of the cardiac conduction system establishes an important unidirectional pathway of excitation/contraction. Throughout its course, large branches of the conduction system are insulated from the surrounding myocardium by connective tissue. This tends to decrease inappropriate stimulation and contraction of cardiac muscle fibers.
The number of functional contacts between the conduction pathway and cardiac musculature greatly increases in the subendocardial network.
Thus, a unidirectional wave of excitation and contraction is established, which moves from the papillary muscles and apex of the ventricles to the arterial outflow tracts.
Impulses begin at the sinu-atrial node, the cardiac pacemaker. This collection of cells is located at the superior end of the crista terminalis at the junction of the superior vena cava and the right atrium (Fig. 3.83A).
This is also the junction between the parts of the right atrium derived from the embryonic sinus venosus and the atrium proper.
The excitation signals generated by the sinu-atrial node spread across the atria, causing the muscle to contract.
Concurrently, the wave of excitation in the atria stimulates the atrioventricular node, which is located near the opening of the coronary sinus, close to the attachment of the septal cusp of the tricuspid valve, and within the atrioventricular septum (Fig. 3.83A).
The atrioventricular node is a collection of specialized cells that forms the beginning of an elaborate system of conducting tissue, the atrioventricular bundle, which extends the excitatory impulse to all ventricular musculature.
The atrioventricular bundle is a direct continuation of the atrioventricular node (Fig. 3.83A). It follows along the lower border of the membranous part of the interventricular septum before splitting into right and left bundles.
The right bundle branch continues on the right side of the interventricular septum toward the apex of the right ventricle. From the septum it enters the septomarginal trabecula to reach the base of the anterior papillary muscle. At this point, it divides and is continuous with the final component of the cardiac conduction system, the subendocardial plexus of ventricular conduction cells or Purkinje fibers. This network of specialized cells spreads throughout the ventricle to supply the ventricular musculature, including the papillary muscles.