VAN 121: Veterinary Gross Anatomy-II (2+2)

CONDUCTING SYSTEM OF THE HEART
(Neuromyocardium)

• The parts concerned in the controlling and conducting mechanism of the heart are composed of myocardial tissue distinguished from the ordinary cardiac muscle, not only because it is less highly differentiated from the original cells, but also because the muscle fibres are intimately associated with numerous nerve cells and fibrils which probably have a share in the initiation and transmission of the rhythmic contraction of the heart pace maker.
• Its parts are named the sinu-atrial (SA) node, atrio-ventricular (AV) node, and atrio ventricular (AV) bundle, which end in the terminal subendocardial network.

• The sinu-atrial (SA) node is a small collection of vascular neuromycardium situated in the wall of the right atrium at the upper end of crista terminalis.
• The atrio ventricular (AV) node is a nodule of the same kind of vascular tissue, situated in the septal wall of the atrium immediately above the opening of the coronary sinus.
• The atrio-ventricular (AV) bundle is a pale bundle of the special muscle fibres with which the nerve fibres are associated.
• It springs from the node, runs forward on the septum and reaches the upper border of the ventricular septum. From here it continues as two chief divisions.
• One of these descends on the right side of the ventricular septum and passes by the moderator band to the lateral papillary muscle.
• The other branch descends on the left side of the septum and ramifies on the wall of the ventricle, through the bases of the papillary muscles.
• The bundle and its branches are surrounded by a fibrous sheath and are subendocardial.
• The terminal subendocardial network is spread out beneath the endocardium of the greater part of both the ventricles.
• It is derived from the branches of the artio-ventricular bundle at the bases of the papillary muscles.