The Structure of the Specialized Impulse-Conducting System of the Steer Heart

The specialized impulse-originating and -conducting system of the steer heart has been analyzed with the light, phase contrast, and electron microscopes after careful dissection of its gross anatomic parts. The specific tissue is composed of cells with distinct cell boundaries. No syncytium exists. The cells of the sino-atrial (S-A) and atrioventricular (A-V) nodes closely resemble those of the common myocardium. They connect end to end via intercalated disks, most of which represent the starting point and termination of numerous myofibrils. The cells of the bundle of His and its distal branches are large and spindleshaped and joined in a staggered fashion. They display a fair number of myofibrils. The cell contact is established by numerous desmosomes that only rarely connect with myofibrils. It is believed that the multiple desmosome type of connection, present in most parts of the specific tissue of the steer heart, indicates that this tissue has maintained its embryologic appearance, to a large extent...