Cardiac muscle cells possess the ability for spontaneous rhythmic contractions. The contraction or beat of the heart is regulated and coordinated by specialized and modified cardiac muscle cells that are found in nodes and muscle bundles. The beat of the heart is initiated at the sinoatrial (SA) node which consists of a group of specialized cardiac muscle cells located at the junction of the superior vena cava in the right atrium. The impulse spreads from this node along the cardiac mus-cle fibers of the atria. The impulse is then received at the atrioventricular (AV) node which is lo-cated on the inner or medial wall of the right ventricle adjacent to the tricuspid valve. Specialized cardiac muscle cells then conduct impulses from the AV node along the ventricular septum and into the ventricular walls. Within the ventricular septum, the specialized cells are grouped into a bundle, the AV bundle (of His). This bundle then divides into two main branches, a left and right bundle branch, the former going to the left ventricle and the latter to the right ventricle. The specialized con-ducting fibers carry the impulse at a rate that is approximately four times faster than the cardiac muscle fibers. They are responsible for the final distribution of the electrical stimulus to the my-ocardium. While the sinoatrial node on its own exhibits a constant or inherent rhythm, it is modu-lated by the autonomic nervous system. Thus, the rate of the heartbeat can be decreased by parasympathetic fibers from the vagus nerve or increased by fibers from sympathetic ganglia. The specialized conducting cells within the ventricles are referred to as Purkinje fibers. The cells that make up the Purkinje fibers differ from cardiac muscle cells in that they are larger and have their my-ofibrils located mostly at the periphery of the cell. Their nuclei are also larger. The cytoplasm be-tween the nucleus and the peripherally located myofibrils stains poorly, a reflection, in part, of the large amount of glycogen present in this part of the cell. ORIENTATION MICROGRAPH: The specimen shown here is a saggital section revealing part of the atrial wall (A) and the ventricular wall (V). Between these two portions of the heart is the atri-oventricular septum (AS). The clear space is the interior of the atrium. AAVVASASAVAS
PLATE 25 • CARDIAC MUSCLE, PURKINJE FIBERS KEY A, atrial wall AS, atrioventricular septum DICT, dense irregular connective tissue Ec, endocardium Ec’, deep endocardium Et, endothelium EtC, endothelial cells ID, intercalated discs M, myofibrils My, myocardium PF, Purkinje fibers SM, smooth muscle cells V, ventricular wall
Purkinje fibers, heart, human, Masson, ×180.
This micrograph shows the area in the rectangle of the orientation figure. At this site, the endocardium (Ec) has been separated by bundles of Purkinje fibers (bundle of His) (PF) coursing along the ventricle wall. Normally, the endocardium consists of three layers. The endothelium (Et) lining the ventricle is the most superficial, but is barely detectable at this magnification.
Purkinje fibers, heart, human, Masson, ×365; inset ×600.
This higher magnification is the boxed area in the above photomicrograph. It reveals the endothelial cells of the endocardium (EtC) and the underlying connective tissue containing smooth muscle cells (SM). Where the Purkinje fibers are cut in cross section or obliquely sectioned, the myofibrils (M) are seen at the periphery of the