FIGURE 13.10 • Photomicrograph of the ventricular wall containing the conducting system. This photomicrograph shows a Mallory-Azan–stained section of the ventricular wall of a human heart. The upper two thirds of the micrograph is occupied by the endocardium (E) containing a thick layer of Purkinje fibers. The free luminal surface of the ventricle (top) is covered by endothelium and an underlying layer of subendothelial connective tissue (stained blue). The deep layer of endocardium contains the Purkinje fibers. Note the intercalated discs in the fibers (arrows). The Purkinje fibers contain large amounts of glycogen, which appear as homogeneous, pale-staining regions that occupy the center portion of the cell surrounded by the myofibrils. The nuclei (N) are round and are larger than the nuclei of the cardiac muscle cells in the myocardium (M). They are frequently surrounded by the lighter-stained cytoplasm, which represents the juxtanuclear region of the cell. Because of the considerable size of the Purkinje cells, the nuclei are often not included in the section. Among the Purkinje fibers course nerves (NF ) that belong to the autonomic nervous system. 320.
the heart, course along both sides of the interventricular septum (see Fig. 13.5), and terminate as Purkinje fibers in the myocardium of the ventricles. The cells that form the Purkinje fibers are larger than ventricular muscle cells. Their myofibrils are located at the periphery of the cell. The nuclei are round and are larger than the nuclei of the cardiac muscle cells in the myocardium. Because of the considerable size of the cells, the nuclei are often not included in the section. Intercalated disks are present in Purkinje fibers, but they are variable in appearance and number depending on their location. They are positive for periodic acid–Schiff (PAS) staining because of the large amount of glycogen they contain. With hematoxylin and eosin (H&E) and most other stains, the glycogen-rich center portion of the cell appears homogeneous and stains pale (see Fig 13.10). Because of the stored glycogen, Purkinje fiber cells are more resistant to hypoxia than are ventricular muscle cells.
Systemic Regulation of Heart Function
As mentioned above, the heart beats independently of any nervous stimulation. This spontaneous rhythm of the heart can be altered by nerve impulses from both sympathetic and parasympathetic divisions of the autonomic nervous system. The autonomic nerves do not initiate contraction of the cardiac muscle but rather regulate the heart rate (a chronotropic effect) according to the body’s immediate needs.
Stimulation of the parasympathetic nerves decreases the heart rate.
The parasympathetic nerve supply to the heart originates in the vagus nerve (cranial nerve X). Presynaptic parasympathetic fibers synapse with postsynaptic neurons within the heart. Their short postsynaptic fibers terminate chiefly at the SA and AV nodes but also extend into the coronary arteries that supply the heart. The release of the neurotransmitter acetylcholine from the terminals of these fibers slows the heart rate (an effect known as bradycardia), reduces the force of the heartbeat, and constricts the coronary arteries of the heart.
Stimulation of the sympathetic nerves increases the heart rate.