Figure 1220

Photomicrograph of a mitral valve. This photomicrograph shows a section through one of the two cusps of the mitral valve near its attachment to the fibrous ring. Both sides of the cusp are covered by the endothelium. Note that the valve exhibits a layered architecture. Beginning at the atrial side (top of the image), the first layer underlying the endothelium Is the spongiosa-not well developed in this part of the cusp. The second layer is the fibrosa, which forms the majority of the dense connective tissue in the core of the valve. The third layer, the ventricularis, is formed by dense connective tissue containing layers of elastic and collagen fibers. X125.

thicken. The valves become rigid and inflexible, which affects their ability to open and close.

Intrinsic Regulation of Heart Rate

Contraction of the heart is synchronized by specialized cardiac muscle fibers

Cardiac muscle can contract in a rhythmic manner without any direct stimulus from the nervous system. The pace of the beating action in the heart is initiated at the sinu-atrial (S-A) node, a group of specialized cardiac muscle cells located near the junction of the superior vena cava and the right atrium (see Fig. 12.17). The S-A node is also referred to as the pacemaker. The pacemaker rate of the S-A node is about 60 to 100/min. The S-A node initiates an impulse that spreads along the cardiac muscle fibers of the atria and along internodal tracts composed of modified cardiac muscle fibers. The impulse is then picked up at the atrioventricular (A-V) node and conducted across the fi brous skeleton to the ventricles by the atrioventricular (A-V) bundle (of His). The bundle divides into smaller right and left bundle branches and then into subendothelial branches commonly called Purkinje fibers.

The A-V bundle, the bundle branches, and the Purkinje fibers are modified cardiac muscle cells that are specialized to conduct impulses (Fig. 12.21). The nodes and A-V bundle and its branches are modified cardiac muscle fibers that are smaller than normal. The Purkinje fibers are modified

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