23 - Chapter 23. Cardiac and smooth muscle A. Myocardium -...

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Chapter 23. Cardiac and smooth muscle A. Myocardium - the specialized muscle of the heart As briefly introduced in Chapter 2, the myofibrils of cardiomyocytes are quite well organized and striations are apparent, just as in skeletal muscle. The cells are not exclusively arranged as a mass of parallel units because that is inappropriate for a hollow organ required to function as a pump. At a gross level, bands of cells are organized in a somewhat spiral fashion and make up several layers. The cells have a single nucleus and possess many obliquely branching processes, the ends of which form intimate contacts with adjacent cells. These contact areas are the intercalated disks , with some direct cytoplasmic continuity provided by gap junctions, mentioned in Chapter 11, and they function in the electrocoupling of adjacent cells. An individual cell makes such contact with about a dozen neighboring cells. APs can therefore pass rapidly from cell to cell, enabling the bulk of cells to be excited almost simultaneously. Functionally, this represents an electrical syncytium. The cardiac cells are arranged around a matrix of connective tissue elements that fuse in selected locations as fibrous bands. There is no insertion onto any skeletal structure. Support is also obtained from the pericardium , a tough membrane that envelops the heart, which additionally serves in providing lubrication of the surface of the myocardium. Myocardiocytes are spontaneously active, even when totally isolated from the body as dispersed cells. The cells have a wide range of intrinsic beating frequencies but because they are so tightly electro- coupled within the heart, pacing of Figure 23-1. The conduction system of the heart. This cut- away view from the ventral side shows the specialized nodes, one of which - the sino-atrial node (1) – serves as the heart pacemaker. The atrio-ventricular node (2) is also located in the wall of the right atrium; it supplies the common His bundle (3). The bundle divides into two major branches (3’) within the interventricular wall that pass towards the apex of the heart. Each branch then arborizes into Purkinje fibers (4), extending cranially into the peripheral walls of the ventricles. excitation is dictated by the cells with highest intrinsic frequency. A group of cells in the sino-atrial node (SA node) generally serves to control excitatory events in the rest of the heart. This constitutes the heart pacemaker (Figure 23-1). In Chapter 34 it will become evident that the SA node is usually subject to inhibitory motor innervation from the autonomic nervous system, in striking contrast to neural control over skeletal muscle, which is exclusively excitatory. Atrial myocardiocytes possess a rich
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This note was uploaded on 03/23/2009 for the course ANSCI 1110 taught by Professor Brucecurrie during the Fall '08 term at Cornell University (Engineering School).

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23 - Chapter 23. Cardiac and smooth muscle A. Myocardium -...

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