8.7 Cardiac Muscle

Cardiac muscle tissue is only found in the heart. Highly coordinated contractions of cardiac muscle pump blood into the vessels of the circulatory system. Similar to skeletal muscle, cardiac muscle is striated and organized into sarcomeres, possessing the same banding organization as skeletal muscle (Figure 8.7.1). However, cardiac muscle fibers are shorter than skeletal muscle fibers and usually contain only one nucleus, which is located in the central region of the cell. Cardiac muscle fibers also possess many mitochondria and myoglobin, as ATP is produced primarily through aerobic metabolism. Cardiac muscle fibers cells also are extensively branched and are connected to one another at their ends by intercalated discs. An intercalated disc allows the cardiac muscle cells to contract in a wave-like pattern so that the heart can work as a pump.

Intercalated discs are part of the sarcolemma and contain two structures important in cardiac muscle contraction: gap junctions and desmosomes. A gap junction forms channels between adjacent cardiac muscle fibers that allow the depolarizing current produced by cations to flow from one cardiac muscle cell to the next. This joining is called electric coupling, and in cardiac muscle it allows the quick transmission of action potentials and the coordinated contraction of the entire heart. This network of electrically connected cardiac muscle cells creates a functional unit of contraction called a syncytium. The remainder of the intercalated disc is composed of desmosomes. A desmosome is a cell structure that anchors the ends of cardiac muscle fibers together so the cells do not pull apart during the stress of individual fibers contracting (Figure 8.7.2).

 

Contractions of the heart (heartbeats) are controlled by specialized cardiac muscle cells called pacemaker cells that directly control heart rate. Although cardiac muscle cannot be consciously controlled, the pacemaker cells respond to signals from the autonomic nervous system (ANS) to speed up or slow down the heart rate. The pacemaker cells can also respond to various hormones that modulate heart rate to control blood pressure.

The wave of contraction that allows the heart to work as a unit, called a functional syncytium, begins with the pacemaker cells. This group of cells is self-excitable and able to depolarize to threshold and fire action potentials on their own, a feature called autorhythmicity; they do this at set intervals which determine heart rate. Because they are connected with gap junctions to surrounding muscle fibers and the specialized fibers of the heart’s conduction system, the pacemaker cells are able to transfer the depolarization to the other cardiac muscle fibers in a manner that allows the heart to contract in a coordinated manner.

Glossary

autorhythmicity

ability of cardiac muscle to initiate its own electrical impulse that triggers the mechanical contraction that pumps blood at a fixed pace without nervous or endocrine control

desmosomes

cellular structures that anchor the ends of cardiac muscle fibers together so the cells do not pull apart when fibers contract

intercalated disc

physical junction between adjacent cardiac muscle cells; consisting of desmosomes, specialized linking proteoglycans, and gap junctions that allow passage of ions between the two cells

gap junctions

form channels between adjacent cardiac muscle fibers that allow the depolarizing current produced by cations to flow from one cardiac muscle cell to the next

pacemaker cells

cluster of specialized myocardial cells that initiate and control heat rate

Glossary Flashcards

Image Descriptions

Figure 8.7.2. This multi-level anatomical diagram illustrates the structural organization of cardiac muscle tissue from the macroscopic to the microscopic level. On the left, a simplified drawing of the whole heart shows its external anatomy. A gray arrow points from the heart to the middle section, which displays several branching cardiac muscle fibers in coral-pink color. These fibers show characteristic features including intercalated discs (labeled at the top where fibers connect end-to-end), centrally located nuclei within the cells, gap junctions (specialized cell-to-cell connections marked with small squares), and blood capillaries running alongside the fibers. The fibers appear cylindrical and branch to connect with adjacent cells, demonstrating the three-dimensional network typical of cardiac muscle. Another gray arrow leads to the right panel, which provides a highly magnified longitudinal view of cardiac muscle cells. This detailed view shows two adjacent cardiac muscle fibers separated by an intercalated disc (the wavy white structure running vertically through the center). The intercalated disc contains desmosomes (labeled at top, appearing as dark connection points) and gap junctions (labeled at bottom, shown as small green circles). Within each fiber, the characteristic striations of cardiac muscle are clearly visible as alternating light and dark horizontal bands representing the sarcomere organization. The yellow structures visible in the lower fiber represent mitochondria, which are abundant in cardiac muscle to meet its high energy demands for continuous contraction. [Return to Figure 8.7.2]

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