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Chapter 8. Muscle Tissue

8.7 Cardiac Muscle

Learning Objectives

By the end of this section, you will be able to:

  • compare and contrast the structure of cardiac and skeletal muscle;
  • describe intercalated discs and gap junctions; and
  • describe a desmosome.

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.

This image is a micrograph of cardiac muscle cells.
Figure 8.7.1 – Cardiac Muscle Tissue: Cardiac muscle tissue is only found in the heart (LM × 1600). (credit: Micrograph provided by the Regents of University of Michigan Medical School © 2012)

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).

 

This image shows the structure of the cardiac muscle. A small image of the heart is shown on the top left of the figure and then a magnified view of the cardiac muscle is shown, with the nucleus and the cardiac muscle fiber labeled. A further magnification shows the structure of the intercalated discs with the desmosome and gap junction.
Figure 8.7.2 – Cardiac Muscle: Intercalated discs are part of the cardiac muscle sarcolemma and they contain gap junctions and desmosomes.

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.

Section Review

Cardiac muscle is only found in the heart and is specialized to pump blood throughout the body. Cardiac muscle is striated, as its actin and myosin filaments are organized into sarcomeres. Cardiac muscle fibers are shorter than skeletal muscle fibers, usually contain only one nucleus, are extensively branched, and are connected to one another at their ends by intercalated discs. Intercalated discs, gap junctions, and desmosomes allow cardiac muscle cells to contract in a wave-like pattern so that the heart can work as a pump. Contractions of the heart are controlled by specialized pacemaker cells.

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

 


This work, Human Physiology, is adapted from Anatomy & Physiology by OpenStax, licensed under CC BY. This edition, with revised content and artwork, is licensed under CC BY-SA except where otherwise noted.

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