Cellular Myocytes
The function of the heart as a pump is ultimately dependent on the coordinated contractions of its chambers to move blood throughout the body. These contractions are produced by cardiac myocytes, the muscle cells of the heart. Understanding of the structu
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Cellular Myocytes Vincent A. Barnett
Abstract
The function of the heart as a pump is ultimately dependent on the coordinated contractions of its chambers to move blood throughout the body. These contractions are produced by cardiac myocytes, the muscle cells of the heart. Understanding of the structure and function of these cells on an individual level provides insights into adaptations of the heart due to normal as well as pathophysiological changes over the course of a lifetime. Keywords
Actin • Action potential • Adenosine triphosphate • Gap junctions • Intercalated disk • Membrane potential • Myofibril • Myosin • Sarcomere • Sarcoplasmic reticulum • Tropomyosin • Troponin • Transverse tubules
12.1 General Cellular Morphology All human cells can be thought of as biological machines that are surrounded by a membrane bilayer (plasma membrane). The plasma membrane has a nominal thickness of ~5 nm (50 Å) and encloses the cellular machinery within the intracellular space whose environment is closely regulated for optimal performance. The average diameter of a nonmuscle cell is approximately 10–20 μm. The encapsulating membrane is primarily composed of a bilayer of phospholipids, bipolar molecules with hydrophilic head groups, and hydrophobic lipid tails (Fig. 12.1). In addition, the plasma membrane is studded with receptors (Fig. 12.1) for various biochemical signaling molecules (hormones, neurotransmitters, etc.). Also resident in the plasma membrane are a number of ion-specific pumps and channels which function to regulate the ionic composition of the internal environment of the cell (endoplasm). The interior of each cell contains V.A. Barnett, PhD (*) Department of Integrative Biology and Physiology, University of Minnesota, 6-125 Jackson Hall, 321 Church St SE, Minneapolis, MN 55455, USA e-mail: [email protected]
enzymes and organelles that are specialized to support a wide array of biological functions. Key organelles include: the nucleus (which contains the genetic blueprint for cellular function), mitochondria (which converts various energy sources to adenosine triphosphate, or ATP), the endoplasmic reticulum (protein and lipid synthesis as well as calcium storage), and the Golgi apparatus (which supports processing of newly synthesized proteins). The cells of each tissue anchor themselves together and to the surrounding connective tissues via membrane-bound anchoring proteins (Fig. 12.1).
12.2 Cardiac Muscle Cell Morphology Muscle cells are similar in that they contain these common organelles but distinct in that they also include an elaborate protein scaffold within the cell that is anchored to the cell membrane and the extracellular matrix of connective tissue (Fig. 12.2). Force generation arising from protein–protein interactions within the internal protein lattice leads to the contraction of the cells and pumping of blood by the heart. Mammalian cardiac cells are roughly cylindrical but may also include short branch-like projections. The cells have an asymmetric profile with diameters in the ran
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