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Cardiac muscle is a type of involuntary mononucleated, or uninucleated, striated muscle found exclusively within the heart. Its function is to "pump" blood through the circulatory system by contracting.
Initiation Unlike skeletal muscle, which contracts in response to nerve stimulation, and like smooth muscle, cardiac muscle is myogenic, meaning that it stimulates its own contraction without a requisite electrical impulse coming from the central nervous system. A single cardiac muscle cell, if left without input, will contract rhythmically at a steady rate; if two cardiac muscle cells are in contact, whichever one contracts first will stimulate the other to contract, and so on. This inherent contractile activity is heavily regulated by the autonomic nervous system. If synchronization of cardiac muscle contraction is disrupted for some reason (for example, in a heart attack), uncoordinated contraction known as fibrillation can result. This transmission of impulses makes cardiac muscle tissue similar to nerve tissue, although cardiac muscle cells are notably connected to each other by intercalated discs. Intercalated discs conduct electrochemical potentials directly between the cytoplasms of adjacent cells via gap junctions, in contrast to the chemical synapses used by neurons. Rate Specialized pacemaker cells in the sinoatrial node normally determine the overall rate of contractions, with an average resting pulse of 72 beats per minute. The central nervous system does not directly create the impulses to contact the heart, but only sends signals to speed up or slow down the heart rate through the autonomic nervous system using two opposing kinds of modulation: Since cardiac muscle is myogenic, the pacemaker serves only to modulate and coordinate contrations. The cardiac muscle cells would still fire in the absence of a functioning SA node pacemaker, albeit in a chaotic and ineffective manner. This condition is known as fibrillation. Note that the heart can still beat properly even if its connections to the central nervous system are completely severed. Role of calcium In contrast to skeletal muscle, cardiac muscle cannot contract in the absence of extracellular calcium ions as well as extracellular potassium ions. Striation Cardiac muscle exhibits cross striations formed by alternation segments of thick and thin protein filaments which are anchored by segments called Z-lines. However, cardiac muscle is not considered striated muscle and should not be referred to by this term. The primary structural proteins of cardiac muscle are actin and myosin. The actin filaments are thin causing the lighter appearance of the I bands in muscle, while myosin is thicker and darker lending a darker appearance to the alternating A bands in cardiac muscle as observed by a light enhanced microscope. Nuclei Cardiac muscle can be distinguished from skeletal muscle because cardiac muscle nuclei are centrally located among the myofibrils, unlike the peripheral nuclei of skeletal muscle. A unique aspect of cardiac muscle is the number of nuclei found inside the cell. Skeletal muscle cells are multinucleated from the fusion of muscle cells and smooth muscle cells are strictly mononucleated, and cardiac muscle cells are mononucleated in humans. In some animal species, the fetus and post parturition infant most cardiac muscle cells are mononucleated. Shortly after birth (within a few months) most cardiac muscles undergo a change of nucleation from mononucleated to primarily binucleated, and some go on to become multinucleated. Generally among species the cardiac muscle is 90% binucleated cells and 5% both mono and multinucleated cells, but exact numbers depend upon the species in question. All cells of a given motor unit are stimulated and contract at the same time. T-Tubules Another histological differences between cardiac muscle and skeletal muscle is that the T-tubules in cardiac muscle are shorter, broader and run along the Z-Discs. Additionally, the triads formed between the T-tubules and the sarcoplasmic reticulum of cardiac muscle is less well defined than those visible in skeletal muscle. Intercalated Disks Under light microscopy, intercalated disks appear as thin, typically dark-staining lines dividing adjacent cardiac muscle cells. The intercalated disks run perpendicular to the direction of muscle fibers. Under electron microscopy, an intercalated disk's path appears more complex. At low magnification, this may appear as a convoluted electron dense structure overlying the location of the obscured Z-line. At high magnification, the intercalated disk's path appears even more convoluted, with both longitudinal and transverse areas appearing in longitudinal section. Gap junctions (or nexus junctions) fascia adherens (resembling the zonula adherens), and desmosomes are visible. In transverse section, the intercalated disk's appearance is labyrinthine and may include isolated interdigitations. Reference See also | |||||||||||||
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