Cardiac muscle 424348 219478051 2008-06-15T12:31:19Z Epbr123 1395162 Reverted edits by [[Special:Contributions/78.146.240.200|78.146.240.200]] to last version by Allstarecho (using [[WP:HG|Huggle]]) {{Merge|myocardium|date=March 2008}} {{Infobox Anatomy | Name = Cardiac muscle | Latin = | GraySubject = | GrayPage = | Image = Glanzstreifen.jpg | Caption = | Image2 = | Caption2 = | System = | MeshName = | MeshNumber = | DorlandsPre = | DorlandsSuf = | }} The '''cardiac muscle''' is a type of involuntary [[sarcomere|striated]] [[muscle]] found in the walls of the [[heart]]. As it [[muscular contraction|contracts]], it propels [[blood]] into the heart and through the blood vessels of the [[circulatory system]]. ==Metabolism== '''Cardiac muscle''' is adapted to be highly resistant to fatigue: it has a large number of [[Mitochondrion|mitochondria]], enabling continuous [[aerobic respiration]], numerous [[myoglobin]]s ([[oxygen]]-storing pigment), and a good blood supply, which provides nutrients and oxygen. The heart is so tuned to aerobic metabolism that it is unable to pump sufficiently in [[ischaemia|ischaemic]] conditions. At [[basal metabolic rate]]s, about 1% of energy is derived from [[anaerobic metabolism]]. This can increase to 10% under moderately [[Hypoxia (medical)|hypoxic]] conditions, but, under more severe hypoxic conditions, not enough energy can be liberated by [[Lactic acid#Exercise and lactate|lactate production]] to sustain [[Ventricle (heart)|ventricular]] contractions.<ref>Ganong, Review of Medical Physiology, 22nd Edition. p81</ref> Under basal aerobic conditions, 60% of energy comes from fat (free fatty acids and triacylglycerols/triglycerides), 35% from carbohydrates, and 5% from amino acids and ketone bodies. However, these proportions vary widely according to nutritional state. For example, during starvation, lactate can be recycled by the heart. This is very energy efficient, because one NAD<sup>+</sup> is reduced to NADH and H<sup>+</sup> (equal to 2.5 or 3 ATP) when lactate is oxidized to pyruvate, which can then be burned aerobically in the TCA cycle, liberating much more energy (ca 14 ATP per cycle). In the condition of diabetes, more fat and less carbohydrate is used due to the reduced induction of [[GLUT4]] glucose transporters to the cell surfaces. However, contraction itself plays a part in bringing GLUT4 transporters to the surface.<ref>S Lund, GD Holman, O Schmitz, and O Pedersen. Contraction Stimulates Translocation of Glucose Transporter GLUT4 in Skeletal Muscle Through a Mechanism Distinct from that of Insulin. PNAS 92: 5817-5821.</ref> This is true of skeletal muscle, but relevant in particular to cardiac muscle, since it is always contracting. Unlike [[skeletal muscle]], which contracts in response to [[nerve]] stimulation, specialized pacemaker cells at the entrance of the right atrium termed the [[sinoatrial node]] display the phenomenon of automaticity and are [[myogenic]], meaning that they are self-excitable without a requisite electrical impulse coming from the central nervous system. The rest of the myocardium conducts these action potentials by way of electrical synapses called [[gap junctions]]. It is because of this automaticity that an individual's heart does not stop when a neuromuscular blocker (such as [[succinylcholine]] or [[rocuronium]]) is administered, such as during [[general anesthesia]]. A single cardiac muscle [[cell (biology)|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 [[myocardial infarction|heart attack]]), uncoordinated contraction known as [[fibrillation]] can result. ===Intercalated disc=== An intercalated disc is an undulating double membrane separating adjacent cells in cardiac muscle fibers. Intercalated discs support synchronized contraction of cardiac tissue. They can easily be visualized by a longitudinal section of the tissue. Three types of membrane junctions exist within an intercalated disc—[[fascia adherens]], [[macula adherens]], and [[gap junctions]]. Fascia adherens are anchoring sites for actin, and connects to the closest sarcomere. Macula adherens stop separation during contraction by binding intermediate filaments joining the cells together, also called a desmosome. Gap junctions allow action potentials to spread between cardiac cells by permitting the passage of ions between cells, producing depolarization of the heart muscle. When observing cardiac tissue through a microscope, intercalated discs are an identifying feature of cardiac muscle ===Rate=== Specialized [[Cardiac pacemaker|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 contract 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: * (1) [[sympathetic nervous system]] (fight or flight response) * (2) [[parasympathetic nervous system]] (rest and repose) Since cardiac muscle is myogenic, the pacemaker serves only to modulate and coordinate contractions. 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. <!-- This is not the definition of fibrillation. This is not how fibrillation develops. The proposed scenario would not even cause fibrillation. Leaving this description as is will give readers a completely incorrect understanding of cardiac fibrillation. The heart is actually fairly resilient to losing the sinoatrial nodal pacemaker. The AV node takes over, as it is the next most rapidly depolarizing site. A dominant AV nodal pacemaker usually results in bradycardia around 40 beats per minute, but it's well organized and not immediately life-threatening as is fibrillation. --> ===Role of calcium=== In contrast to [[skeletal muscle]], cardiac muscle cannot contract in the absence of extracellular [[calcium]] ions as well as extracellular sodium ions. In this sense, it is intermediate between [[smooth muscle]], which has a poorly developed sarcoplasmic reticulum and derives its calcium across the sarcolemma; and [[skeletal muscle]] which is activated by calcium stored in the [[sarcoplasmic reticulum]] (SR). The reason for the [[calcium]] dependence is due to the mechanism of [[calcium-induced calcium release]] (CICR) from the SR that must occur under normal excitation-contraction (EC) coupling to cause contraction. ==Appearance== ===Striation=== Cardiac muscle exhibits cross striations formed by alternation segments of thick and thin protein filaments which are anchored by segments called [[T-lines]]. 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. ===T-Tubules=== Another histological difference between cardiac muscle and skeletal muscle is that the [[T-tubules]] in cardiac muscle are larger, broader and run along the Z-Discs. There are fewer T-tubules in comparison with Skeletal muscle. Additionally, cardiac muscle forms dyads instead of the triads formed between the T-tubules and the [[sarcoplasmic reticulum]] in skeletal muscle. ===Intercalated Discs=== Under [[light microscopy]], intercalated discs appear as thin, typically dark-staining lines dividing adjacent cardiac muscle cells. The intercalated discs run perpendicular to the direction of muscle fibers. Under electron microscopy, an intercalated disc'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 disc's path appears even more convoluted, with both longitudinal and transverse areas appearing in longitudinal section.<ref>{{BUHistology|22501loa}}</ref> [[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. ==References== <references/> *[http://web.indstate.edu/thcme/mwking/muscle.html Indiana State University, Muscle action] * {{GeorgiaPhysiology|2/2ch7/2ch7line}} == See also == *[[Myocardium]] * [[Heart]] * [[Circulatory system]] * [[Cardiac action potential]] * [[Calcium sparks]] * [[Troponin]] {{muscular system}} {{Muscle tissue}} [[Category:Cardiac anatomy]] [[Category:Muscular system]] [[da:Hjertemuskel]] [[de:Herzmuskel]] [[es:Músculo cardíaco]] [[pt:Músculo cardíaco]] [[sk:Srdcová svalovina]] [[sv:Hjärtmuskulatur]] [[th:กล้ามเนื้อหัวใจ]] [[vi:Cơ tim]] [[tr:Kalp kası]]