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Muscle fiber types in skeletal musclebased on the predominant pathway of ATP synthesis, amount of myoglobin (oxygen transport protein)1. Slow-twitch fibers (ST fibers)also called red fibers, because they have increased amount of myoglobinwhich gives them a red color (increased oxygen capability)increased number of mitochondriacontain slow acting ATPase on myosin headuse mainly aerobic pathwaysendurance type muscles – prolonged contractions2. Fast-twitch fibers (FT fibers)small amounts of myoglobin (white fibers)few mitochondriafast ATPase enzymedepend on anaerobic energy pathwaysbest suited for rapid, intense movements, but will fatigue quicklyIntermediate fibers (Fast Oxidative-Glycolytic)oContain fast ATPase like FT fibers, but are more dependent on oxygen and contain higher levels ofmyoglobin like ST fibersCardiac muscleLocated in the heartFunction: pumps blood through circulatory systemMicroscopic anatomycells are short, branched and striatedusually have one centrally located nucleuscells are interconnected by Intercalated discsdesmosomes5
gap junctionsoion movement through gap junctions coordinates the contraction of the muscle cellsoi.e., direct electrical connection between cells Contractioncardiac muscle cells do not require neural input to contractthe heart contains specialized muscle cells called pacemaker cellsthat self-depolarize generating an action potentialneural innervation only alters the pace set by these cellsE-C coupling in cardiac muscleSource of excitation stimulusoPacemaker cellsSource of CalciumoAP triggers the influx of calcium from extracellular fluidoCalcium influx triggers the release of calcium from SR (Ca2+-induced Ca2+release) Amount of calcium released from SRoAmount of SR calcium release dependent on the amount of calcium that enters from the extracellular spaceIn skeletal muscle, SR Ca2+ release is maximal and increases in tension development of whole muscle is based on increasing the amount of fibers recruitedoAll-or-none principle of heart muscle applies to entire muscle not individual fibersIn skeletal muscle, all-or-none principle applies to muscel fibers within a motor unit Smooth muscleLocations:Arterial wallsWalls of hallow organs such as the bladderAlong respiratory, digestive, urinary, and reproductive tractsAnatomyoSmall, spindle-shaped, non-striatedoSingle nucleusoContain actin and myosin filaments, but not arrange in orderly sarcomeresoAlso contain intermediate filamentsContraction:oCalcium source: extracellular and SRoStimulus for contraction:Respond to local factors such as pH, O2and CO2levelsInnervated by autonomic nervous system oFor example: sympathetic division of autonomic nervous system causes contraction of smooth muscle within arteries (vasoconstriction)6