Occurs in cisternae of inner mitochondrial membrane Muscle fiber types in skeletal muscle based 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 myoglobin which gives them a red color (increased oxygen capability) increased number of mitochondria contain slow acting ATPase on myosin head use mainly aerobic pathways endurance type muscles – prolonged contractions 2. Fast-twitch fibers (FT fibers) small amounts of myoglobin (white fibers) few mitochondria fast ATPase enzyme depend on anaerobic energy pathways best suited for rapid, intense movements, but will fatigue quickly Intermediate fibers (Fast Oxidative-Glycolytic) o Contain fast ATPase like FT fibers, but are more dependent on oxygen and contain higher levels of myoglobin like ST fibers Cardiac muscle Located in the heart Function: pumps blood through circulatory system Microscopic anatomy cells are short, branched and striated usually have one centrally located nucleus cells are interconnected by Intercalated discs desmosomes 5
gap junctions o ion movement through gap junctions coordinates the contraction of the muscle cells o i.e., direct electrical connection between cells Contraction cardiac muscle cells do not require neural input to contract the heart contains specialized muscle cells called pacemaker cells that self-depolarize generating an action potential neural innervation only alters the pace set by these cells E-C coupling in cardiac muscle Source of excitation stimulus o Pacemaker cells Source of Calcium o AP triggers the influx of calcium from extracellular fluid o Calcium influx triggers the release of calcium from SR (Ca 2+ -induced Ca 2+ release) Amount of calcium released from SR o Amount of SR calcium release dependent on the amount of calcium that enters from the extracellular space In skeletal muscle, SR Ca 2+ release is maximal and increases in tension development of whole muscle is based on increasing the amount of fibers recruited o All-or-none principle of heart muscle applies to entire muscle not individual fibers In skeletal muscle, all-or-none principle applies to muscel fibers within a motor unit Smooth muscle Locations: Arterial walls Walls of hallow organs such as the bladder Along respiratory, digestive, urinary, and reproductive tracts Anatomy o Small, spindle-shaped, non-striated o Single nucleus o Contain actin and myosin filaments, but not arrange in orderly sarcomeres o Also contain intermediate filaments Contraction: o Calcium source: extracellular and SR o Stimulus for contraction: Respond to local factors such as pH, O 2 and CO 2 levels Innervated by autonomic nervous system o For example: sympathetic division of autonomic nervous system causes contraction of smooth muscle within arteries (vasoconstriction) 6
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