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Smooth Muscle and Nervous System

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BIOL 1117
Christopher Richardson

L21-Smooth Muscle and Nervous System 10/30/13 • Smooth Muscle o composed of myocytes that have a fusiform shape with only one nucleus, located near the middle of the cell o no visible striations  thick and thin filaments are present, but not aligned with each other o z discs are absent and replaced by dense bodies  ordered array of protein masses in cytoplasm o sarcoplasmic reticulum is limited and there are no T tubules o injured smooth muscle regenerates itself – through mitosis o Of the two types of smooth muscle, single-unit type occurs in walls of most blood vessels and in the walls of organs of digestive, respiratory, urinary, and reproductive tracts. o The more common single single unit type affects the flow of food, flow of air, the flow of blood, the flow of urine and more o Single unit type has myocytes that are connected with electrical connections via gap junctions; this allows the cells to directly stimulate each other which means they can contract together as a single unit o Smooth muscle contraction is involuntary o In contrast to skeletal muscle, in which membrane activation is dependent on somatic neurons, many different inputs to the plasma membrane of smooth muscle which can alter the contractile activity of the smooth muscle which can lead to either increasing contraction or decreasing contraction • Stimulation of Smooth Muscle o some smooth muscle can contract or relax in response to temperature and chemical stimuli o some smooth muscle contracts in response to stretch o some smooth muscle spontaneously depolarizes o All inputs alter contractile activity by altering the cytostolic concentration of calcium o Some smooth muscle lacks nerve fibers: even contracting without any change in membrane potential o Most smooth muscle is innervated by autonomic nerve fibers like cardiac muscle but not somatic motor nerve fibers o autonomic nerve fibers  can trigger and modify contractions of smooth muscle  stimulate smooth muscle with either acetylcholine or norepinephrine  These neurotransmitters have contrasting effects in different locations • Contraction of Smooth Muscle 2+ o contraction is triggered by Ca , energized byATP, and achieved by sliding thin past thick filaments o **Most Ca mostly enters the cell from ECF, not from sarcoplasmic reticulum  voltage, ligand, and mechanically-gated Ca channels  During relaxation, pump calcium back out of the cell o calcium binds to calmodulin on thick filaments  Smooth muscle has no troponin but does have tropomyosin which never blocks the active sites on actin  activates a kinase which usesATP to add a phosphate to regulatory protein on myosin head  This forces myosin head up from thick filament which allows it to bind to actin on thin filament.  Akey difference from skeletal muscle is calcium in smooth muscle changes the thick filament, not the thin filament o myosinATPase hydrolyzies ATP  extending out myosin head as part of preparing it for force generation during power stroke like skeletal muscle  However, unlike skeletal muscle myosin head is not extended out before attaching to actin • When myosin head is phosphorylated it can attach to actin but extends out at a later time o Thus, split ofATP once and release of energy (exergonic) to transfer a phosphate to myosin head to activate the head (endergonic) o Then split oneATP per cycle to extend out head (endergonic) which can be extended out repeatedly as each cycle repeats as long as the head stays phosphorylated o thick filaments pull on thin ones, thin ones pull on dense bodies and membrane plaques o force is transferred to plasma membrane and entire cell shortens • Contraction and Relaxation o contraction and relaxation very slow in comparison to skeletal muscle  MyosinATPase is much slower in smooth using less energy  calcium removal from cell is much slower in smooth than skeletal muscle  As calcium levels start to fall in cell, myosin is dephosphorylated by an enzyme which means myosin’s ability to hydrolyzeATP decreases and is unable to bind to actin o latch-bridge mec
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