Circular layer of smooth muscle shows longitudinal views of smooth muscle

Circular layer of smooth muscle shows longitudinal

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Circular layer of smooth muscle (shows longitudinal views of smooth muscle fibers) Longitudinal layer of smooth muscle (shows smooth muscle fibers in cross section) Smooth Muscle Found in walls of most hollow organs (except heart) Usually in two layers (longitudinal and circular)
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Microscopic Structure Spindle-shaped fibers - thin and short compared with skeletal muscle fibers Lacks connective tissue sheaths; endomysium only Sarcoplasmic Reticulum Pouchlike infoldings ( caveolae ) of sarcolemma sequester Ca 2+ No sarcomeres, myofibrils, or T tubules
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Microscopic Structure of Smooth Muscle Fibers Longitudinal layer Circular layer Structure allows for peristalsis
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Figure 9.26 Innervation of smooth muscle. Varicosities Autonomic nerve fibers innervate most smooth muscle fibers. Smooth muscle cell Synaptic vesicles Mitochondrion Varicosities release their neurotransmitters into a wide synaptic cleft (a diffuse junction).
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Contraction of Smooth Muscle Slow, synchronized contractions Cells electrically coupled by gap junctions Some cells self-excitatory
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Contraction of Smooth Muscle Actin and myosin interact by sliding filament mechanism Final trigger is intracellular Ca 2+ ATP energizes sliding process
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Figure 9.28 Sequence of events in excitation-contraction coupling of smooth muscle. Extracellular fluid (ECF) Plasma membrane Cytoplasm Ca 2+ binds to and activates calmodulin. Inactive calmodulin Activated calmodulin activates the myosin light chain kinase enzymes. Inactive kinase The activated kinase enzymes catalyze transfer of phosphate to myosin, activating the myosin ATPases. Activated myosin forms cross bridges with actin of the thin filaments. Shortening begins. Thick filament Activated (phosphory- lated) myosin molecule Activated kinase Activated calmodulin Sarcoplasmic reticulum Inactive myosin molecule Calcium ions (Ca 2+ ) enter the cytosol from the ECF via voltage- dependent or voltage- independent Ca 2+ channels, or from the scant SR. Thin filament Ca 2+ Ca 2+ Ca 2+ 1 2 3 4 5
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