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Unformatted text preview: 101L - Sp09 EXCITATION- CONTRACTION COUPLING IN SMOOTH MUSCLE Two types of Smooth Muscle Visceral or Single Unit or Unitary GI Tract, Uterus, Bladder, Ureters, Arterioles Multiunit Large Blood Vessels, Sphincters, Ciliary Muscles Lack gap or low resistance junctions Extensive gap, low resistance junctions - electrically behaves as a unit. 1) Actin, Tropomyosin but no Troponin. 2) Myosin has 6 subunits : 2 heavy chains (each with myosin head and tails) and 4 light chains with 2 light chains attached to each myosin head. Light chains regulate contraction. 3) 10 actin molecules for each myosin (cf. skeletal where 2 actin for each myosin. 4) Contractile proteins not organized into sarcomeres. 5) Lacks T tubules and lateral sacs. Poorly developed SR. Coordinated activation dependent on neural input or action of local mediators Interstitial Cells of Cajal exhibit pacemaker activity - Generate Basal Electrical Rhythm (BER) Neural input has little affect on BER but does contribute to amplitude of the BER &amp; activation of cells. Other regulatory proteins, in addition to myosin light chains, include (1) calmodulin &amp; (2) caldesmon Contractile and Regulatory Proteins in Smooth Muscle Ex8ExcitContracSmMus.isf Page 1 Binds to Calmdulin (1) Binds to Caldesmon Changes Configuration of (2) Activates enzyme binds to Ongoing Dephosphorylation Coupling occurs through Calcium. Sarcoplasmic Calcium increases through ICa, Ca-Calmodulin Complex has 2 roles in E-C coupling Caldesmon-Ca- Calmodulin Caldesmon- Tropomyosin complex on actin myofilaments Unmasks myosin binding site on actin Myosin Light Chain Kinase....
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- Winter '09