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WSU BIOLOGY 251 - Topic 13

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BIO 251 1st Edition Lecture 13 Outline of Last Lecture I. Events in muscle contraction II. Factors of whole muscle tensionIII. Muscle metabolism & fiber types a. Skeletal fiber types IV. Control of motor movement Outline of Current Lecture I. Skeletal, smooth, cardiacII. Structure of smooth muscle III. Molecular basis of smooth muscle contractionIV. Multiunit vs. single unit smooth muscle V. Multiunit VI. Single unit VII. Smooth muscle mechanics VIII. Modification of contractile strength IX. Cardiac muscle X. Top hat questionsXI. Diseasea. Muscle dystrophy These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.i. Duchenne Current Lecture-Skeletal, Smooth & CardiacoAll have contractile system composed of thin actin filaments that slide past stationary thick myosin filaments in response to increase in Ca++oAll use ATP directly as energy sources for cross-bridge cycle oBut structure and organization of the 3 muscle fibers are different oBut mechanisms of excitations different oBut coupling of excitation and contraction different oBut contraction responses different -Smooth muscle structureoStructure of smooth muscle cells (12.34)-Found in walls of hollow organs and tubes -Contraction causes -forward movement of contents of tube-Examples: digestive tract & blood vessels -Spindle shaped, single nucleus-Smaller than skeletal muscle cells -Don't extend full length -Groups of smooth muscle cells are arranged in sheetsoSubcellular structure of smooth muscle -3 types of smooth muscle cell filaments-Thick myosin filaments -Longer than skeletal muscle -Thin filaments composed of-Actin -Tropomyosin -NO TROPONIN -10 to 15 thin filaments/thick filament instead of 6 in skeletal -Intermediate filaments only support cell shape -Myofibrils not formed, no sarcomeres arrangement -NO Z LINES -Smooth muscle has dense bodies made of same protein that make up z lines -Dense bodies found throughout cell and anchored to cell membrane -Actin filaments are anchored to dense bodies-No T tubule -Underdeveloped SR-Molecular basis of smooth muscle contraction (12.35)oCa++ channels on smooth muscle membrane open, Ca++ from the ECF diffuses into cell oEntering Ca++ causes SR to release small amounts of Ca++ functionally not very important -b/c smooth cells are so much smaller in diameter than skeletal cells, SR & T-tubules not needed to deliver Ca++ deep into muscle oCa++ activates enzyme called calmodulinoActivated calmodulin activates enzyme myosin light chain kinase (MLCK)oActivated MLCK phosphorylates myosin by splitting a phosphate off of ATP oPhosphorylates myosin binds w/ actin & cross bridge cycling beings oWhen ca++ removed by active transport out of smooth muscle cell, calmodulin and in turn MLCK return to inactive form, and enzyme called phophatase removes phosphate from myosin. Hence myosin because unphophorylated and no longer binds to actin and muscle cell relaxes oMechanism of stimulation of smooth depends on whether it is multiunit or single unit smooth muscle -Multiunit vs. single unit smooth muscle oMulti (rare)-Different, independent, functional units -Stimulated by ANS -Large airways, blood vessels, eye muscles oSingle (common)-Single unit -Fibers connected by gap junctions -Autonomic or myogenic stimulation-Multiunit smooth muscle (rare) (12.36)oOrganization -Smooth cells within smooth muscle are organized into different functional units-Each unit separately stimulated by nerves of Autonomic nervous system -These contractions are called neurogenic (nerve produced)-Each unit functions independently of other units-Similar to skeletal muscle -Rare, found in -Walls of large blood vessels -Large airways to lungs-Eye muscles related to distance vision -Iris of eye-Base of hair follicles (goose bumps)-Single unit smooth muscle (common) (12.36) oMuscle fibers in muscle make up a single unit & contract together oFibers linked by gap junctions-An AP anywhere in muscle propagates via gap junctions to all fibers, so whole muscle contracts together -EX: uterine walls need to contract together to expel baby from uterus, stomach walls also need to contract togetheroStimulated by -ANS-Myogenic activity (12.37)-Pacemaker activity -Pacemaker cell, membrane repolarized on its own b/c of Autonomic changes in channel permeability -Once AP fired in pacemaker it spreads to rest of smooth muscle cells via gap junctions -Slow wave protection-Gradual alternating hyperpolarizing and depolarizing swings in potential caused by cyclical changes in rate at which Na+ is actively transported across membrane -Threshold not always reached, but when it is AP's follow-Smooth muscle mechanics oModification of contraction strength in single unit smooth muscle -Fiber tension modified by varying cytosolic Ca++ concentration -As cytosolic Ca++ increases so does # of cycling cross bridges -Many single unit smooth muscles maintain low levels of cytosolic Ca++-Means low level of contraction always occurring (muscle tone)-Modification of Contractile Strength oSingle unit-Modify Ca++ in cytosol oMultiunit -Functional unit recruitment -Modify ca++ in cytosol oBoth: modify ca++ in cytosol-ANS -Hormones -Metabolites -Mechanical stretch oModification of contraction strength in multiunit smooth muscle -Functional unit recruitment -Motor unit recruitment of skeletal muscle -Varying cytosolic Ca++ concentration within a cell oFactors that modify cytosolic Ca++ -Contraction strength for both single and multiunit smooth muscle -Both branches of ANS-Hormones -Metabolites -Mechanical stretch -Drugs oConsiderable stretched smooth muscle can still develop tension -Urinary bladder-When full, stretched, but you need to develop more tension b/c you have to contract bladder to empty it oCan relax even when stretched -Caused by re-arrangement of cross bridges after stretching oSlow and economical -Slow rate of ATP use -Cross bridges latch onto thin filaments longer oBOTTOM LINE-Smooth muscle is highly specialized to economically maintain tension for longer periods w/o fatigue-Can accommodate variation in contents volume w/ little change in muscle tension-Cardiac MuscleoStructure -Striated-Thick and thin filaments highly organized-Contains troponin and tropomyosin


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