I. Introduction!A. Muscle tissue-->movement of the body and changes in size and shape of internal organs. They are elongated cells in parallel arrays.!B. 3 types of muscle tissue!1. Skeletal muscle!- striated!- voluntary control!- large, multinucleated, up to 1 m in length!- periphally located nuclei!- limited reg. via satellite cells!- controls body movements!- attached to bone, extraocular eye muscle, tongue!2. Cardiac muscle!- striated!- involuntary control!- smaller than skeletal muscle!- branching cells w/centrally located nuclei!- limited regeneration!- walls of heart and base of large veins entering the heart!3. Smooth muscle - changes the shape and size of internal organs!- non-striated!- involuntary control!- smallest!- spindle shaped w/central nuclei!- can regenerate via cell division!- walls of hollow organs (blood vessels, reproductive organs, bladder, GI, resp. tract) and skin (arrector pili) and eye (ciliary muscle)!II. Skeletal muscle A. Development of skeletal muscle cells= muscle fibers!1. Mesenchymal cells (myoblasts) fuse to form multinucleated myotubes--> synthesize muscle specific proteins (actin + myosin) --> differentiate into skeletal muscle cells = muscle fibers!2. Mature skeletal muscle cells are multinucleated and can't divide (post-mitotic)!3. Some myoblasts don't fuse into myotubes-->stay mono-nucleated satellite cells (muscle stem cells) on the external surface of muscle fibers -->basis of regeneration B. Skeletal muscle regeneration 1. Muscle cells (all types) surrounded by external lamina (sim. basal lam.)!2. Satellite cells are within external lamina!- injury-->activate sat. cells -->proliferate and form new myoblasts -->fuse to form new myotubes (if the exterminal lamina is intact)!- if external lamina disrupted-->fibroblasts repair injured site -->scar tissue!C. Organization of skeletal muscle (e.g. bicep)!1. Each muscle cell surrounded by external lamina + a thin layer of CT (endomysium), contains reticular fibers + small blood vessels + small nerve fibers!2. Group of muscle cells/fibers organized into muscle fascicle!- fascicle is surrounded by thicker CT perimysium (w/larger blood vessels and nerves)!3. Group of muscle fascicles= a muscle (e.g. bicep)!- muscle is surrounded by dense, irregular CT epimysium w/large blood vessels and nerves!D. Organization of skeletal muscle cells (= muscle fibers= myofibers)!1. Myofilaments= myosin thick fils + actin thin fils!2. Myofibrils = contractile units = myofils + proteins!- cylindical, contracile bundles, highly ordered arrangment of overlapping myofils and proteins!- run parallel to the long axis of a muscle cells, extend entire length of cell, most of cell V!- show cross-striations in LM!- made up of end to end arrangement of sarcomeres!3. Sarcolemma= the pm of a muscle cell!- contains ion channels (voltage gated Na+ channels)!- can rapidly depolarize and propagate action potentials!- extends into cell as T-tubules!4. Sarcoplasm= cytoplasm of muscle cell!5. Sarcoplasmic reticulum= the ER of a muscle cell!- spec. for storage and release of Ca2+--> regulates contraction!- org. in tubular netowrk around the myofibrils!III. Organization of myofibrils in striated muscle A. Actin thin filaments (F-actin)!1. Subunits= G-actin (globular, polar [+ and - end] polypeptide monors!- assemble head to tail to form polar actin protofilament!- ATP-bound monomers add to the + end of growing actin fil= ATP cap!- After addition--> ATP hydrolyzed to ADP-->loss of ATP cap-->depolymerization!- Dynamic instability (length fluctuations) on smaller scale than MTs!2. 2 parallel protofils wrap around each other in R handed helix --> polar actin filament!B. Myosin motors = motors on actin!1. Multi-protein complexes, made of heavy and light chains that walk along actin fils!2. N terminal globular motor domains of heavy chains !- bind actin filaments and ATP, ATPase activity!- Use E of ATP hydrolysis to move towards + end of actin fils (except myosin VI) 3. C terminus coiled tail domains of heavy chains - bind cargo!- some form coiled coils w/other heavy chain tails!4. Can be monomers (1 heavy chain) or dimers (2 heavy chains)!5. Myosin I molecules !- often have membrane binding site in tail domains!- involved in organelle transport along actin fils and attachment of actin fils to the membrane of microvilli!6. Myosin V!- involved in vesicle and organelle transport!- transports melanosomes (pigment containing vesicles in melanocytes) along actin fils!C. Myosin II = what we have in muscle!1. Forms thick filaments (only isoform that can)!2. Functions in contraction of muscle and non-muscle cells (and cytokineses and whole cell locomotion)!3. Exists as dimers: 2 heavy chains + 2 copies of eaach of 2 light chains!- C term. coiled domains of 2 myosin IIs wrap around each other-->dimer w/ coiled coil tail and 2 globular motor domains!- N term. heads form the motor domain that binds to actin and binds/hydrolyzes ATP !4. Thick filaments of muscle cells formed by assoc. of myosin II dimers!- in striated muscle cells, form large, bipolar thick filaments w/200-300 myosin heads!- self-assemble into thick filaments!- coiled tail dimers wrap around each other in regular and staggered array!- head portions project out in a regular helical pattern from the actin filament!- are oriented in opp. directions in each half-->bipolar thick fil!- isolated motor domain heads = S1 heads - central bare zone= free of heads, just myosin tails!- myosin S1 heads reveal actin polarity!+ end = barbed end!- end = pointed end!D. Sarcomeres!1. basic contractile units of myofibril!2. made of overlapping arrays of myofilaments (thin actin fils + thick myosin fils) !3. form the portion of a myofibril between Z line to Z line!4. + ends of thin actin fils attached to Z discs at each end, - ends extend towards middle of sarcomere where they overlap with thick myosin fils!5. Banding pattern of cross striations!- A bands (anisotropic)= dark bands, length of thick fils, a portion contains overlapping thin fils - I bands (isotropic)= portions of thin fils which don't overlap thick fils - Z lines/discs= cap + ends and bundle thin fils - Sarcomere= basic contractile unit of myofibril runs from Z line to Z line - H band= portion of A band w/only thick fils - M lines= link thick fils into lattic!6. Myofibrils are composed of end-to-end arrangment of sarcomeres, extend the length of the fiber!- Myofibrils are cylindrical bundles of overlapping myofilaments, w/in myofibrils are
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