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Muscle tissue There are four characteristics associated with muscle tissue: Excitability - Tissue can receive & respond to stimulation Contractility - Tissue can shorten & thicken Extensibility - Tissue can lengthen Elasticity - After contracting or lengthening, tissue always wants to return to its resting state Types of muscle tissue: Skeletal muscle tissue • Associated with & attached to the skeleton • Under our conscious (voluntary) control • Microscopically the tissue appears striated • Cells are long, cylindrical & multinucleated Connective tissue component Dense irregular CT Dense irregular CT Surrounded by perimysium 10-100 muscle fibers Reticular fiber Surrounded by epimysium Surrounded by endomysium Microanatomy of a Muscle Fiber (Cell) Transverse (T) tubules: (invaginations of the sarcolemma), spread of action potential Sarcoplasm: contains myoglobin (oxygen binding and release) Sarcoplasmic sarcolemma (plasma membrane) reticulum: (Ca+2 Terminal stores) cisternae: Dilated sacs mitochondria thick myofilament thin myofilament Myofibril: arranged in compartments called sarcomeres nuclei triad Sacromere A band: The darker middle part of the sarcomere, which extends the entire length of thick filaments. I band: is a lighter, less dense area that contains thin filaments but no thick filaments H zone: narrow, in the center of each A band contains thick filaments but no thin filaments. M line: proteins that hold the thick filaments together at the center of the H zone form middle of the sarcomere. Z disc: Narrow,plate-shaped regions of dense protein material Structural Proteins Titin, half sarcomere, from a Z disc to an M line (Elastic) Myomesin: M line. holds the thick filaments Nebulin: anchor thin filaments to Z discs Dystrophin: links thin filaments integral membrane proteins of the sarcolemma. α-actin: Z disc Proteins Structural: Titin, Myomesin, Nebulin, Dystrophin and α-actin Contractile: Actin (thin) and Mysin (thick) Regulatory: Tropomyosin and Troponin * * In relaxed muscle, myosin is blocked from binding to actin because strands of tropomyosin cover the myosin binding site on actin. The tropomyosin strand is held in place by troponin. When Ca+2 bind to troponin, it undergoes a change in shape; this change moves tropomyosin away from myosin-binding sites on actin, allowing myosin to bind to actin and muscle contraction to begin. Sliding Filament Theory • Myosin heads attach to actin molecules (at binding (active) site) • Myosin “pulls” on actin, causing thin myofilaments to slide across thick myofilaments, towards the center of the sarcomere • Sarcomere shortens, I bands get smaller, H zone gets smaller, & zone of overlap increases • As sarcomeres shorten, myofibril shortens. As myofibrils shorten, so does muscle fiber (filament itself do not shorten) • Once a muscle fiber begins to contract, it will contract maximally • This is known as the “all or none” principle Cardiac muscle tissue Striated like skeletal muscle Shorter in length and less circular (squarish) in transverse section exhibit branching; Y shape appearance. Centrally located nucleus Makes up myocardium of heart Unconsciously (involuntarily) controlled Cardiac muscle tissue intercalated discs: thickenings of the sarcolemma 1) Desmosomes 2) Gap junctions Have endomysium, but no epimysium same arrangement of actin and myosin, and the same bands (T) tubules of cardiac muscle are wider but less abundant than those of skeletal muscle. one T tubule per sarcomere, located at the Z disc. Smooth muscle tissue • Makes up walls of organs & blood vessels • Tissue is non-striated & involuntary • Cells are short, spindle-shaped & have a single nucleus • Tissue is extremely extensible, while still retaining ability to contract Smooth muscle tissue smaller than skeletal muscle fibers The sarcoplasm contains both thick filaments and thin filaments, not arranged in sarcomeres as in striated muscle. Contain intermediate filaments (desmin), structural protein lack transverse tubules Have endomysium, but no perimysium or epimysium dense bodies, attached to intermediate filaments, which are functionally similar to Z discs Smooth muscle tissue The tension generated by the thick and thin filaments during contraction is transmitted to intermediate filaments. pulling the dense bodies. (shortening of the muscle fiber) Calcium ions flow into smooth muscle sarcoplasm from both the interstitial fluid and sarcoplasmic reticulum smooth muscle contraction starts more slowly and lasts much longer