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Skeletal Muscle Notes

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Skeletal muscle Embryological development
-notochord expresses SHH, which stimulates the somites to segregate into sceleretome and dermamyotome
-muscle develops from the myotome from the dermamyotome
-signalling molecule, neurotrophin, wingless family 1,3 from neural tube and ectoderm stimulates the expression of PAX 3 and PAX 7 which stimulates the formation of myoblasts- committed stage
-myoblasts proliferate and fuse to from multinucleate myotubes
-the myotubes are stimulated to express muscle specific proteins- activation of Myf5, MyoD, myogenin
-some of the myoblasts remain attached to the basal lamina- satellite cells Function of muscle
-produce movement of the joints of the skeleton
-maintain posture
-support soft tissues
-protect anatomical openings
-maintain body temperature via shivering

SturctureThe whole muscle, bundles of fascicles, all surrounded by the epimysiumdense connective tissue Bundles of myofibres, fascile, that are held together by connective tissue known as permysium. Each myofibre is surrounded by the endomysium Each myofibre is made up of many myofibrils Each myofibrils is made up of an end to end arrangement of sarcomeres that consist of myofilaments arranged as an interdigitating structure of thick and thin filaments which gives skeletal a striated appearance under a microscope . Each sarcomere is a repeating unit between Z disks

Histological appearance of skeletal muscle-striated and multinucleate
-most of the myocyte is stained pink as there are a large number of proteins
-nuclei is stained purple and is found around the edge of the cell

Ultrastructure
-Thick filaments-

10nm wide and 1.5 micrometer long Myosin- consists of 2 heavy chains- each chain has an alpha helix and a globular head. The alpha helices of the chains are supercoiled to give a rigid rod like tail. The globular heads protrude to form cross bridges. It also consists of 2 light chains that are associated with the globular heads The tails of the myosin are arranged in parallel bundles Three pairs of heads protrude from the parallel bundles of myosin tails, and are 120 degrees apart and form a structure known as the crown The myosin filaments are attached to the M line

-Thin filaments-5nm wide and 1micrometer long Actin- globular subunits that polymerise into a filamentous structure- F actin. The filamentous strand is made up of 2 alpha helical strands that wrap around each other. The actin is bound to the Z line via actin binding proteins: alpha actinin The actin is associated with tropomyosin, 2 alpha helices twist around the grooves formed by the actin and they block the myosin binding sites in the actin groove Troponin has 3 subunits, troponin T binds to tropomyosin, troponin I binds to actin and troponin binds to Ca ion

-StriationStriations are due to banded appearance of dark and light regions Isotropic band, is made up of only actin filaments, I bands shorten during contraction Anisotropic band- made up of an overlap of both actin and myosin, A length remains unchanged

Muscoskeletal coupline Intracellular coupling- within the sarcomere
-forces generated within each sarcomere due to the interactions between myosin and actin - cross bridge cycling- shortens the sarcomere as actin are linked to Z lines and generates tension

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