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PG1006 Lecture 4 Skeletal Muscle Dr. Neil Docherty My Teaching Objec/ves 1. To describe the hierarchical structure of skeletal muscle fibres 2. To relate the structure of myofibrils to force genera:on thorugh the descrip:on of sarcomeres and the sliding filament theory 3. To introduce the concept of how skeletal muscle excitability translates into contrac:lity The Role of Skeletal muscle Skeletal movement and support Support of the body cavi:es Reflex control of body temperature Mechanical Proper/es of Skeletal Muscle Contrac:lity Tensile strength effect on tendons Series elas:city Skeletal Muscle from Outside-‐In Note packeting of functional units and compare with peripheral nerve packaging Skeletal Muscle fibres -‐ H&E stain F = fasiculi P = perimysium En = endomysium E = epimysium Longitudinal and transverse sec/on of muscle fibres – H&E stain Transverse sec/on through skeletal muscle fibres -‐ iron haematoxylin Very high magnifica:on (x1200) Only 1 nucleus visible Individual packets of filaments within muscle fibre These are the myofibrils Muscle Fibre Organisa/on Muscle Fibre (>100µM diameter, >0.75m length) Composed of Myofibrils(1µm diameter whole length of muscle) Repeating units of Sarcomeres Regular arrays of Thick filaments (myosin polymer) 18nm diameter 1.6µm length Thin filaments (actin polymer) 5-8nm diameter 1µm length Muscle Fibre Structure Transmission Electron Microscopy Regular striated patterns of filaments and organelles hint at functionality The Sarcomere Visualised A band-stacked thick filaments and overlapping regions of thin filaments H-zone-Central thick filament only Contains M line I band-Peripheral thin filament only Terminates at Z line Area between Z-lines =SARCOMERE 2.5µm length (resting) Adapted from Human Physiology Cells to Systems (7th Ed.) Sherwood L The Sarcomere Order Hints at Func/on -T.E.M. patterning reflects molecular composition Adapted from Human Physiology- Cells to Systems (7th Ed.) Sherwood L and Wheater’s Functional Histology (5th Ed.) Burkitt H.G. Young B. & Heath J.W. Sarcomere Geometry Depending how you look at it! -Thin filaments form a hexagon around a thick filament -Thick filaments triangulate a central thin filament A single thick filament can cross bridge to six thin filaments Adapted from Human Physiology Cells to Systems (7th Ed.) Sherwood L. Myosin-‐The Thick Filament Protein Several hundred myosin molecules/thick filament Mirror Image arrangement Cross bridging to actin by globular head Crucial! -ATPase site -Actin binding site Adapted from Human Physiology Cells to Systems (7th Ed.) Sherwood L Ac/n-‐The Primary Thin Filament Protein Spherical polymer of actin monomers Two polymeric actins intertwine Each actin monomer contains a myosin binding site Adapted from Human Physiology Cells to Systems (7th Ed.) Sherwood L. Sliding filament theory of contrac/on Sarcomere – thick and thin filaments - Thick filaments are composed of myosin - Thin filaments are composed of actin During contraction the thick and thin filaments slide over each other, shortening the sarcomere N.B. the filaments do not shorten, by sliding over each other the space between them is Shortened and the fibril contracts/shortens Organisa/on of Neuromuscular Communica/on The Motor Unit Motor unit Adapted from Human Physiology Cells to Systems (7th Ed.) Sherwood L. Conduc/ng system for contrac/le s/muli T tubule system - allows synchronous contraction of all sarcomeres in a muscle fibre - Extends transversely through into muscle cell to surround each myofibril - continuous with extracellular space Conduc/ng system – longitudinal sec/on, EM x33,000 A and I bands Td = tubular triads, tubule (T) of T system and terminal cisternae (TC) of sarcoplasmic reticulum SR = tubular elements connecting TC M = mitochondria Aerobic and Anaerobic Fibres succinate dehydrogenase (x200) A = aerobic (type I) muscle fibres slow-twitch fibres An = anaerobic (type II) muscle fibres fast-twitch Myosin ATP-‐ase (x600) Your Learning from Today Should focus on being able to; 1. Outline structural organisation of skeletal muscle how peripheral nerves are packaged 2. Describe skeletal muscel force generation via the sliding filament theory 3.Provide a basic description of the motor unit structure of neuromuscular communication