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MUSCLE TISSUE: Sylabus for Foreign Students Department of Histology and Embryology, P. J. Šafárik University, Medical Faculty, Košice MUSCLE TISSUE: Sylabus for foreign students Author: doc. MVDr. Iveta Domoráková, PhD. Revised by: prof. MUDr. Eva Mechírová, CSc. MUSCLE TISSUE Function: muscle tissue is specialized for contraction Types of muscle tissue: 1. Striated skeletal muscle 2. Striated cardiac muscle 3. Smooth muscle STRUCTURE OF MUSCLE IN THE LIGHT MICROSCOPE 1. SKELETAL MUSCLE Function: contraction - voluntary and rapid - body movement, muscle tissue in the tongue (speech, mixing of food), breathing, voice BASIC MORPHOLOGIC UNIT in LM: multinucleated skeletal muscle fiber with cross striation. Nuclei are situated below the sarcolemma. Fig. 1 – longitudinal and transverse section of skeletal muscle fiber 2. CARDIAC MUSCLE Function: contraction - involuntary; rapid and rhythmic - in the heart (myocardium) BASIC MORPHOLOGIC UNIT in LM: cardiac muscle cell (cardiomyocyte) with cross striation. Nuclei (1-2) are situated in the centre of the cell. Cardiomyocytes are connected by intercalated discs. Fig. 2. – longitudinal and transverse section of cardiomyocytes 3. SMOOTH MUSCLE Function: contraction is involuntary; weak and slow - in the wall of hollow organs (stomach, small intestine) BASIC MORPHOLOGIC UNIT in LM: spindle shaped smooth muscle cell. Cytoplasm has no cross striation. Oval or rod-like nuclei in the centre. Fig. 3. – longitudinal and transverse section of smooth muscle cells Cytoplasm of muscle fibers and muscle cells is eosinophilic. ORIGIN Most of muscle tissue develops from mesoderm that gives rise to mesenchymal cells. • Skeletal muscle develops from paraaxial mesoderm, organized into myotomes in somites. Muscles of the head develop from mesenchyme of branchial arches. • Cardiac muscle develops from cardiogenic mesoderm. • Smooth muscle develops from splanchnic mesoderm - except of iris where smooth muscle arises from neuroectoderm. Fig. 4 Development of multinucleated muscle fiber from myoblasts. ORGANISATION OF SKELETAL MUSCLE Each muscle fiber is surrounded by endomysium - network of reticular fibers, also contains blood and lymphatic capillaries and nerves. Groups of muscle fibers (anywhere between 10 to 100 or more) form fascicles (bundles). Muscle fascicle is surrounded by perimysium - sheath of connective tissue. Entire muscle is surrounded by epimysium - composed of dense irregular connective tissue. It is continuous with fascia and other connective tissue wrappings of muscle including the endomysium and perimysium. It is also continuous with tendons. Fig. 5. Organisation of skeletal muscle SEQUENCE OF ORGANISATION from muscle to molecular structure (Fig.6.) Muscles are composed of muscle fascicles. They are composed of skeletal muscle fibers (visible in LM). Muscle fiber contains myofibrils (visible in EM). Myofibrils create the biggest part of the sarcoplasm. They are oriented longitudinally with long axis of muscle fiber. Myofilbrils are composed of thin actin filaments (myofilaments) and thick myosin filaments (myofilaments). Actin filament on molecular level is composed of: F-actin, troponin complex (Tn subunits: TnC, TnI, TnT) and tropomyosin. Actin in the sarcomere is bound to the Z-line by an α-actinin. Myosin filament is composed of hundreds of myosin molecules (golf stick shape). Fig.6. Organisation of skeletal muscle from muscle to molecular structure Fig. 7. Sarcomere is a functional unit of striated muscles. Sarcomeres are repeated functional unites visible on the myofibrils in EM. Sarcomere is present between two Z-lines. Z-line serves for attachment of actin filaments. I band is formed only by actin filaments. A band is composed of myosin filaments and partly of actin filaments. H band is present in the centre of A band and is composed of myosin filaments. Width of H band changes during contraction of sarcomere. H band is shorter after contraction and longer after relaxation in dependence how much actin filaments penetrate between myosin filaments. STRUCTURE OF SKELETAL MUSCLE FIBER in ELECTRON MICROSCOPE Muscle fiber (Fig. 8) is covered by sarcolemma. Sarcoplasm is filled predominantly with myofibrils surrounded by sarcoplasmic reticulum (sER). Between myofibrils are mitochondria – sarcosomes - energy for contraction. Oval nuclei are situated beneath the sarcolemma. Sarcoplasm contains glycogen granules and pigment myoglobin. Fig.8 – Structure of muscle fiber in EM TRIAD 3) 2 terminal cisternae of sarcoplasmic reticulum +1 T-tubule (invagination of sarcolemma; encircle each myofibril at the border of A-band and I band). Function of T-tubules: Fast transmission of nerve impulses by depolarization of membranes (sarcolemma, membranes of sarcoplasmic reticulum Function of sarcoplasmic reticulum: storrage and release of calcium ions necessary for contraction. Calcium ions are bound to the troponin complex of TnC subunit. MYOFIBRILS – structural and functional units of muscle fiber - sarcoplasm contains 80% of myofibrils - oriented longitudinally - surrounded by SER, mitochondria (2 %) - composed of myofilaments (actin, myosin) In LM and EM are visible alternating isotropic: I-bands (light) and anisotropic: A- bands (dark) SARCOMERES – basic functional units of striated muscle (skeletal, cardiac). Sarcomere is limited by two Z- lines. Z-line is in the centre of isotropic band. Contains α- actinin, protein that binds actin filaments to Z-line. ACTIN and MYOSIN filaments form myofibril. ACTIN FILAMENT 1. F-actin- double helix filament composed of G-actin monomeres 2. Tropomyosin – double helix peptide chain; runs in the groove of F-actin chains 3. Troponin complex (3 globular proteins - subunits): - Troponin C (TnC) – binds calcium ions → contraction - Troponin T (TnT) – attachment of troponin to tropomyosin - Troponin I (TnI) – inhibits actin-myosin interaction MYOSIN FILAMENT - composed of hundreds of myosin molecules - golf stick shape - rod-like straight part (heavy chain; double helix) - myosin head (flexible; binds to actin filament) Myosin head has: - actin binding site, - ATP binding site, - ATP-ase activity Mechamisms of contraction: Adamkov – Functional Histology, lecture + included figures. During contraction occures shortening of: 1. sarcomere 2. I-band 3. H-band (in maximal contraction could disapear) !!! No changes in lenght of : A-band actin and myosin filaments INNERVATION OF SKELETAL MUSCLE - efferent motor nerve endings Motor end-plate or myoneural junction Myelinated motor nerves form terminal arborisation on the surface of skeletal muscle fibers. At their terminal parts they loose myeline sheath and form dilated axonal bouttons covered only by a thin layer of Schwann cell cytoplasm. Bouttons contain axoplasm rich in mitochondria and synaptic vesicles with acetylcholine (ACh). Here the axolemma forms presynaptic membrane. Between the axon and muscle fiber is a synaptic cleft. Sarcolemma at the site of junction forms many invaginations that increase the postsynaptic surface. Sarcoplasm directly below the folds does not contain myofibrils. Postsynaptic membrane - sarcolemma contains acetylcholine receptors. When action potencial reach the motor end plate, acetylcholine is released from the axon terminal to the synaptic cleft and is bound to acetylcholine receptors of the sarcolemma. Sarcolemma becomes more permeable to Na+ and this resultes in membrane depolarization. At each triad , the depolarization signal is passed to the sarcoplasmic reticulum and results in Ca2+ release (calcium ions are bound to troponinC). When the depolarization stops, the Ca2+ is transported back to the sarcoplasmic reticulum and muscle relaxes. In the synaptic cleft is an active acetylcholinesterase that brake down excess of Ach. CARDIAC MUSCLE CELLS – 3 types 1. Contractile cells 2. Impuls generating and conducting cells (initiate heart beat) 3. Myoendocrine cells (production of hormone for regulation of : Na+ , K+ balance and water in the body) Intercalated discs (ID) Intercalated discs (ID) are connections between cardiac muscle cells. On the transverse site are: fasciae adherentes (α – actinin) – serves for actin filament attachment desmosomes that serve for strong connection between cardiac muscle cells On the lateral site are: gap junctions (nexus) – serve for the transport of ions, spreading of impulses and metabolism Cardiac muscle cells in LM: ID - intercalated discs, nuclei (1-2) in the centre of cell Transverse and longitudinal section of smooth muscle EM: differences between smooth muscle cell and skeletal muscle fiber 1. actin and myosin filaments oriented „criss-cross“ (in striated muscles longitudinally with long axis of the cell) 2. No myofibrils !!! 3. Thin actin filaments have no troponin complex 4. Intermediate filaments: desmin and vimentin 5. Dense bodies ( function like „Z-line“, contain α-actinin) a) cytoplasmic b) below sarcolemma 6. Sarcolemma – invaginations involved in transport of Ca2+ from extracellular space into smooth muscle cell cytoplasm – pinocytotic vesicles 7. Mitochondria, rER and Golgi apparatus 8. Basal lamina + reticular fibers on the cell surface (produced by cell) 9. Nexus – smooth muscle cells spread ions through gap junctions Function: 1. Contraction (smooth muscle in the hollow organs forms compact layers that regulate diameter of the lumens) 2. Secretion (rER, GA): synthesis of colagen type III, elastic fibers, proteoglycans CONTRACTION OF SMOOTH MUSCLE CELL CONTRACTION • criss cross orientation of myofilaments give special shape to the cell after contraction • after contraction rod-like nucleus is changed to cork-screw Contraction of smooth muscle cells is involuntary 1. inervation by autonomic nerve system 2. hormonal stimulation (e.g. smooth muscle in the uterus during pregnancy): oxytocin, estrogen. .
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