5-muscle2324.pptx

Spierweefsel Spiercellen bevatten contractiele eiwitten => Beweging binnenin de cellen => Beweging van het lichaam Morfologisch en functioneel 3 types: (10-1) (1) Skeletspier: - bundels - lang - cylindrisch - meerkernig - dwarsstreping - snel > contractie: - krachtig - wilsafhankelijk (2) Hartspierw...

Spierweefsel Spiercellen bevatten contractiele eiwitten => Beweging binnenin de cellen => Beweging van het lichaam Morfologisch en functioneel 3 types: (10-1) (1) Skeletspier: - bundels - lang - cylindrisch - meerkernig - dwarsstreping - snel > contractie: - krachtig - wilsafhankelijk (2) Hartspierweefsel : - dwarsstreping - lange, vertakte individuele cellen - cellen parallel - speciale celcontacten: intercalaire schijven (zorgen voor dat niet alles tegelijk wordt geactiveerd) > contractie: - autonoom - krachtig - ritmisch (3) Gladde spiercellen: - spoelvormige cellen - geen dwarsstreping - traag > contractie: - niet onderworpen aan de wil Figure 10–1. Structure of the 3 muscle types. The drawings at right show these muscles in cross section. Skeletal muscle is composed of large, elongated, multinucleated fibers. Cardiac muscle is composed of irregular branched cells bound together longitudinally by intercalated disks. Smooth muscle is an agglomerate of fusiform cells. The density of the packing between the cells depends on the amount of extracellular connective tissue present. Three types of muscle. Light micrographs of each type, accompanied by labeled drawings. (a) Skeletal muscle is composed of large, elongated, multinucleated fibers that show strong, quick, voluntary contractions. (b) Cardiac muscle is composed of irregular branched cells bound together longitudinally by intercalated discs and shows strong, involuntary contractions. (c) Smooth muscle is composed of grouped, fusiform cells with weak, involuntary contractions. The density of intercellular packing seen reflects the small amount of extracellular connective tissue present. (a, b: X200; c: X300; All H&E) Development of skeletal muscle. Skeletal muscle begins to differentiate when mesenchymal cells, called myoblasts, align and fuse together to make longer, multinucleated tubes called myotubes. Myotubes synthesize the proteins to make up myofilaments and gradually begin to show cross-striations by light microscopy. Myotubes continue differentiating to form functional myofilaments, and the nuclei are displaced against the sarcolemma. Part of the myoblast population does not fuse and differentiate but remains as a group of mesenchymal cells called muscle satellite Jargon: cytoplasma = sarcoplasma SER = sarcoplasmatisch reticulum Celmembraan = sarcolemma SKELETSPIER Bestaat uit spiervezels > Vezels vormen bundels > lang (tot 30 cm) > meerkernige cellen: (syncytium) >> ovale nuclei >> net onder het sarcolemma (itt hart en gladspierweefsel) Organisatie van de skeletspier (10-2) (10-3) > Epimysium: - straf bindweefsel omgeeft de volledige spier > Perimysium: - septa of tussenschotten van bindweefsel > rond bundels van vezels > Endomysium: (10-4) -laag bindweefsel rond spiercellen -> lamina basalis en reticulaire vezels Bindweefsel: overbrengen van de krachten gegenereerd in de spieren >collagene vezels van pezen insereren in een complexe organisatie van plooien van het plasmalemma van de spiervezels (10-7) Bloed vaten : in de septa van het perimysium > rijk capillair netwerk (10-5) Figure 10–2. Structure and function of skeletal muscle. The drawing at right shows the area of muscle detailed in the enlarged segment. Color highlights endomysium, perimysium, and epimysium. Organization of skeletal muscle. An entire skeletal muscle is enclosed within a thick layer of dense connective tissue called the epimysium that is continuous with fascia and the tendon binding muscle to bone. Large muscles contain several fascicles of muscle tissue, each wrapped in a thin but dense connective tissue layer called the perimysium. Within fascicles individual muscle fibers (elongated multinuclear cells) are surrounded by a delicate connective tissue layer, the endomysium. Skeletal muscle. (c) Longitudinal section of a myotendinous junction. Tendons develop together with skeletal muscles and join muscles to the periosteum of bones. The dense collagen fibers of a tendon (T) are continuous with those in the three connective tissue layers around muscle fibers (M), forming a strong unit that allows muscle contraction to move other structures. (X400; H&E) Figure 10–3. Cross section of striated muscle stained to show collagens type I and III and cell nuclei. The endomysium is indicated by arrowheads and the perimysium by arrows. At left is a piece of epimysium. Picrosirius-hematoxylin stain. High magnification. Figure 10–4. Cross section of striated muscle immunohistochemically stained for laminin, a protein component of the endomysium, which appears in various shades of brown. In the upper right corner is a slightly oblique section of a small nerve. Laminin is also present around nerve fibers. Figure 10–5. Longitudinal section of striated muscle fibers. The blood vessels were injected with a plastic material before the animal was killed. Note the extremely rich network of blood capillaries around the muscle fibers. Giemsa stain. Photomicrograph of low magnification made under polarized light. Figure 10–6. Striated skeletal muscle in longitudinal section (lower) and in cross section (upper). The nuclei can be seen in the periphery of the cell, just under the cell membrane, particularly in the cross sections of these striated fibers. H&E stain. Medium magnification. Figure 10–7. Striated skeletal muscle in longitudinal section. In the left side of the photomicrograph the insertion of collagen fibers with the muscle is clearly seen. Picrosirius–polarized light (PSP) stain. Medium magnification. Striated skeletal muscle in longitudinal section. Longitudinal sections reveal the striations characteristic of skeletal muscle. (a) Parts of three muscle fibers are separated by very thin endomysium that includes one fibroblast nucleus (F). Muscle nuclei (N) are found against the sarcolemma. Along each fiber thousands of dark-staining A bands alternate with lighter I bands. (X200; H&E) Striated skeletal muscle in longitudinal section. (b) At higher magnification, each fiber can be seen to have three or four myofibrils, here with their striations slightly out of alignment with one another. Myofibrils are cylindrical bundles of thick and thin myofilaments which fill most of each muscle fiber. (X500; Giemsa) Striated skeletal muscle in longitudinal section. (c) TEM showing one contractile unit (sarcomere) in the long series that comprises a myofibril. In its middle is an electron-dense A band bisected by a narrow, less dense region called the H zone. On each side of the A band are the lighter-stained I bands, each bisected by a dense Z disc which marks one end of the sarcomere. Mitochondria (M), glycogen granules, and small cisternae of SER occur around the Z disc. (X24,000) Organisatie van spiervezels (10-6), (10-7) (10-8) (10-9) Dwarse bandering: (10-10) (10-11) (10-12) A-band: donker I. band: licht Z-line: in het midden van de I-band. Repetitieve subeenheden (van Z tot Z) = Sarcomeer Myofibrillen: > lange cylindrische filamenteuze bundels in het sarcoplasma > aaneenschakeling van sarcomeren >myofibrillen evenwijdig met het bandenpatroon in register Figure 10–8. Longitudinal section of skeletal muscle fibers. Note the dark-stained A bands and the light-stained I bands, which are crossed by Z lines. Giemsa stain. High magnification. Figure 10–9. Skeletal muscle in longitudinal section. Note the striation in the muscle cells and the moderate amount of collagen (yellow). PSP stain. High magnification. Figure 10–10. Electron micrograph of skeletal muscle of a tadpole. Note the sarcomere with its A, I, and H bands and Z line. The position of the thick and thin filaments in the sarcomere is shown schematically in the lower part of the figure. As illustrated here, triads in amphibian muscle are aligned with the Z line in each sarcomere. In mammalian muscle, however, each sarcomere exhibits 2 triads, one at each A–I band interface (see Figure 10–16). x35,000. (Courtesy of KR Porter.) Sarcomeer: > dikke filamenten in het centrale gedeelte > dunne filamenten tussen en parallel aan de dikke >> 1 uiteinde vast op de Z-lijn => I band: gedeelte waar de dunne filamenten geen overlap hebben met de dikke => H-band: > in het midden van de A –band > enkel dikke filamenten => M-lijn:in het middenvan de H-band: > laterale verbindingen tussen de dikke filamenten > M-lijn: creatine kinase In de A-band: overlap tussen dikke en dunne filamenten => 1 dik omgeven door 6 dunne filamenten (hexagon) Figure 10–11. Structure and position of the thick and thin filaments in the sarcomere. The molecular structure of these components is shown at right. (Drawing by Sylvia Colard Keene. Reproduced, with permission, from Bloom W, Fawcett DW: A Textbook of Histology, 9th ed, Saunders, 1968.) Molecules composing thin and thick filaments. Myofilaments, which include both thick and thin filaments, consist of contractile protein arrays bundled within myofibrils. (a) A thick myofilament, overall diameter 15 nm, contains 200-500 molecules of myosin. (b) A thin filament, diameter 8 nm, contains F-actin, tropomyosin, and troponin. The positional relationships between thick and thin myofilaments as well as their relative sizes are depicted in Figure 10-8. Structure of a myofibril: A series of sarcomeres. (a)The diagram shows that each muscle fiber contains several parallel bundles called myofibrils. (b)Each myofibril consists of a long series of sarcomeres, separated by Z discs and containing thick and thin filaments which overlap in certain regions. (c)Thin filaments are actin filaments with one end bound to α-actinin in the Z disc. Thick filaments are bundles of myosin, which span the entire A band and are bound to proteins of the M line and to the Z disc across the I bands by a very large protein called titin, which has springlike domains. (d)The molecular organization of the sarcomeres produces staining differences which cause the dark- and light-staining bands seen by light microscopy and TEM. (X28,000) (e)With the TEM an oblique section of myofibrils includes both A and I bands and shows hexagonal patterns that indicate the Figure 10–12. Transverse section of skeletal muscle myofibrils illustrating some of the features diagrammed in Figure 10– 11. I, I band; A, A band; H, H band; Z, Z line. x36,000. Belangrijkste proteïnen in de filamenten: Actine, tropomyosine, troponine en myosine (10-13) (10-14) Actine:> lange filamenteuze polymeren (F-actine) > 2 strengen van globulaire monomeren (G-actin) >> structureel asymmetrische moleculen >> back to front verbindingen => polariteit >> bevatten een bindings site voor myosine > Gewonden in een helix >actine filamenten loodrecht op de Z-lijn vastgehecht vertonen een tegengestelde polariteit >> in de Z-lijn  -actinine en desmine (IFprotein) >>> verbinden aanliggende sarcomeren >>> houden de myofibrillen op hun plaats Tropomyosine: > lange, dunne molecule > 2 polypeptides (hoofd - staart) > ligt over de actine subeenheden Troponine: > 3 subeenheden >> TnT: hecht op tropomyosine >> TnC: bindt calcium ionen >> TnI: inhibeert actine-myosine interactie > Hecht op specifieke plaats op tropomyosine molecules Myosine: > groot complex >> 2 identische zware ketens >>> dunne staafvormige molecules rond elkander gewonden >>> kleine globulaire uitstulpingen vormen de hoofdjes >>>> ATP bindings plaatsen >>>> ATP-ase capaciteit >>>> bindt actine >> 2 paren lichte ketens >>> verbonden met het hoofd van de zware ketens > overlap van myosine molecules in de dikke filamenten >> overlap van de staafvormige gedeeltes >> globulaire hoofdjes gericht naar de Z-lijn >> globulaire hoofdjes steken uit de staafvormige gedeeltes Bruggen tussen de myosine en actine filamenten: > Omzetting van chemische in mechanische energie Figure 10–13. Schematic representation of the thin filament, showing the spatial configuration of 3 major protein components—actin, tropomyosin, and troponin. The individual components in the upper part of the drawing are shown in polymerized form in the lower part. The globular actin molecules are polarized and polymerize in one direction. Note that each tropomyosin molecule extends over 7 actin molecules. TnI, TnC, and TnT are troponin subunits. Figure 10–14. Muscle contraction, initiated by the binding of Ca2+ to the TnC unit of troponin, which exposes the myosin binding site on actin (cross-hatched area). In a second step, the myosin head binds to actin and the ATP breaks down into ADP, yielding energy, which produces a movement of the myosin head. As a consequence of this change in myosin, the bound thin filaments slide over the thick filaments. This process, which repeats itself many times during a single contraction, leads to a complete overlapping of the actin and myosin and a resultant shortening of the whole muscle fiber. I, T, C are troponin subunits. (Reproduced, with permission, from Ganong WF: Review of Medical Physiology, 20th ed. McGraw-Hill, 2001.) Sarcoplasmatic reticulum en het Transversaal tubulus systeem (T-buizensysteem) Depolarisatie van de membraan van het sarcoplasmatisch reticulum: > Door natrium ionen > Gestart aan de myoneurale verbinding (oppervlakte van de spiercel) het transverse buizensysteem (T-tubules) laat een uniforme depolarisatie toe(10-15) (10-16) (10- 17) > vingervormige instulpingen van het sarcolemma > anastomoserend netwerk > Omcirkelen ter hoogte van de A-I band iedere sarcomeer > in elke myofibril Aanliggend aan de T-tubulen: terminal cisternen van het sarcoplasmatic reticulum > Vormt triades: T tubulus met 2 laterale gedeeltes van het SR In de Triaden wordt de depolarisatie van het sarcolemma (T-Tubulus) doorgegeven aan het SR. Het SR reguleert de Ca2+ flow nodig voor snelle contractie en relaxatie >na depolarisatie wordt het gestapelde Ca2+ vrijgesteld uit het SR naar de dikke en dunne filamenten >> Ca2+ bindt op troponine >>verbinding toegelaten tussen actine en myosine > na depolarisatie werkt het SR als een reservoir 2+ weer in de voor door actief transport wordt Ca 2+ Ca cisternen gepompt >> stoppen van de spiersamentrekking Organization of a skeletal muscle fiber. Skeletal muscle fibers are composed mainly of myofibrils. Each myofibril extends the length of the fiber and is surrounded by parts of the sarcoplasmic reticulum. The sarcolemma has deep invaginations called T-tubules, each of which becomes associated with two terminal cisternae of the sarcoplasmic reticulum. A T-tubule and its two associated terminal cisternae comprise a “triad” of small spaces along the surface of the myofibrils. Contractie Mechanisme Rust: gedeeltelijke overlapping van dikke en dunne filamenten Contractie: toename in de overlap => glijdende filament hypothese Rust: >ATP gebonden op de ATP-ase site op het myosine kopje >> geen hydrolyse >>> actine is nodig als co-factor >De bindingssite voor myosine op actine is bedekt door troponine-tropomyosine Vrijstelling van Ca2+ : > Ca2+ bindt op de TnC eenheid >> verandering in de ruimtelijke configuratie van de troponine subeenheden >>> verplaatsing van de tropomyosine molecule >>>> toegang tot de myosine bindings site op actine De verbinding veroorzaakt een activering van de ATP hydrolse en vormt ADP en Pi + E > E veroorzaakt een conformatie verandering van het myosine kopje >> actine wordt verplaatst langs het myosine filament (dunne filament dieper in de A-band) > actine-myosine bruggen worden verbroken na binding van een nieuwe ATP molecule >> myosine kopje terug in oorsprongkelijke toestand >>> zonder ATP is het actine-myosine complex stabiel Spiercontractie: >Het resultaat van honderden brug vormende/ verbrekende cycli > complete overlap tussen dikke en dunne filamenten >Na het verwijderen van Ca2+ zal het troponine – tropomyosine complex weer de myosine bindingssite bedekken >> verwijderen van Ca2+ gebeurt door een actief transport mechanisme Figure 10–15. Electron micrograph of a transverse section of fish muscle, showing the surface of 2 cells limiting an intercellular space. Note the invaginations of the sarcolemma, forming the tubules of the T system (arrows). The dark, coarse granules in the cytoplasm (lower left) are glycogen particles. The section passes through the A band (upper right), showing thick and thin filaments. The I band is sectioned (lower left), showing only thin filaments. x60,000. (Courtesy of KR Porter.) Transverse tubule system and triads. Transverse tubules are invaginations of the sarcolemma that penetrate deeply into the muscle fiber around all myofibrils. (a)TEM cross section of fish muscle shows portions of two fibers and the endomysium (E) between them. Several transverse or Ttubules (T) are shown, perpendicular to the fiber surface, penetrating between myofibrils (M). (X50,000) (b)Higher-magnification TEM of skeletal muscle in longitudinal section shows four membranous triads (Tr) cut transversely near the A-band–I-band junctions. Each triad consists of a central transverse tubule (T) and two adjacent terminal cisterns (TC) extending from the sarcoplasmic reticulum. Centrally located is the Z disc. Besides elements of the triad, sarcoplasm surrounding the myofibril also contains dense glycogen granules (G). Components of the triad are responsible for the cyclic release of Ca2+ from the cisternae and its sequestration again which occurs during muscle contraction and relaxation. The association between SR cisternae and T-tubules is shown diagrammatically in Figure 10–16. Electron micrograph of a longitudinal section of the skeletal muscle of a monkey. Note the mitochondria (M) between adjacent myofibrils. The arrowheads indicate triads—2 for each sarcomere in this muscle—located at the A–I band junction. A, A band; I, I band; Z, Z line. x40,000. (Reproduced, with permission, from Junqueira LCU, Salles LMM: UltraEstrutura e Função Celular. Edgard Blücher, 1975.) Figure 10–17. Segment of mammalian skeletal muscle. The sarcolemma and muscle fibrils are partially cut, showing the following components: The invaginations of the T system occur at the level of transition between the A and I bands twice in every sarcomere. They associate with terminal cisternae of the sarcoplasmic reticulum (SR), forming triads. Abundant mitochondria lie between the myofibrils. The cut surface of the myofibrils shows the thin and thick filaments. Surrounding the sarcolemma are a basal lamina and reticular fibers. (Reproduced, with permission, from Krstíc RV: Ultrastructure of the Mammalian Cell. Springer-Verlag, 1979.) Figure 10–18. Ultrastructure of the motor end-plate and the mechanism of muscle contraction. The drawing at the upper right shows branching of a small nerve with a motor end-plate for each muscle fiber. The structure of one of the bulbs of an end-plate is highly enlarged in the center drawing. Note that the axon terminal bud contains synaptic vesicles. The region of the muscle cell membrane covered by the terminal bud has clefts and ridges called junctional folds. The axon loses its myelin sheath and dilates, establishing close, irregular contact with the muscle fiber. Muscle contraction begins with the release of acetylcholine from the synaptic vesicles of the end-plate. This neurotransmitter causes a local increase in the permeability of the sarcolemma. The process is propagated to the rest of the sarcolemma, including its invaginations (all of which constitute the T system), and is transferred to the sarcoplasmic reticulum (SR). The increase of permeability in this organelle liberates calcium ions (drawing at upper left) that trigger the sliding filament mechanism of muscle contraction. Thin filaments slide between the thick filaments and reduce the distance between the Z lines, thereby reducing the size of all bands except the A band. H, H band; S, sarcomere. Impulsoverdracht (10-18) Gemyelineerde motorische zenuwen vertakken in het perimysium en bereiken via het endomysium de spiervezel > motor eind plaat of myoneurale verbinding > in het axon einde >> vele mitochondria >> synaptische vesicles (acetylcholine) > synaptische spleet met verbindingsplooien >> in het sarcoplasma (thv de verbinding - subneuraal apparaat): >>> nuclei >>> mitochondria >>> ribosomen >>> glycogeen korrels Als de AP de MEP bereikt > acetylcholine vrijstelling > binden op receptoren in het sarcolemma >> permeabiliteit voor Na+ neemt toe >>> membraan depolarisatie > > Figure 10–11. Events of muscle contraction. Figure 10–12. Sliding filaments and sarcomere shortening in contraction. Diagrams and TEM micrographs compare changes in the striations of skeletal muscle fibers according to the sliding filament mechanism. (a): In their relaxed state the sarcomere, I band and H zone are at their expanded length. The spring—like action of titin molecules, which span the I band, help pull thin and thick filaments past one another in relaxed muscle. (b): The Z discs at the sarcomere boundaries are drawn closer together during contraction as they move toward the ends of thick filaments in the A band. Titin molecules are compressed during contraction. (c): The neuromuscular junction (NMJ). (b)An SEM shows the branching ends of a motor axon, each covered by an extension of the last Schwann cell and expanded terminally as an MEP embedded in a groove in the external lamina of the muscle fiber. (c)Diagram of enclosed portion of the SEM indicating key features of a typical MEP: synaptic vesicles of acetylcholine (ACh), a synaptic cleft, and a postsynaptic membrane. This membrane, the sarcolemma, is highly folded to increase the number of ACh receptors at the MEP. Receptor binding initiates muscle fiber depolarization, which is carried to the deeper myofibrils by the T-tubules. Andere componenten van het sarcoplasma Glycogeen => korrels  brandstof Myoglobine => zuurstof bindend eiwit Kleine hoeveelheden RER Klein aantal ribosomen Figure 10–21. Section of tongue, an organ rich in striated skeletal muscle fibers. These fibers appear brown because the section was immunohistologically stained to show myoglobin. The light-colored areas among and above the muscle fibers contain connective tissue. In the upper region of the section, stratified and cornified epithelium can be seen. Nuclei are stained by hematoxylin. Low magnification. Skeletal muscle fiber types. Cross section of a skeletal muscle stained histochemically for myosin ATPase at acidic pH, which reveals activity of the “slow” ATPase and shows the distribution of the three main fiber types. Slow oxidative (SO) or type I fibers have high levels of acidic ATPase activity and stain the darkest. Fast glycolytic (FG) or type IIb fibers stain the lightest. Fast oxidative-glycolytic (FOG) or type IIa fibers are intermediate between the other two types (X40). ATPase histochemistry of unfixed, cryostat section, pH 4.2. Hartspierweefsel (10-22), (10-23), (10-24), (10-25), (10-26) (10-27) Een of twee nuclei: centraal gelegen Dicht tegen elkaar Niet in register Dwarsstreping Intercalaire schijven: transversale lijnen >over de hartspi ercel op Figure 10–22. Drawing of a section of heart muscle, showing central nuclei, cross-striation, and intercalated disks. Figure 10–23. Photomicrograph of cardiac muscle. Note the cross-striation and the intercalated disks (arrowheads). Pararosaniline–toluidine blue (PT) stain. High magnification. Figure 10–24. Longitudinal section of portions of 2 cardiac muscle cells. The transversely oriented parts of the intercalated disk consist of a fascia adherens and numerous desmosomes. The longitudinal parts (arrows) contain gap junctions. Mitochondria (M) are numerous. Fibrils of reticular fibers are seen between the two cells. x18,000. (Reproduced, with permission, from Junqueira LCU, Salles LMM: Ultra-Estrutura e Função Celular. Edgard Blücher, 1975.) Verbindingsspecialisaties: (1) Fasciae adherentes: in het transversale gedeelte >verankeringssites voor actine filamenten van de terminale sarcomeren = hemi Z-banden (2)Maculae adherentes (desmosomes): in het transversale gedeelte > cellen aan elkander vastzetten (3) Gap junctions: in het laterale gedeelte > ionische continuïteit >> individuele cellen werken als een syncytium >>> golfvormige contractie van de hartspier Sarcoplasmatisch reticulum: niet zo goed ontwikkeld als bij de skeletspier Diaden ipv triaden. Veel mitochondria (40% van het cytoplasmic volume > < 2% in skeletspier) Belangrijkste brandstof: vetzuren => vetdruppels in hartspiercellen => Kleine hoeveelheid glycogeen Figure 10–25. Ultrastructure of heart muscle in the region of an intercalated disk. Contact between cells is accomplished by interdigitation in the transverse region; contact is broad and flat in the longitudinal plane (LP). A, A band; I, I band; Z, Z line. (Redrawn and reproduced, with permission, from Marshall JM: The heart. In: Medical Physiology, 13th ed, Vol 2, Mountcastle VB [editor]. Mosby, 1974. Based on the results of Fawcett DW, McNutt NS: J Cell Biol 1969;42:1, modified from Poche R, Lindner E: Zellforsch Mikrosk Anat 1955;43:104.) Figure 10–26. Junctional specializations making up the intercalated disk. Fasciae (or zonulae) adherentes (A) in the transverse portions of the disk anchor actin filaments of the terminal sarcomeres to the plasmalemma. Maculae adherentes, or desmosomes (B), found primarily in the transverse portions of the disk, bind cells together, preventing their separation during contraction cycles. Gap junctions (C), restricted to longitudinal portions of the disk—the area subjected to the least stress—ionically couple cells and provide for the spread of contractile depolarization. Figure 10–27. Electron micrograph of a longitudinal section of heart muscle. Note the striation pattern and the alternation of myofibrils and mitochondria rich in cristae. Note the sarcoplasmic reticulum (SR), which is the specialized calciumstoring smooth endoplasmic reticulum. x30,000. Cardiac muscle. (a)The diagram of cardiac muscle cells indicates their characteristic features. The fibers consist of separate cells in a series joined at interdigitating regions called the intercalated discs, which cross an entire fiber between two cells. The transverse regions of the steplike intercalated disc have abundant desmosomes and other adherent junctions for firm adhesion, while longitudinal regions of the discs are filled with gap junctions. Cardiac muscle cells have central nuclei and myofibrils which are usually sparser and less well-organized than those of skeletal muscle. Also, the cells are often branched, allowing the muscle fibers to interweave in a more complicated arrangement within fascicles which helps produce an efficient contraction mechanism for emptying the heart. (b)Light microscopy of cardiac muscle in longitudinal section show nuclei (N) in the center of the muscle fibers and widely spaced intercalated discs (I) that cross the fibers. These irregular intercalated discs should not be confused with the repetitive, much more closely spaced striations (S), which are similar to those of skeletal muscle but less well-organized. Nuclei of fibroblasts in endomysium are also present. (X200; H&E) (c)TEM showing an electron-dense intercalated disc with a steplike structure along the short interdigitating processes of adjacent cardiac muscle cells. As shown here transverse disc regions have many desmosomes (D) and adherent junctions called fascia Cardiac muscle. (b)Light microscopy of cardiac muscle in longitudinal section show nuclei (N) in the center of the muscle fibers and widely spaced intercalated discs (I) that cross the fibers. These irregular intercalated discs should not be confused with the repetitive, much more closely spaced striations (S), which are similar to those of skeletal muscle but less well-organized. Nuclei of fibroblasts in endomysium are also present. (X200; H&E) (c)TEM showing an electron-dense intercalated disc with a steplike structure along the short interdigitating processes of adjacent cardiac muscle cells. As shown here transverse disc regions have many desmosomes (D) and adherent junctions called fascia adherentes(F) which join the cells firmly. Gap junctions (G) joining the cells physiologically are abundant in other regions of the disc. The sarcoplasm has numerous mitochondria (M) and myofibrillar structures like those of skeletal muscle but slightly less organized. Glad spierweefsel (10-29) (10-30)(10-31) (10-32) (10-33) > Relatief lange, niet gestreepte cellen > Omgeven door: - een lamina basalis - netwerk van reticulaire vezels  Spoelvormig: dikst in het midden en dunner naar de uiteinden >1 kern in het centrum van de cel >Dicht opeengepakt: dunne gedeelte van een cel aanliggend aan het bredere deel van een andere cel Inhoud: > Mitochondria > Polyribosomen > RER > GC > Rudimentair sarcoplasmatic reticulum Figure 10–29. Photomicrographs of smooth muscle cells in cross section (upper) and in longitudinal section (lower). Note the centrally located nuclei. In many cells the nuclei were not included in the section. PT stain. Medium magnification. Figure 10–31. Transverse section of smooth muscle impregnated with silver to stain the reticular fibers. These fibers form a network that surrounds the muscle cells that are not stained by this method. At the right is an arteriole surrounded by thicker collagen fibers. x300. Figure 10–30. Drawing of a segment of smooth muscle. All cells are surrounded by a net of reticular fibers. In cross section, these cells show various diameters. Contractiele activiteit: bundels myofilamenten vormen een ruitvormig netwerk > dunne filamenten: actine en tropomyosine > dikke filamenten: myosine Gelijktijdig of golfvormige contractie Figure 10–32. Electron micrograph of a transverse section of smooth muscle. The cells are sectioned at various diameters and have many subsurface vesicles in their cytoplasm. Thick and thin filaments are not organized into myofibrils, and there are few mitochondria (M). Note the collagen fibrils of the reticular fibers and a small unmyelinated nerve (N) between the cells. x6650. Figure 10–33. Smooth muscle cells relaxed and contracted. Cytoplasmic filaments insert on dense bodies located in the cell membrane and deep in the cytoplasm. Contraction of these filaments decreases the size of the cell and promotes the contraction of the whole muscle. During the contraction the cell nucleus is deformed. Figure 10–21. Smooth muscle contraction. Most molecules that allow contraction are similar in the three types of muscle, but the filaments of smooth muscle are arranged differently and appear less organized. (a): The diagram shows thin filaments attach to dense bodies located in the cell membrane and deep in the cytoplasm. Dense bodies contain α—actinin for thin filament attachment. Dense bodies at the membrane are also attachment sites for intermediate filaments and for adhesive junctions between cells. This arrangement of both the cytoskeleton and contractile apparatus allows the multicellular tissue to contract as a unit, providing better efficiency and force. (b): Contraction decreases the length of the cell, deforming the nucleus and promoting contraction of the whole muscle. The micrograph shows a region of contracted tissue in the wall of a urinary bladder. The long nuclei of individual fibers assume a cork—screw shape when the fibers contract, reflecting the reduced cell length at this time. X240. Mallory trichrome. Smooth muscle. Cells or fibers of smooth muscle are long, tapering structures with elongated nuclei centrally located at the cell’s widest part. (a)In most of the digestive tract and certain similar structures smooth muscle is organized into two layers which contract in a coordinated manner to produce a wave that moves the tract’s contents in a process termed peristalsis. In smooth muscle of the small intestine wall cut in cross section, cells of the inner circular (IC) layer are cut lengthwise and cells of the outer longitudinal layer (OL) are cut transversely. Only some nuclei (arrows) of the latter cells are in the plane of section so that many cells appear to be devoid of nuclei. (X140; H&E) (b)Section of smooth muscle in bladder shows interwoven bundles of muscle fibers in cross section (XS) and longitudinal section (LS) with the same fascicle. There is much collagen in the branching perimysium (P), but the endomysium can barely be seen by routine staining. (X140; Mallory trichrome) REGENERATIE van SPIERWEEFSEL Hartspier: geen regeneratieve capaciteit >defecten (infarct) worden vervangen door bindweefsel Skeletspier: beperkte regeneratie vanuit satellietcellen > spoelvormige cellen in de lamina basalis rond de cellen > inactieve myoblasten >> worden geactiveerd na letsel > ook betrokken bij hypertrofie >> fusie met de spiercel om de spiermassa te doen toenemen Glad spierweefsel: kan hypertrofiëren