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Department of Human Morphology and Developmental Biology
Muscle tissue Agnes Nemeskeri 2015 Department of Human Morphology and Developmental Biology
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A/ Smooth muscle tissue
Types of muscle tissue - muscle cells (myocytes) -energy from the hydrolysis of ATP (converting chemical energy into mechanical work) A/ Smooth muscle tissue -no cross-striations B/ Striated muscle tissue -repeating elements give cross-striated appearance
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Striated muscle
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Striated muscle SKELETAL b) VISCERAL c) CARDIAC
- histological unit: multinucleated (100s) muscle fiber grouped in bundles - originating and inserting on skeletal elements - contraction needs nervous stimulus b) VISCERAL - histological unit: multinucleated muscle fiber grouped in bundles - independent of bones (intrinsic muscles of tongue, upper third of esophagus - contraction needs nervous stimulus c) CARDIAC - histological unit: cardiac muscle cell (mono- or binucleated - branching network of individual cells – functions as a unit - intrinsically capable of rhythmic contraction
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Size of muscle cells Striated muscle fiber Cardiac muscle cell
Smooth muscle cell in uterine wall at the end of gestation Smooth muscle cell in blood vessel wall
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Light microscopy of skeletal and visceral striated muscle
esophagus
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Skeletal and visceral striated muscle
Myofibrils are composed of repeated sections of sarcomeres muscle fiber thin thick line sarcomere 1 muscle cell from a biceps may contain ~100,000 sarcomeres
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Striated muscle fiber- myofibril
– as long as the muscle fiber - 1 muscle fiber – 100s – 1000s striated muscle fiber striated muscle fiber Sarcomere = – 2.2 µm long myofibril Sarcomeres are composed of regularly aligned thin and thick filaments 1 myofibril may contain ~ sarcomeres
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Muscle fever micro tears Inflammation – leukocytes, monocytes
cytokins – directly activtvate nociceptive receptors
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Cross striation at molecular level
thin filament thick filament A - anisotropic - because in a polarizing light microscope, the dark bands are birefringent I - isotropic - because in a polarizing microscope, these bands are much less birefringent than the A bands
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Thin filament I Thin filament – 6-9 nm diameter
F-actin resembles a two-stranded cable Thin filament – 6-9 nm diameter - pointed end – actin capping protein: tropomodulin - barbed ends of actin filaments are anchored to the Z-bands Actin – major component of thin filament - cellular actin has two forms: a) monomeric globules called G-actin - linear polymer - G-actin globular subunits – polymerization b) polymeric filaments called F-actin (filaments made up of many G-actin monomers) - 2 strands of fibrous actin filaments are coiled on each other, form double helix – filamentous ~ 67 x 40 x 37 Å in size, has a molecular mass of 41,785 Da globules consists of 2 lobes separated by a cleft: “ATP-ase fold” binds ATP and Mg2+ and hydrolyzes the ATP to ADP plus phosphate in addition to its nucleotide binding site, each G-actin contains a high-affinity myosin head-binding site each G-actin is orientated in the same direction which makes up a polar character
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Thin filament II. 1. Actin 2. Tropomyosin – filamentous protein (over 7 G-actins) - thin filament contains tropomyosin molecule - 40 nm long protein that is wrapped around the F-actin helix - during the resting phase the tropomyosin covers the actin’s active sites so that actin-myosin interaction cannot take place and produce muscular contraction - blocking the myosin binding site on actin 3. Troponin complex 1. Actin 2. Tropomyosin 3. Troponin complex - complex of 3 regulatory proteins - troponins bound to the tropomyosin: 3 polymers: troponin I, troponin T and troponin C a) TnT is a tropomyosin-binding subunit - regulates interaction of troponin complex with thin filamentum b) TnC is a Ca2+ - binding subunit - Ca2+ increases – TnC binds Ca2+ - tropomyosin is rolled away from myosin-binding sites on actin c) TnI binds to actin to hold the actin-tropomyosin complex in place because of it, myosin cannot bind actin in relaxed muscle when Ca2+ binds to the TnC it causes conformational changes which lead to dislocation of TnI
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Thin filament III 1. Actin 2. Tropomyosin 3. Troponin complex 4. Nebulin - nebulin is essential for maintaining the structural integrity of myofibrils - very large protein ( kDa) and binds as many as 200 actin monomers - its length is proportional to thin filament length - it is believed that nebulin acts as a thin filament "ruler" and regulates thin filament length
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Thick filament Each head cycles approximately five times per second with a movement of 110 Å per cycle. However, because hundreds of heads are interacting with the same actin filament, the overall rate of movement of myosin relative to the actin filament may reach 80,000 Å per second Thick filament – nm - myosin: ATP dependent motor protein - one myosin molecule is composed of 4 chains - 2 heavy chains - coiled heavy chain: tail - N terminal globular head, - 2 light chains - neck region between the head and tail creates angle - hundreds of myosin molecules form a thick filament
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Structural and mechanical integrity of the sarcomeric elements
Desmin - a muscle-specific, type III intermediate filament - forms a scaffold around the Z-disk of the sarcomere -connects the Z-disk to the subsarcolemmal cytoskeleton -it links the myofibrils laterally by connecting the Z-disks -desmin may also connect the sarcomere to the extracellular matrix (ECM) through desmosomes which could be important in signalling between the ECM and the sarcomere which could regulate contraction and movement - desmin links the mitochondria to the sarcomere Desminopathy J Clin Invest. 2009;119(7):1806–1813. doi: /JCI38027. Desmin related cardiomyopathy
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Structural and mechanical integrity of the sarcomeric elements
Titin molecule – extends from Z-disc to M-line - large protein - associates with thick filament -maintains central position of thick filaments Nebulin – extends from Z-disc along the length of actin filament - template for the regulation of thin filament length Alpha actinin- component of Z-disc - anchors the barbed end of of actin filaments to Z-disc Myomesin + M-protein - involved in anchoring the myosin filaments to other filaments (titin) - myomesin protects sarcomere, keeps it stable during intense or sustained stretching
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Sarcoplasmic reticulum + T-tubule system
-major role in excitation-contraction coupling -sarcoplasmic reticulum - stores and pumps calcium ions - smooth endoplasmic reticulum T-tubule - deep invagination of the sarcolemma TRIAD=T-tubule + terminal cistern sarcoplasmic reticulum releases calcium ions during muscle contraction and absorb them during relaxation
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Resting muscle at molecular level
ATP binding – weakens the actin-myosin cross-bridge – without nerve stimulus – no Ca++ around the myofilaments - myosin head detaches – myosin binding actin spots are covered by tropomyosin ATP relaxation myosin head detaches
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Contraction at molecular level „Sliding filament model”
- nervous stimulus – action potential rapidly spreads along the muscle cell and enters the cell through the T-tubule - action potential opens gates in the muscle's calcium store (sarcoplasmic reticulum) calcium ions flow into the cytoplasm calcium ions bind to troponin-tropomyosin molecules troponin C changes shape and slides tropomyosin out of the groove, exposing the actin-myosin binding sites activation of a myosin light chain ATP-ase enzyme regulatory part exposed actin-myosin binding sites myosin interacts with actin myosin ATPase activated and ATP hydrolyzed release of phosphate from the myosin molecule after the ATP hydrolysis while myosin is tightly bound to actin release of phosphate results in a conformational change in the molecule that pulls against the actin - myosin cocks – the head moves the + end of actin filament combined effect of the myriad power strokes causes the muscle to contract during each cycle, myosin moves 5 – 25 nm and one ATP is hydrolyzed - ADP is released - ATP binding site is empty - ATP binding - myosin head detaches
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Ca++ in contraction muscle contractions are caused by calcium's bonding to troponin unmasking the binding sites covered by the troponin-tropomyosin complex on the actin myofilament allows the myosin cross-bridges to connect with the actin
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Cardiac muscle
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Light microscopy of the cardiac muscle
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EM of cardiac muscle Eberth’s line = intercalated disc
fascia adherens - anchoring sites for actin, connect to sarcomere desmosoma - stop separation during contraction by binding intermediate filaments, joining the cells together nexus - allow action potentials to spread between cardiac cells by permitting the passage of ions between cells producing depolarization of heart muscle -cardiomyocytes connected by intercalated discs to work as a syntitium
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Sarcoplasmic reticulum and T-tubule system
DIAD - at the sarcomere Z-line - a single T-tubule paired with a terminal cisterna of the sarcoplasmic reticulum - juxtaposing an inlet for the action potential near a source of Ca2+ ions
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Smooth Muscle
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Smooth muscle – light microscopy
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Smooth muscle - EM plaque thin thick filaments dense body dense bodies nucleus plaques Dense bodies - actin filaments of contractile units are attached to dense bodies Dense bands (or dense subsarcolemmal plaques) - are circumfering the smooth muscle cell in a rib-like pattern areas alternate with regions of membrane containing numerous caveolae
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Contraction of smooth muscle cells
Unlike cardiac and skeletal muscle smooth muscle does not contain calcium-binding protein troponin Transient receptor potential channels (TRP) a large group of ion channels TRP channels modulate ion entry driving forces and Ca2+ and Mg2+ transport machinery in the plasma membrane
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Myofascial integration
Color code: connective tissue muscle fiber muscle pain in humans ion channels to detect pH changes: acid-sensing ion channels (ASICs) - lactate in extreme active muscle - ATP in the low concentrations The fascia seems to be the structure most heavily innervated by nociceptors, and most likely involved with eliciting muscle pain in human muscle Muscle pain and sensory muscle fatigue ameliorate quickly once exercise is ended, but increase again in untrained muscles 12–24 hours later and can last 48 hours or longer before diminishing. The pain has been studied extensively and is often called delayed-onset muscle soreness (DOMS) - exercise of untrained muscle causes micro-tears that result in inflammation - acute viral infection could cause myalgia and fatigue through cytokine pathways
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REFERENCES
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