Histology Of Muscle Lecture PDF

HISTOLOGY OF MUSCLE  OVERVIEW AND CLASSIFICATION  A tissue characterized by the aggregation of specialized elongated cells arranged in a parallel fashion  Main function of myocytes is contraction  Contraction is mediated by the interaction between myofilaments  Tw...

HISTOLOGY OF MUSCLE  OVERVIEW AND CLASSIFICATION  A tissue characterized by the aggregation of specialized elongated cells arranged in a parallel fashion  Main function of myocytes is contraction  Contraction is mediated by the interaction between myofilaments  Two principal myofilaments ▪ THIN filaments (6 - 8nm diam) : composed of ACTIN. A polymer of fibrous actin formed from globular actin (G-actin) ▪ THICK filaments (15nm diam) : composed of MYOSIN II protein    OVERVIEW AND CLASSIFICATION  The thin and thick filaments occupy the cytoplasm (sacroplasm)  Importantly, actin and myosin also function in other cell types to mediate cytokinesis, exocytosis and cell migration  Classification ▪ Striated muscle : cells exhibit cross-striations ▪ Smooth muscle : cells lack cross-striations  The cross-striations are due to the architectural organization of the actin and myosin myofilaments  OVERVIEW AND CLASSIFICATION  Striated muscle is further sub classified ▪ Skeletal muscle – attached to bone, produces skeletal movement and maintains posture ▪ Visceral Striated muscle – morphologically indistinct from skeletal muscle, localized to tongue, pharynx, diaphragm and upper oesophagus ▪ Cardiac muscle STRIATED VS. SMOOTH SKELETAL MUSCLE  SKELETAL MUSCLE  Composed of multinucleated syncytia formed by the fusion of multiple individual myoblast cells that vary in length  Plasma membrane (sarcolemma) with nuclei immediately adjacent  Held together by connective tissue ▪ Endomysium : reticular fibres surrounding each muscle fibre ▪ Perimysium : a connective tissue layer surrounding groups of fibres to form bundles/fascicles ▪ Epimysium : dense connective tissue that surrounds collections of fascicles to form muscle  SKELETAL MUSCLE  Three types of muscle fibres, typically all types present to varying extents in any skeletal muscle:  Type I / slow oxidative: ▪ small fibres that appear red ▪ Many mitochondria, large amounts of myoglobin and cytochromes ▪ Slow-twitch fatigue-resistant motor units ▪ Low myosin ATP-ase activity ▪ Principal fibres in long muscles of the back – they are adapted to the long slow contraction to maintain erect posture ▪ High endurance athletes  SKELETAL MUSCLE  Type IIa/ fast-oxidative ▪ Intermediate color in fresh tissue ▪ High numbers of mitochondria and myoglobin ▪ They contain large amounts of glycogen and perform anaerobic glycolysis ▪ Fast-twitch fatigue resistant  Type IIb / fast glycolytic ▪ Paler, contain fewer mitochondria than Type I or IIa fibers ▪ Contain high amounts of glycogen and high anaerobic activity ▪ Low levels of oxidative enzymes ▪ Fast-twitch fatigue prone units that generate high tension ▪ Adapted for rapid contraction and precise fine movements  MYOFIBRILS AND MYOFILAMENTS  Muscle fibers are composed of longitudinally -arrayed structural units called myofibrils  They are composed of bundles of myofilaments that extend through the entire length of myocytes  Myofilaments are the individual filamentous polymers of myosin II (thick) and actin (thin) and their associated proteins  They are the actual contractile units in skeletal muscle SARCOMERE STRUCTURE  SARCOMERE STRUCTURE  The sarcomere is the functional unit of the myofibril, defined by the segment of the myofibril between two adjacent Z -lines  A -band and I-band represent alternating dark and light bands respectively under phase contrast microscopy  Z-disc traverses I-band  M-line traverses and bisects the A -band  SARCOMERE STRUCTURE  The myosin-containing thick filaments are localized to the central portion (A band) of the sarcomere.  The thin actin filaments attach to the Z lines and extend into the A band  The I band is the portion of two adjacent sarcomeres that contains only thin filaments.  The thin filaments are composed of F-actin, troponin and tropomyosin. The thick filaments are composed of myosin II only.  SKELETAL MUSCLE TISSUE Contractile Proteins ▪ Myosin ▪ Thick filaments ▪ Function as a motor protein which can achieve motion ▪ Convert ATP to energy of motion ▪ Projections of each myosin molecule protrude outward (myosin head) ▪ Actin ▪ Thin filaments ▪ Actin molecules provide a site where a myosin head can attach ▪ Tropomyosin and troponin are also part of the thin filament ▪ In relaxed muscle ▪ Myosin is blocked from binding to actin ▪ Strands of tropomyosin cover the myosin-binding sites ▪ Calcium ion binding to troponin moves tropomyosin away from myosin- binding sites ▪ Allows muscle contraction to begin as myosin binds to actin  SKELETAL MUSCLE TISSUE Structural Proteins ▪Titin ▪ Stabilize the position of myosin ▪ accounts for much of the elasticity and extensibility of myofibrils ▪Dystrophin ▪ Links thin filaments to the sarcolemma  CONTRACTION AND RELAXATION OF SKELETAL MUSCLE The Sliding Filament Mechanism ▪Myosin heads attach to and “walk” along the thin filaments at both ends of a sarcomere ▪Progressively pulling the thin filaments toward the center of the sarcomere ▪Z discs come closer together and the sarcomere shortens ▪Leading to shortening of the entire muscle CONTRACTION AND RELAXATION OF SKELETAL MUSCLE CONTRACTION AND RELAXATION OF SKELETAL MUSCLE  CONTRACTION AND RELAXATION OF SKELETAL MUSCLE The Contraction Cycle ▪The onset of contraction begins with the SR releasing calcium ions into the muscle cell ▪Where they bind to actin opening the myosin binding sites ▪(SR – sarcoplasmic reticulum found in muscle cells similar to smooth ER – stores calcium ions )  CONTRACTION AND RELAXATION OF SKELETAL MUSCLE The contraction cycle consists of 4 steps ▪1) ATP hydrolysis ▪ Hydrolysis of ATP reorients and energizes the myosin head ▪2) Formation of cross-bridges ▪ Myosin head attaches to the myosin-binding site on actin ▪3) Power stroke ▪ During the power stroke the cross-bridge rotates, sliding the filaments ▪4) Detachment of myosin from actin ▪ As the next ATP binds to the myosin head, the myosin head detaches from actin ▪ The contraction cycle repeats as long as ATP is available and the Ca ++ level is sufficiently high ▪ Continuing cycles apply the force that shortens the sarcomere CONTRACTION AND RELAXATION OF SKELETAL MUSCLE  1 Myosin heads Key: hydrolyze ATP and = Ca2+ become reoriented and energized ADP P 2 Myosin heads bind to actin, forming P crossbridges ATP Contraction cycle continues if ATP is available and Ca2+ level in ADP the sarcoplasm is high ATP ADP 4 As myosin heads bind ATP, the crossbridges detach 3 Myosin crossbridges from actin rotate toward center of the sarcomere (power stroke)  CONTRACTION AND RELAXATION OF SKELETAL MUSCLE Excitation–Contraction Coupling ▪ An increase in Ca ++ concentration in the muscle starts contraction ▪ A decrease in Ca ++ stops it ▪ Action potentials cause Ca ++ to be released from the SR into the muscle cell ▪ Ca ++ moves tropomyosin away from the myosin- binding sites on actin allowing cross-bridges to form ▪ The muscle cell membrane contains Ca ++ pumps to return Ca ++ back to the SR quickly ▪ Decreasing calcium ion levels ▪ As the Ca ++ level in the cell drops, myosin-binding sites are covered and the muscle relaxes CONTRACTION AND RELAXATION OF SKELETAL MUSCLE Copyright 2009, John Wiley & Sons, Inc.  MUSCLE METABOLISM Production of ATP in Muscle Fibers ▪A large amount of ATP is needed to: ▪ Power the contraction cycle ▪ Pump Ca ++ into the SR ▪The ATP inside muscle fibers will power contraction for only a few seconds ▪ATP must be produced by the muscle fiber after reserves are used up ▪Muscle fibers have three ways to produce ATP ▪ 1) From creatine phosphate ▪ 2) By anaerobic cellular respiration ▪ 3) By aerobic cellular respiration MUSCLE METABOLISM  MUSCLE METABOLISM Creatine Phosphate ▪Excess ATP is used to synthesize creatine phosphate ▪Creatine phosphate transfers its high energy phosphate group to ADP regenerating new ATP ▪Creatine phosphate and ATP provide enough energy for contraction for about 15 seconds  MUSCLE METABOLISM  Anaerobic Respiration ▪ Series of ATP producing reactions that do not require oxygen ▪ Glucose is used to generate ATP when the supply of creatine phosphate is depleted ▪ Glucose is derived from the blood and from glycogen stored in muscle fibers ▪ Glycolysis breaks down glucose into molecules of pyruvic acid and produces two molecules of ATP ▪ If sufficient oxygen is present, pyruvic acid formed by glycolysis enters aerobic respiration pathways producing a large amount of ATP ▪ If oxygen levels are low, anaerobic reactions convert pyruvic acid to lactic acid which is carried away by the blood ▪ Anaerobic respiration can provide enough energy for about 30 to 40 seconds of muscle activity  MUSCLE METABOLISM  Aerobic Respiration ▪ Activity that lasts longer than half a minute depends on aerobic respiration ▪ Pyruvic acid entering the mitochondria is completely oxidized generating ▪ ATP ▪ carbon dioxide ▪ Water ▪ Heat ▪ Each molecule of glucose yields about 36 molecules of ATP ▪ Muscle tissue has two sources of oxygen ▪ 1) Oxygen from hemoglobin in the blood ▪ 2) Oxygen released by myoglobin in the muscle cell ▪ Myoglobin and hemoglobin are oxygen-binding proteins ▪ Aerobic respiration supplies ATP for prolonged activity  MUSCLE METABOLISM Muscle Fatigue ▪Inability of muscle to maintain force of contraction after prolonged activity ▪Factors that contribute to muscle fatigue ▪Inadequate release of calcium ions from the SR ▪Depletion of creatine phosphate ▪Insufficient oxygen ▪Depletion of glycogen and other nutrients ▪Buildup of lactic acid and ADP ▪Failure of the motor neuron to release enough acetylcholine  MUSCLE METABOLISM Oxygen Consumption After Exercise ▪After exercise, heavy breathing continues and oxygen consumption remains above the resting level ▪Oxygen debt ▪ The added oxygen that is taken into the body after exercise ▪This added oxygen is used to restore muscle cells to the resting level in three ways ▪ 1) to convert lactic acid into glycogen ▪ 2) to synthesize creatine phosphate and ATP ▪ 3) to replace the oxygen removed from myoglobin  CONTROL OF MUSCLE TENSION Twitch Contraction ▪The brief contraction of the muscle fibers in a motor unit in response to an action potential ▪Twitches last from 20 to 200 msec ▪Latent period (2 msec) ▪ A brief delay between the stimulus and muscular contraction ▪ The action potential sweeps over the sarcolemma and Ca ++ is released from the SR ▪Contraction period (10–100 msec) ▪ Ca ++ binds to troponin ▪ Myosin-binding sites on actin are exposed ▪ Cross-bridges form  CONTROL OF MUSCLE TENSION ▪Relaxation period (10–100 msec) ▪ Ca ++ is transported into the SR ▪ Myosin-binding sites are covered by tropomyosin ▪ Myosin heads detach from actin ▪ Muscle fibers that move the eyes have contraction periods lasting 10 msec ▪ Muscle fibers that move the legs have contraction periods lasting 100 msec ▪Refractory period ▪ When a muscle fiber contracts, it temporarily cannot respond to another action potential ▪ Skeletal muscle has a refractory period of 5 milliseconds ▪ Cardiac muscle has a refractory period of 300 milliseconds CONTROL OF MUSCLE TENSION  CONTROL OF MUSCLE TENSION Types of Contractions ▪Isotonic contraction ▪ The tension developed remains constant while the muscle changes its length ▪ Used for body movements and for moving objects ▪ Picking a book up off a table ▪Isometric contraction ▪ The tension generated is not enough for the object to be moved and the muscle does not change its length ▪ Holding a book steady using an outstretched arm SMOOTH MUSCLE  SMOOTH MUSCLE  Occurs as bundles of elongated fusiform cells that have tapered ends  Cells vary in size in dif ferent tissues 20 -200microns  Interconnected by gap junctions – to enable synchronous contraction of bundles of smooth muscle cells  Cells have characteristic nuclei in longitudinal section – they appear elongated and have tapering ends - match the shape of the cell SMOOTH MUSCLE  SMOOTH MUSCLE  Contractile apparatus in smooth muscle contains thick and thin filaments  The cells possess a cytoskeleton containing desmin and vimentin intermediate filaments  The sarcoplasm of SM cells contains labile thick myosin filaments that are lost during tissue preparation  SMOOTH MUSCLE  SM is specialized for prolonged contraction without fatigue  They can ▪ Produce peristaltic movements by contracting in a wave -like manner ▪ Produce extrusive movements – as in the urinary bladder, gallbladder or uterus ▪ Exhibit spontaneous contractile activity in the absence of nerve stimuli  Contraction of SM is regulated by post -ganglionic fibers of the autonomic nervous system  Most SM is directly innervated by SNS and PNS  SMOOTH MUSCLE  In the GIT – the enteric division of the ANS (autonomic nervous system) is the primary source of innervation to smooth muscle in the gut  Ca 2+ enters the sarcoplasm during depolarisation by voltage- gated Ca channels  Some hormones act as ligands on ligand-gated Ca channels to initiate SM contraction – e.g. oxytocin and ADH from the posterior pituitary during birth  Gap junctions between smooth muscle cells propagate contraction through the muscle layer MUSCLE-TENDON JUNCTION Oblique-cut skeletal muscle inserting into tendon SKELETAL MUSCLE X400 Parallel muscle fibers cut in longitudinal section SKELETAL MUSCLE X260  CS of striated muscle  Polygonal muscle fibres with peripheral nuclei  Perimysium visible between bundles of muscle fibers that contains blood vessels SKLETAL MUSCLE X640  Higher magnification showing cross section through several muscle cells  Myofibrils account for the stippled appearance SMOOTH MUSCLE X 900  Spindle shaped fusiform SM cells from intestine with corkscrew appearance of nuclei  CS ➔ SMOOTH MUSCLE X 160  Uterine SM arranged as interlacing bundles separated by connective tissue  Unclear separation between SM and connective tissue  Interrupted lines show SM bundles arranged in dif ferent orientations  CARDIAC MUSCLE  Similar arrangement and same type of contractile filaments as skeletal muscle  Cells exhibit cross-striations under LM  Dense-staining cross-bands known as intercalated discs cross the muscle fibers  These represent highly specialized attachments between adjacent cells  Not a syncitium but composed of end-to-end alignment of individual cells CARDIAC MUSCLE X 160  Intercalated discs arranged linearly or cross-step fashion.  Cardiac myocyte nuclei located in the center of the fiber.  Branching of myocyte fibers Horizontal muscle fibers with cross striations. Intercalated discs are not always visible in routine H&E sections  STRUCTURE OF CARDIAC MUSCLE  Unlike skeletal muscle, cardiac myocyte nuclei lie at the cell center  Myofibrils in cardiac myocyte separate to pass around the nucleus in TEM (transmission electron micrograph)  Cell organelles are located in a biconical juxtanuclear region, especially mitochondria  In the atria – granules that contain ANP (atrial natriuretic peptide) and BNP (brain natriuretic peptide) are located in the juxtanuclear region  These regulate fluid balance through natriuresis – the excretion of sodium in the urine which is controlled by ANP and BNP (natriuretic peptides)  STRUCTURE OF CARDIAC MUSCLE  Cardiac myocytes have many large mitochondria with numerous cristae packed between the myofibrils  Numerous glycogen granule stores are also present between the myofibrils.  Intercalated discs appear as dense-staining linear structures that are oriented transversely to the muscle fiber.  The intercalated disc represents the attachment site between individual cardiac myocytes  INTERCALATED DISCS  Under TEM (transmission electron microscopy), the intercalated disc is composed of ▪ A transverse component – crosses the fiber perpendicular to the myofibrils ▪ A lateral component, not visible under LM, lies parallel to the myofibrils  Adhering junctions are the major constituent of the transverse component of I.D. ▪ Responsible for staining in H&E sections ▪ Attaches individual myocytes to form a functioning fiber ▪ The thin filaments of the terminal sarcomere attach to adhering junctions to anchor into the plasma membrane.  INTERCALATED DISCS  Gap junctions ▪ Constitute the main element in the lateral component of the I.D. ▪ Provide ionic continuity between individual myocytes, enabling the cells to behave as a syncitium  THE SER  The sER in cardiac myocytes is organised into a single network along the sarcomere that extends from Z -line to Z- line  Only one T-tubule (extension of cell membrane that penetrates into the centre of skeletal and cardiac muscle cells) per sarcomere in cardiac muscle CARDIAC MUSCLE X 400  Linear contractile units called myofibrils  These separate to pass around nuclei and delineate a perinuclear region that is free from myofibrils  Many cells are binucleate