Muscle Tissue PDF

MUSCLE TISSUE Lec. Gözde ÖĞÜTCÜ Faculty of Medicine Department of Histology and Embryology  Muscle tissue is responsible for movement of the body and its parts and for changes in the size and shape of internal organs This tissue is cha...

MUSCLE TISSUE Lec. Gözde ÖĞÜTCÜ Faculty of Medicine Department of Histology and Embryology  Muscle tissue is responsible for movement of the body and its parts and for changes in the size and shape of internal organs This tissue is characterized by aggregates of specialized, elongated cells arranged in parallel array that have the primary role of contraction  Properties of Muscles 1. Excitability (responsiveness) respond to chemical, mechanical or electrical stimuli. 2. Conductivity initiate events that lead to contraction. 3. Contractility ability to shorten substantially 4. Extensibility able to stretch between contractions. 5. Elasticity ability to return to original length after stretching. Myofilaments, which are responsible for the contraction of muscle fibers, are of 2 types: Thin filaments: They are 6-8 nm in diameter and 1.0 microns in length and consist of actin protein. This fibrous actin (F- actin) structure is formed by the polymerization of globular actin molecules (G-actin). Thick filaments: They are approximately 15 nm in diameter and 1.5 microns in length and consist of myosin proteins. They show a regular arrangement, parallel to each other, with their long tails and heads facing outwards. Muscle terminology Myofiber Or Myocyte: a muscle cell Sarcolemma: the plasma membrane of a muscle cell Sarcoplasm: the cytoplasm of the muscle cell Sarcoplasmic Reticulum: the endoplasmic reticulum of a muscle cell Sarcosome: the mitochondria of a muscle cell Origin With the exception of some smooth muscle tissue, the muscular system develops from the mesodermal germ layer and consists of skeletal, smooth and cardiac muscle. Myogenesis is the formation of muscle fibers, called myotubes. These fibers are multinucleated and are capable of contraction. First step in their formation is alignment of the myoblasts into the myotubes and the second stage is the fusion itself. Types of Muscle Tissue Three type; Striated skeletal mucle Voluntary, serebrospinal nerves,cross striated quick forceful contractıon Striated cardiac muscle İnvoluntary ,autonomic, cross striated ,vıgorus rhytmic contractıon Smooth muscle İnvoluntary ,autonomic ,no striation, slow conractıon SKELETAL MUSCLE Skeletal muscle is attached to bone and is responsible for movement of the axial and appendicular skeleton and for maintenance of body position and posture. BASIC MORPHOLOGIC UNIT in LM: multinucleated skeletal muscle fiber with cross striation. Elongated nuclei are situated below the sarcolemma. SKELETAL MUSCLE Muscles attached to the skeletal bone Muscle layer in the upper half of the esophagus diaphragm muscles tongue muscle extra ocular muscles Mimic muscles of the face 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 form fascicles Muscle fascicle is surrounded by perimysium - sheath of connective tissue Entire muscle is surrounded by epimysium - composed of dense irregular connective tissue. Organization Within Muscle Fibers  Myofibrils and Myofilaments The structural and functional subunit of the muscle fiber is the myofibril Myofibrils are visible in favorable histologic preparations and are best seen in crosssections of muscle fibers Myofibrils extend the entire length of the muscle cell. Myofibrils are composed of bundles of myofilaments. Myofilaments are the individual filamentous polymers of myosin II (thick filaments) and actin (thin filaments) and its associated proteins. Myofilaments are the actual contractile elements of striated muscle and surrounded by sarcoplasmic reticulum. Myofibrils and Myofilaments In light microscopy, the cytoplasm of each striated muscle fiber is observed as alternating light and dark discs or bands in longitudinal section. When examined with a polarized microscope, it is seen that the dark bands are anisotropic (Birefringent = double refracts polarized light) and the light bands are isotropic (Monorefringent = single refracts polarized light). Therefore, they are called A and I bands.  What are these bands and lines composed of? Dark A bands are composed of thick (myosin) filaments overlapping thin (actin) filaments. Light I bands contain only thin filaments. Z(Zwischenscheibe) lines (disks) bisect I bands. Alpha actinin attaches actin filaments to the Z line. H(Hensen disk) bands are in the center of A bands. They lie between the free ends of the thin filaments and contain only myosin filaments. M(middle stripe-mittel scheibe) lines bisect H bands. They are where adjacent thick filaments connect. Sarcomere: actin, myosin,z lines,H zone Mline A band, I band The functional unit of the myofibril is the sarcomere, the segment of the myofibril between two adjacent Z lines The sarcomere is the basic contractile unit of striated muscle The A band marks the extent of the myosin filaments. Actin filaments extend from the Z line into the region of the A band, where they interdigitate with the myosin filaments  SARCOMERE During contraction, the fibrils are thicker and sarcomeres are shorter. The distance between the two Z bands shortens as contraction increases. As the I bands get shorter, the ends of the A bands get closer to the Z line. As a result, the A and I bands become indistinguishable, but the length of the A bands remains constant during contraction.  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 Accessory proteins maintain precise alignment of thin and thick filaments Titin, that anchors thick filaments in the Z lines. preventing excessive stretching of the sarcomere Alfa-Actinin, a short actin-binding protein, bundles thin filaments into parallel arrays and anchors them at the Z line. Desmin,intermediate filament, forms a lattice that surrounds the sarcomere at the level of the Z lines, attaching them to one another and to the plasma membrane, thus forming stabilizing cross-links between neighboring myofibrils. Myomesin, myosin-binding protein, holds thick filaments in register at the M line.  C protein: Attaches myosin filaments to the M line G-Actin: Forms fibrillar actin filaments, carries out contraction together with myosin Nebulin: Located in the Z disc, helps α-Actinin molecules bind to actin filaments Dystrophin: It is the protein that connects laminin located in the external lamina of the muscle cell to actin filaments. The genetic disorder associated with severe muscle weakness in the absence of this protein is called Duchenne's muscular dystrophy. Since dystropin is encoded on the X chromosome, it is observed only in men. Sarcoplasmic reticulum and Transverse Tubule System ıs the smooth endoplasmic reticulum in skeletal muscle Specialized to sequester and release calcium ions Surrounds each myofibril Sarcoplasmic reticulum Sarcoplasmic reticulum is a special type of agranular endoplasmic reticulum and consists of membrane- surrounded, continuous tubules. The tubules surround each myofibril like a network, enveloping the A band longitudinally along its entire length.  Transfer (T) Tubules ,Terminal cisterna When the longitudinal tubules reach the junction of the A-I bands from both sides, they unite to form transverse tubules with bulging ends, called terminal cisterna. Ca+2 is released into the sarcoplasm from the terminal cisternae. Paired terminal cisternae are separated from each other by a smaller central transverse tubule (T tubule). The location of the T tubule is the junction of A-I bands. The T tubule is not part of the sarcoplasmic reticulum, but is a tubular invagination of the sarcolemma towards the junction of the A-I bands. Triad In this way, the formation formed by the two terminal cisterns of the sarcoplasmic reticulum and the T-tubule is called Triad. In mammalian striated muscles, there are two triads associated with each sarcomere. Transfer (T) Tubules ,Terminal cisterna and triad At each A-I band junction, a tubular invagination of the sarcolemma termed a transverse (T) tubule penetrates the muscle fiber and lies next to the surface of myofibrils T tubules make contact with the sarcoplasmic reticulum An expansion of the sarcoplasmic reticulum called a terminal cisterna lies on each side of the T tubule. A triad is a complex of 2 terminal cisternae with a T tubule in between. Cross section of skeletal muscle Mechanism of Contraction During contraction muscle fiber or cell shortens about one third of its original length The length of the actin and myosin does not change but slide past each other. So that sarcomere decreases in length Z lines come closer to the A band The length of the A band does not change but I band shortens in length, because thin filaments are pushed towards the center of sarcomere by myosin heads while doing this actin filaments pull Z lines together with them. H band gradually decrease in length, thin filaments penetrate deeper into the sarcomere  Contraction in the Sarcomere A band stays the same I band gets smaller H zone gets smaller Sarcomere shortens Skeletal Muscle Fiber Types Skeletal muscle is composed of 3 different fiber types: The fiber type is based on the biochemical process for making ATP and how fast the fibers contract. Red or slow oxidative fibers [dark staining (R)] Intermediate or Fast oxidative fibers [lighter (I) staining) Fast glycolytic fibers [white (W) or non staining] Skeletal Muscle Fiber Types Fiber type characteristics Slow oxidative (R): Dark staining red in color = abundant myoglobin Having many mitochondria and much myoglobin Contract slowly and are more resistant to fatigue. Ex. back muscles and support muscles Intermediate Fast oxidative (I): Stains less darkly than Red but slightly larger Produce ATP via aerobic metabolism like slow Contract faster and more powerfully than slow Abundant in lower limbs = contract for long periods  Fiber type characteristics White or Fast glycolytic fibers (W): Stain a pale color due to little myoglobin. Largest in diameter of all three fiber types Depend on anaerobic glycogenolysis to make ATP. Contract rapidly and fatigue easily. Rapid contractions lead to rapid fatique as lactic acid produced by glycolysis. They form the extraocular muscles and the finger muscles. Short distance sprinters, weightlifters, and other athletes have high concentrations of these fibers.  It is not possible to separate these fibers in hematoxylin and eosin staining. Can be demonstrated by histochemical reactions (especially succinic dehydrogenase reactions) Oxidative enzyme activity was demonstrated by histochemical method.  Neuromuscular Control Skeletal muscle contraction is controlled by a nerve impulse (action potential) transmitted by the motor nerve from the brain or spinal cord. A motor unit consists of all the muscle fibers controlled by a single motor neuron. Fine control muscles (i.e. eyelid muscles) have fewer muscle fibers/ nerve (2:1). A contraction is initiated by an action potential (nerve impulse) and followed by the release a chemical neurotransmitter at the neuromuscular junction (NMJ). Neurotransmitter for skeletal muscle is acetylcholine.  Neuromuscular control Each muscle fiber is innervated by a single motor neuron Contractions may be graded or full due to the number of muscle fibers that respond to the stimulus. The more fibers, the greater the muscle contraction Synapse – functional connection between a nerve fiber and its target cell. Neuromuscular junction – synapse between a motor nerve and a muscle fiber.  Every muscle contraction is preceded by a nerve impulse from the CNS. Motor nerve Muscle fibers Motor innervated by unit single motor neuron Neuromuscular junctions  Neuromuscular Junction Synaptic knob, terminal or bouton – bulbous swelling at the end of a motor nerve above the motor end plate on the muscle fiber. Synaptic cleft – gap between the synaptic knob and the motor end plate. Synaptic vesicles – small packets of neurotransmitter chemical (e.g. acetylcholine, norepinephrine, etc.) Neuromuscular junction SEM of Neuromuscular Junction CARDIAC MUSCLE BASIC MORPHOLOGIC UNIT in LM: cardiac muscle cell (cardiomyocyte) with cross striation. Cells are often branched Nuclei (1-2) are situated in the centre of the cell. Cardiomyocytes are connected by intercalated discs.  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 Functions of Cardiac Muscle Tissue Cardiac muscle tissue plays the most important role in the contraction of the atria and ventricles of the heart. it causes the rhythmical beating of the heart, circulating the blood and its contents throughout the body as a consequence. influenced involuntarily by endocrine (hormones) and autonomic nervous systems Cardiac muscle with plump nuclei , intercalated discs , and striations Smooth Endoplasmic Reticulum The sER of cardiac muscle is not as well organized as that of skeletal muscle. The T tubules in cardiac muscle penetrate into the myofilament bundles at the level of the Z line, between the ends of the sER network. there is only one T tubule per sarcomere in cardiac muscle. Small terminal cisternae of the sER are in close proximity to the T tubules to form a dyad at the level of the Z line The T tubules of cardiac muscle are much larger than the T tubules of skeletal muscle and carry an investment of external lamina material into the cell.  SMOOTH MUSCLE Smooth muscle generally occurs as bundles or sheets of elongated fusiform cells, consists of centrally located and single-nucleus with finely tapered ends Smooth muscle controls slow, involuntary movements of the walls of hollow organs; the stomach, GIT, uterus,glands, blood vessels… Do not exhibit cross striations because the myofilaments do not achieve the same degree of order in their arrangement SMOOTH MUSCLE In the wall of the digestive system from the middle of the esophagus to the anus In the walls of arteries, veins and large lymphatics, In the urinary and genital duct walls In the walls of the respiratory tract, such as tracheal-alveolar ducts Arrector pylorum muscle (extends from the skin to the hair follicle) Dartos muscle (scrotum) Iris and corpus ciliare (eye) Smooth Muscle Tissue Contains both thick and thin filaments, and a cytoskeleton of desmin and vimentin intermediate filaments. Not arranged in orderly in sarcomeres Smooth muscle cells are interconnected by gap junctions, the specialized communication junctions between the cells Caveola in the sarcolemma instead of T tubules Caveolae can trigger Ca2+ release from nearby segments of ER  Dense bodies provide an attachment site for thin filaments and intermediate filaments. Dense bodies are intracellular analogs of the striated muscle Z lines. They play an important role in transmitting contractile forces generated inside the cell to the cell surface, altering the cell’s shape During contraction, the filaments pull on the dense bodies causing a shorthening of the muscle fiber These structures are distributed throughout the sarcoplasm in a network of intermediate filaments containing the protein desmin and vimentin. PROTEINS ESSENTIAL FOR CONTRACTION Myosin light chain kinase (MLCK) is important in the mechanism of contraction in smooth muscle. It initiates the contraction cycle after its activation by Ca2+–calmodulin complex. Active MLCK phosphorylates one of the myosin regulatory light chains, enabling it to form a cross-bridge with actin filaments. Calmodulin, Ca2+-binding protein, is related to the TnC found in skeletal muscle, which regulates the intracellular concentration of Ca2+. Ca2+calmodulin complex binds to MLCK to activate this enzyme. It may also, with caldesmon, regulate its phosphorylation and release from F-actin. Alfa-actinin, a 31-kilodalton protein, provides structural component to dense bodies.  REGENERATION OF MUSCLE TISSUE Skeletal muscle cells cannot undergo mitosis, show limited regeneration. Regeneration can occur in skeletal muscle with the satellite cells that can proliferate, fuse and form new muscle fibers. Skeletal muscle fibers in adults contain small, single-nucleated satellite cells. These are located between the sarcolemma and the endomysium (external lamina). Cells can divide and play a role in repair and regeneration. REGENERATION OF MUSCLE TISSUE Cardiac muscle shows very little regeneration capacity. Damaged to heart muscle are generally replaced by proliferating fibroblasts and growth of connective tissue, forming myocardial scars. Smooth muscle is capable of more active regenerative response. After injury, smooth muscle cells undergo mitosis and replace the damaged tissue.