Skeletal Muscle Histology Lesson 1-2 PDF

Skeletal Muscle Assist. Prof. Dila ŞENER AKÇORA Department of Histology and Embryology February 2024 Muscle tissue is basic tissue type composed of cells that optimize the cell property of contractility. Actin microfilaments and associated proteins generate the forces necessary for the muscle contra...

Skeletal Muscle Assist. Prof. Dila ŞENER AKÇORA Department of Histology and Embryology February 2024 Muscle tissue is basic tissue type composed of cells that optimize the cell property of contractility. Actin microfilaments and associated proteins generate the forces necessary for the muscle contraction. Functions of muscle tissue: movement, body posture, transport of blood (cardiac muscle), lymph flow (skeletal muscle), heat generation (85% due to muscle contractions) Three types of muscle tissue can be distinguished on the basis of morphologic and functional characteristics with the structure of each adapted to its physiologic role. In all types of muscle, contraction is caused by the sliding interaction of thick myosin filaments along thin actin filaments. All muscle cells are of mesodermal origin. Cardiac muscle originates in splanchnopleuric mesoderm, Most smooth muscle is derived from splanchnic and somatic mesoderm, Most skeletal muscles originate from somatic mesoderm. Skeletal muscle contains bundles of very long, multinucleated cells with cross-striations. Skeletal muscle attached to bones. Found in tongue. Some portions of esophagus. Muscle Tissue Terminology Certain muscle cell organelles with special names Muscle cell: muscle fiber (MF) The cytoplasm of muscle cells: sarcoplasm Smooth ER: sarcoplasmic reticulum, Muscle cell membrane and its external lamina: sarcolemma Mitochondria: Sarcosome Muscle fibers (= cells) are typically: Long Cylindrical Multinucleated (peripherally located beneath sarcolemma) Striated Diameter: 10-100 um, hypertrophy Strength of fiber ~ diameter Strength of entire muscle ~ number & thickness of fibers Mesoderm-myotomemyoblast Myoblasts fuse with each other: myotubes Produce myofibrils (composed of myofilaments) Muscle fibers are arranged parallel to each other Continuous capillaries in intercellular spaces Small satellite cells Single nucleus Regenerative cells Located in shallow depressions of muscle cell’s surface Share muscle fiber’s external lamina Chromatin network of nucleus denser & coarser than that of muscle fiber Pink-red due to: Rich vascular supply Myoglobin pigments O2-transporting protein Resemble but smaller than hemoglobin Capillaries of skeletal muscle: blood vessels were injected with a dark plastic polymer before the muscle was collected and sectioned longitudinally. A rich network of capillaries in endomysium surrounding muscle fibers is revealed by this method. Giemsa with polarized light. Light micrograph of skeletal muscle: Cross and longitudinal sections, Masson’s trichrome. Light micrograph of skeletal muscle: Longitudinal and cross sections, Hematoxylen & Eosin Classification Fiber diameter Quantity of myoglobin Number of mitochondria Extensiveness of sarcoplasmic reticulum Concentration of enzymes Rate of contraction Red White Intermediate which statment is correct regarding skeletal muscles ? Comparision of skeletal muscle fiber types Characteristics Red muscle fibers White muscle fibers Vascularization Rich Poorer Innervation Smaller nerve fibers Larger nerve fibers Fiber diameter Smaller Larger Contraction Slow but repetitive Not easily fatigued Weaker contraction Fast Easily fatigued Stronger contraction Sarcoplasmic reticulum Not extensive Extensive Mitochondria Numerous Few Myoglobin Rich Poor Enzymes Rich in oxidative enzymes Poor in adenosine triphosphatase Poor in oxidative enzymes Rich in phosphorylases and adenosine triphosphatase Examples Gross anatomical muscle (biceps) All 3 types in relatively constant proportion, characteristic of that particular muscle Chicken: Thigh MFs predominantly red Breast MFs predominantly white Innervation determines fiber type. If experimentally switched, fiber accomodates itself to new nerve supply Organization of a Skeletal Muscle (Investments) Thin layers of connective tissue (CT) surround and organize the contractile fibers in all three types of muscle, CT elements: Interconnected Contractile forces produced by muscle cells are transferred to CT Continuous with tendons & aponeuroses, which connect muscle to bone and to other tissues perimysium is denser than epimysium. false Epimysium Entire muscle Dense irregular collagenous connective tissue Septa of this tissue extend inward, carrying the larger nerves, blood vessels, and lymphatics of the muscle. Perimysium Surrounds bundles (fascicles) of MFs Less dense collagenous connective tissue derived from epimysium Endomysium Surrounds each muscle cell (fiber) Reticular fibers (intermingle with those of neighboring cellsfibroblasts) and an external lamina (basal lamina) Collagen (type 1 & 3) in these connective tissue layers of muscle serve to transmit the mechanical forces generated by the contracting muscle cells Fıgure 10-4 Copyright © McGraw-Hill Companies Some skeletal muscles taper at their ends, where the epimysium is continuous with the dense regular connective tissue of a tendon at myotendinous junctions. Here the cells become tapered and fluted. Ultrastructural studies show that in these transitional regions, collagen fibers from the tendon insert themselves among & deep into muscle fibers and become continuous with reticular fibers of endomysium. Myotendinous junction, Mallory azan Myofibril: Much of the skeletal muscle cell is composed of longitudinal arrays of cylindrical myofibrils Each are 1 to 2 μm in diameter. Myofibrils composed of interdigitating, rod-like thick & thin myofilaments Myofibrils extend the entire length of the cell and are aligned parallel with their neighbors. This arrangement of the myofibrils is responsible for the crossstriations of light and dark banding that are characteristic of skeletal muscle viewed in longitudinal section. Fine structure of skeletal muscle fibers The sarcoplasm has little RER and contains primarily long cylindrical filament bundles, myofibrils, running parallel to the long axis of the fiber Organization Within Muscle Fibers Dark bands: A bands Light bands: I bands H band: At the center of A band Pale area Devoid of thin filaments Bisected by a thin M line myomesin, C protein interconnect thick filaments In TEM, each I band is bisected by a thin dark line: Z disk (Z line) Sarcomere: Region of the myofibril between 2 successive Z disks 2.5 um in length in resting muscle Contractile/ functional unit of skeletal muscle fibers Structural organisation of Myofibrils: thick and thin myofilaments The dark bands = thick myofilaments= A bands (anisotropic or birefringent in polarized light). 15 nm in diameter and 1.5 μm long. Includes myosin II and titin proteins. The light bands= thin myofilaments= I bands (Izosotropic, do not alter polarized light). 7 nm in diameter and 1.0 μm long. Includes actin, troponin (I,C,T), tropomyosin, tropomodulin and nebulin proteins. Electron micrograph of cross section of skeletal muscle fiber. Asterisks represent thick and thin filaments. gly, glycogen; m, mitochondria; pm, plasma membrane. Protein Functions Myosin II Major protein of thick filament. Its interaction with actin hydrolizes ATP & produces contraction Titin Forms an elastic mesh & anchors thick filaments to Z disc G actin Interaction of g-actin with with myosin II, assists in hydrolizing ATP, result in contraction Troponin C Troponin T Troponin I Binds calcium Binds to tropomyosin Bind to actin and inhibit actin-myosin interaction Tropomyosin Occupies grooves of the thin filaments Tropomodulin Caps the minus end of thin filaments/α-Actinin: Anchors plus ends of thin filaments to Z disk Nebulin Z disk protein that may assist α-actinin to anchor thin filaments to Z disk During muscle contraction: I band becomes narrower Width of A band remains unaltered H band disappeared Z disks move closer together Individual thick & thin filaments do not shorten; instead, 2 Z disks are brought closer together as thin filaments slide past the thick filaments Huxley’s sliding filament theory Huxley’s sliding filament theory Thin filaments move toward the center of sarcomere A greater overlap between thick & thin filaments is created As a result, width of I and H bands reduces but width of A band does not influence. Myofibrils are held together by intermediate filaments desmin and vimentin Secure periphery of Z disks of neighboring myofibrils to each other Myofibrils attached to cytoplasmic aspect of sarcolemma by proteins such as dystrophin, a protein which binds to actin The structural organization of myofibrils is maintained largely by five proteins: Titin, α-Actinin, Cap Z, Nebulin and Tropomodulin Titin: Forms an elastic mesh & anchors thick filaments to Z disc. Thick filaments are positioned within the sarcomere by assistance of titin. α-Actinin: Anchors plus ends of thin filaments to Z disk. Component of the Z disk and thin filaments held in register through this protein. Cap Z: The plus end of the thin filament is held in place by this protein. Nebulin: Acts as a “ruler,” ensuring the precise length of the thin filament. It is supported in this function by the protein tropomodulin. Sarcoplasmic Reticulum (SR) & Transverse Tubule System In skeletal muscle fibers the smooth ER, or sarcoplasmic reticulum, is specialized for Ca2+ sequestration. SR forms a meshwork around each myofibril & displays dilated cisternae at each A-I junction Depolarization of the sarcoplasmic reticulum membrane, which causes release of calcium, is initiated at specialized motor nerve synapses on the sarcolemma. NEUROMUSCULAR JUNCTION-NMJ (NERVE – MUSCLE BINDING = SYNAPS) Sarcolemma is folded into a system of transverse or T tubules, and trigger Ca2+ release from SR throughout the fiber simultaneously and cause uniform contraction of all myofibrils. These long, fingerlike invaginations penetrate deeply into the sarcoplasm and encircle every myofibril near the aligned A- Iband boundaries of sarcomeres. Thus, T tubules facilitate the conduction of depolarization waves along the sarcolemma. Adjacent to each side of every T tubule are expanded terminal cisterns of the sarcoplasmic reticulum. In longitudinal TEM sections, this complex of a T tubule with two closely associated terminal cisterns of sarcoplasmic reticulum on each side is known as a triad (2 SR + 1 T-tubule: TRIAD) Permit depolarization wave to spread immediately from sarcolemma throughout cell, reaching terminal cisternae, which have voltage-gated calcium release channels in their membrane After depolarization of the sarcoplasmic reticulum membrane, calcium ions are released through Ca2+ channels into cytoplasm surrounding the thick and thin filaments. Ca2+ binds troponin and allows bridging between actin and myosin molecules. Innervation of Skeletal Muscle Each skeletal muscle receives at least 2 types of nerve fibers: motor & sensory – Motor: contraction – Sensory: to muscle spindles Autonomic fibers supply vascular elements of muscle Each motor neuron & the muscle fibers it controls, form a motor unit Fibers of motor unit contract together and follow all-or-none-law Impulse transmission at myoneural junction Motor fibers branch & loses myelin sheath Terminal of fiber dilated & overlies motor end plate of individual muscle fibers Muscle-nerve junction: myoneural junction – Axon terminal, Synaptic cleft, Muscle cell membrane (postsynaptic membrane) Muscle cell membrane Modified Forms primary synaptic cleft, occupied by axon terminal Secondary synaptic clefts (junctional folds) open into primary clefts Both clefts lined by basal lamina Sarcoplasm in vicinity of secondary cleft rich in glycogen, nuclei, ribosomes, mitochondria Axon terminal covered by Schwann cells Houses mitochondria, SER, synaptic vesicles (ACh) Transmit stimulus from nerve to muscle cell Figure 8-12 Electron micrograph of a mouse neuromuscular junction. (From Feczko D, Klueber KM: Cytoarchitecture of muscle in a genetic model of murine diabetes. Am J Anat 182:224-240, 1988.) Muscle spindles & Golgi tendon organs The neural control of muscle function requires the capability of inducing or inhibiting muscle contraction & monitor the status of the muscle and its tendon during muscle activity. This monitoring is performed by two types of sensory receptors. Information from these receptors are produced at unconscious levels within the spinal cord. The information also reaches the cerebellum and cerebral cortex, so we can sense muscle position. Muscle spindles (encapsulated sensory receptor) Continuously provide feedback about the changes and rate of alteration in muscle length. When muscle is stretched, it normally undergoes reflex contraction, or stretch reflex. Encapsulated sensory receptor located among, and in parallel with the muscle cells. Each muscle spindle is composed of 8 to 10 elongated, narrow, very small, modified muscle cells called intrafusal fibers, surrounded by the fluid-containing periaxial space, which in turn is enclosed by the capsule. CT elements of the capsule are continuous with the collagen fibers of the perimysium and endomysium. Skeletal muscle fibers surrounding the muscle spindle are unremarkable and are called extrafusal fibers. Intrafusal fibers are two types: nuclear bag fibers (static, dynamic) & more numerous, thinner nuclear chain fibers. The nuclei of both types of fibers occupy the centers of the cells; their myofibrils are located on either side of the nuclear region, limiting contraction to the polar regions of these spindleshaped cells. The central regions of the intrafusal fibers do not contract. The nuclei are aggregated in the nuclear bag fibers, but they are aligned in a single row in nuclear chain fibers. Golgi tendon organs (neurotendinous spindles) Monitor the tension and the rate at which the tension is being produced during movement (intensity of contraction). Cylindrical structures, 1 mm in length, 0.1 mm in diameter. Located at the myotendinous junction and are positioned in series with the muscle fibers. Composed of wavy collagen fibers and the nonmyelinated continuation of a single type Ib axon that branches as free nerve endings in interstices btw collagen fibers. When the muscle contracts, it places tensile forces on the collagen fibers, straightening them with a consequent compression and inducing of the entwined nerve endings. Proprioception The ability of a person to touch his or her nose in absolute darkness is due to the integrative activities of muscle spindles and Golgi tendon organs. These structures provide feedback about the amount of tension placed on the muscle and tendon and input to the cerebellum and cerebral cortex supplying information about the body’s position in three-dimensional space: this ability is referred to as proprioception. REGENERATION OF MUSCLE SKELETAL MUSCLE No mitotic activity Can regenerate due to satellite cells – Mitotic activity…result in hyperplasia subsequent to muscle injury – Muscle building: satellite cells fuse with existing muscle cells, increasing muscle mass during hypertrophy Skeletal muscle cells regulate their number and size by secretion of myostatin Copyright © McGraw-Hill Companies