Muscle Tissue PDF

 ‫بسم هللا الرحمن الرحيم‬ ‫كلية الطب البيطري‬ ‫جامعة الزقازيق‬ Muscle Tissue Presentrd by Dr. Rasha Ragab Beheiry Assistant professor of Histology and Cytology Zagazig U...

 ‫بسم هللا الرحمن الرحيم‬ ‫كلية الطب البيطري‬ ‫جامعة الزقازيق‬ Muscle Tissue Presentrd by Dr. Rasha Ragab Beheiry Assistant professor of Histology and Cytology Zagazig University  Definition: Muscle tissue is one of the four basic tissue in the body composed of muscle fibers (cells) that are specialized for contraction (shortening) and relaxation and supported by C.T which connect muscle cells together and it carries nerve fibers and blood capillaries to them. Made up of cells that are called myocytes. Because muscle cells are much longer and narrow, muscle cells are called muscle fibers. 4  Smooth Skeletal muscle Muscle Cardiac Un Striated (Involuntary) Striated Muscle (Voluntary) Striated (Involuntary) Key words describing the components of muscle fiber Plasma MembraneSarcolemma Cytoplasm Sarcoplasm Smooth Endoplasmic Reticulum: Sarcoplasmic reticulum Mitochondria Sarcosomes Nearly all muscle cells are of mesodermal origin muscle fibers, (cells) or myocytes  Skeletal Muscle Striated, skeletal & voluntary muscle. Skeletal because its contraction moves the skeleton. Striated because it shows alternate dark & light bands (cross-striations). Present mainly in limbs and in relation to body wall. Shape: Elongated, Cylindrical, multinucleated cells Varies in length from a few mm to few cm. Mostly originate from somatic mesoderm. Basic unit is long, cylindrical fiber.  line end to end Myoblasts Myotubes The Myotubes manufacture cytoplasmic myofilaments, which are distributed as myofibrils responsible for the contractile capability of the cell. Skeletal muscle is pink to red because of: a) its rich blood supply b) the presence of oxygen-transporting proteins (myoglobin pigments). Depending on the quantity these pigments, number of mitochondria, concentration of various enzymes, and rate of contraction, muscle fibers are classified as red, white and intermediate. Organization The epimysium, an external sheath of dense connective tissue surrounding the entire muscle. Perimysium, a less dense collagenous, continuous with epimysium, surrounding muscle bundles. Endomysium; reticular fibers and an external lamina (basal lamina) surrounding each muscle fiber.  Histological structure of Skeletal muscle fiber L/M structure Longitudinal section of muscle fiber: Shape: long &cylendrical Nucleus: multinucleated, peripherally located. Each fiber is surrounded by endomysium. Small Myosatellite cells, with a single nucleus (has a denser and coarser chromatin network than those of muscle fiber), are located in shallow depressions on the outer sarcolemmal surface, act as regenerative cells. Skeletal muscle L.S Skeletal muscle C.S  Cytoplasm: longitudinal arrays of cylinder-shaped myofibrils, each 1 to 2 um in diameter. They extend in parallel arrangement inside the fiber, which are responsible for the appearance of Cross striations due to presence of light and dark bands  1- Dark band(A band) * Double refraction of polarized light. * Subdivided by H band(H disk) and bisected by a thin M line. *Contain both thin and thick filaments * Also called anisotropic band 2-Light band( I band) * No refraction to polarized light * Subdivided by Z line or Z disk *Contain only thin filaments The region of the myofibrils between two successive Z disks, known as a sarcomere, is 2.5 um in length and considered the contractile unit of striated muscle fiber.  Fine Structure (E/M) The cytoplasm of Muscle fiber is occupied with contractile filaments called myofibrils Myofillaments are formed from different protein molecules (thick and thin filaments) Structural Organization of Myofibrils: Interdigitating thick and thin rod-like myofilaments. The thick filaments (15 nm in diameter and 1.5 um long) are composed of myosin, whereas the thin filaments (7 nm in diameter and 1.0 m long) are composed primarily of actin.  Sarcomere Is the contractile unite of muscle cell. Is located between two Z lines. It is formed from whole dark A band surrounded by two halves of I band on both sides. Thick filaments (myosin) occupy the A band. The thin filaments run between thick filaments and have one end attached to the Z line that has a characteristic zigzag appearance. I band is light as it mainly composed of thin filaments. A band is dark as they are mainly composed of thick and overlapping portions of thin filaments.  The center of each A band is occupied by a pale H band, containing only myosin thick filaments and bisected by a thin M line The M line catalyzes the transfer of ADP to ATP to provide energy for muscle contraction. Each I band is bisected by a thin dark line, the Z disk (Z line).  Thick Filaments (Myosin) Every thick filament consists of 200 to 300 myosin molecules. Each myosin molecule; is composed of two identical heavy chains and four (two pairs) of light chains. The heavy chains resemble two golf clubs, whose rod-like polypeptide chains are wrapped around each other. The heavy chains can be cleaved by trypsin-into a rod-like tail, light meromyosin, and a globular head, heavy meromyosin. Heavy meromyosin is cleaved by papin into: Two globular (S1) moieties Short helical rod-like segment (S2) N.B: S1 bind ATP has role in the formation of cross-bridges between thick & thin myofilament.  Thin Filament Thin filaments are composed of: a) F-actin, associated with b) tropomyosin, which also forms a long fine polymer, and c) troponin, a globular complex of three subunits. * Myosin and actin together represent 55% of the total protein of striated muscle.  F-actin: * consists of long filamentous polymers containing two strands of globular (G-actin) monomers, 5.6 nm in diameter, twisted around each other in a double helical formation. * G-actin molecules are asymmetric and polymerize to produce a filament with polarity. * Each G-actin monomer contains a binding site for myosin. Actin filaments, which are anchored perpendicularly on the Z line by the actin-binding protein -actinin, exhibit opposite polarity on each side of the line  Tropomyosin Each subunit is a long, thin molecule about 40 nm in length containing two polypeptide chains, which assembles to form a long polymer located in the groove between the two twisted actin strands A single Troponin molecule, composed of three G-polypeptides; a) TnT subunit binds the entire troponin molecule to tropomyosin; b) TnC has a great affinity for calcium; and c) Tnl binds actin, inhibit actin and myosin interaction. Troponin molecule is attached to and on tropomyosin at regular distances. When calcium binds to TnC, a conformational shift in tropomyosin happens, exposing the previously masked active sites on the actin filament, so that myosin heads can bind.  T Tubules and Sarcoplasmic Reticulum, T (transverse) tubules: These are transeverse invaginations that extend from sarcolemma deeply inside the fiber. T-tubules run in association with the terminal cisternae of sarcoplasmic reticulum. Sarcoplasmic reticulum: is the SER has sarcotubules that associated with this system of T tubules. Triad: It is formed of one central t-tubule and two peripheral terminal cisternae (storing calcium) of SR. It is found close to the junction of I&A bands.  Triad. This arrangement permits spreading of a depolarization wave from sarcolemma to and throughout the cell, reaching the terminal cisternae, which have voltage- gated Ca+ release channels.  Muscle Contraction and Relaxation For muscle contraction, actin molecules should interact with the heads of myosin molecules and sliding occur Sequence of events leads to contraction in skeletal muscle 1) Motor signal < sarcolemma < T tubules < cisternae of the sarcoplasmic reticulum, leading to calcium release to sarcoplasm (cytosol). 2) Calcium ions bind to the TnC subunit of troponin, altering its conformation. 3) Conformational change in troponin shifts the position of tropomyosin deeper into the groove, exposing the active myosin-binding site on actin molecule. 4) ATP + Ca ADP + P. 5) P is released, resulting not only in an increased bond strength between the actin and myosin but also in a conformational alteration of the S, fragment. 6) ADP is also released, and the thin filament is dragged toward the center of the sarcomere ("power stroke").. 7) A new ATP molecule binds to the S1 fragment of myosin heads, which causes the release of the bond between actin and myosin. Energy Sources for Muscle Contraction: ATP, creatine phosphate, Lipid droplets and glycogen During Muscle Contraction; Calcium ions are released from SR Calcium prevents the action of regulatory proteins(troponin& tropomyosin) so free G-actin can cross-bridge with myosin head. During Muscle Relaxation; Calcium ions are absent. In the absence of Calcium, the action regulatory proteins prevent cross-bridge of G-actin with myosin head.  Innervation of Skeletal Muscle Motor Autonomic (motor end-plate) Sensory supply the (neuromuscular to muscle spindles vascular junction) elements of skeletal muscle  Impulse Transmission at the Myoneural Junctions ⮚ The terminal of each arborized twig becomes, dilated and overlies the motor end plate of individual muscle fibers. Each of these muscle-nerve junctions, known as a myoneural junction, is composed of an axon terminal, synaptic cleft, and sarcolemma. ⮚ The axon terminal (covered by Schwann cells) houses mitohondria, smooth endoplasmic reticulum, and as many as 300,000 synaptic vesicles containing the neurotransmitter acetylcholine. ⮚ The function of the myoneural junction is to transmit a stimulus chemically from the nerve fiber to sarcolemma and T-tubules. Diagram indicating key features of a typical neuromuscular junction: 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 NMJ. Receptor binding initiates muscle fiber depolarization, which is carried to the deeper myofibrils by the T tubules. Myotendinous junction. Tendons join muscles to the periosteum of bones. The collagen fibers of tendons are continuous with those in the connective tissue layers in the muscle, forming a strong unit that allows muscle contraction to move the skeleton. In this image: The longitudinal section shows part of a tendon (T) inserted into the endomysium and perimysium of a muscle.  Muscle Spindles & Tendon Organs Striated muscles and myotendinous junctions contain sensory receptors that are encapsulated proprioceptors Muscle Spindles They are encapsulated sensory receptor (stretch detectors) located among the voluntry muscle fibers. Structure: Each muscle spindle is composed of 8 to 10 elongated, narrow, very small, modified muscle cells called intratusal fibers, surrounded by the fluid-containing periaxial space, which is enclosed by the capsule. When muscle is stretched it normally undergoes reflex contraction, known as the stretch reflex. This protective response, preventing the tearing of muscle fibers.  Diagram shows both a muscle spindle and a tendon organ. Muscle spindles have afferent sensory and efferent motor nerve fibers associated with the intrafusal fibers, which are modified muscle fibers. TEM cross-section near the end of a muscle spindle shows the capsule (C), sensory myelinated axons (MA), and the intrafusal muscle fibers (MF). These thin fibers differ from the ordinary skeletal muscle fibers in having essentially no myofibrils. Their many nuclei can either be closely aligned (nuclear chain fibers) or piled in a central dilatation (nuclear bag fibers). Satellite cells (SC) are also present within the external lamina of intrafusal fibers. Muscle spindles detect contraction of neighboring (extrafusal) muscle fibers during body movement and participate in the nervous control of body posture and the coordinate action of opposing muscles. The tendon organ collects information about the degree of tension among tendons and relays this data to the CNS, where the information is processed with that from muscle spindles to protect myotendinous junctions and help coordinate fine muscular contractions. Intrafusal fibers are of two types: - Nuclear bag fibers (aggregated nuclei) - Thinner nuclear chain fibers (more numerous & nuclei in a single row). -Their nuclei occupy the center of fiber. − Their myofibrils are located in both non-nuclear sarcoplasm. Sensory nerve fibers form primary nerve endings (type la, rapidly adapting, respond to rate & stretch), which Wrap spirally around the nuclear regions of both types of intrafusal fibers.  On either side of these nerve endings, secondary nerve endings (type IIa, slowly adapting, respond to duration) also wrap around the nuclear region. Additionally, slowly conducting axons of small γ- efferent (motor) neurons terminate on motor endplates on sarcolemma of non- nuclear parts the of the intrafusal fibers. Extrafusal fibers receive motor α-efferent neurons. Golgi Tendon Organs (Neuro-tendinous Spindles); Golgi tendon organs are encapsulated thin intrafusal collagen fibers that receive the nerve endings of type-Ib sensory neurons.  Cardiac muscle Def: It is the striated involuntry muscle found only in the heart and in the base of blood vessels joining the heart. At LM: - Shape: the fibers branched and anastomosed to form network (15 µm in diameter and 80 µm long). - Cytoplasm: have cross striations. - Nucleus: single, oval, prominent centrally located. ⮚Heart muscle differs from skeletal and smooth muscles that it possesses an inherent rhythmicity as well as the ability to contract spontaneously. ⮚A system of modified cardiac muscle cells (Sino-atrial node, Atrioventricular node and Atrioventricular bundle, Purkinje fibers) has been adapted to ensure the coordination of its contractile actions. At EM: is similar to skeletal except: 1) Mitochondria are large and numerous due to the continous energy needs of cardiac muscle. 2) The t-tubules are found at Z-lines (instead of A-I junction) 3) Ca+ must be actively transported into the cardiac muscle cell from the fluid surrounding cardiac fibers. 4) Each T-tubule is in contact with only one cisternae of sarcoplasmic reticulum forming Diad not triad. 5) Presence of intercalated discs.  6) Because the oxygen requirement of cardiac muscle cells is high, they contain an abundant amount of myoglobin. 7) Atrial muscle cells are smaller than those of the ventricles. These cells also house granules (especially in the right atrium) containing atrial natriuretic peptide (ANP), which function to lower blood pressure.  Intercalated Disks Cardiac muscle cells form highly specialized end-to-end junctions, referred to as intercalated disks that appear as dark lines that run transeversely in a step-wise manner across the fibers at the level of Z lines. - Intercalated disks have transverse portions, where fasciae adherenes and desmosomes abound, as well as lateral portions rich in gap junctions. - On the cytoplasmic aspect of the sarcolemma of the intercalated disks, thin myofilaments attach to the fasciae adherentes, which are thus analogous to Z disks (1/2 Z). - Gap junctions permit rapid flow of molecules amoung adjacent fibers, also form in regions where cells lying side by side come in close contact with each other.  Smooth Muscle Smooth &Unstriated, does not possess striations. No T- tubules. Smooth muscle is not under voluntary control; instead, it is regulated by the autonomic nervous system, hormones. Hence, smooth muscle is also referred to, as involuntary muscle. Site: a) in the walls of hollow viscera (e.g., the gastrointestinal, some of the reproductive, and the urinary tracts), b)walls of blood vessels. c)larger ducts of compound glands. d) respiratory passages. e) bundles within the dermis of skin. Mostly originate from somatic mesoderm At LM: -Shape: fusiform, elongated cells and tapers at either ends, (spindle- shaped) (15 µm in diameter and 0.2 mm long). Nucleus -Nucleus: an oval-shaped nucleus, with two or more nucleoli. During muscle shortening the Muscle fiber nucleus assumes a characteristic corkscrew appearance. -Each smooth fiber is surrounded by an external lamina containing numerous reticular fibers. - Cytoplasm: no cross striations. -Under H & E staining, the cytoplasm of smooth muscle fibers appears unremarkable ; however, iron hematoxylin stain demonstrates the presence of dense bodies adhering to the cytosolic side of sarcolemma. -Thin longitudinal striations may be evident in the sarcoplasm representing clumped myofilaments.  Smooth muscle - Sheets of smooth muscle cells are frequently arranged in two layers, perpendicular to each other, as in the digestive and urinary systems. - This arrangement permits waves of peristalsis.  In C.S: - Various diameters may be noted, some with nuclei, some not. - Cross-sections without nuclei represent the tapered ends of smooth muscle cells as they interdigitate with the other smooth muscle fibers. E/M The cytoplasm contains numerous mitochondria, Golgi apparatus, rough and smooth endoplasmic reticulum, and inclusions such as glycogen. Filled with thin (7 nm) and thick (15 nm) filaments are present. The thin filaments are composed of actin with its associated tropomyosin but with absence of troponin. The thick filaments are composed of myosin.  An additional system of intermediate filaments (IF), vimentin and desmin in vascular smooth muscle, and desmin (only) in non-vascular smooth muscle. These IFs as well as thin filaments insert into dense bodies, formed of α actinin and other Z disk-associated proteins. Dense bodies may be located in the cytoplasm or associated with the cytoplasmic aspect of the sarcolemma and are believed to resemble Z disks in function.  Lying just beneath the cell membrane are structures associated with the sparse sarcoplasmic reticuium, known as caveolae (sarcolemmal vesicles). These vesicles may function in the release and sequestering of calcium ions.  Skeletal Cardiac Smooth site Attached to In the heart B,vs, internal skeleton organs action voluntry involuntry involuntry striations striated striated unstriated sarcomere present present absent myofibrils present present absent nuclei multinucleated single Single, spindle shaped T tubules Present at A-I Present at Z- absent junction lines Troponin present present absent