Histology of Muscular Tissue PDF

Dr Sahar Galal Objectives - General characteristics of muscular tissue - Structure of skeletal muscle LM EM: transverse triad – myofibrils -- myoglobin Structure of sarcomere - Structure of smooth muscle (LM – EM) General characteristics of muscular tissue: - The cells of muscle...

Dr Sahar Galal Objectives - General characteristics of muscular tissue - Structure of skeletal muscle LM EM: transverse triad – myofibrils -- myoglobin Structure of sarcomere - Structure of smooth muscle (LM – EM) General characteristics of muscular tissue: - The cells of muscle tissue are long narrow called muscle fibers - The cell membrane of muscle fiber is called sarcolemma, the cytoplasm is called sarcoplasm, the smooth endoplasmic reticulum is called sarcoplasmic reticulum, the mitochondria is called sarcosome. - Muscle tissue is specialized for producing movements so they show great development of the function of contractility - Muscle fibers are classified on both a structural and a functional basis - Structurally, muscle fibers are striated or non-striated (according to whether or not they have transverse striations at the level of light microscope) - Functionally, muscle fibers are either voluntary or involuntary Sarcolemma Sarcoplasm  General characteristics of There are 3 types of muscle fibers: muscular tissue 1) Skeletal muscle 2) Cardiac muscle 3) Smooth muscle Skeletal muscle Cardiac muscle Smooth muscle Striated voluntary Striated involuntary Smooth involuntary Site: Muscles Site: Myocardium of Site: Viscera & Blood attached to skeleton Heart vessels Skeletal Muscle - Striated, Voluntary Site: Muscles attached to the skeleton (skeletal), upper part of the digestive system (tongue, pharynx and upper part of oesophagus), and extraocular muscles of the eye. Light microscopy (LM) Each skeletal muscle fiber is characterized by: Long cylindrical cells (1-40mm long) Large diameter (thick) [up to 0.1mm in diameter] Non-branching (except rare sites: face & tongue) Acidophilic cytoplasm with transverse striations (striated) due to presence of myofibrils in the cytoplasm Nucleus: Multiple (Multinucleated cell), flat, situated peripherally under the sarcolemma  Skeletal Muscle Transverse striations of alternating dark (A) and light (I) bands Each (I) band is bisected by dark line called (Z) line Each (A) band is bisected by pale disc called H zone H zone is bisected by a dark line called M line Distance between 2 (Z) lines is named a “Sarcomere” “Sarcomere” is the contractile unit of the skeletal muscle  Skeletal Muscle In a transverse section (TS) of skeletal muscle, the muscle fibers appear rounded or polygonal in shape. The sarcoplasm contains the myofibrils which appear as dark dots arranged in groups called Cohnheim’s areas among the sarcoplasm LS TS  Skeletal Muscle Electron microscopy (EM) of skeletal muscle fiber The Sarcoplasm contains:  Myofibrils  Transverse triad (T-triad): T-tubule & sarcoplasmic reticulum (sER)  Plenty of mitochondria arranged peripherally & inbetween the myofibrils  Myoglobin  Glycogen Skeletal Muscle  Skeletal Muscle 1) T-tubules, Sarcoplasmic reticulum [ T-triad] Transverse (T) tubule Invagination of the sarcolemma into the sarcoplasm forming transverse (T) tubules that encircle the myofibrils at the junction of A and I bands Sarcoplasmic reticulum Terminal cisternae of sarcoplasmic reticulum (sER) surround T-tubule 2 Terminal T- tubule Transverse Triad (T-triad) cisternae A-I junction One T-tubule + 2 terminal cisternae constitute the T-triad  Skeletal Muscle Transverse Triad (T-Triad) One T-tubule + 2 terminal cisternae Each sarcomere has 2 T-triads Function of T-triad: - T-triad allow a wave of depolarization 2 terminal T- tubule to spread from the surface of the cisternae sarcolemma through the T-tubule, to A-I junction reach the terminal cisternae. - Terminal cisternae pump Ca + ions into the myofibrils. - Ca+ ions allow actin to interact with myosin thus initiating shortening of the sarcomere (muscle contraction)  Skeletal Muscle 2) Myofibrils Thread-like structures present in bundles in the sarcoplasm, oriented longitudinally parallel to each other & parallel to the long axis of muscle fiber - Each myofibril show alternating dark (A) and light (I) bands - Each (I) band is bisected by dark line called (Z) line - Each (A) band is bisected by pale disc called H zone I-band A-band - The regular arrangement of myofibrils (parallel to each other) is responsible for the transverse striations of the muscle fiber seen by LM  Skeletal Muscle Sarcomere Sarcomere is the contractile unit of the skeletal muscle fiber It is the distance between 2 (Z) lines Sarcomere includes whole A-band and ½ of I-band on each side Z Z ½I A-band  Skeletal Muscle Myofilaments The sarcomere is formed of 2 types of myofilaments: 1) Thick myofilament (myosin): Present in the A-band only 2) Thin myofilament (Actin): Attached to Z-line, extends into I-band and A-band ½I A-band Light band (I-band): Formed of actin only Dark band (A-band): Formed of actin and myosin H – zone: Formed of myosin only  Skeletal Muscle Molecular structure of myofilaments Thin filaments (Actin) - Formed of two chains of F-actin filaments (polymer of globular G- actin) wrapped around each other associated with tropomyosin and troponin complex (TnT, TnC, TnI) - Tropomyosin, a 40-nm-long coil of two polypeptide chains located in the groove between the two twisted actin strands. - Troponin, a complex of three subunits: TnT, which attaches to tropomyosin; TnC, which binds Ca2+; and TnI, binds to actin.  Skeletal Muscle Molecular structure of myofilaments Thick filaments (myosin) - Formed of 200-300 myosin II molecules - Each myosin molecule is formed of head and tail - Each myosin molecule is formed of two polypeptide heavy chains and four polypeptide light chains  Skeletal Muscle Sliding filament theory of muscle contraction 1) An impulse generated along the sarcolemma is transmitted through T-tubule to the terminal cisternea of sarcoplasmic reticulum 2) Ca+ ions are released from the terminal cisternae 3) Ca+ ions bind to troponin altering its conformation 4) Conformational changes in troponin shifts the position of tropomyosin, thus, unmasking the myosin binding sites on the actin molecule N.B.: In the resting state tropomyosin covers the sites of interaction between actin and myosin which is present on actin filaments Skeletal Muscle  Skeletal Muscle 5) ATP binds to head of myosin and is hydrolysed into ADP + energy. Then myosin head binds to myosin binding site on actin filament 6) ADP and energy is released resulting in myosin head bends and pulls the actin filamants towards the M-line 7) A new ATP molecule is attached to the myosin head. Myosin is separated from actin 8) ATP is hydrolysed and the cycle is repeated  Skeletal Muscle EM of Sarcomere Z Z ½I ½I Sarcomere Relaxed sarcomere Contracted sarcomere H-zone decreases I-band decreases  Skeletal Muscle 3) Myoglobin (oxygen –binding protein) Pigment protein (red to brown color) provides oxygen for muscle Red muscle fibers White muscle fibers Fiber diameter Smaller Larger Vascularity Rich vascular supply Poor vascular supply Myoglobin Rich Poor Mitochondria Numerous Few Slow, but repetitive, not Fast, stronger contraction, Contraction easily fatigued easily fatigued There are 3 types of skeletal muscle fibers; red, white, and intermediate. All 3 types are present in any given muscle but with different proportions.  Skeletal Muscle Myoglobin (oxygen –binding protein) Red mf White mf  Skeletal Muscle Connective tissue coverings of the skeletal muscle: 1) Epimysium: Cover the whole muscle Epimysium 2) Perimysium: Surround bundle of muscle fibers Perimysium 3) Endomysium: Surround individual (single) muscle fiber Endomysium Myofibril Muscle fiber  Skeletal Muscle Regeneration of the skeletal muscle: - Regeneration of skeletal muscle fibers occurs through satellite cells - Satellite cells are flattened cells present between the sarcolemma of skeletal muscle fiber and its basement membrane - Satellite cells differentiate into myoblasts (mononucleated, no myofibrils). Myoblasts fuse to form new skeletal muscle fibers (multinucleated, develop myofibrils) Smooth Muscle - Smooth, Involuntary Site: Wall of blood vessels & viscera - The digestive tract from the oesophagus to anus - The respiratory passages - The urinary and genital ducts Light microscopy: Short spindle-shaped Small diameter Single rod-shaped nucleus No transverse striations (no sarcomere) No branching or anastomosis No intercalated discs Regeneration by mitosis (no satellites)  Smooth Muscle Electron microscopy (EM) of smooth muscle fiber No myofibrils or sarcomere in smooth muscle fiber The sarcoplasm contains: - Thin filaments (Actin) - Thick filaments (myosin) - Intermediate filaments The 3 types of filaments run longitudinally in the sarcoplasm overlapping each other in irregular way. (filaments are Not arranged in sarcomere) The intermediate and actin filaments insert into cytoplasmic and sarcolemma-associated dense bodies. Dense bodies contain α-actinin and are functionally similar to the Z discs of the skeletal muscle During contraction: Myosin filaments slide over actin filaments pulling them, and actin filaments pull on intermediate filaments thus changing the shape of the cell  Smooth Muscle The sarcoplasmic reticulum is less developed T-tubules are Not found. Instead of T-tubules, invaginations of the sarcolemma termed Caveolae are responsible for carrying the impulse to the inside of the muscle fiber Electron micrograph of smooth muscle fiber BEST WISHES FOR SUCCESS