Muscle Tissue Xing Wenying Introduction n Components l

Muscle Tissue Xing Wenying 邢文英

Introduction n Components: l Muscle cells(muscle fibers) • Elongated, thread-like, containing myofilaments and being contractile • cell membrane → sarcolemma; cytoplasm → sarcoplasm; SER → sarcoplasmic reticulum l CT: contain blood vessels, lymphatics and nerves.

n classification : According to the structure and function structure striated unstriated

Skeletal muscle

n Histological features of skeletal muscle fibers l l l Long cylindrical, 1 -40 mm long, 10 -100 um in diameter Multinucleate, ovoid nuclei, just under the sarcolemma Cross-striations: alternating dark and light bands

n Histological features of skeletal muscle fibers l Dark bands--A bands： H band, M line Light bands--I bands： Z line

sarcomere: • The sarcomere is the basic contractile unit of striated muscle. It is the segment of a myofibril between two adjacent Z line. • 1 sacromere = 1/2 I band+1 A band+1/2 I band l

l Myofibrils: Sarcoplasm is filled with long cylindrical filamentous bundles called myofibrils.

n Ultrastructure of skeletal muscle fibers Myofibril Sarcoplasmic reticulum Mitochondrium Transverse tubule

l myofibrils and myofilaments • Myofibrils are composed of bundles of myofilaments. • Thick filaments and thin filaments.

Thick filaments ü 1. 5μm long and 10 nm in diameter ü Occupy A band ü Made up of myosin molecules: --rods: overlap --heads: cross bridges, have ATPase activity.

Thin filaments 1 um long, 5 nm in diameter ü situated in both ending ü sides of each sarcomere; Run between and parallel to thick filaments; One end attached to the Z line, the other is free at H band ü composed of actin, tropomyosin, troponin

Thin filaments A. Actin: binding site for myosin.

Thin filaments B. Tropomyosin

Thin filaments C. Troponin: is a complex of three sub-units. Tn T(troponin T) Tn C(troponin C) Tn I(troponin I)

Arrangement ü I band -- only thin filaments ü A band -- both thick and thin filaments ü H band -- only thick filaments ü Z line -- anchor for thin filaments ü M line – fixation of thick filaments

l Transverse tubule (T tubule) u Definition: A transverse distributed tubular system formed by Sarcolemma invaginating into sarcoplasm, encircling each myofibrils. u Location: A-I junctional part u Function: Rapidly conduct impulses for contraction to every myofibrils

l Transverse tubule (T tubule) Sarcolemma with opening to T-tubule T tubule

l Sarcoplasmic reticulum (SR) u Definition: A longitudinal distributed tubular system formed by SER, encircling each myofibril between 2 adjacent T tubules. u Terminal cisternae: Ends of L tubule dilate and fuse to form flattened sac.

l Sarcoplasmic reticulum (SR) Triad: 1 T tubule + 2 terminal cisternae u Function: store and release calcium ions regulating concentration of Ca 2+ within sarcoplasm u

l Other organelles and inclusion Mitochondria Glycogen lipid droplet providing the energy necessary for the reactions involved in contraction.

n Mechanism of contraction -- sliding filament hypothesis The thin filaments slide past the thick filaments and insert further into the A band. u I band sarcomere become shorter, H band shortens or disappears, whereas A band remains the same length. u Ca 2+ and ATP play an important role. u

Cardiac muscle l Mainly distributed in the wall of heart; l Has more connective tissue and capillaries; l Some specialized as Purkinje fibers.

n Microstructure of cardiac muscle l Short cylindrical, 80 -150 um long, 10 -20 um in diameter, with branches, associated with each other; 1 or 2, ovoid, centrally-located pale-staining nuclei; l Exhibit cross striations, but less distinct; l

l Intercalated disk: unique characteristic

n Ultrastructure of cardiac muscle l Myofibril have different diameter, the boundary of myofibril is not very clear. l T tubules are larger, located at Z-line level.

n Ultrastructure of cardiac muscle l Sarcoplasmic reticulum is not well-developed, forms less and smaller terminal cisternae, so Diads are common consisting of 1 T tubule and 1 terminal cisternae on one side.

n Ultrastructure of cardiac muscle Intercalated discs • Specialized cell junctions at Z lines. l

n Ultrastructure of cardiac muscle l rich in mitochondria, glycogen, pigment.

Smooth muscle Seen in blood vessels and hollow viscera, arranged in layers.

n Structure of Smooth Muscle fiber

Summary n n n The structure of skeletal muscle. The structure of cardiac muscle. The structure of smooth muscle. Mechanism of contraction -- sliding filament hypothesis

Summary n The structure of cardiac muscle.

Summary n The structure of smooth muscle.

Homework Ø Review the characteristics of skeletal muscle and cardiac muscle. Ø Prepare for nerve tissue.