MUSCLE TISSUE

CLASSIFICATION OF MUSCLE Muscular tissue is classified on the basis of the appearance of the contractile cells Smooth muscle Striated muscle - skeletal - cardiac A skeletal muscle consists of striated muscle fibers held together by connective tissue called endomysium A skeletal muscle fiber is a multinucleated structure, loaded with specific organels of contraction - myofibrils In between muscle fibers are located stem cells called myosatellitocytes

Terms related to the skeletal Terms related to the skeletal muscle fiber: - Sarcoplasm - Sarcoplasmic reticulum - Sarcosomes - Sarcomeres - Myofibrils - Myofilaments - Actin - Myosin - Tubulus transversus - Triade

STRUCTURE OF THE MUSCLE FIBER Subunit of the muscle fiber – myofibril Myofibrils are composed of myofilaments Thick filaments (consist of myosin) Thin filaments (consist of actin, troponin, tropomyosin) Structural and functional unit of myofibril is called sarcomere Sarcomere includes: Light, Isotropic band – I-band Dark, Anisotropic band – A-band Z-line – bisects the I-band H-zone – light zone, that bisects the A-band M-line – can be seen in the middle of the H-zone Repetition of these units cause cross-striations of muscle fiber

SARCOMERE – A STRUCTURAL AND FUNCTIONAL UNIT OF MYOFIBRIL Sarcomere – segment of the myofibril between two Z-lines The thick filaments – central portion of the sarcomere – A-band The thin filaments – attach to the Z-line, extend into the A-band to the edge of the H-zone I-band – portions of two sarcomeres on either side of a Z-line Thick and thin filaments overlap in the lateral portions of the A-band Each thick filament is surrounded by six thin filaments

The sarcomere and its components

Structural components of a sarcomere Actin myofilaments - G-actin polymerized chains - Tropomyosin - Tropomodulin - Troponin Nebulin Myosin myofilaments (myosin II molecules = consist of heavy and light meromyosin subunits) Titin Alpha-actinin cross-linking protein Cap Z cross-linking protein

EM of myofibrils and sarcomeres

ACTIN-MYOSIN INTERACTION The hydrolysis of ATP uncouples the head of the myosin from the actin filament Regulation of contraction: calcium, sarcoplasmic reticulum and T-tubules T-tubule (tubulus transversus) + 2 terminal cisternae of sarcoplasmic reticulum form triad of muscular fiber Calcium must be available for the reaction between actin and myosin (Huxley’s sliding filament theory) For relaxation after contraction, Ca2+ must be removed from the sarcoplasm (due to calsequestrin activity) The sarcoplasmic reticulum serves as the reservoir and regulator of the Ca2+

Cardiac muscle is nonvoluntary striated muscle limited to myocardium of heart and the proximal portions of pulmonary veins Consist of myocardiocytes: contractil (typical), conducting (atypical) and secretory (hormon-producing) Intercalated discs are contacts in between adjusting contractile myocardiocytes Nuclei are located in the central area of contractile myocardiocytes Abundant mitochondria and blood capillaries

LM of cardiac muscle in longitudinal section displaying intercalated discs

Cardiac muscle schematic with an intercalated disc structure

EM of atrial cell with granules containing atrial natriuretic peptide

EM of an intercalated disc

SMOOTH MUSCLE Smooth muscle is the intrinsic muscle of the alimentary canal, respiratory tract, blood vessels, genitourinary tract, and other hollow or tubular organs Smooth muscle generally occurs as bundles or sheets of elongate fusiform cells Cytoplasm is filled with an extensive array of interweaving thin (7 nm, actin) filaments, thick (15 nm, myosin) filaments, as well as with intermediate (10 nm, desmin and vimentin) filaments The dense bodies contain α-actinin, other Z-disk-associated proteins, into which thin and intermediate filaments are inserted Smooth muscle have no T-system, instead are present caveole Smooth muscle is specialized for slow, prolonged, spontaneous contraction