1. MUSCLE TISSUE

3. 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

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

7. 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

9. 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

10. The sarcomere and its components

11. 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

13. EM of myofibrils and sarcomeres

17. 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+

20. 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

21. LM of cardiac muscle in longitudinal section displaying intercalated discs

23. Cardiac muscle schematic with an intercalated disc structure

25. EM of atrial cell with granules containing atrial natriuretic peptide

26. EM of an intercalated disc

27. 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