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
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 Zlines
• 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