Page 78 - Headlines Histology2024_Neat

• Each tubule is associated with single cisternae of sarcoplasmic reticulum
forming a structure called diad.

• Mitochondria are numerous with closely packed cristae rich in oxidative
enzymes.

• The sarcoplasm contains larger amount of glycogen.
• The perinuclear sarcoplasm of cardiac myofibers contains a large number of

secretory granules that have recently been proven to be the source of
polypeptide hormone called ANP (Atrial Natriuretic Peptide).
• The cardiac myofibers are formed of several cells connected end-to-end by
intercalated discs. Each intercalated disc crosses a cardiac fiber at the Z-line
level in a stepwise manner. It consists of three types of cell junctions: fascia
adherens, desmosomes and gap junction. The ntercalated discs bind the cell,
transmit forces of contraction and provide areas of low electrical resistance for
the spread of excitation throughout the myocardium.
Purkinje fibers

• They are modified cardiac muscle fibers designed for rapid conduction
of nerve impulses. They differ from the ordinary cardiac muscle fibers in
the following aspects:
o They are larger in size.
o The nucleus is smaller and eccentricity situated.
o The cytoplasm is paler, vacuolated because it is rich in glycogen.
o The myofibrils are fewer and concentrated at the periphery.
o The T tubules are absent.

Contraction of skeletal muscle (Sliding filament theory)

• In relaxed sarcomer, the thick (myosin) filaments are located in the center
region forming the A-band. Both ends of the thick filaments are free.

• The thin filaments (actin) have only one end free and the other end is
attached to a Z line. Their free ends extend toward the middle of the
sarcomer to interdigitate with the thick filaments.

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