Page 325 - Atlas of Histology with Functional Correlations

sarcoplasmic reticulum

When muscle contracts, I and H bands shorten, whereas A bands stay the
same
Muscle contraction and shortening draw Z lines closer together and shorten
sarcomere

CARDIAC MUSCLE

Located in the heart and large vessels attached to the heart

Cross-striations of actin and myosin form similar I bands, A bands, and Z
lines as in skeletal muscle
Characterized by dense junctional complexes called intercalated discs that
contain gap junctions

Contain one or two central nuclei; fibers are shorter and show branching
T tubules are located at Z lines and are larger in skeletal muscle
The sarcoplasmic reticulum is less well developed than in skeletal muscles
Mitochondria are larger and more abundant in cardiac fibers

Gap junctions couple all fibers for rhythmic contraction and form the
functional syncytium
For contraction, calcium is imported from outside the cell and from the
sarcoplasmic reticulum

Exhibit autorhythmicity and spontaneously generate stimuli
The autonomic nervous system innervates the heart and influences heart rate
and blood pressure

SMOOTH MUSCLE

Found in hollow organs and blood vessels

Zonula adherens binds muscle cells, whereas gap junctions provide
functional coupling
Contain actin and myosin filaments without cross-striation patterns
Fibers are fusiform in shape and contain single central nuclei

In intestines, muscles are arranged in concentric layers and in blood vessels
in a circular pattern
Actin and myosin filaments are present, but they do not show regular
arrangement or striations

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