HISTOLOGY OF
THE CARDIAC
MUSCLE
Pamela BL
Introduction
 The
cardiac muscle forms the muscular
wall of the heart that is it forms the
myocardium.
 Some of the cardiac muscle is also
present in the walls of the aorta,
pulmonary vein and superior vena cava.
 Contractions
of the cardiac muscle is not
under voluntary control.
 Heart rate is regulated intrinsically by a
pacemaker composed of special cardiac
muscle fibers that are influenced by the
autonomic nervous system.
 Cardiac
muscle fibers are chains of
cardiac muscle cells joined end to end by
cell junctions.
 Cardiac muscle is also a type of “striated”
muscle because of its appearance under
the microscopy.
 Most often the terms “striated” or
“striped” refer to voluntary skeletal
muscle.
 Cardiac
muscle fibers have the same
general pattern of striations as the skeletal
muscle fibers, however, they are traversed
at intervals by intercalated discs(disks)specialized end to end junctions that are
unique to cardiac muscle fibers.
 Most cardiac muscle cells have a single
nucleus, but some contain two.
 During
embryonic development, the
splanchnic mesoderm cells of the
primitive heart tube align into chain like
arrays.
 Rather than fusing into syncytial cells as in
the development of skeletal muscle,
cardiac cells form complex junctions
between their extended processes.
 Cells
within the chain bifurcate, or branch
and bind to cells in adjacent chains.
 Consequently, the heart consists of tightly
knit bundles of cells interwoven in a
fashion that provides for a characteristic
wave of contraction that leads to
wringing out of heart ventricles.
 Mature
cardiac cells are approximately
15 nanometers in diameter and from 85 to
100 nanometers in length.
 Mature cardiac cells do not divide and
thus destroyed cells are not replaced by
new cells.
 An injury to cardiac muscle tissue is
repaired by formation of fibrous
connective tissue.
 Unlike
multinucleated skeletal muscles
with their nuclei under the sarcolemma at
the periphery of the cell, each cardiac
muscle cell possesses only one or two
centrally located nuclei.
 Surrounding the muscle cells is a delicate
sheath of endomysial connective tissue
containing a rich capillary network.
A
unique and distinguishing characteristic
of cardiac muscle is the presence of darkstaining transverse lines that cross the
chains of cardiac cells at irregular
intervals.
 These
intercalated disks represent
junctional complexes found at the
interface between adjacent cardiac
muscle cells.
 The junctions may appear as straight lines
or may exhibit a step like pattern.
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The structure and function of the contractile
proteins in the cardiac cells are virtually the
same as in the skeletal muscle.
The T-tubule system and the sarcoplasmic
reticulum, however, are not as regularly
arranged in the cardiac myocytes.
The T tubules are more numerous and larger in
ventricular muscle than in the skeletal muscle.
 Cardiac
muscle cells contain numerous
mitochondria, which occupy 40% or more
of the cytoplasmic volume, reflecting the
need for continuous aerobic metabolism
in heart muscle.
 By comparison, only about 2% of skeletal
muscle fiber is occupied by mitochondria.
 Fatty
acids, transported to cardiac
muscle cells by lipoproteins, are the major
fuel of the heart.
 Fatty acids are stored as triglycerides in
the numerous lipid droplets seen in
cardiac muscle cells.
A
small amount of glycogen is present
and can be broken down to glucose and
used for energy production during periods
of stress.
Clinical notes
 Hypertrophy
of the myocardium:
Hypertrophy is an increase in the size of cells
resulting in increase in the size of the organ.
Thus the hypertrophied organ has no new
cells just larger cells.
The increased size of cells is due not to
cellular swelling but to the synthesis of more
structural components.
 In
the heart, the stimulus for hypertrophy is
usually chronic hemodynamic overload
resulting from either hypertension or faulty
valves.
 Synthesis of more proteins and filaments
occurs, achieving a balance between
the demand and the cell’s functional
capacity.
Summary
 Location:
muscle of heart and adjacent
portion of the great vessels.
 Appearance: branching and
anastomosing shorter fibers with
transverse striations running parallel and
connected end to end by intercalated
discs, single central nucleus
 Type
of activity: strong, quick continuous
rhythmic contraction, acts to pump blood
from the heart
 Stimulation: involuntary;
intrinsically(myogenically) stimulated and
propagated, rate and strength of
contraction modified by autonomic
nervous system.
 Thank
you for listening!