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48216659 physiology chap9 cardiac muscle

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9 Cardiac Muscle
right heart: pumps blood through the lungs
left heart: pumps blood through the peripheral
organs
each of these hearts is a pulsatile two-chamber
pump composed of an atrium and a ventricle.
Each atrium is a weak primer pump for the
ventricle, helping to move blood into the ventricle.
The ventricles then supply the main pumping
force that propels the blood either (1) through the
pulmonary circulation by the right ventricle or (2)
through the peripheral circulation by the left
ventricle.
cardiac rhythmicity - Special mechanisms in the
heart cause a continuing succession of heart
contractions
Physiology of Cardiac Muscle
3 major types of cardiac muscle:
1. atrial muscle
2. ventricular muscle
3. specialized excitatory and conductive
muscle fibers
Physiologic Anatomy of Cardiac Muscle
striated
myofibrils that contain actin and myosin filaments
Cardiac Muscle as a Syncytium
intercalated discs dark areas crossing the
cardiac muscle fibers; cell membranes that
separate individual cardiac muscle cells from one
another
2 syncytiums:
1. atrial syncytium - constitutes the walls of the
two atria
2. ventricular syncytium - constitutes the walls
of the two ventricles
The atria are separated from the ventricles by
fibrous tissue that surrounds the atrioventricular
(A-V) valvular openings between the atria and
ventricles.
Normally, potentials are conducted only by way
of a specialized conductive system called the A-V
bundle, a bundle of conductive fibers several
millimeters in diameter
This division of the muscle of the heart into two
functional syncytiums allows the atria to contract
a short time ahead of ventricular contraction
Action Potentials in Cardiac Muscle
Action potential = 105 mV
intracellular potential rises from a very negative
value, about -85 millivolts, between beats to a
slightly positive value, about +20 millivolts, during
each beat
After the initial spike, the membrane remains
depolarized for about 0.2 second, exhibiting
a plateau; presence of this plateau in the action
potential causes ventricular contraction to last as
much as 15 times as long in cardiac muscle as in
skeletal muscle.
What Causes the Long Action Potential and the
Plateau?
At least two major differences between the
membrane properties of cardiac and skeletal
muscle account for the prolonged action potential
and the plateau in cardiac muscle.
1. action potential of skeletal muscle is caused
almost entirely by sudden opening of large
numbers of so-called fast sodium
channels; remain open for only a few
thousandths of a second
- In cardiac muscle, the action potential is
caused by opening of two types of channels:
(1) the same fast sodium channels and (2)
another entirely different population of slow
calcium channels (calcium-sodium
channels); slower to open, remain open for
several tenths of a second
- maintains a prolonged period of
depolarization, causing the plateau in the
action potential; calcium ions that enter
during this plateau phase activate the
muscle contractile process, while the calcium

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