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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48216659 physiology chap9 cardiac muscle