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Execise 31

Heart contraction results from...

...a series of electrical potential changes (depolarization waves) that travel through the heart preliminary to each beat.

Functional syncytium

Cardiac muscles cells are electrically connected by gap junctions - the entire myocardium behaves like a single unit.

The ability of cardiac muscle to beat is intrinsic. True or False

True. It does not depend on impulses from the nervous system to initiate its contraction and will continue to contract rhythmically even if all nerve connections are severed.

Autonomic nervous system effects on the heart?

Accelerate or decelerate the heartbeat depending on which division is activated.

Intrinsic conduction system

Nodal system. Ensures that heart muscle depolarizes in an orderly and sequential manner and the heart beats as a coordinated unit.

Components of the intrinsic conduction system

Sinoatrial (SA) node
Atrioventricular (AV) node
Atrioventricular bundle
Bundle branches
Purkinje fibers

Sinoatrial (SA) node

Pacemaker - sets the rate. Generates impulses - provides the stimulus for contraction.

Atrioventricular (AV) node

Where the impulses pause for 0.1 seconds allowing the atria to complete their contraction.

Atrioventricular (AV) bundle

Bundle of His. Connects the atria to the ventricles.

Bundle branches

Conducts the impulses through the interventricular septum.

Purkinje fibers

Long strands of barrel-shaped cells called Purkinje myocytes. Depolarize the contractile cells of both ventricles.

Electrocardiogram (ECG or EKG)

The graphic recording of the electrical changes (depolarization followed by repolarization) occuring during the cardiac cycle. A record of voltage and time - nothing else.

P-R interval represents

The time between the beginning of atrial depolarization and ventricular depolarization.

Stroke volume increases with physical conditioning.



Heart rate above 100 beats/min


Heart rate below 60 beats/min


A condition of rapid uncoordinated heart contractions which makes the heart useless as a pump.

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