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True or False. Cardiac muscle cells are electrically connected by gap junctions and behave as a single unit.


Because it sets the rate of depolarization for the normal heart, the _________ node is known as the pacemaker of the heart.

sinoatrial (SA)

True or False. Activity of the nerves of the autonomic nervous system is essential for cardiac muscle to contract.


The typical ECG has normally how many recognizable deflection waves?


In a typical ECG, the __________ wave signals the depolarization of the atria immediately before they contract

P wave

True or False. Repolarization of the atria is usually masked by the large QRS complex.


A heart rate over 100 beats per minute is known as


How many electrodes or leads will you place on your subject for today's activity if using a standard ECG apparatus?


True or False. Electrical activity recorded by any lead depends on the location and orientation of the recording electrodes.


intrinsic conduction system

AKA nodal system of the heart, consisting of specialized noncontractile myocardial tissue. Components are the sinoatrial (SA) node, atrioventricular (AV) node, AV bundle (bundle of His), and bundle branches, and Purkinje fibers.

sinoatrial node (SA node)

has highest rate of discharge and provides stimulus for contraction. From SA node, the impulse spreads throughout the atria and to the AV node, which is immediately followed by atrial contraction.

atrioventricular node (AV node)

the impulse is momentarily delayed, allowing the atria to complete their contraction. Then passes through AV bundle

Electrocardiogram, ECG

the graphic recording of the electrical changes in heart activity, depolarization followed by repolarization, occuring during the cardiac cycle. Divided into segments and intervals. It is a record of voltage and time.

P-R interval

-time between the beginning of atrial depolarization and ventricular depolarization.
-Transmission of the impulse to contract from the SA node to the AV node and through the ventricles
-Longer interval suggests a part AV heart blocked caused by damage to AV node.

QRS complex

when prolonged may indicate a right or left bundle branch block when one ventricle is contracting later than the other.

Q-T interval

-period from the beginning of ventricular depolarization through repolarization and includes the time of ventricular contraction (S-T segment)
-Time required for the ventricles to undergo a single cycle of depolarization and repolarization


rate below 60 beats per minute


prolonged tachycardia leads to fibrillation. A condition of uncoordinated heart contractions which makes the heart useless as a pump. Uncoordinated myocardial activity.

stroke volume

amount of blood ejected by a ventricle with each contraction, increases with physical conditioning.

P wave

atrial depolarization

QRS complex

ventricle depolarization

T wave

ventricle repolarization

Intrinsic conduction system in order

SA node --> AV node --> AV bundle --> left and right bundle branches --> Purkinje fibers

At what structure in the transmission sequence is the impulse temporarily delayed? why?

AV node, because it allows for completion of atrial contraction between initiation of ventricular systole (contraction)

What is the role of the the nodal system?

it ensures that depolarization proceeds in order from atria to ventricles; and accelerates and coordinates heart activity to effectively pump blood.

Why does heart rate increase during running?

exercise raises heart rate through the sympathetic nervous system. Sympathetic nervous fibers release NE on the heart and the pacemaker fires more rapidly.

Immediately before P wave

the heart is in relaxation

During P wave

depolarization of the atria

Immediately after P wave, the P-R segment

contraction of the atria

during QRS complex

depolarization of ventricles

Immediately after QRS wave, the S-T segment

contraction of ventricles

during T wave

repolarization of ventricles

what would be more serious, an atrial or ventricular fibrillation? Why?

Ventricular fibrillation is more serious because the ventricles have major responsibility for pumping blood from the heart to the lungs and all other organs of the body.

Heart valve abnormalities can be detected by auscultation, not ECG. Why?

Valve problems are detected by stethoscope because they are not part of depolarization pathway of the heart, so an ECG would not be able to record changes.

SA node

pacemaker of heart, don't contract, but has pacemaker cells that automatically generates impulses to contract right atrium, generates impulses to AV node

AV node

pauses to allow completion of atrial contraction between initiation of ventricular systole (contraction). Sends impulse to AV bundle

AV bundle

connects the atria to the ventricles. Sends impulse to bundle branches.

bundle branches

conduct impulses through the interventricular septum

purkinje fibers

depolarize the contractile cells of both ventricles, causing the heart to beat.

stroke volume

the amount of blood ejected by a ventricle with each contraction. It can increase with physical conditioning.

"lub" sound

heard when AV valves close after ventricular depolarization

"dub" sound

heard when semilunar valves close from ventrical relaxation

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