Electrical and Contractile Activity of the Heart

T/F: The SA node does not have a stable resting potential.

True - allows for easy spontaneous firing

What phase involves the pacemaker action potential beginning at -60 mV and drifting upward due to slow sodium inflow via HCN channels?

Phase 4

What is another term for gradual depolarization?

Pacemaker potential

Around -50 MV, what opens to allow for further depolariation?

Transient (T-type) Ca2+ channels

What happens when it reaches threshold of -40 mV?

Voltage-gated long-lasting (L-type) Ca2+ and Na+ chennels open

What phase occurs when faster depolarization peaks at 0 mV?

Phase 0

During phase 3, what channels will open and allow this molecule to leave the cell?

Potassium

When K+ channels open and K+ leaves the cell, what does this cause?

Repolarization

What channels inactivate in close during phase 3?

L-type Ca2+ channels

How frequently does the SA node fire?

Typically every 0.8 seconds

What is another name for the primary type of sodium channel open during the pacemaker potential?

'Funny current' channel
AKA hyperpolarization-activated cyclic nucleotide-gated (HCN) channel

What is the resting potential of a cardiomyocyte?

-90 mV

T/F: Cardiomyocytes have a stable resting potential of -90 mV.

True

Why do cardiomyocytes have a stable resting potential?

Depolarize only when stimulated

What are the 4(5) stages of cardiomyocyte action potential?

Phase 0: Depolarization
Phase 1: Early repolarization
Phase 2: Plateau phase
Phase 3: Repolarization
Phase 4: Resting phase

In the cardiomyocyte action potential, what does a stimulus open?

Voltage-regulated Na+ gates

In the cardiomyocyte action potential, what is the peak?

+30 mV

In the plateau phase of cardiomyocyte AP, what molecule is transiently flowing outward to allow for slight repolarization?

Potassium

In the plateau phase of cardiomyocyte AP, what happens when voltage-gated Ca2+ channels open?

Ca2+ channels open on the sarcoplasmic reticulum

How is contraction triggered in the plateau phase of cardiomyocyte AP?

Ca2+ (mostly from sarcoplasmic reticulum) binds to troponin

What happens to the channels in phase 3 of cardiomyocyte AP?

Ca2+ channels close
Voltage-gated K+ channels open - rapid diffusion of K+ out of the cell

What is the purpose of the long refractory period of cardiomyocyte contraction?

Prevent wave summation and tetanus, which would stop the pumping action of the heart

In the cardiomyocyte action potential, what happens in phase 4?

Only K+ channels are open
Efflux of K+ establishes a negative RMP (-90 mV)

What is the impulse conduction order?

1. SA node fires
2. Signal from SA node goes to cardiomyocytes of both atria to depolarize
3. Signal reaches AV node
4. Signal slows thru AV node so atria can contract
5. Signal spreads thru ventricles via Bundle of His and Purkinje fibers
6. Both ventricular myocardium depolarize and contract in near unison

What is the term for normal heartbeat triggered by the SA node?

Sinus rhythm

What is the result of vagal damage?

Increased heart rate

What are the abnormal pacemaker sites within the heart that display automaticity?

Ectopic focus

T/F: The ectopic focus sites are normally suppressed by the higher rate of the SA node.

True

What is the term for abnormal heart rhythm that originates from AV node or His bundle?

Junctional/nodal rhythm

What is the rate of heart beat when set by AV node?

40-50 bpm

What is the heart beat rate set when set by the AV bundle and Purkinje fibers?

20-40 bpm

What does an EKG measure?

electrical current in heart

Where are the electrodes placed for an EKG?

4 on limbs
6 on chest

What is the term for a HR above 100 bpm?

Tachycardia

What is the term for a HR below 50 bpm?

Bradycardia

What does the P wave represent?

SA node fires
Atria depolarize and contract
Atrial systole begins 100 ms after SA signal

What does the QRS complex represent?

ventricular depolarization and atrial repolarization

What does the ST segment represent?

Ventricular systole and contraction

What does the T wave represent?

Ventricular repolarization and relaxation

What is the term for any abnormal cardiac rhythm or rate?

Arrhythmia

What can arrhythmia cause?

Heart enlargement
Myocardial infarction

What can result from serious arrhythmia?

Ventricles contract rapidly and in uncoordinated manner

What is a hallmark of a MI?

ventricular fibrillation

What is the term for strong electrical shock with intent to depolarize entire myocardium and return to sinus rhythm?

Defibrillation

What is the term for uncoordinated contraction of atria?

Atrial fibrillation

What does atrial fibrillation increase the risk for?

Heart attack
Stroke
Blood clots
Dementia
Heart failure

T/F: Atrial fibrillation may resolve on its own.

True

What is the term for a group of conditions that occur when there is a delay or blockage of the electrical signal resulting in it to be slowed or completely stopped?

Heart blocks

Which heart block is the most clinically significant?

AV nodal block

What are the hallmarks of a 1st degree AV block?

Signals slowed from SA to AV node
Longer PR interval
Normal QRS complex

What are the hallmarks of a 3rd degree AV block?

Complete heart block
Atria and ventricles are depolarizing independently
Regular P wave
very slow QRS

What degree heart block requires a pacemaker?

3rd degree AV heart block