Cardiology Intro/Electrophysiology

Question 1

What are the 3 types of cardiac cells?

Answer 1

Myocardial cells
Pacemaker cells
Conduction cells (purkinje)

Question 2

Function of myocardial cells

Answer 2

Contraction and impulse conduction

Question 3

What type of cells exhibit automatic rhythmical electrical discharge in the form of action potentials? (dictate heart rhythm)

Answer 3

Pacemaker cells

Question 4

What type of cells conduct the action potentials through the heart, providing an excitatory system that controls rhythmic beating?

Answer 4

Conduction cells (purkinje)

Question 5

What makes the myocardium a functional syncytium?

Answer 5

Doesn’t morph together to form one multinucleated cell like skeletal muscle does, but is able to function as a single unit due to presense of intercalated discs.

Question 6

What are the crossbands at the end of each myocardial cell called?

Answer 6

Intercalated discs

Question 7

Types of junctions at intercalated discs

Answer 7

Gap junctions: longitudinal surface. Allow rapid diffusion of ions and AP to travel from cell to cell (functional syncytium)

Desmosomes: transverse surface. Hold cells together and provide mechanical strength.

Question 8

What are the main Ca++ sources for cardiac muscle contraction?

Answer 8

Ca++ is derived from both the ECF and the SR. SR is not developed enough to provide enough Ca++ on its own.

Question 9

Different action potentials in the heart

Answer 9

Pacemaker potential

Atrial/ventricular cell potential

Question 10

Characteristics of pacemaker potentials

Answer 10

Potential gradually becomes less negative until it reaches the threshold.

Question 11

Characteristics of atrial/ventricular potentials

Answer 11

AP are longer than those in nerve and other muscle cells (300ms vs 3ms in skeletal)

Question 12

What is the normal pacemaker of the heart?

Answer 12

Sinoatrial (SA) node located in the right atrium

initiates action potential

Question 13

Where are the backup pacemakers of the heart?

Answer 13

AV node

His-purkinje system

Question 14

Why is the SA node the main pacemaker?

Answer 14

Discharges faster than the other cells (70-80x per min)
AV node: 40-60x per min
Purkinje fibers: 15-40x per min

Question 15

Where is the SA Node located?

Answer 15

In the right atrium, below the opening for the cranial vena cava.

Fibers are continuous with atrial muscle fibers, allowing AP to spread immediately into the atrial muscle wall.

Question 16

Where does the action potential go after the SA node?

Answer 16

Through atrial muscle wall to the AV node, where it is delayed to allow the atria to completely empty their blood into the ventricles before ventricular systole.

Question 17

Where is the AV node located?

Answer 17

Posterior wall of the right atrium, behind the tricuspid valve.
Has less gap junction permeability, so it more resistant to conduction (delayed response)

Question 18

What cells transmit the AP after the AV node?

Answer 18

Purkinje fibers lead impulse from the AV node to the AV bundle (bundle of His), and into the ventricles.

Question 19

How does transmission of the AP in the ventricles compare to that of the atria?

Answer 19

Much faster in the ventricles. Not delayed as it is in the atria.

*Transmission through the remainder of the ventricular muscle is instantaneous

Question 20

What keeps the electrical impulse from traveling through abnormal routes?

Answer 20

A fibrous skeleton surrounds the atrial muscle.

Impulses can only be transmitted from atrium to ventricle.

Question 21

How far do the branches of the AV bundle extend?

Answer 21

Down beneath the endocardium to the apex of the ventricles and wrap around, heading back up to the base of the heart.

*Purkinje fibers penetrate and become continuous with the muscle.

Question 22

Because the cardiac muscle wraps around the heart in a double spiral, how do the atria and ventricles contract?

Answer 22

Atria squeeze inward

Ventricles shorten upward

Question 23

What is responsible for the slow depolarization of the sinus node?

Answer 23

Pacemaker cells lack normal, fast Na+ channels.

Funny Na+ channels (lf or f) close during AP and open spontaneously after AP finishes, which starts the cycle of depolarization again.

Cause progressive increase in Na+ permeability, and Na+ enters from ECF to lead membrane to threshold again.

Question 24

How does the closing of the K+ channels assist in depolarization in the sinus node?

Answer 24

Closing of K+ channels at the end of each repolarization keeps K+ inside the cell, making the potential more positive, and leading to another AP.

Question 25

What is the role of Ca++ channels in depolarization of the sinus node?

Answer 25

Fast Ca++ channels open, Ca++ enters the cell, and gives the final push toward the threshold.

Question 26

What channels have a role in repolarization of the sinus node?

Answer 26

Ca++ channels close: no Ca++ going in
K+ channels open: K+ travels out of cell

Potential becomes more negative

Question 27

What ion is primarily responsible for AP in pacemaker cells?

Answer 27

CALCIUM

Question 28

What kind of AP occurs in myocardial cells?

Answer 28

Rapid depolarization, plateau, abrupt repolarization

Question 29

What is responsible for the AP in myocardial cells lasting 100x longer than in nerves or skeletal muscle?

Answer 29

Plateau due to slow Ca++ channels (slow influx of Ca++ and Na+) and delayed efflux of K+ prevents the return of AP to resting potential.

Question 30

What is the purpose of the plateau in myocardial AP?

Answer 30

Absolute refractory period. Gives ventricles time to empty and refill before next contraction. Don’t want tetnization!

Question 31

What 2 channels are involved in myocardial AP?

Answer 31

Fast Na+ channels to depolarize membrane.

Slow Ca++ channels to allow Ca++ and Na+ to flow in and maintain prolonged depolarization.

Ca++ ions help activate the contractile process