A & P - Cardiovascular System, The Heart - Cardiac Electrophysiology

Front 1

What structural feature do cardiac muscle fibres have that is different than skeletal muscle?

Back 1

Cardiac muscle fibers branch, giving individual fibers a step-stair appearance

Front 2

What is the name of the structure that holds cardiac muscle fibers together?

Back 2

Desmosome

Front 3

Which areas of the myocardium possess automaticity?

Back 3

Cells of the SA node and AV node, the so-called "pacemaker" cells

Front 4

What percentage of cardiac muscle fibers possess autorhythmicity?

Back 4

1%

Front 5

What is the technical name for the Bundle of His?

Back 5

Atrioventricular (AV) bundle

Front 6

Where is the SA node located?

Back 6

Right atrial wall just inferior and lateral to the opening of the SVC

Front 7

Where is the AV node located?

Back 7

Interatrial septum

Front 8

What structure does the action potential enter after the AV node?

Back 8

Bundle of His/Atrioventricular bundle

Front 9

What is the only pathway through which action potentials can conduct from the atria to the ventricles?

Back 9

Atrioventricular bundle/Bundle of His

Front 10

Where are the right and left bundle branches located?

Back 10

Interventricular septum

Front 11

If other cardiac muscle fibers in the conduction system display autorhythmicity, why is the SA node the "natural pacemaker of the heart"?

Back 11

On their own SA fibers initiate an action potential about 100 times per minute, faster than any other autorhythmic fibers. These SA node-generated action potentials spread through the conduction system and stimulate other areas before they are able to generate action potentials of their own.

Front 12

Through what structures doe the Purkinje fibers conduct action potentials?

Back 12

The ventricular walls, beginning at the apex and upward to the remainder of the ventricular myocardium

Front 13

Describe the route of an action potential as it travels through the conduction system

Back 13

1) SA node
2) Atrial myocardium
3) AV node
4) AV bundle (bundle of His)
4) Right and left bundle branches
5) Purkinje fibers
6) Ventricular myocardium

Front 14

Describe the phases of an action potential passing through a CONTRACTILE fiber of the heart:

Back 14

Phase 4:
-Resting potential (-90 mV)
-Na+ fast channels closed

Phase 0:
-Depolarization (0 mV or into positive voltage range)
-Na+ fast channels open, Na+ influx

Phase 1:
-transient, small-scale repolarization
-K+ channels open, K+ efflux

Phase 2:
-activation of L-type calcium channels, influx of Ca++ into cell
-"plateau" phase because Ca++ coming in and K+ still leaving

Phase 3:
-repolarization
-continued K+ efflux until resting potential attained

Front 15

What is the resting membrane potential of a contractile cardiac fiber?

Back 15

-90 mV

Front 16

What is autorhythmicity?

Back 16

Autorhythmicity is self-initiated depolarization in a rhythmic fashion.

Front 17

Why do the fast Na+ channels, so essential to depolarization of contractile fibers, remain permanently closed in pacemaker cells?

Back 17

Pacemaker cells never reach the -90 mV resting potential of the contractile cells. Rather, they are chronically at less negative membrane potentials. The effect of this on fast Na+ channels is to deactivate them

Front 18

What is the maximum negative voltage of pacemaker cells?

Back 18

-60 mV

Front 19

What is the pacemaker current? What's another "funny" name for it? What physical events account for this current?

Back 19

The pacemaker current is a gradual spontaneous ionic influx that occurs in pacemaker cells. It is also called the funny current.

The pacemaker current results from the inflow of Na+ ions through the pacemaker channel, which is different than the voltage gated fast Na+ channel.

The pacemaker current draws the membrane potential towards a threshold potential when Ca++ channels open to cause further depolarization.

Front 20

Describe the phases of an action potential as it is generated in a cardiac pacemaker cell:

Back 20

Phase 4
-pacemaker channels are open and a slow Na+ influx causes gradual depolarization (cell membrane becoming less negative)

Phase 0:
-depolarization
-pacemaker current brings membrane potential to threshold, Ca++ channels open and Ca++ pours into the cell

Phase 1:
K+ outflow brings potential back towards -60 mV

Front 21

Which double helix protein, lying in the grooves between actin filaments, prohibits myosin heads from binding to actin?

Back 21

Tropomyosin

Front 22

Which protein does Ca++ bind to in order to initiate muscle contraction?

Back 22

Troponin

Front 23

Describe the mechanism of calcium-induced calcium release.

Back 23

The small influx of Ca++ through L-type Ca++ channels during phase 2 of an action potential is not enough to get a good contraction going.

Ca++ influx, however, triggers further Ca++ release from the terminal cisternae of the sarcoplasmic reticulum.

This process is mediated by ryanodine receptors.

Front 24

Describe how depolarization of the mycocyte results in contraction:

Back 24

During phase 2 of the action potential, L-type Ca++ channels open and Ca++ flows into the cell.

Ca++ binds to ryanodine receptors on SR, triggering further release of Ca++ stores in the terminal cisternae ("calcium-induced calcium release").

Cytosolic Ca++ binds to troponin, which induces tropomyosin to move out of the way.

Myosin heads then bind to actin and away we go.

Front 25

What is a ryanodine receptor?

Back 25

Receptor in the sarcoplasmic reticulum (SR)

Ca++ from original influx binds to ryanodine receptors, triggering a blast of Ca++ release from the terminal cisternae of the SR.

Front 26

What is the job of the sarcoendoplasmic reticulum calcium ATPase (SERCA)?

Back 26

Sequester cytosolic Ca++ back into the terminal cisternae of the sarcoplasmic reticulum

Front 27

How does cytosolic Ca++ return to the terminal cisternae of the sarcoplasmic reticulum, thus ending muscle contraction?

Back 27

Sarcoendoplasmic reticulum calcium ATPase (CERCA)