Physiology of Cardiac Muscle Flashcards
What are the major characteristics of cardiac muscle?
- Mononuclear
- Striations faintly visible
- INTERCALATED DISK
Which three cell junctions make u intercalated discs?
Gap Junctions
Desmosomes
Adhering junctions
What level of heart excitiation is being represented in the picture?
SA node generates impulses; atrial excitation begins
What level of heart excitiation is being represented in the picture?
Impluse delayed at AV node
What level of heart excitation is being represented in the picture?
Impulse passes to heart apex; ventricular excitation begins
What level of heart excitation is being represented in the picture?
Ventricular excitation complete
What is the sequence of heart excitation?
- SA node generates impulses at about 75 times/min
- AV node delays the by 0.1 second
- Impulse passes from atria to ventricles via the AV bundle of His
- AV bundle splits into two pathways in the interventricular septum
- Bundle branches carry the impuse toward the apex of the heart
- Purkinje fibers carry the impulse to the heart apex and ventricular walls
Why is it important that the AV node delays the impulse by approximately 0.1 seconds?
It ensures that the atria have ejected their blood into the ventricles first before the ventricles contract.
Pacemaker cells use _________ influx for rising phase of the action potential.
Calcium (as opposed to sodium)
Pacemaker cells have unstable resting potentials called _____________.
Pacemaker potentials
Where are pacemaker cells normally found?
In the SA node
The main ions found outside the cell at rest are ______________ and ___________, whereas inside the cell it is mainly _____________.
Sodium and chloride; potassium
What are the 5 phases of the ventricular myocyte membrane potential?
Phase 0- Depolarization
Phase 1- Transient
Phase 2- Plateau
Phase 3- Repolarization
Phase 4- Resting Potential
What occurs in phase 4 of the ventricular myocyte membrane potential?
- Phase 4 occurs when the cell is at rest, in a period known as diastole.
- Voltage ~~ -90mV
- Membrane is most permeable to K+
NOTE: Pacemaker cells are never at rest
What occurs in phase 0 of the ventricular myocyte membrane potential?
- Depolarization
- In non-pacemaker cells (i.e. ventricular cells), this is produced predominantly by the activation of Na+ channels, which increases the membrane conductance (flow) of Na+.
- These channels are activated when an action potential arrives from a neighbouring cell, through gap junctions.
NOTE: In pacemaker cells (e.g. sinoatrial node cells), however, the increase in membrane voltage is mainly due to activation of L-type calcium channels.
What occurs in phase 1 of the ventricular myocyte membrane potential?
- Transient
- Phase begins with the rapid inactivation of the Na+ channels by the inactivation gate
- Reduces the movement of sodium into the cell.
- At the same time potassium channels open and close rapidly, allowing for a brief flow of potassium ions out of the cell, making the membrane potential slightly more negative.
- This is referred to as a ‘notch’ on the action potential waveform.
NOTE: There is no obvious phase 1 present in pacemaker cells.
What occurs in phase 2 of the ventricular myocyte membrane potential?
- Plateau
- The membrane potential remaining almost constant, as the membrane very, very slowly begins to repolarize.
- Phase is important in preventing irregular heartbeat
NOTE: There is no plateau phase in pacemaker cells
The cardiac action potential is also known as _________.
dV/dt
What does the membrane potential remain relatively constant in the plateau phase?
- During this phase delayed rectifier potassium channels allow potassium to leave the cell while L-type calcium channels (activated by the flow of sodium during phase 0), allow the movement of calcium into the cell.
- This calcium, binds to and opens more calcium channels (called ryanodine receptors) located on the sarcoplasmic reticulum within the cell, allowing the flow of calcium out of the SR.
- This calcium influx also activates chloride channels, which allow Cl− to enter the cell. Together the movement of both Ca2+ and Cl− oppose the voltage change caused by K+.
- The increased calcium concentration also increases the activity of the sodium-calcium exchanger, and the increase in sodium entering the cell increases activity of the sodium-potassium pump. The movement of all of these ions results in the membrane potential remaining relatively constant.
What occurs in phase 3 of the ventricular myocyte membrane potential?
- Repolarization
- The L-type Ca2+ channels close, while the slow delayed rectifier (IKs) K+ channels remain open as more potassium leak channels open.
In a healthy sinoatrial node, the pacemaker potential is the main determinant of the___________
heart rate.
What are the steps to the pacemaker potential?
- Decrease in potassium permeability accompanied by slow sodium entry
- Fast calcium channels open
* Increase in calcium permeability - Decrease in calcium permeability and an increase in potassium permeability
- Slow depolarization
- Fast calcium channels open
- Action potential
- Threshold
The rate of automaticity of pacemaker cells is determined by:
- Rate of diastolic depolarization
- Magnitude of maximum diastolic potential
- threshold potential level
How can the automaticity of pacemaker cells be explained?
These cells begin leaking sodium into the cell, as soon as they return to their resting state
