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Flashcards in Test 2 week 3 Deck (275)
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What are the properties of the heart

Automaticity, excitability, conductivity, contractility


What is automaticity

cells can depolarize without and outside impulse. (self excitation)


What is excitability?

Cells can respond to an electrical impulse


What is conductivity?

cells can propagate the electrical stimulus from cell to another


What is contractility?

the specialized ability of cardiac muscles to contract


Electrical activity _____ mechanical activity

always precedes


Why are there different action potential shapes in the heart?

Because there are different permeabilities in the heart, because parts of the heart has different ion channel compositions


What are the two categories of action potentials

Fast and slow response


Examples of a fast response AP

Atrial, and ventricular myocytes, and purkinje fibers


Example of slow response AP

SA and AV nodes


What are the phases of a fast response ventricular muscle AP

Phase 0-4

- Phase 1: the peak where K channels open


What happens in Phase 0

rapid upstroke caused by fast Na channels opening then slow Ca channels at the end of the depolarization


What happens in phase 1

The peak/ initial drop where K channels open, causing a rapid re-polarization of the AP


What happens in phase 2

Ca channels open some more. Net flux of Ca goes from the outside to the inside. Plateau occurs here


What happens in phase 3

K channels open some more, and Ca channels start to close. K moves from the outside to the inside, countering the influx of Ca Rapid repolarization


What happens in phase 4

the restoration of the resting membrane potential


The genesis of phase 0 (upstroke)

- Depolarization of the membrane from- 90 mV to -65 mV increases Na+ conductance by opening of voltage gated Na+ channels.
• Na+ influx causes the rapid upstroke (INa).
• Once opened, Na+ channels inactivate quickly.
• They will remain in the inactive state until Vm is restored to repolarized state.
• Properties of the Na+ channel underlie the basis of AP refractory periods(when inactivation channels are closed).

"closed resting, open active, closed inactive"


In slow response AP phase 0 is driven by _____

A calcium current


What is the effect of tetrodotoxin on fast response channels?

Phase 0/ rapid upstroke starts to diminish.

Turns the fast upstroke to a slow one

Inhibitor of Na channels


Genesis of Early Repolarization (Phase 1)

- Early, brief period of repolarization.
• Caused by activation of a transient outward current (Ito) carried by K+.
• Also due to total inactivation of INa or ICa.
• These channels are activated by depolarization, but are rapidly inactivated as well.


Rapid depolarization is caused by ____

transient outward K current


In a fast response AP ____ current also contributes to the repolarization



In a slow response AP ____ current also contributes to the repolarization

Na and Ca


Genesis of the Plateau (Phase 2)

- L-type Ca channels (L for “long-lasting”) open at the end of depolarization but are inactivated much more slowly than fast Na+ channels.

• Influx of Ca++ (ICa) is counterbalanced by efflux of K+ (IK).

• Ca+ channel blockers such as verapamil and amlodipine act on these channels

• β-adrenergic stimulation(e.g.catecholamines) have the opposite affect and enhance Ca++ conductance through L-type Ca++ channels, thus enhancing contractility (CICR).

• During the end of phase 0, delayed rectifier (IK) channels are opened very slowly. Activation of these channels increases K+ conductance and K+ efflux balances Ca++ influx therefore producing the plateau.


What are delayed rectifier channels?

The K channels / current that open up in the plateau phase and are responsible for balancing the Ca influx


Two main types of delayed rectifier channels

- Slow activated (IKs)
- Rapid activating (IKr)


The duration of an atrial muscle is ____ than a ventricular muscle duration



The duration of the ventricular muscle AP is determined by

slow(IKs) and rapid (IKr) activating channels


Effect of decreasing Ca channels on AP

When the Ca channels are blocked,(blockers such as verapamil and amlodipine) the amplitude f the plateau of AP will decrease and shortens the duration of the AP.

The force produced by the heart muscle will be alot less


Conductance of the heart is decreased and decrease contractility of the heart because

Ca channels are blocked, hence less Ca is coming in giving the delayed rectifier K channel an edge are able to re-polarize the heart a bit faster