Lecture 4: Cardiac Action Potential Flashcards
What is the Na+ (f) gate specific for and when is it activated?
- Funny voltage-gated Na+ channels
- Specific to SA and AV node where it causes slow depolarization during resting phase
- Activated when the SA or AV node repolarizes
What is the Eq. Pot. for Na+?
+30 mV
What is the Eq. Pot for K+?
-90 mV
What is the Eq. Pot. for Ca2+?
+40 mV
During what phase are the Na+ (f) gates open?
Phase 4 (SA or AV node repolarization)
What constitutes phase 0 of AP for ventricles, atria, and purkinje system?
Rapid upstoke (depolarization) caused by crossing threshold (RMP) and voltage-gated Na+ channels opening
What constitutes phase 1 of AP for ventricles, atria, and purkinje system?
Small repolarization caused by start of the Na+ (m) gates closing and some K+ (a) gates opening
Function of the K+ (a) channels opening during phase 1?
Cause a very rapid partial repolarization
What occurs during phase 2 of AP for ventricles, atria, and purkinje system; what is this phase referred to as?
- Plateau phase
- Sustained by slow Ca2+ channels opening and closing of special, voltage gated K+ (b) channels
Function of the Ca2+ channels opening in phase 2?
Cause a slow inward current of Ca2+ to go along with the outward current of K+. Membrane potential remains at a stable depolarization (plateau)
Other than helping to stabilize membrane potential during phase 2, what is another important function of Ca2+ influx?
Causes the release of more Ca2+ from intracellular stores for excitation-contraction coupling.
What occurs during phase 3 of AP for ventricles, atria, and purkinje system?
Complete repolarization is caused by slow Ca2+ channels closing and K+ channels opening
What are the 3 characteristics of the AP in ventricles, atria, and the purkinje system?
1) Stable resting potentials
2) Rapid upstroke
3) Long duration
The resting potential (phase 4) is sustained by the high conductance of?
K+ (c) - the K+ leak channel
How does the phase 4 of AV node differ from that of the SA node?
Slower depolarization during phase 4, typically doesn’t reach threshold until it receives a triggering signal from the SA node.
What is special about the Na+ (f) channels?
Causes the slow, intrinsic and spontaneous depolarization of the SA and AV nodes. This “automaticity” is why the SA node is known as the pacemaker.
What occurs during phase 0 in the SA/AV node?
Depolarization due to: opening of slow Ca2+ channels and closing of special K+ (b) channels
Why is there no phase 1 and phase 2 in SA/AV node?
Due to scarcity of traditional, voltage-gated Na+ (m) channels
What occurs during phase 3 in the SA/AV node?
Repolarization due to: closing of Ca2+ gates and opening of special K+ (b) gates
If phase 4 of the AV node fires before that of the SA node what happens to the heart rate?
The rate will be much slower
What are considered the latent pacemakers?
AV node, bundle of His, and Purkinje fibers
What occurs to bundle of His and purkinje fibers if not stimulated for an extended period?
Will begin to spontaneously depolarize during phase 4
How does the rate of the bundle of Purkinje fibers compare to that of the AV node?
Slower
The SA node will naturally depolarize to reach threshold without?
Electrical stimulation
The higher the inward current (Na+ (m)) or faster the Ca2+ current, does what for conduction velocity?
Higher the velocity
How does fiber size dictate the conduction velocity?
Larger the fiber = faster transmission of AP = faster the velocity
Where is the conduction velocity the greatest?
Purkinje (and atrial pathways): 2-4 m/s
Where is conduction velocity the slowest?
AV node: 0.01-0.05 m/s
What is the conduction velocity in the atria and ventricles?
1 m/s
What is the importance of the slow conduction velocity of the AV node?
Allows the atria to fully contract, and for the ventricles to receive the maximum amount of blood before contraction.
What is the refractory period?
The electrolyte gates have not ‘reset’ sufficiently to allow a second AP to be generated
What is the refractory period important for in cardiac muscle?
To help prevent arrhythmias
How does the refractory period in cardiac cells compare to that of neurons?
Longer in cardiac cells
What is the absolute refractory period (ARP)?
No matter how large the stimulus is, there can be no AP generated
What is the relative refractory period (RRP)?
An AP can be generated, but will required a stimulus that is greater than normal
What is a supranormal period (SNP)?
When the cell is more excitable than normal, a smaller stimulus than normal can generate an AP
If an AP is generated during the RRP, how will it compare to a normal AP?
Will be weaker, have an abnormal configuration, and a shortened plateau phase
What are chronotropic effects?
The effects of the Autonomic Nervous System on HR
How do chronotropic effects act on the heart?
Change rate of depolarization of SA node and therefore heart rate (HR)
What are dromotropic effects?
The effects of the Autonomic Nervous System on conduction velocity
Stimulation by what part of the autonomic nervous system causes positive chronotropic effects; affect on HR?
Sympathetic nervous system; increases HR
Stimulation by what part of the autonomic nervous system causes negative chronotropic effects; affect on HR?
Parasympathetic nervous system; decreases HR
What NT is released by the parasympathetic nervous system and what does it bind to in the SA node?
Releases ACh, which binds to muscarinic receptors
How does the parasympathetic nervous system act on phase 4 in SA and AV nodes?
Decreases opening of special Na+ (f) gates
How does the parasympathetic nervous systems affect on phase 4 impact the heart?
- Decreased depolarization rate - Decreased HR (- chronotropy)
What does the effect of the parasympathetic nervous system do to conduction velocity through the AV node?
Decrease conduction velocity
Which parts of the heart are impacted by the parasympathetic nervous system?
AV nodes, SA nodes, and to a significant extent contractile myocytes
What NT is released by the sympathetic nervous system and what does it bind to in the SA node?
Norepinephrine, which binds to Beta-1 receptors
How does the parasympathetic nervous system act on phase 4?
Increases the opening of special Na+ (f) gates
How does the sympathetic nervous systems affect on phase 4 impact the heart?
- Increases the rate of depolarization - Increase HR (+ chronotropy)
What does the effect of the sympathetic nervous system do to conduction velocity through the AV node?
Increased conduction velocity
What is a PVC?
Premature ventricular heartbeat - development of new pacemaker signal (focus) in the ventricles
What is a PAV?
Premature atrial heartbeat - can be from development of new pacemaker signal (focus) in atrium away from SA node (e.g., left atrium)
What are effects of ectopic pacemakers?
Can cause arrhythmias, such as atrial fibrillation
A reduction in the amount of Ca2+ entering the cell does what to myocytes?
Slow things down.
What does this image depict?
Action potential through the Ventricle
What does this image depict?
Action potential through the Atrium
What does this image depict?
Action potential through the SA node
