06. Physiology - Cardiac Electrophysiology & Conduction

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Questions and Answers

What effect does PKC phosphorylation have on sodium current?

It completely blocks sodium current.

It reduces the sodium current. (correct)

It has no effect on the sodium current.

It increases the sodium current.

Which medication is classified as a sodium channel blocker?

Isoproterenol

Verapamil

Lidocaine (correct)

Nifedipine

What is the effect on phase 0 of the action potential if the sodium current is reduced?

The repolarization occurs slower.

The depolarization occurs faster.

There is no change to phase 0.

The depolarization occurs slower. (correct)

What is an effect of PKA on calcium current?

It enhances the calcium current. Signup and view all the answers

Which of the following drugs is a calcium channel blocker?

Nifedipine Signup and view all the answers

What initiates the depolarization phase of the myocyte action potential?

Opening of fast voltage gated sodium channels Signup and view all the answers

In which phase does a brief early repolarization occur?

Phase 1 Signup and view all the answers

Which current flows more rapidly during the action potential upstroke?

INa Signup and view all the answers

Which of the following currents is specifically responsible for early repolarization in myocytes?

INa Signup and view all the answers

At approximately what membrane potential does the depolarization phase trigger?

-60 mV Signup and view all the answers

What ion movement occurs during the depolarization phase of a cardiac action potential?

Calcium ions move in Signup and view all the answers

Which ion channels primarily influence automaticity in cardiac cells?

Calcium channels Signup and view all the answers

What best describes the escape phenomenon in non-pacemaker cells?

Reversal of resting membrane potential due to ion channel activity Signup and view all the answers

What is the status of the ion channels during the repolarization phase of the cardiac action potential?

Potassium channels are open, calcium channels are closed Signup and view all the answers

Which ion exchangers are involved in cardiac cellular electrophysiology?

Na+/Ca2+ exchanger Signup and view all the answers

During which phase of a cardiac action potential are both sodium and potassium channels inactive?

Phase 2 Signup and view all the answers

What is the effect of increased potassium ion concentration on cardiac action potentials?

Decreased excitability of the cell Signup and view all the answers

What describes the state of ion channels during the resting potential of cardiac cells?

Sodium channels are inactive, potassium channels are open Signup and view all the answers

What happens to phase 2 of the action potential if the calcium current is reduced or delayed?

The phase 2 plateau may be reduced. Signup and view all the answers

During which phase does the Na+-K+-ATPase show its highest activity?

Phase 4 Signup and view all the answers

If the inward rectifying potassium channel (IK1) is inhibited, what is likely to occur?

The resting membrane potential may not be maintained properly. Signup and view all the answers

How does enhancing Ito affect the phases of the action potential?

It shortens phase 1. Signup and view all the answers

Which ion's current contributes to the early repolarization during phase 2 of the action potential?

Calcium Signup and view all the answers

What effect does suppressing the Na+-K+-ATPase due to low ATP availability have on cardiac cells?

It increases intracellular sodium concentration. Signup and view all the answers

What happens to the action potential phases when an ion current is modified?

The phases can change depending on the ion's role. Signup and view all the answers

What is the role of L-type calcium currents during the action potential?

They contribute to the plateau during phase 2. Signup and view all the answers

What characterizes the absolute refractory period?

Channels are inactive or haven’t fully closed. Signup and view all the answers

Why is the action potential during the relative refractory period typically smaller?

Not all channels are restored to a closed state. Signup and view all the answers

What is a significant difference between cardiac and skeletal muscle action potentials?

Skeletal muscle can be tetanized while cardiac muscle cannot. Signup and view all the answers

What happens to ion channels during Phase 3 of the cardiac action potential?

Some channels are inactivated while others are ready to reopen. Signup and view all the answers

What role does the annulus fibrosus play in cardiac conduction?

Separates atrial and ventricular myocytes preventing conduction. Signup and view all the answers

How does cardiac muscle differ from skeletal muscle regarding force generation?

Cardiac muscle generates force without tetanus. Signup and view all the answers

What happens if ion current changes during the cardiac action potential?

It can lead to arrhythmias. Signup and view all the answers

Which mechanism is crucial in understanding cardiac arrhythmias?

Reentry of conduction currents. Signup and view all the answers

Study Notes

Cardiac Action Potentials

Cardiac action potentials have five phases:
- Phase 0: Depolarization (upstroke) due to fast voltage-gated sodium channel opening, creating INa.
- Phase 1: Brief rapid early repolarization, as INa slows.
- Phase 2: Plateau phase, resulting from a balance between slow inward calcium current (ICaL) and outward potassium currents (IK1).
- Phase 3: Repolarization, as ICaL declines and outward potassium currents increase.
- Phase 4: Resting membrane potential, maintained by IK1.
Sodium, calcium, and potassium channel activity determines the shape and duration of each phase.
Automaticity: The ability of some cardiac cells to spontaneously generate action potentials.
- Modified by specific ion channels, mainly the inward I_f (funny) current.
Escape: When a non-pacemaker cell spontaneously generates action potentials due to altered electrophysiological conditions.

Sodium Current

Sodium current is essential for rapid depolarization (Phase 0).
Can be regulated by:
- Protein Kinase C (PKC) phosphorylation: Reduces sodium current.
- Sodium channel blockers: Reduce sodium current, examples include:
  - Tetrodotoxin (TTX)
  - Lidocaine
  - Ranolazine
  - Quinine
Inhibiting sodium current slows depolarization, delaying action potential initiation.

Calcium Current

Calcium current contributes to plateau phase (Phase 2) and is involved in early repolarization (Phase 1).
Regulated by:
- Protein Kinase A (PKA): Increases calcium current.
- Calcium Channel Blockers: Decrease calcium current, examples include:
  - Verapamil
  - Nifedipine
Enhancing calcium current alters Phase 1 and 2. Decreasing calcium current reduces plateau phase and may affect early repolarization.

Non-voltage-gated Channels

Na+-K+-ATPase: Maintains resting membrane potential (Phase 4) by exchanging sodium and potassium ions.
- Impairment of Na+-K+-ATPase affects resting membrane potential maintenance.
Key to understanding ion current effects:
- Consider the phases where the current is most active (channels are open).
- Think about the direction of ion movement – potassium out = polarization, sodium and calcium in = depolarization.

Differences between Myocytes

There are variations in ion channel expression and activity among different myocytes, leading to variations in action potential characteristics.

Refractory Period

Absolute refractory period: No action potential can be generated due to channel inactivation.
Relative refractory period: Some channels are starting to close, but not all are ready to reopen, resulting in weaker action potentials.
Importance of refractory period for cardiac electrophysiology:
- Essential for understanding arrhythmias and conduction abnormalities.

Cardiac vs. Skeletal Muscle

Skeletal muscle: Can be tetanized (multiple depolarizations lead to force summation).
Cardiac muscle: Cannot be tetanized due to differences in action potentials and ion channels.
- This prevents sustained contraction and allows for normal heart function.

Conduction

Sequence of activation:
- Atria and ventricles mainly composed of myocytes connected by gap junctions.
- Annulus fibrosus isolates atria from ventricles, preventing direct conduction between chambers.
Different cardiac cell types:
- Have different activation sequences, conduction velocities, pacemaker rates.

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Description

This quiz explores the five phases of cardiac action potentials, detailing the processes of depolarization, repolarization, and the role of ion channels. It also covers concepts such as automaticity and escape mechanisms in cardiac cells. Test your understanding of these critical physiological processes.