Electrophysiology

Electrophysiology of the heart

Rosfaiizah Siran

Remember that cardiac muscles are interconnect by desmosomes mechanically and gap junctions electrically. The gap junctions allow the ions to be transferred from one cardiac cells to another.

The two main types of cardiac cells are pacemaker or conducting cells which have their own percentage in cardiac tissues and their own properties.

Remember the properties of the cardiac cells. The conducting cells and contractile cells are differentiate by these properties!

Multiple Select

Choose 2 statements that are true regarding conducting cells:

• It consists of 1% in cardiac cells.

• Contractility is its main property

• Automaticity is its main property

• Ventricular cells are the example.

Multiple Select

Choose 2 statements that are true regarding contractile cells:

• It consists of 99% in cardiac cells.

• Contractility is its main property

• Automaticity is its main property

• SA node cells are the example.

Multiple Choice

Both conducting and contractile cells are excitable.

• True

• False

Multiple Choice

Both conducting and contractile cells have automaticity property.

• True

• False

Remember that SA node is our pacemaker cells. It determines the heart rate and rhythm because of it is the fastest that generate action potential compare to the other conducting cells.

Because of that, the spread of action potential or impulses occur in sequence for normal heart. From SA node down to the purkinje fibers.

Multiple Choice

Choose the correct sequence of impulse spread in normal conducting pathways:

• SA node --> AV nodes --> Internodal pathway --> Bundle of HIS --> Bundle branches --> Purkinje fibers

• SA node --> AV nodes --> Internodal pathway --> Bundle of HIS --> Purkinje fibers --> Bundle branches

• SA node --> Internodal pathway --> AV nodes--> Bundle of HIS --> Bundle branches --> Purkinje fibers

Remember there are many shapes of action potential of conducting cells. As it goes further down, the shape of the action potential of purkinje fibers are nearly the same as ventricular muscle cells which are contractile cells. The one that I described later is the action potential of SA node cells.

The three major ions that involve in generation of action potential of SA node are sodium, calcium and potassium.

Multiple Choice

Choose the correct event that occur during pacemaker potential.

• Influx on sodium ions

• Influx of calcium ions

• Efflux of potassium ions

Multiple Choice

The most important event that responsible for depolarization in action potential of SA node cells is

• Influx on sodium ions

• Influx of calcium ions

• Efflux of potassium ions

There are three important events that before and during the action potential generation.

Pacemaker potential is important for the automaticity of the pacemaker cells. The opening of voltage gated sodium channels allow slow depolarization that then reach the threshold.

When reaching threshold, there is opening of LONG LASTING calcium ion channel that allow depolarization.

Repolarization occurs when the voltage gated potassium ion channels open, after the closure of the calcium ion channels.

Multiple Choice

Choose the correct labelling for the phase.

• 1 is pacemaker potential, 2 is repolarization, 3 is depolarization

• 1 is funny currents, 2 is depolarization, 3 is repolarization

• 1 is depolarization, 2 is repolarization, 3 is pacemaker potential

Multiple Choice

Choose the channels that OPEN during the phase.

• 1 : voltage gated calcium channels

• 2. voltage gated sodium channels

• 3. voltage gated potassium channels

Multiple Select

Choose the channel that CLOSE during the phase.

• 1 : voltage gated potassium channels

• 2. voltage gated long lasting calcium channels

• 3. voltage gated short lasting calcium channels

Remember this drawing, as we can ask everyone to draw a schematic diagram on how the electrical impulses are conducted in the heart

This slide is to show the relationship of electrical impulse conduction with the activity of the heart.

Multiple Choice

Choose one correct statement of electrical activity with cardiac event.

• SA node depolarization occurs concurrent with atrial contraction

• SA node depolarization leads to spread of impulses within the atrial

• SA node depolarization occurs concurrent with AV node depolarization

How about the electrical activity of contractile cells?

This slide explains the basis of how SA node cells trigger the electrical activity in the contractile cells.

The shape of the action potential in atrial and ventricular cells are different from SA node cells and AV node cells.

The main four type of ions that responsible for the action potential of contractile cells are sodium, potassium and calcium ions.

Multiple Choice

Choose ONE correct event that occur during action potential of contractile cells.

• Phase 0 : Influx on sodium ions

• Phase 1: Efflux of calcium ions

• Phase 2: Efflux of potassium ions

• Phase 3: Influx of calcium ions

Phase 0 is depolarisation, and phase 1 is small repolarization.

Phase 2 is plateau phase, phase 3 is repolarization and phase 4 is when the membrane potential reaching resting state.

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This slide is just to recap the differences between the action potentials of SA node and contractile myocardial cells.

The process that occur in excitation contraction coupling of cardiac cells are the nearly the same as musculoskeletal cells.

Kindly note refractory period for cardiac muscle fibers lasts as most as long as muscle contraction, in comparison to skeletal muscle fibers.

Parasympathetic stimulation decreases the heart rate. However sympathetic stimulation increases heart rate and force of contraction.

Remember the mechanism on how the parasympathetic and sympathetic neurons decreases or increases the heart rate.

This slide to summarize again the detail on the effect of parasympathetic and symptathetic stimulation.

Multiple Choice

In relation to the diagram, choose the correct order of event below:

1. Action potential along T tubule

2. Current spreads through gap junctions

3. Ca2+ release from sarcoplasmic reticulum

4. Ca2+ channels open

• 1,2,3 and 4

• 2,1,3 and 4

• 2,1,4 and 3

• 4,3,2 and 1

Multiple Select

The effect(s) of parasympathetic stimulation to the heart is(are)

• decreases the rate of depolarization of SA node

• decreases excitability of cardiac cells

• decreases AV nodal delay

• decreases contractility of ventricular muscle

Multiple Select

The effect(s) of sympathetic stimulation to the heart is(are)

• increases the rate of depolarization of SA node

• increases excitability of cardiac cells

• increases AV nodal delay

• increases contractility of atrial muscle

Multiple Choice

In relation to the diagram, choose the correct order of event below:

1. Action potential along T tubule

2. Current spreads through gap junctions

3. Ca2+ release from sarcoplasmic reticulum

4. Ca2+ channels open

• 1,2,3 and 4

• 2,1,3 and 4

• 2,1,4 and 3

• 4,3,2 and 1

Thank you

All the best