Electrophysiological Properties Of Cardiac Myocytes

Electrophysiological Properties of Cardiac Myocytes 1) Automaticity 2) Conductivity 3) Excitability 4) Contraction

1) Automaticity i. Normal:

• Automaticity is the ability to initiate its own beat. • High to low rhythmicity: Sinus node, interatrial tract, internodal tract, atrial-ventricular node, intraventricular tracts, purkinje cells.

• Pacemaker: SA node ,which has the highest rhythmicity.

ii. Abnormal:

• Ectopic pacemaker: other regions of the heart that initiate beats under special circumstances are called ectopic pacemaker.

• When: a. Their own rhythmicity becomes enhanced; b. The rhythmicity of the higher order becomes depressed; c. Conduction pathways with a higher degree of rhythmicity become blocked.

2) Conductivity i. Normal:

• Specialized conducting system: consists of SA node, interatrial tracts, internodal tract, atrialventricular node, intraventricular tracts, purkinje cells.

Normal AP Conduction in the Heart Sino-Atrial (SA) Node

Atrio-Ventricular (AV) Node Bundle of HIS Left Bundle

Right Bundle

Branch

Branch

Electrical Pathways

Purkinje fibers

ii. Abnormal:

•

Conduction block:

a. Sinoatrial block; b. Atrioventricular block;

3) Excitability i. Normal:

•Refractory of Cardiac myocytes a. Effective refractory period b. Relative refractory period c. Supernormal period

a. Effective refractory period

 phase 0 to –60mV. No matter how strong the stimulus is, it cannot produce action potential.

b. Relative refractory period:

 -60mV to –80mV. With a stronger stimulus than normal an action potential may be evoked.

c. Supernormal period -80 to –90mV. The threshold for excitaiton is slightly lower than normal.

Long refractory (250ms) overlays with almost total duration of contraction

Significance: The long refractory period means that cardiac muscle cannot be restimulated until contraction is almost over & this makes summation of cardiac muscle impossible. This is a valuable protective mechanism because pumping requires alternate periods of contraction & relaxation.

ii. Abnormal:

• premature systole • Compensatory pause

Premature systole: If the cardiac muscle is stimulated by an extra stimulus given in the relative refractory period, it can contract earlier. This contraction is called premature systole.

During the contraction, premature systole can not be evoked by the electric stimulus because of the effective refractory period.

Compensatory pause: The extra stimulus did not disturb the natural rhythm of the normal pacemaker and a prolonged interval, the compensatory pause, followed the extrasystole.

The reason for the compensatory pause after a extrasystole is that the cardiac impulse from the normal pacemaker is blocked by the refractory period of the ventricle induced by the extrasystole, and the following ventricular contraction does not occur. After the compensatory pause, the normal pacemaker resumes its control.

4) Contraction Most of the muscle cells in the heart are contractile cells, which can make the atria and ventricle to systole and diastole.

The Beat of a Single Cell

3.Electrocardiogram (ECG) 1) Definition 2) Normal ECG

1) Definition With an electrocardiograph, electrical activity of the heart can be recorded from the surface of the body. This electrical activity is called electrocardiogram(ECG). That is to say, ECG is the record of spread of electrical activity through the heart .

At any instant of the cardiac cycle the ECG indicates the net electrical field that is the summation of many weak elcetrical fields being produced by individual cardiac cells at that instant. When a large number of cells are simultaneously depolarizing or repolarizing ,large voltages are observed on ECG.

2)Normal ECG

P wave = caused by atrial depolarization QRS complex = caused by ventricular depolarization T wave = caused by ventricular repolarization ECG = useful in diagnosing abnormal heart rates, arrhythmias, & damage of heart muscle .

Start of ECG Cycle

Early P Wave

Later in P Wave

Early QRS

Later in QRS

S-T Segment

Early T Wave

Later in T-Wave

Back to where we started

Abnormal EKGs