Electrical and mechanical events of the heart.

Electrical and mechanical events of the heart.

Objectives:
1- Define resting membrane potentials of cardiac cells.
2- Define action potential of cardiac cells.
3- Describe action potential of ventricle, atria, and the purkinje system (Fast
response).
4- State the benefit of plateau to cardiac cells.
5- Action potential in SA node (Slow response).
6- Latent pacemaker.
7- Refractory period.
8- Mechanical events of left ventricle.
Resting membrane potentials of cardiac cells:
The resting membrane potential is expressed in milli volts (mV). Intracellular potential is expressed relative to extracellular potential. The resting membrane potential of cardiac cells is determined primarily by potassium ions. The conductance (permeability) to K ion at rest is high, while the conductance to sodium is low. The normal concentration of K in the cell is about 150 mM, and about 4 mM out the cell. While the normal concentration of Na is about 20 mM inside the cell and 145 mM outside the cell. Myocardial cells have a resting membrane potential of approximately – 85, instead – 90 mV. Changes in membrane potential are caused by flow of ions into or out of the cell.
Action potential of cardiac muscles:
The action potential of cardiac muscle differs from skeletal muscles in prolonged action potential (Plateau) and in strength of contraction (more calcium ions enter cardiac muscle cells from ECF and sarcoplasmic reticulum). Depolarization causes the membrane potential to become less negative. It occurs when there is movement of positive charge (sodium or calcium ions) into the cell.
Action potential of ventricle, atria, and the purkinje system (Fast response):
Action potential of single cardiac muscle cell in these tissues is characterized by rapid depolarization due to opening fasting sodium channels and a slow depolarization during a plateau. The action potential duration is long, varies from 150 ms in the atria to 250 ms in the ventricle to 300 ms in purkinje system and consequently the long refractory period. The resting membrane potential in these tissues is stable. The initial depolarization is due to Na ion influx through rapidly opening Na ion channels. The Ca ions influx through more slowly opening Ca ion channels produces the plateau phase, and repolarization is due to K ion efflux through three types of K ion channels.