What is cardiac excitability?

Cardiac excitability refers to the ability of the heart muscle (myocardium) to generate and respond to electrical impulses that initiate and coordinate the contraction and relaxation of the heart. Here are the key characteristics and factors influencing cardiac excitability:

1. Resting Membrane Potential:

Cardiac cells, like other excitable cells, have a resting membrane potential (RMP). This is the difference in electrical potential between the inside and outside of the cell when it is at rest and not actively generating an electrical impulse. In cardiac myocytes, the RMP is typically around -85 to -95 millivolts (mV).

2. Action Potential:

Cardiac excitability involves the generation of an action potential, which is a rapid change in the membrane potential of the cardiac cell. When a stimulus reaches a certain threshold, it causes the membrane potential to rapidly depolarize (become less negative) and reach a peak positive potential (upstroke). This is followed by a repolarization phase, where the membrane potential returns to its resting state (downstroke).

3. Refractory Periods:

After an action potential, cardiac cells enter different refractory periods:

- Absolute refractory period: During this period, no stimulus, no matter how strong, can trigger another action potential.

- Relative refractory period: During this period, a stronger-than-normal stimulus can elicit an action potential, but it requires more energy.

4. Threshold Potential:

The threshold potential is the minimum membrane potential that must be reached to initiate an action potential. In cardiac cells, the threshold potential is typically between -60 and -70 mV.

5. Action Potential Duration:

The action potential duration (APD) refers to the time it takes from the start of depolarization to the end of repolarization. Prolonged APD can lead to arrhythmias.

6. Ion Channels:

Cardiac excitability is tightly regulated by the activity of various ion channels within the cell membrane. These channels, such as sodium, potassium, and calcium channels, control the flow of ions into and out of the cell, influencing the membrane potential and action potential waveform.

7. Autonomic Nervous System:

The autonomic nervous system, particularly the sympathetic and parasympathetic divisions, can modulate cardiac excitability. Sympathetic stimulation increases excitability and heart rate, while parasympathetic stimulation decreases excitability and heart rate.

8. Heart Disease and Drugs:

Cardiac excitability can be affected by various heart conditions and medications. For instance, certain arrhythmias may arise due to abnormal excitability, and some drugs used in cardiac therapy target ion channels to influence excitability.

Overall, cardiac excitability is a crucial factor in maintaining the regular rhythm and coordinated contraction of the heart. Dysregulation of excitability can lead to various cardiac arrhythmias and impact overall heart function. Understanding cardiac excitability and its regulation is essential in the study of cardiac electrophysiology and the development of treatments for heart rhythm disorders.