Cardiac Muscle Physiology: Action Potentials & Heart Cycle
Cardiac Muscle Physiology: Action Potentials
The resting membrane potential of normal myocardium is -85 to -95 millivolts. The action potential recorded in ventricular muscle is 105 millivolts. This means the membrane potential rises from its normal, very negative value to a positive value of +20 millivolts.
Cardiac vs. Skeletal Muscle Action Potential
There are at least two important differences between the membrane properties of cardiac and skeletal muscle that explain the prolonged action potential plateau of cardiac muscle:
- Role of Slow Channels: The action potential of skeletal muscle is almost entirely caused by the sudden opening of large quantities of so-called fast Na+ channels. In myocardial action potential, it is caused by two types of channels: the fast Na+ channels and slow Ca2+ channels. The latter remain open longer and allow entry of Ca2+ and Na+, which maintains a more prolonged period of depolarization.
- Decreased Permeability to Potassium: During the action potential, the membrane’s permeability to K+ decreases about five times. This effect is absent in skeletal muscle, where K+ output decreases rapidly. This prevents the potential from returning to its resting level during the plateau phase.
Understanding the Cardiac Cycle
Each cardiac cycle begins with the spontaneous generation of an action potential in the sinus node. The action potential travels rapidly through both atria and then through the AV bundle to the ventricles. There is a delay of 0.1 seconds (1/10 sec) in the transmission of cardiac stimulation from the atria to the ventricles. This crucial delay allows the atria to contract before the ventricles, effectively pumping blood into them prior to the ventricles’ strong contraction. Thus, the atria act as primer pumps for the ventricles. The ventricles then serve as the main source of power to move blood through the circulatory system.
Phases of the Cardiac Cycle
Atrial Primer Pump Function
The atria function as primer pumps, simply increasing ventricular pump efficiency.
Ventricular Filling Phase
During ventricular systole, large amounts of blood accumulate in the atria because the AV valves are closed. When ventricular systole finishes and ventricular pressures fall back to low diastolic values, the elevated atrial pressure causes the AV valves to open, allowing blood to flow into the ventricles. This is called the rapid ventricular filling phase.
Ventricular Ejection Period
When left ventricular pressure rises above 80 mmHg (right ventricular pressure > 8 mmHg), these ventricular pressures drive the opening of the semilunar valves. This initiates the flow of blood from the ventricles. Approximately 70% of the blood is ejected in the first third of this period, and the remaining 30% in the subsequent two-thirds. The first two-thirds is called the rapid ejection period, and the remaining one-third is the slow ejection period.
Heart Valve Function
Atrioventricular (AV) Valves
The AV valves (mitral and tricuspid) prevent the backflow of blood from the ventricles to the atria during systole.
Semilunar Valves
The semilunar valves (aortic and pulmonary) prevent blood from flowing back from the aorta and pulmonary arteries into the ventricles during diastole.
All these heart valves open and close passively, driven by pressure gradients. They close when a retrograde pressure gradient pushes blood backward, and open when an antegrade pressure gradient pushes blood forward.