QS Study

Origin of cardiac impulse –

A cardiac impulse is a cardiac excitation that travels in a wavelike motion and initiates the cardiac cycle. The cardiac cycle refers to the rhythmic beating of the heart. The heart is therefore capable of beating rhythmically in the absence of external stimuli; this is called autorhythmicity.

Myocardial fibers have a resting membrane potential of approximately -90mV.

The individual fibers are separated by membranes, but depolarization spreads radially through them as if they were a syncytium, because of the presence of gap junctions.

Because of high concentration of in the ECF (outside the nodal fiber) as well as the negative electrical charge inside the resting sinus nodal fibers, the positive Na+ outside the fibers even normally tend to leak to the inside. Furthermore, the resting nodal fibers have a moderate number of channels that are already open to Na+. Therefore between the heartbeats, an influx of positively charged Na+ causes a slowly rising resting membrane potential.

The resting potential gradually rises between every two heartbeats. When the potential rises to a threshold voltage of about -40mV, the Ca++ – Na+ channels become activated, leading to rapid entry of both Ca2++ and Na+. This action potential is produced.

Therefore, basically the inherent leakiness of the sinus nodal fibers to Na- causes their self-excitation.

Fig: The origin and spread of cardiac impulse

The trans-membrane action potential of single cardiac muscle cells is characterized by rapid depolarization, a plateau, and slow repolarization process.

The initial depolarization is due to Na+ influx through rapidly opening Na+ channel (the Na+ current).

Ca++ influx through more slowly opening Ca++ channels (the Ca++ current) produces the plateau phase, and repolarization is due to K+ efflux.