Sinoatrial (SA) node fibers possess the ability to depolarize spontaneously until a threshold potential of about −40 mV is reached, which generates a new action potential (Fig. 2.3). These pacemaker action potentials spread to working myocardial fibers, resulting in working myocardium action potentials (Fig. 2.3).

When threshold is reached at SA node what channels open causing further depolarization of the membrane?

When threshold is reached at the SA node (an autorhythmic cell), what channels open causing further depolarization of the membrane? Fast calcium channels. Yes, unlike nerve cells or cardiac muscle cells, fast calcium channels are responsible for the depolarization phase of the autorhythmic cell action potential.

What happens when SA node reaches threshold?

When the pacemaker potential in the SA node reaches threshold, an action potential is generated.