The slow depolarization of the pacemaker cells is caused by the opening of the:.

The slow depolarization of the pacemaker cells is caused by the opening of the non selective cation channels.

Explanation:

Pacemaker cells are specialized cardiac muscle cells with the task to create the rhythmic pulses that make the cardiac muscle contract properly, functioning as a whole and not as independent contractile cells.

One of the things that differentiate pacemaker cells from regular cardiomyocytes is that pacemaker cells slowly depolarize by themselves. This happens because when these cells are in phase 4, in a state of hyperpolarization, the 'funny current' appears to begin depolarizing the cell again.

Hyperpolarization usually occurs because there's an outflux of potassium ions (which makes the inside of the cell much more negative than the outside of the cell) but, in pacemaker cells, the membrane becomes less and less permeable to potassium, which ends up decreasing the outflux of this ion. Along with this change in potassium's electrical conductance, the funny current increases the permeability of sodium, which means that there's also a sodium influx happening at the same time, helping in the depolarization process.

This mixed sodium and potassium permeability is one of the key features of the funny current, and is possible thanks to the opening of non-selective cation channels in the membrane that allow the passing of any cation following their specific chemical gradients.

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