The first cells to initiate an electrical signal in the heart is the

Answer is Sinoatrial node, SA.

Explanation:

The sinoatrial node, SA, is known to be a group of cells which are found at the top of the heart.

The electrical signal that is initiated by the sinoatrial node bring about or stimulates the atria and then the ventricle of the heart.

It should be noted that, the sinoatrial node are regarded as the pacemaker cells.