The right answer is The first image (the left one).
The contraction of the atrium is done with the help of the sinus node which synchronously emits a discharge which is diffused in the heart such is shown in the image 1.
The sinus node, or node of Keith and Flack (nodus sinuatrialis) is a set of cells located in the upper wall of the right atrium of the heart (right atrium), whose depolarization controls the normal heart rhythm, called "sinus rhythm" .
The depolarization of the action potential is conducted step by step in the cells of the right and left atria, accompanied by a contraction of the muscle fibers.
