Conversation so far:
what node serves as the pacemaker of the heart
The sinoatrial node or sinus node is the heart's natural pacemaker.
what controls the firing of the sa node
The autonomic nervous system controls the firing of the SA node to trigger the start of the cardiac cycle.
where is the sa node
The SA node consists of a cluster of cells that are situated in the upper part of the wall of the right atrium
what does the sa node look like
The SA node represents a crescent like shaped cluster of myocytes divided by connective tissue, spreading over a few square millimeters

Question: what about the cells
Self-contained rewrite: what do the sa node cells look like