(A). Preparation of rear wall of right atrium containing sinus node (SN) and right ring. Boxes shows position of mapping arrays covering area of SN (8×8 extracellular electrodes) and right ring (6×10 extracellular electrodes). Leading pacemaker site (identified as site of earliest activation) located in SN, and anterograde conduction in right atrial free wall. Arrows show directionof AP conduction from SN to rest of preparation. (B). Preparation containing right ring only (SN and AVN removed), the box shows position of mapping array (6×10 extracellular electrodes) and activation map shows retrograde conduction from pacemaker site to remainder of right atrium. (C). Typical intracellular action potentials recorded from pacemaker sites in right ringand SN.