Figure 2: (a), Simultaneously recorded action potential (top) and transient (bottom) of a rabbit SAN cell. (b) Time derivatives of the action potential (top) and transient (bottom) traces shown in panel (a). Arrows indicate the timing of the maximum rate of change in membrane potential () and intracellular concentration (d). Horizontal bar indicates the time lag of ms. (c) Time lag between the occurrence of and d versus beating frequency of the 7 cells measured. Solid line is the linear regression line (slope ). Note the absence of an appreciable frequency dependence of the time lag.