Figure 2. The “optical APclamp”: Experimental protocol to extract opsin current during an optically-triggered action potential.

The schematic illustrates the three steps in a classical APclamp (in black) and the added elements to achieve an “optical APclamp” (in blue). Importantly, in step 2b, identical light pulse(s) to the one(s) used in step 1 must be delivered synchronously (lag time τ) with the action potential clamp. The traces are used for illustration only but they are actual recordings from ChR2-expressing adult guinea pig cardiomyocytes, as shown in⁷.