Figure 4. Optimal isolation of somatic Na⁺ currents.

Two voltage clamp protocols designed to determine optimal conditions for axial current removal. Illustrative data obtained from the same neuron as shown in Fig. 1. A, The prepulse potential should be chosen as the minimum voltage that quickly evokes the uncontrolled current spike (−40 mV). A relatively low test potential (−30 mV) should result in a current with smooth rise and approximately single exponential decay (red trace). B, The axial current, initially eliminated by the prepulse, recovers within a few ms at −55 mV. C & D, Simulations of the compartmental model shown in Fig 2, in response to voltage clamp protocols similar to those in (A) and (B), respectively. The dashed red lines represent the somatic Na⁺ current, approximately equal to the total (“measured”) current. The internal solution contained 50 mM Na⁺, and traces were TTX-subtracted.