Transient inward current is carried by TTX-sensitive sodium channels. A, Whole-cell membrane current in response to a voltage-clamp step from −70 to 0 mV, given at the arrow. The thick trace shows the current in normal external solution, the thin traceshows the response during superfusion with solution in which sodium ions were replaced with choline, and the gray traceshows recovery of the current after cessation of 0 Na⁺ superfusion. B, Whole-cell membrane current in response to a voltage-clamp step from −70 to −15 mV, given at the arrow. The thick traceshows the control response, the thin trace illustrates the response during superfusion with external solution containing 100 nm tetrodotoxin (TTX), and thegray trace shows current recorded 160 sec after removal of TTX.