1. A device for applying a desired current to a tissue membrane, said device comprising: an input lead electrically connected to the tissue membrane to provide a sensed signal indicative of voltage across the membrane; an input circuit that includes an analog-to-digital converter, and is responsive to said sensed signal and provides a digitized signal indicative of said sensed signal; a digital signal processor that executes selected program instructions to operate in a selected one of (i) a voltage clamp mode, (ii) a current clamp mode or (iii) a dynamic clamp mode based upon the state of a mode selection signal indicative of the selected operating mode, wherein said digital signal processor provides a digital current command signal indicative of the current to be applied to the tissue membrane; an output circuit that receives said digital current command signal, and generates a current signal indicative thereof; and an output lead electrically connected to the tissue membrane, which receives and applies said current signal to said tissue membrane.