When searching for the rheobase, a proper current amplitude range must be chosen. If the maximum current used is too small, no spikes will be produced. If too large, cell health might be compromised. Before starting the search, the cell's membrane input resistance (from negative current injections) can be measured and used to estimate the current necessary to activate the cell (e.g. if -10pA reduces the potential by 20mV, then a cell that rests at -60mV will likely spike at least once in response to +30pA injections).