While current sources or sinks are abstract elements used for analysis, generally they have physical counterparts in real-world applications; e.g. the anode or cathode in a battery.
When positive electric charges of dissolved ions, for example, flow quickly across a plasma membrane to the inside of a cell, this leaves behind a transient cloud of negativity in the vicinity of the cell exterior, a so-called "sink".
Put another way, the electrode internal solution will donate some of the positive charge needed to compensate the negativity caused by the current sink.
Thus, by measuring a negativity relative to ground, the electrode indirectly reports the presence of a nearby current sink.
Current sources and sinks have proven to be valuable in the study of brain function.
Some special considerations are needed to apply these methods to brain tissue, where the electrical resistance is non-uniform.