Current source density analysis (which could more accurately be called current source and sink density analysis) is the practice of placing a microelectrode in proximity to a nerve or a nerve cell to detect current sourcing from, or sinking into, their plasma membranes. When positive charges, for example, flow quickly across a plasma membrane to the inside of a cell (sink) this creates a transient cloud of negativity in the vicinity of the sink. This is because the flow of positive charges into the interior of the cell leaves behind uncompensated negative charges. A nearby micro-electrode with substantial tip resistance (on the order of 1 MΩ) can detect that negativity because a voltage difference will develop across the tip of the electrode (between the negativity outside the electrode, and the electroneutral environment inside the electrode). 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, the inside of the electrode will become negative relative to ground for as long as the extracellular negativity persists. The extracellular negativity will persist as long as the current sink is present. Thus, by measuring a negativity relative to ground, the electrode indirectly reports the presence of a nearby current sink. The size of the recorded negativity will vary directly with the size of the current sink and inversely with the distance between the electrode and the sink.