Current sources and sinks
From Wikipedia, the free encyclopedia
Current sources and sinks are local electrical currents that flow from a location where they can't be detected into a location where they can be detected (current source) or vice versa (current sink). By analogy to the flow of water, a current source would be like a mountain spring. A current of water flows from a hidden location underground to the surface where it can be easily seen. A sink on the other hand would be like water flowing down a drain. It flows from where you can see it to where you can't. Electrical current can have the same basic properties. Current flowing from the "live" wires of an electrical outlet would be a source, while current flowing back along the common would be a sink (although with AC circuits this distinction gets a little murkey).
There is no actual physical difference between a current source and a current sink. Whether you call the flow of current a source or a sink depends entirely on your point of reference. If you were a troll living by an underground stream, the aforementioned spring would be a sink from your point of view, and a source from the point of view of an observer on the surface, even though it is the same current of water.
Current sources and sinks have particular relevance in current flow across biological membranes.
Shown at right is a general two-compartment model to help illustrate the definition of current sources of sinks. In this two compartment model, the compartments are separated by a semi-conductive barrier (gray). An observer, symbolized by the eye, can "see" only one compartment at a time. Red arrows indicate the direction of flow of positive charges, while black arrows indicate the direction of flow of negative charges. The pink and green backgrounds are meant to symbolize different times, time 1 when currents are flowing in one direction and time 2 when they are flowing in the opposite direction. The difference between the left and right panels is simply the location of the "eye".
A source or a sink is defined by which compartment is viewable by the observer.
- A source is:
- A flow of positive charges from the "invisible" to the "visible" compartment (i.e. "toward the eye"), or...
- A flow of negative charges from the visible to the invisible (away from the eye).
- A sink is:
- A flow of positive charges "away from the eye", or...
- A flow of negative charges "toward the eye".
In biology, the "barrier" would be a cell membrane. The two compartments are the inside and the outside of the cell. Generally speaking your point of observation will be outside the cell. Thus any flow of positive charges flowing into the cell will be a sink, and any positive charges flowing out of the cell will be a source. Note that when considering the flow of negative charges, the definitions are reversed.
[edit] Current sources and sinks in Neurobiology
Current sources and sinks have proven to be very valuable in the study of brain function. The study of sources and sinks falls into two general categories: Electroencephalography (EEG) and Current source density analysis (field potentials).
[edit] Current Source Density Analysis
Current source density analysis (which could more accurately be called current source and sink density analysis) is the practice of placing a microelectrode in close proximity to a nerve or a nerve cell to detect current sourcing from or sinking into their plasma membranes. When charges, let's say positive charges, 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 megaohm) 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 (the amplitude will decrease as the square of the distance).
