Minutes of Lab Meeting, 1/28/00
Given that you have two electrodes, one near your neuron in question, and one at a distant reference (equipotential), the following phenomena will ensue as you approach the cell.
A source is where positive charges emanate, and move towards your electrode. (A veritable fountain of postive charge.)
A sink is where positive charges are sucked up, hence they are moving away from your electrode.
Whether a source or sink is active depends on whether the site experiences change in conductance. Hence if an action potential is initiated at the axon initial segment near the soma, the soma can be considered an active sink, and the dendrites and axons would be called passive sources.
By understanding sources and sinks and making the assumption that the extracellular fluid is of constant non-zero resitivity, and that the electrode approaches an axon, you can predict the following extracellular potential shape as the action potential conducts:
Orthodromic potentials are due to synaptic inputs upon the soma and dendrites and propagate down the axon.
Antedromic potentials are caused by invasion of the soma by action potentials travelling 'backwards' up the axon into the soma.
Both are considered to originate at the axon initial segment.
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