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I am currently reading "Electric Fields of the Brain" by Paul Nunez, and encountered this passage:

The synaptic inputs to a neuron are of two types: those that produce excitatory postsynaptic potentials (EPSPs) across the membrane of the target neuron, thereby making it easier for the target neuron to fire an action potential and the inhibitory postsynaptic potentials (IPSPs), which act in the opposite manner on the output neuron. EPSPs produce local membrane current sinks with corresponding distributed passive sources to preserve current conservation. IPSPs produce local membrane current sources with more distant distributed passive sinks.

Can somebody explain why EPSPs would produce current sinks instead of sources (and likewise why IPSPs would produce a source)? I was under the impression that EPSPs would propagate a signal, instead of terminating them, making them act more like a source.

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Sinks and sources just refer to the sign of the local field potential measured with extracellular electrodes.

Excitation involves positive charges entering cells, depolarizing them. When positive charges move into a cell, there is less positive charge outside the cell where the electrode is, so it becomes more negative. This is called a "sink" because the electrode records a negative deflection.

Inhibition is a bit more complicated because shunting inhibition does not involve much net movement of ions, rather it involves opening chloride channels with reversal potential near the resting membrane potential, which prevents cation channels from contributing to depolarization (sodium coming in the cation channels is balanced by chloride coming in the inhibitory channels).

Charge flows in circuits, so when you get negativity in one place you get positivity elsewhere and vice versa. If you stimulate a cell near its soma such that positive charge enters the cell there, the positive charge will flow into the rest of the cell but also attract negative charges towards it. The result is that membranes out on distal portions of the cell become slightly (because this is over a wide space) more positive inside, which in turn repels positive charge outside the membrane so an electrode near the fringes of a cell but not near the stimulation site will actually record a source. Generally in CSD recordings these types of sources dominate over inhibition, it's very difficult to record a "current source" and attribute it to inhibition.

I attached a few references below; I think the Olejniczak paper is a good place to start. It's focused on EEG but will help understand how what goes on inside neurons affects what we measure outside of them. Nicholson & Freeman talk about the theory behind current source density and is probably a straightforward read if you're familiar with the physics/math of electricity. Mitzdorf is a longer review article that talks more about the information you can get from a CSD and talks about the difficulty in interpreting sources that I mention.


Mitzdorf, U. (1985). Current source-density method and application in cat cerebral cortex: investigation of evoked potentials and EEG phenomena. Physiological reviews, 65(1), 37-100.

Nicholson, C., & Freeman, J. A. (1975). Theory of current source-density analysis and determination of conductivity tensor for anuran cerebellum. Journal of neurophysiology, 38(2), 356-368.

Olejniczak, P. (2006). Neurophysiologic basis of EEG. Journal of clinical neurophysiology, 23(3), 186-189.

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