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With electrostimulation or functional electrostimulation, a response from the neuron is generated by electrical impulses.

But I always read, for example, that a stimulating electrode is above position xy of the neuron (e.g. above the axon), but in order for current to flow, there must be a closed circuit? i.e. if cathodic stimulation is used (e.g. with a -0.5mA pulse lasting 0.05ms), then there must be an anodic pole somewhere so that the current can flow, or am I misunderstanding something?

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Current flows from a high potential to a low potential, so if you inject a great deal of current via an electrode it will still flow into the body. However, even in the case of unipolar setups for deep brain stimulation (DBS), a second electrode is used, but relatively far away from the first one. Doing it this way is quite uncommon though: most electrical stimulation uses two poles (or even more), as it limits the spread of current to unwanted areas.

Monopolar vs bipolar deep brain stimulation

Transcranial direct current stimulation (tDCS) will for instance use an electrode on one side of the scalp and one on the other side.

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  • $\begingroup$ There's always a second pole, even in monopolar stimulation, it just may not be particularly close to the first one. $\endgroup$
    – Bryan Krause
    Nov 24, 2021 at 14:53
  • $\begingroup$ @BryanKrause Although physically the current would flow, my answer was a bit misleading with respect to what you say, as a second electrode is indeed used even for monopolar stimulation, so I've amended it. $\endgroup$
    – David Cian
    Nov 24, 2021 at 15:48

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