What has changed in your brain after electroconvulsive "therapy" (ECT)? From this Wikipedia article:

Electroconvulsive therapy (ECT), formerly known as electroshock therapy, is a psychiatric treatment in which seizures in the brain (without muscular convulsions) are electrically induced in patients to provide relief from mental disorders.1 Typically, 70 to 120 volts are applied externally to the patient's head resulting in approximately 800 milliamperes of direct current passed through the brain, for 100 milliseconds to 6 seconds duration, either from temple to temple (bilateral ECT) or from front to the back of one side of the head (unilateral ECT).

This procedure is applied to people (&quot;patients&quot;) who suffer from depression or bi-polarity (though I think the last is not a disorder, as is very exemplified in the movie &quot;One flew over the cuckoo's nest&quot;; it's a common feature of all human beings and we are taught at an already young age to keep our attention, concentration, actions, etc. between unnatural boundaries). Depression can emerge from such conditioning, from exterior impulses (seeing that Nature gets less and less, for example), maybe there is a genetic component. Exogenous and endogenous depression.
Most people relapse (as can be read in the article) before a year after the treatment and there are side effects (the most common adverse effects are confusion and transient memory loss).
In short: it's not an effective therapy in the long run.
In this advertisement of the Mayo Clinique (where ECT is still used) it is said:

ECT is much safer today. Although ECT may still cause some side effects, it now uses electric currents given in a controlled setting to achieve the most benefit with the fewest possible risks.

Of course, they put it that way out of commercial interest.
I think a combination of medicines with therapy does a better job in the case of depression.
All that I've written is just a preliminary for my question:
What happens on the neuron-level if ECT is applied to someone's brain?
There is actually quite some information on this. After a cursory Google Scholar search using the keywords &quot;long-term neuronal effect of ECT&quot; I found the following effects on the brain:

A week or two after the ECT, gray matter of depressed patients seemed to be increased in volume throughout the brain (Ousdal, 2020);
In rats, ECT resulted in behavioral improvements and neurogenesis in the hippocampus. Quite some of these new neurons survived up to 12 months (Vestergaard et al, 2017);
In mice, a single ECT triggered cortical snaptogenesis and a sustained neurogenesis, accompanied by behavioral improvement. Relapse occurred 40 days later. However, a 5-week continuation protocol following the initial ECS treatment led to a more persistent improvement of behavior and better survival rates of new neurons, but the authors did not investigate how long effects last after ECT was stopped fully (Jonckheere et al., 2018);

References
- Jonckheere et al., Brain Stim (2018); 11(6): 1336-47
- Ousdal et al., Biol Psych (2020); 87(5): 451-61
- Vestergaard et al., Hippocampus (2017); 27(1): 52-60