So let's say you can force a neuron, or group of neurons, to open their ion pumps and release an electric signal. If a neuron suddenly fired off a signal, would this cause a small disruption in the nervous system and stun the victim, or would it cause something like a seizure?
As a medical student: A single neuron in the brain will go almost completely unnoticed, if you could fire off several neurons in a coordinated fashion you could cause everything from a light twitch of a muscle to weird sensory experiences to a full grown seizure.
A full epileptic seizure is in fact just a chain reaction starting with a small area of the brain firing its neurons uncontrollably then spreading to the rest of the brain resulting in a state referred to as status epilepticus. It would be a very effective method of rendering someone temporarily harmless.
There are also other kinds of seizures which could result from this ability and I would suggest you look into the different kinds of epileptic attacks there is and choose whichever you see fits best for the purpose of your story. :)
So from what I can tell, a single neuron, or even a lot of them misfiring would not cause what you want, unless it happened in just the right place.
Neurons actually misfire somewhat regularly, and while they can send mixed messages to your body, they don't usually cause you to be stunned or worse.
There is evidence that frequent heavy misfiring is a contributing factor for OCD, as well as autism, Parkinson's, epilepsy, and other mental diseases, depending on where and how they are happening.
If you were to target a specific site, such as the cerebellum to effect motor control, or the mesial (middle) part of the temporal lobe, which could cause an epileptic seizure, and get a large enough group of them to go off, then it might happen like you want.
Essentially if you want to immobilize an attacker through neuron attacks you're basically talking about a stun gun or something with similar effects.
If you manage to get a large enough collection of neurons firing in the brain you could get anywhere from no effect to a heart attack to a seizure to curing the victim's depression or making them make a funny face (heh), etc... You'll be taking a gamble. Most likely though, if you get enough of them going in the brain, you'd cause a seizure. There are types of seizures that lead to just momentary unconsciousness, see absence seizures, if you could somehow magically control that.
You might want to try targeting e.g. the lower spinal cord instead, go for the legs. Or target muscles, like the aforementioned stun gun.