Is it possible to temporarily paralyze someone through the use of electric signals or focused ultrasound? I've been reading about how it is possible to send signals to the brain using focused ultrasound or electrical impulses. It is possible to make someone see a certain shape or color by stimulating different areas of the brain. I was wondering whether or not it would be possible to temporarily shut off a person's motor functions by for example, sending signals to the cerebellum or perhaps intercepting them before they can travel down the spinal chord.

With in-depth study and research, it would be possible on 1 person. However, it's unlikely that this type of technology could ever be weaponized.

Nerves and neural networks are particularly known for having unique calibrations per individual. Even if you can decipher every neural communication protocol on a test subject, the signal protocol would be at least slightly different on others, even for crude movement control.

Mind control would be several orders of magnitude harder - each person's brain is different.

Most neurons communicate through neurotransmitters rather than electricity, completely ignoring your control scheme.

Even if none of these barriers existed, people could always block them using "tinfoil hats".