I don’t know the muscles but maybe someone here can chime in with the specific names from this very unscientific description: on each side of my head kind of near my temples and on each side of my jaw near the corner(?) bone. (Really regretting never having taken an anatomy class...) If you put your finger on your earlobe and pull it maybe an inch forward I would say it was about there. My doctor kindly used some sort of very cold topical numbing spray before the injections which meant I couldn’t feel much. So in addition to my description being very unscientific it could also be slightly inaccurate.

I think my doctor was somewhat unique (at least at the time) for doing the temple injections to treat TMJ and I think the temple is one of the injection locations used now when someone gets Botox for migraines. I know there were additional possible injection sites he used for more severe cases or where there wasn’t complete relief but I can’t recall where they were because I didn’t end up needing them.

Could you be referring to the masseters? Those are the muscles you use for chewing.

Are there any negative side effects from this procedure, short term and long term?

How frequently does it need to be repeated?

We usually do masseter and temporalis muscles.

Any idea if it works for tinnitus?

Try and see whether you can relax your jaw (if you do it right your tongue would hang lower than usual and it may feel like saliva is produced at a faster rate), and see if tinnitus retreats after a while. If it does, I would speculate that the shots would then also work, though it may be helpful to find a psychosomatic approach first. (I'm not a doctor but familiar with such cases).