Ideally you want to have around 4-5 letters prepared:
2 science professor **
1 non-science **
[OR if your school offers a committee rec letter this replaces the 2sci/1 nonsci]
1-2 letters from professionals (volunteer coordinator/employer/research PI/physician you've worked closely with/etc.)
You want to get rec letters from professors who know you most and can speak to your academic abilities/ research/ involvement with campus or community. It seems like you already have a good one (cognitive neuroscience) but you'll need one more science. Also, when you go in to ask for these letters ask them if they can write you a STRONG letter of rec. The wording is important because you need the best letters you can get. Letters of rec can make a difference when it comes to receiving an II or getting that coveted acceptance. It would be awful if you received a rec letter with a professor saying they honestly didn't know you and only included your rank and grade in the class...
Also, AMCAS has instructions on how to submit the rec letter which you should print out and give to your writers. I made a personal guide on how to submit the letter out of a youtube video I found online. You should give your writers as much as you can to work off of, your personal statement, grades (this is if you want them to see your academics), resume/CV, and maybe include a page that says things relevant to your relationship with them.
Sorry...ALSO!! If you've gotten very close to a TA that knows you well, maybe you can talk to them about writing your letter alongside the professor. It could make for a strong letter!