I'm in a biology PhD program... I focus on cell bio, but a lot of my peers do neuroscience, so I can answer questions if you have them

There's no need for an MD if you want to go into research. If you want to run your own lab someday you'll need a PhD (5+ years) and several years as a postdoc. Some people do MD-PhDs, a special program that gives both degrees, if they really want to focus on medical research, but that's not necessary.

To get into a PhD program you need a BS in a related field, some kind of research experience usually (required to get into the better programs, but not required for all programs... some people spend time as a lab tech or do a masters to get this experience if they weren't involved in research in undergrad), and reference letters. Oh and probably retake the GRE. I didn't take the subject GRE, just the general one, though a good score on the subject GRE could help if other areas of your application aren't as strong.

The good news is most PhD programs in STEM fields fund you. The program pays for your tuition and gives you a stipend. It varies a lot by program, and mine is one of the better funded ones, but for example I get all my tuition paid, really good health insurance paid for, and an additional stipend of $33k to spend on rent, food, etc (though I'm in an area with a high cost of living... this is a lot lower in areas with a lower cost of living, and lower at less well funded universities... I would say the average is maybe around $20-25k)

The lab work in a PhD program isn't lab work like a class, it's research work. Usually you "rotate" in the first year, which is like speed dating with different labs, and at the end of the year you pick which lab you want to be in for your PhD (the PI/prof who runs the lab also has to agree to take you). It's not really like a BS where you can take the courses ahead of time without being in the program, you have to get accepted to the program to do the course work.