I submitted 3 letters of recommendation-- two from professors of neurology and one from a professor of ob-gyn. One of the letters (in neurology) was from a professor with whom I had spent two summers doing research. From what others have told me, this is the strongest letter, most likely because this individual best knew me and my work. As long as you have at least one letter from neurology, I am not convinced that the letter writer's field of expertise is most important. I think it is crucial that the individual really knows you. While it might be useful to have a prominent neurologist at your institution write a letter for you, I believe that it is virtually useless if the letter does not indicate that he/she knows you. Another bit of advice-- individuals who have been around the institution and in academics longer tend to write better letters.