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As an undergraduate student you can major in whatever you want. I'm not sure your major as an undergraduate will have much influence on your career as a neurosurgeon or another kind of doctor. Your goal as an undergraduate is to get into medical school, so any major in the sciences would work. What could help you become a neurosurgeon as an undergraduate though would be to get involved in some kind of research in that field if possible.
After completion of 4 years of med school, you are required to complete a minimum of 6 year usually 7 years of a residency program in Neurological surgery to become a neurosurgeon.