Since undergraduate (= bachelor's) comes before medical school and medical school before neurology residency, you can actually major in anything! (Medical school itself has no "major".) You just need to do very well in your pre-med courses (biology, chemistry, basic physics, and usually a bit of math.)
That being said, if people are already interested in neurology as undergraduates, they typically obtain a B.S. in Biology, or even Neuroscience if their college offers such a degree. But you should really just get a bachelors in something you love. Neurology residency programs are going to look at how you did in medical school, and will be much less interested in your B.S. or B.A.
I am a neurologist and I double majored, B.S. Biology and B.A. Philosophy.
I received a BS in combined science. Many premed students get a degree in a science field such as biology. Medical school was 4 years. I did an internship, which is the first after graduation from medical school in internal medicine. The neurology residency is then 3 more years. I then did a one year fellowship in Neurophysiology. This fellowship gave me more expertise in the field of epilepsy.