Why does pain from a myocardial infarction radiate to the neck and sometimes down the arm?

It can: It can do either or both or neither.

Several reasons: The sensory nerves that innervate the heart are not like the nerves in finger tips-the pain is "visceral" and cannot be localized like a pin prick. It's perceived as spreading out from its origin into adjacent structures. Moreover, the inner arm shares the same dermatome, ie, the nerves send signals to the same segment of the spinal cord and then to the brain which interprets it as the same.