This is a great question. I do believe that there should be a pay disparity for several reasons.
First, the amount of training differs. Nurses can go to nursing school and finish in 4 years and get their nursing licence. To be sure this is a hard program, but the program for doctors differ. After an undergraduate course of study, there is four years of medical school and then residency, which is a minimum of three years. For a specialized course of study, you can add a few more years.
Second, there are different types of nurses and the pay scale differs, which is an important point to keep in mind. Nurse practitioners have additional education and hence they make more money. There are also people called physican's assistants who can make more money as well. The general rule is the more education, the more the salary.
Finally, since doctors are ultimately reponsible, they should get paid the most.