I would have to disagree with "Iyela." But this is just my opinion. I think you can also have certain advantages applying to med school as a nurse. It really depends on many factors. First of all, 1. what college you graduated from 2. overall GPA & GPA in premed coursework 3. MCAT scores. These are the things that will allow you to make the first round of cuts and actually get an interview at all.
Next they will look at what makes you stand apart. As far as volunteer work--well if you work in a clinical setting, that will most likely hold the same weight, or a lot more I think, than someone who works as a volunteer. In addition, you have direct experience in the field, where as a volunteer really does not. You actually know why you want to be a doctor and not everyone does.They may have some patient contact, but not clinical experience.
Good Med schools require 3-5 letters. If you have doctors that you have worked with directly write some of these for you, and attest to your potential, that is going to be invaluable. Most undergrad science majors that are "hospital volunteers" will not have that. Their strength often times comes from academia and research experiences.
Where you feel that you may be weak on your application, you can make up for in other parts. If you did not go to a pregistious university, then you should take the extra time to ace your premed coursework and MCATs, even if is means waiting another year to apply. Each applicant is different, and diversity is critical in making yourself stand apart. There are more 22-year-old bio majors and less nurses, that is a fact that makes you more "diverse."
There really is no such thing as a "pre-med" degree. I graduated from a UC with a Biology major. It just so happens that the first 50 units of my course work overlapped with med school prequisites. It also happens that I sat next to 100s of people each day that were all "aspiring doctors." 1000s of them. 22-year-old Biology majors that have good grades, lab experience, and volunteer a couple of hours a week so that they can put it in their application. They are not more competitive just because they have done these things!
Sorry to write so much. This is something I have been thinking about too much lately. To Med school or not to Med school ...