So after hours of researching myself and finding the name of the company that develops the TPMS for the 2007 Murano (and for a lot of the cars in the American market), the valve stem, unfortunately, is integrated with the TPMS transmitter itself. If the battery dies, you need to replace the unit. If the valve stem is not working properly, you need to replace it, etc etc.
Fortunately, my warranty covered for the replacement, but for those who's don't, it is expensive as hell to replace (it ranges from $80 to $100). Even if you were to replace the transmitter(s) yourself, you would still need to purchase an Test & Learn tool for about $300 to have the new transmitters assigned to your car or go to a Nissan dealership and have it done there.
Hope this post helps de-mystifies some of the information about the TPMS on the 07 Murano. And here is the company's website
http://www.schraderbridgeport.com/