Rebecca, I conduct pain research, although not on descending control.
A good place for you to look is in a new book by Alan Light, "The Initial
Processing of Pain and Its Descending Control", Karger, 1992. The last
chapter is a review of descending control mechanisms related to nociception.
The book is outrageously priced (>$200), so I would recommend a library first.
But, this is a very good review, as recent as you can expect. Joel Greenspan
