If you want to know how the body works, you have to know about anatomy, or how the body is structured, says Jannini, who has co-authored several studies on the G spot.
But 12 years ago, when he turned his attention to the topic, even the anatomy of this storied erogenous zone was a big question mark.
"We realized that the anatomical description of the female body was absolutely not correct and not studied for 500 years — something like that. Nobody was really coming back, using modern technologies on the female body," he says.
The G spot is popularly known as a sensitive area felt through the front (bellyside) vaginal wall that responds to erotic stimulation, but sex researchers now increasingly tend to view it as a series of related structures that can be stimulated, directly or indirectly, by means of pressure on the front vaginal wall.
Recent studies, including autopsies and microscopic studies by Jannini and other researchers, outline a G spot area that includes tissue from the inner clitoris, which extends upward in two branches, embracing the urethra and vagina, as well as glands muscles, and nerves from the front vaginal wall. The urethra appears to play a major role. The G spot area may begin to swell when stimulated and produce intense orgasms, Jannini wrote in a 2010 overview on the topic in Controversies in Sexual Medicine. And Jannini says the region contains the same biochemical machinery of excitation as the male sexual organs, including enzymes linked to male arousal.
The G spot as a center of erotic sensation remains controversial, but in a study of 20 women in the Journal of Sexual Medicine in 2008, Jannini and his co-authors found that women with a thicker urethrovaginal space (basically, a G spot area minus the inner clitoris) were more likely to report experiencing vaginal orgasms.
"I'm sure something is there," Jannini says of the G spot area, which he calls the clitoris-urethrovaginal complex, but it's not necessarily there for all women, and certainly not in the same way. Jannini says some women find G spot pressure uncomfortable or painful, and he's autopsied normal women with G spots that were missing muscles, vessels, glands or biochemical mechanisms that are known to contribute to sexual pleasure.
He adds that the G spot variations he's seen in autopsies were dramatic, and the external clitoris remains the most reliable route to arousal for women.
"The women that have (a sexually responsive G spot) are not more lucky — they are just different. Like the color of the eyes, like all the differences that are present in the human body," Jannini says.
Questionnaires suggest that 40 percent to 54 percent of women have experienced the expulsion of fluid at orgasms, according to a 2007 study in the Journal of Sexual Medicine.
So are they ejaculating? The phenomenon remains controversial among scientists. A 2011 study published in the Journal of Sexual Medicine suggested that there is a difference between the expulsion of clear fluid through the urethra ("squirting") and the expulsion of a small amount of whitish fluid through the urethra ("female ejaculation"). Researchers collected fluid from a volunteer who emitted both substances, and found that while the clear "squirting" fluid resembled watered-down urine and probably came from the bladder, the "ejaculate" bore a biochemical resemblance to semen and appeared to come from the Skene's glands, which were officially renamed the "female prostate" in 2002, according to the Federative International Committee on Anatomical Terminology.
The bottom line: Even the definition of female ejaculation is still up for debate.