The predictable swimming sperm featured in health class videos are rather uncharacteristic portraits of the life of these sex cells.

Spermatozoa actually spend most of their time resting up in the male reproductive tract so they can make a dash to the egg once the opportunity arises. It has long been known that the little swimmers get their cue to start moving from a big change in pH levels, but a new study explains just how that shift occurs: a specific molecule in the sperm cells' tails releases protons when the time is right.

"The concentration of protons is extremely high at all times while the sperm are in the male reproductive tract, which makes the intracellular sperm environment acidic and inhibits the activity of the sperm cell," Yuriy Kirichok, of the University of California, San Francisco, said in a prepared statement. So releasing many of those protons from inside of the sperm cells lowers the internal acidity, a cue to the sperm cells to start flipping their flagella.

But how does that proton release happen? Previously, researchers had been unsure because they were not able to measure the membrane conductance on human sperm. But by using a whole-cell patch-clamp method to measure electrophysiological changes, Kirichok and his team were able to pinpoint the specific molecule—called Hv1—that opens up proton channels on the sperm cell membranes to release the extra protons. "The Hv1 channel, when opened, can allow protons to exit, and activates a cascade of biochemical reactions that cause the spermatozoa to move, mature and prepare to fertilize the egg," said Kirichok. This channel is signaled by increased pH and decreased zinc—both relative characteristics of the female reproductive tract compared to that of the male.

Another change in the environment that seems to spur on the sperm is an endocannabinoid that is common in both male and female reproductive tracts. The role of the endocannabinoid system also raises questions about the reported link between marijuana, which activates cannabinoid receptors, and male infertility. "Marijuana likely activates sperm prematurely, leaving them burnt out in a matter of hours," said Kirichok, who is a co-author of the new study, published online February 4 in the journal Cell.

A better understanding of sperm cells' swimming tactics might open the door to different methods of contraception. "You can imagine now that we know the molecule responsible we could block it to prevent activation and fertilization," Kirichok noted. Conversely, those spermatozoa needing a boost might some day get a little help with their Hv1 channels.

Image courtesy of iStockphoto/wdi2007