However, the molecular steps subsequent to CatSper activation by progesterone are obscure, though the involvement of trans-membrane adenylyl cyclase, cAMP and protein kinase A as well as soluble guanylyl cyclase, cGMP, inositol trisphosphate receptor and store-operated Ca2+ channel was proposed. Physiology Chemotaxis is a short-range guidance mechanism. As such, it can guide spermatozoa for short distances only, estimated at the order of millimeters. It is, therefore, believed that, in mammals, sperm chemotaxis occurs in the oviduct, close to the oocyte. First spermatozoa may be chemotactically guided to the oocyte-cumulus complex by the gradient of progesterone, secreted from the cumulus cells. In addition, progesterone may inwardly guide spermatozoa, already present within the periphery of the cumulus oophorus. Spermatozoa that are already deep within the cumulus oophorus may sense the more potent chemoattractant that is secreted from the oocyte and chemotactically guide themselves to the oocyte according to the gradient of this chemoattractant. It should be borne in mind, however, that this scenario may be an oversimplification. In view of the increasing number of different chemoattractants that are being discovered, the physiology of chemotaxis in vivo might be much more complex. References Further reading Michael Eisenbach (with contribution from Joseph W Lengeler) (2004). "Chemotaxis." Imperial College Press, London. Michael Eisenbach and Laura Cecilia Giojalas (2006). "Sperm guidance in mammals - an unpaved road to the egg". Nat. Rev. Mol. Cell Biol.'' 7, 276–285. Alvarez, L., Friedrich, B.M., Gompper, G., Kaupp. U.B. (2013). "The computational sperm cell". Trends in Cell Biol. Cell biology