channels open, allowing positive ions to rush into the cell (the so-called funny current), triggering another depolarization event and subsequent cardiac contraction. This gives the heart its automaticity. The primary cyclic nucleotide operating in conjunction with the HCN channel is cAMP. Olfactory sensory neurons Almost all responses to odorants in olfactory sensory neurons (OSNs) are facilitated by CNG channels. When an odorant binds to its specific receptor in the chemosensitive cilia membrane, it activates a G protein, which causes a downstream reaction activating the enzyme adenylyl cyclase (AC). This enzyme is responsible for an increase in cAMP concentration within the OSN. cAMP binds to the CNG channels in the OSN membrane, opening them, and making the cell highly permeable to Ca2+. Calcium ions flow into the cell causing a depolarization. As in all other cell types, CNG channels in OSNs also allow Na+ to flow into the cell. Additionally, the increased Ca2+ concentration inside the cell activates Ca2+-dependent chloride (Cl−) channels, which causes intracellular Cl− ions to also flow out of the cell augmenting the depolarization event. This depolarization stimulates an action potential that ultimately signals the reception of the odorant. In addition to cAMP gated ion channels, a small subset of OSNs also has cGMP-selective CNG channels that contain the CNGA3 subunit. Spermatozoa cAMP and cGMP mediate several cellular responses such as acrosomal exocytosis, or the fusion of the male sperm to the female egg, and chemotaxis. In the sea urchin species, Strongylocentrotus purpuratus, speract, a short peptide, was studied. Speract activates a receptor-type guanylate cyclase (GC) and stimulates a rise in intracellular cGMP concentrations. Speract also increases the concentration of calcium. Although there has yet to be any establishment of a direct causal relationship, the previously mentioned observations suggest that cGMP activates calcium conductance. CNG channels are prime candidates for the calcium-entry pathway, due to their high calcium permeability. CNG channels have yet to be detected by homology screening. In mammals, the testicular CNG channel subunits that are expressed are A3, B1, and B3. Heterologous expression of the A3 subunit was cloned from testis and produced channels that were cGMP sensitive and selective. It is possible that these channels are involved in a cGMP-stimulated calcium flux into the sperm. However a more extensive characterization of the channel has not been accomplished due to the low success rate of detecting channel activity. Since A3 subunit knockout mice are fertile,