1. My Dog is Broken! A Case Study in Cell Signaling. By: Philip Wang and Emad Madha

2. How Erections Occur Nitric Oxide binds to a cell receptor (Guanylyl Cyclase) inside the cell. Guanylyl Cyclase is activated and GTP produces cGMP. cGMP binds to a regulatory subunit (PKG) and activates it. The activated PKG uses ATP and proteins to create phospho-proteins. The phospho-proteins help lower the concentration of calcium ions in the cell. This decrease of calcium ions causes the smooth muscle cells in the penis to relax, letting blood flow into the penis. Erection occurs.

3. Ginseng's Effect on Nitric Oxide Ginseng helps the body produce more nitric oxide. Since nitric oxide is the first messenger, having ginseng would help increase the nitric oxide available for use. This would lead to an increase of cGMP being activated, which would lead to an increase of PKG being activated. The increase of active PKG would decrease even more calcium ions, which would lead to relaxation of the smooth muscle cells. Erection occurs.

4. Viagra's Effect on PDE5 PDE5 is short for phosphodiesterase. Phosphodiesterase normally accepts cGMP and breaks it down as a regulator for this pathway. PDE5 inhibitors prevent phosphodiesterase from breaking down cGMP This results in an increase in cGMP, which would result in an increase of PKG Calcium ion levels would decrease and the smooth muscle cells would relax. Erection occurs.

5. Another Possible Cause If both ginseng and Viagra don't work, then the only protein left that would consequently affect the entire process would be PKG. PKG is a protein kinase and it has many essential functions that it carries out to induce erection.