Often times the breakdown or damage in the sequence affects the arteries, muscles, and surrounding tissues of the penis, and this breakdown is most commonly the result of a disease. Diseases that commonly cause ED include: searching for cheap viagra? canadian pharmacy viagra for sale What better, Viagra or Cialis? canadian viagra for sale online Erection begins with sexual stimulation. Sexual stimulation can be tactile (for example, by touching the penis), or mental (for example, by having sexual fantasies). Sexual stimulation generates electrical impulses along the nerves going to the penis and causes the nerves to release nitric oxide, which in turn increases the production of cyclic GMP (cGMP) in the smooth muscle cells of the corpora cavernosa. The cGMP causes the smooth muscles of the corpora cavernosa to relax, and allow rapid blood flow into the penis. The in-coming blood fills the corpora cavernosa, making the penis expand. If one can naturally engage in behaviors that increase blood flow to the penis, it will have a positive effect in preventing erectile dysfunction, Sadeghi-Nejad said. Limited effectiveness