The penis contains two chambers called the corpora cavernosa, which run the length of the penis. A spongy tissue fills the chambers. The corpora cavernosa chambers are surrounded by a membrane, called the tunica albuginea. The spongy tissue contains smooth muscles, fibrous tissues, spaces, veins, and arteries. The urethra, which is the channel for urine and male ejaculation, runs along the underside of the corpora cavernosa and is surrounded by the corpus spongiosum. An erection begins with either sensory or mental stimulation, or both. Impulses from the brain and local nerves cause the muscles of the corpora cavernosa to relax, allowing blood to flow in and fill the spaces. The blood creates pressure in the corpora cavernosa, causing the penis to expand into an erection. The tunica albuginea helps trap the blood in the corpora cavernosa, thereby sustaining an erection. When muscles in the penis contract to stop the inflow of blood and open outflow channels, the erection is reversed.