The enlargement of the penis fascinates every boy when it happens for the first time. It involves several body mechanisms to come together. The hypothalamus in the brain sends electrical signals of sexual arousal to the lower part of the spinal cord. Nerves in this area signal nerves in the pelvis to send blood through the arteries into the penis. If you want to directly arouse yourself, stimulating the genitals can also cause an erection. The male sex hormone, testosterone, helps trigger and sustain an erection.