But, during the rigid erection phase, contraction of the ischiocavernosus muscle and bulbocavernosus muscle do compress the spongiosum and penile veins and result in a measurable change in warmth and tone in the head of the penis. Without the thick tunica covering against which the blood veins of the corpora cavernosa are flattened, there is little increased pressure in the glans and in the third penile cylinder. However, the absence of any tunica albuginea in the covering of the glans penis and the presence of only a very thin tunica covering over the corpus spongiosum means there is no significant venous occlusion. During erection, blood flow certainly also increases to the corpus spongiosum tissue around the urethra and to the glans penis. Keep in mind that the major elements of the erection process are occurring in the two erectile cylinders, the corpora cavernosa. The penis then returns to the flaccid phase. Very rapidly the venous outflow is re-established. Sympathetic (adrenergic) stimulation causes contraction of the trabecular smooth muscle . A rhythmic contraction of the latter muscle is necessary for ejaculation. The somatic penile nerves, primarily in the engorged, supersensitive glans penis, contribute to the sacral nerves to stimulate the pudendal motor nerve which is responsible for contraction of both the ischiocavernosus and bulbocavernosus muscles. During this phase the pressure inside the corpora cavernosa may temporarily increase to several hundred mm of Hg due to contraction of the pelvic floor muscle outside the penis (ischiocavernosus muscle). This allows cGMP levels to remain high and continue their function as a smooth muscle relaxant that potentiates and maintains the penile erection.