Whenever I have an erection I can make my penis basically "bounce". What muscle is that and why does it happen?
The muscle is called the bulbospongiosus muscle. An older name for the same muscle is the bulbocavernosus muscle. It is shown in the cut-away image on the right in red. The muscle has several functions:
- When you urinate, after the bladder empties, this muscle contracts to help expel the last of the urine from the urinary tract. You probably noticed it as a slight shudder right at the end.
- The muscle also contributes to erections by squeezing the vein that drains the penis, as well as keeping pressure on the cavernosium chambers to keep the blood pressure in the external part of the penis higher, thus keeping the penis stiffer.
- After ejaculation, this muscle also contracts in rhythmic waves during orgasm to squeeze any left over semen in the urinary tube.
This muscle is semi-autonomous; that is, it operates automatically, but you can cause it to contract with conscious thought.