Nipples don’t have muscle inside them — the erection of the nipple happens because of tiny smooth muscle fibers in the areola that contract in response to cold, touch, or sexual arousal.
The brain of adult men is approximately 10% larger than that of women. Because men are typically taller and have more muscle mass than women, their brains require more neurons to control the body.
Gonadal hormones like testosterone and estrogen play a major role in activating and maintaining libido (sexual desire) in both men and women. The influence of these hormones changes with age, and central (brain) factors become more important over time.
