Different sexual physiological responses of the breasts (illustrated)

The first evidence of increased response to sexual tension in the excitement phase of the sexual response cycle is the erection response of the nipples. This is the result of the abundant smooth muscle fibers in the nipple undergoing involuntary contraction after being subjected to sexual stimulation. The response of the two nipples often does not appear synchronously; one may have already reached full erection and swelling, while the other exhibits a lag phenomenon.