groups of nerves and blood vessels that run together. In men, bundles of nerves and vessels run along each side of the prostate and help the penis become erect. Removal or injury of these bundles during surgery, or damage from radiation therapy, can lead to impotence. Women also have these bundles, which run along both sides of the vagina and affect erectile tissue in the genital area. See also impotence, nerve-sparing prostatectomy, prostate.