''The skin of the nipple is rich in a supply of special nerves that are sensitive to certain stimuli. When stimulated, the nerves send a message to the smooth muscle in that area to contract. When the muscle contracts, the nipple gets tighter, producing a nipple "hard-on" The stimulus can be physical (a loosely fitting shirt that moves back and forth over the nipple), or it can be a drop in temperature. It can even be a psychological stimulus, like thinking about something sensual or sexual. These special nerves are part of the autonomic nervous system, which is instrumental in helping the body prepare for sexual activity.
Whether or not one has erect nipples also depends on the individual. Some people experience them more often than others, just as some men have erect penises more frequently than other men. Levels of various hormones can also play a role, as they have effects on the autonomic nervous system, which is the part of the nervous system that controls the involuntary functions of internal body processes, such as heart rate and blood pressure.'' Phew! ;o)