|MadSci Network: Neuroscience
The reason you are getting this reaction is a natural protection mechanism of the body to stress. The body releases adrenalin in response to stress, which causes several reactions in the body, including the contraction of the arrector pili muscles at the base of each hair, which is often called "goosebumps." The reaction is caused by the sympathetic nervous system to the adrenalin. In animals with a lot of hair, this reaction can result in more insulation from cold (which is why humans get "goosebumps" when they are shivering in cold environments), and it can also make animals look more menacing. But, humans do not have a lot of hair, so the function of this reaction in humans is not clear, and may just simply be the result of evolution. A person can get this reaction of "hair standing on end," or "goosebumps," in many types of situations. It can be caused by any kind of stress, including fear and anxiety, or even from emotional reactions to pleasurable stimuli (such as being kissed on the ear). What you are experiencing is a stress reaction, even though it is pleasurable. The smaller hairs on the ear are highly sensitive, since the ear is a very valuable sense organ. Thus, whenever something brushes against those hairs on your ear, your body detects a "foreign invader" and you get the "fight or flight" reaction of adrenalin. Evidently, you are getting this reaction more strongly on the side of your body that is being "invaded" so the reaction will be stronger. But adrenaline does circulate to stimulate the sympathetic nervous system all over your body. Therefore, you are probably also getting the reaction on the other side of your body, too. But you notice the reaction more on the side that is being "invaded," because you have the highest level of need to avoid the "invasion" and your body will put your brain on notice of the stimulus on the same side as your kissed ear, so you can direct your body away from the stressful stimulus. (Remember, the bodily reaction, and the adrenaline, does not cause a physical sensation of pain or pleasure. The pleasure of such a stimulus is a psychological reaction, not a physical reaction.)
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