The human body is a spectacular thing. While pain may not be its most popular feature, our ability to sense pain is vital–we avoid danger and discomfort with this crucial sense. Not all body parts feel pain equally, as anyone who has stubbed his toe can tell you.
While it may seem like we feel pain in our body parts, pain actually is felt in the somatosensory cortex of the brain. When pain receptors in our body receive the signal that something is wrong, neurons carry the message up to our brain, where the pain is felt. The neurons from each body part communicate with their own specific section of the brain. Neurologists can map out these sections on the somatosensory cortex, and it turns out that the body parts communicating with a larger slice of the cortex are more able to feel sensation and pain. For instance, the fingertips have a high nerve density; these nerves travel up to the brain to a large area of the cortex. This large area is proportional not to the size of the body part but to the density of sensory nerves.
The body parts most sensitive to pain are the lips, face, hands and fingers, feet and toes, and tongue.
The back is the least sensitive part of the body. To test it out, have a friend place one, two or three fingers on the palm of your hand. Without looking, try to guess how many fingers are pressing into your hand; it should be easy to guess correctly. Now have your friend place fingers against the middle of your back, and then try to guess how many fingers he’s using. You’ll notice it’s much more difficult. This is because the back has only a tiny section of the somatosensory cortex dedicated to its sensations.