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I find what you say about itch interesting. There is a lot of talk about itch being psychological, but that doesn't mean its "all in the head." Being stressed or anxious causes physical processes in the body to occur. It is thought that stress can cause mast cells to release histamine, which is a cause of itch. In this way, the sensory neurons are receiving input, but not from a direct external source. Although it is interesting to speculate on the reasons for why simply thinking about itch can cause one to experience it. For example, I took a Pain class last semester, and we spent a class discussing itch. If you looked around the class, everyone was surreptitiously scratching themselves. It was one of the hardest classes to focus in because of the distraction thinking of and experiencing this itch caused.

Like mentioned in a reply to this post, pain (i.e. scratching) can inhibit itch. There is a pathway that is used to detail this, which is included on this page:

http://www.nature.com/neuro/journal/v4/n1/full/nn0101_9.html

This is one of the reasons why when patients are given opiates for chronic pain, they tend to develop itch. When mechanical stimuli and pain can inhibit the itch pathway, but when they are inhibited, the itch pathway is free to transit to the dorsal horn and up to the brain. Some patients stop taking the opiates because they say that constant itch is even more miserable than the constant pain. Some people actually have what is called chronic pruritus, where they experience constant itch.