Is this due to pressure differentials in the surrounding tissue? (Is it possible to have a pinched nerve without compression of the surrounding tissues, and does this cause pain?)
What are the molecules/cell types involved in this process? Is the pain dependent on the compression of axons from mechanosensitive or nociceptive neurons?
Typically nerve compression or a "pinched" nerve is due to inflammation in the tissue through which a nerve passes. The gap is already quite narrow so any inflammation is quite potent. Inflammation causes you to feel things as more painful (hyperalgesia). In a similar way to how of you burn your hand then poke it, it hurts, when cells are damaged the receptors surrounding them are sensitised to be more reactive. Binding of many ligands involved in inflammation leads to higher than normal sodium influxes which means you feel pain as this impulse is sent. I could be a lot more detailed but not sure what level you want, however for much more detail see this review.
As AndroidPenguin described the nociceptive pathways are activated by inflammation or noxious chemicals.
Sometimes pain can arise independent of active nociceptive pathways. Most evident in cases of Neuralgia and perhaps in case of Pseudoneuromas.
In certain cases the injured nerve causes disinhibition of the pain pathways arising from the dorsal horn of spinal cord. This disinhibition is because of loss of the nerve function. Sometimes increased nociceptor drive can activate dorsal horn of spinal cord which will now respond to normal mechanosensation also [1].
Phenotypic switching is also possible [2].
PS: Both these articles are inaccessible to me at this moment. Refer them for more details.
