Why does mechanical pressure stimulate nerves?

Hi there,

I am not talking about mechanoreceptors or nerves that connect to mechanoreceptors.  

Rather I am talking about something like sciatica (where mechanical pressure on the sciatic nerve due to spinal disc herniation causes pain).  Or, another phenomenon that I think might be an example of the same thing, when I apply light mechanical pressure to my eyes it stimulates my optic nerves (right?) and I see "light".

So it seems that mechanical pressure stimulates nerves.  But how, in terms of the resting potential and action potential mechanisms of nerve cells, does this occur?  Does mechanical pressure cause voltage-gated channels to open?  Or is there some other mechanism?