The thecal sac is the casing that holds out brain and spinal cord. Kinda like a casing of a sausage. It sounds like the dsic protusion is putting pressure on the dura but not the cord whch is better. THe dura contains nerves and therefore you might feel a neurot sensation from this pressure. Sometimes when you get a sensation up and down the spinal cord it is a sensation going up and odwn the dura. The dura is a closed sac so pressure on one area means tension on another part.

THe nerve root pressure is probably what Jo described..a bone spur from the facet joint or vertebrae puting pressure on the nerve root.

Dont' know much about the foraminal space though.

Hope that helps.