The journal Nature printed a study into the possible effects of marijuana on Multiple Sclerosis (MS). MS involves the body attacking its own myelin, the sheath that insulates nerves. This produces tremors, paralysis, loss of vision and pain. There is no known cure.
Six different synthetic cannabinoids were injected into mice with a condition similar to MS, and five of them reduced the rodents' tremors and spasticity.

The compounds stimulated cannabinoid receptors on nerve cells, but not those within the brain, so presumably the mice didn't get high.

"We've tapped into and exposed this natural system," said David Baker of University College in London, who led the study. "That's why this is important."