How do muscle relaxants work? Do they act directly on the muscle and actually relax muscle tissue and ease spasms, or do they just prevent your brain from receiving signals that inform you of tight muscles? In the latter case, it seems like your brain would sort of become "immune" to the pain even though the muscle is still in a spasm. I'm obviously not a biologist, just curious about this. How do muscle relaxants work?

Check out the muscle relaxant article on Wikipedia, it's pretty straight forward. In short, there are two main types: Neuromuscular blockers, than act at the junction between the neuron and the muscle; and spasmolytics/antispasmodics, which (mainly) act on the central nervous system to reduce excitation or increase inhibition. Most of the ones I've heard of, such as diazepam, are in the latter category.