What cut of beef is "plat nerveux"?
Different countries can have very different names for their cuts of meat, and in some cases, there isn't an obvious/direct equivalent from a French cut to an American or English cut. If you look at a French butcher's diagram, you'll see the lines and cuts don't correspond directly to a US diagram.

I believe that what you saw labeled as "plat nerveux" is likely "gite de noix" which in the US is "eye of round roast". "Nerveux de gite" is a braised beef dish that is kind of like an American pot roast. 

Eye of round can have a shape similar to a tenderloin, but is a tougher, less expensive cut of meat.