Hmm, I am by no means knowledgeable in matters of biology, but it seems to me you are somewhat confused about at least the terminology, if not the functionality.
The blood-brain barrier has no direct relation to nerve signal transmission. Its function is to essentially be a screen for molecules that have no business being in the brain.
The dermis is just a fancy name for skin. It also has nothing to do with receiving muscle signals; those signals, as should be probably obvious, terminate in the muscle they are supposed to actuate.
Now, if your *actual* question is "how can you measure the voltage difference between the brain origin of the signal, and its terminus in the muscle, the simple answer is: electrodes.
If the question is, can you detect a nerve signal from outside the body, there's a lot of variables involved. Maybe a question better asked in a biology forum.