Measuring the voltage used in nerve signal transmission from the brain to a muscle is complex and not directly related to the blood-brain barrier, which primarily regulates molecule access to the brain rather than nerve signal transmission. The dermis, or skin, does not play a role in receiving muscle signals, as these signals terminate directly at the muscle. To measure the voltage difference between the brain and muscle, electrodes can be used. However, detecting nerve signals from outside the body involves various factors and may require insights from biology experts. The discussion has been moved to a biology forum for further exploration.