Neuron activity on body parts measurable?
To measure single-unit neuron activity you need to use a microelectrode system, but it is very invasive. How useful the information collected depends on what you searching for. If you interest in studying the neurophysiology of neurons its good way to go. If on the other hand, you are interested actual outcomes stimulation then the average neural response is actually more useful.

Computational work often uses these neural averages as 'the' signal. In the end synchronicity of neurons working together is what really matters in achieving higher order functions. 

A good book I have found  " Theoretical Neuroscience: Computational and Mathematical Modeling of Neural Systems " by Dayan P., Abbott, LF., (2005) 
really insightful into how brain activity is measured and analyzed. Prior knowledge in Calculus and Statistics will greatly help make sense of the math, but even without it is enlightening.