In my view, while the single neuron is the basic anatomical and information processing-signaling unit of the brain, it is not capable of generating behaviors and, ultimately, thinking. Instead, the true functional unit of the central nervous system is a population of neurons, or neural ensembles or cell assemblies.
I think the word 'soul' - it depends on what you define by 'soul.' If you're defining it as a mystical aura that is outside the body, I cannot measure that. If you're talking about soul as human nature, the rank of spectrum of behaviors and reactions that we know humans produce under certain circumstances, that is measurable.