Is magnetic resonance microscopy appropriate for investigating ion-channel flow?
Clarification
I am unclear of your aim. What feature of elecrophisiology do you want to characterize?
What do you mean by large scale? E.g. Micrometers (μm),  Milimeters (mm) or Centimetres (cm).
Magnetic resonance microscopy imagings at Micrometers (μm).
Answer
An established way to observe neuron propagation is the use of calcium imaging. You can take a video under a fluorescent microscope to observe (Not measure) elecrophisiology. Also I believe this technique may provide the 'larger scale' you are seeking.
More On Calcium Imaging:
https://www.cell.com/neuron/comments/S0896-6273(12)00172-9