What is the maximum electric potential that can occur in the human body?
To the best of my knowledge, the potential difference across the hair cells in the inner ear (cochlea) (Fig. 1) is the highest in the (human) body. It is about 120 mV, mainly due to the exceptionally high positive potential of the scala media as provided by the stria vascularis, referred to as the endocochlear potential. This potential is mainly built up by the high amount of K+ pumped into the scala media by ion pumps located in the marginal cells and basal cells (Salt et al, 1987) in the stria vascularis. Adding the potential difference of -40 mV of the hair cell it yields a total difference of 120 mV (Fig. 2).

References
- Morrill &amp; He, J Otology (2017); 12: 151-64
- Salt et al., Laryngoscope (1987); 97(8 Pt 1): 984-91


Fig. 1. The cochlea. source: Morrill &amp; He (2017)


Fig. 2. The potential difference across the hair cells is 120 mV, due to the endocochlear potential of +80 mV in the scala media. source: University of Minnesota Dulluth