Neurons can generate a voltage, if there is an action potential. Also, membranes where an ion concentration gradient is present, generate some membrane potential. What I wish to know is, which cellular element in the human body creates the greatest potential differences? And how high can they be so high? Are there also biochemical reactions, where significantly high electric/electrostatic voltages are important? How strong are these?
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).
Fig. 1. The cochlea. source: Morrill & 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
The inner membrane of the mitochondria has a huge membrane potential generated by the pumping of protons as part of the respiratory chain.
In rat cortical neurons this value is around -140 mV and can variate between -108 and -158 depending on the metabolic activity of the cell.