how do we feel electricity on skin even though merkel cells are pressure sensitive only?

Question

Note: Here I am not talking about electric shock (one feels when a lot of electricity passes through), I am talking about smaller sensations one feel directly on the skin.

Note: I am not able to find any correlation graph between feature of electricity that determines the intensity of feeling i.e., voltage, current or frequency. So is there any research paper or anything else around that can shed light on this? This will be very helpful.

I have studied that humans sense touch because of the "Merkel cells" in the skin, and Merkel cells are essentially pressure sensors. I don't think electricity leads to pressure changes, So then how do we feel the sensation of electricity on skin. Is it because electricity directly interface with neurons some how?

If I am not able to explain the question here is a more practical way of thinking about it:

If I touch two electrode close on the skin (or maybe possibly one even though I am not sure one can do the trick) and pass electricity how will skill feel it.

Answer 1

In the skin, there are 4 types of tactile receptors, dependent on whether it is glabrous or hairy skin;

1. Meissner's corpuscles
2. Merkel cells
3. Ruffini endings
4. Pacinian corpuscles

Ruffini endings are thought not to be sensitive to electrical stimulation, the other 3, however are. These other 3 can readily be electrically activated and each type can be separately targeted by changing stimulus and placement parameters. Activation is normally due to pressure differences. However, the nerve endings are basically axons, and these can be activated electrically as well Kajimoto et al. (2002).

Some parts of the body have high densities of these tactile receptors, such as the fingertips and the tongue. That's why electrical stimulation has been applied as a means to convey information via the skin. Take for instance the BrainPort device (Fig. 1); this device consists of a camera hooked to a tongue-display unit (TDU). The TDU translates camera pixels into tactile images on the tongue, brought about by 144 electrodes. It is designed to substitute vision for blind people. (Stronks et al., 2016).

^{Fig. 1. BrainPort device. Source: Nau et al. (2014)}

<sub>References
- Kajimoto et al., Electronics and Communications in Japan, 85(6) 120-8
- Nau et al., Front Hum Neurosci (2014): fnhum.2014.00291
- Stronks et al., Exp Rev Med Dev (2016); 13(10): 919–31</sub>