All muscles and all neurons use transmembrane currents as signals for further cellular activity, contraction for muscles and neurotransmitter release for neurons.
These currents only go across the cell membrane and are carried by specific ions which pass through special proteins (channel proteins, some receptors) in the cell membrane.
In 1780, Galvani
stimulated in frog legs with electricity, causing them to contract.
The
electric organ in things like electric fish is is a stack of modified muscle cells where the ions only go out one side of the cell so that it operates like a battery. Because many are stacked in the same direction, they can provide significant voltages.
There are also obscure currents that occur during different developmental events:
Fertilization calcium wave in eggs.
Currents also flow through amputated limbs in some animals. Newts, which regenerate their limbs, have current flow through particular parts of an amputated stump. Frogs, which don't regenerate their limbs have current flow through different parts of the amputation stump.
There has been research on this (
for example) which shows an involvement with the regeneration process, but I am not current on it.
Applying current to organs and other things will have different effects depending on what you apply it to, if it is DC or AC, the voltage and amperage, frequency if DC.
Too much will kill cells and
can even cook things.
Strong enough AC currents will break cell membranes which will kill the cells if not rapidly corrected (too much calcium getting in can mess up the cells).
Electroporation used this in a very controlled manner to make small holes in cell membranes through which things like DNA can be introduced into the cell for experimental purposes.