Electric organs are stacks of modified muscle cells, and each electrocyte face is essentially an enormous synapse. That concentrates acetylcholine receptors and the machinery of chemical transmission far beyond anything obtainable from ordinary muscle.
The first isolation, in 1970, used the electroplax of the electric eel Electrophorus electricus and a snake-venom toxin that binds the receptor tightly. Torpedo tissue followed within a year and then took over, because its postsynaptic membrane is denser in receptor than any other known material. Purification at scale, the sequencing of the subunits, and the structure resolved by electron microscopy of Torpedo membranes all rest on the ray. Synaptic vesicles isolated from the same organ underpinned much of what is known about neurotransmitter release.
The rays involved were mainly Torpedo marmorata in Europe and Torpedo californica in North America. The result is that a receptor central to myasthenia gravis, to nerve agents and to nicotine addiction was worked out largely not in a mammal but in a flat, slow, nocturnal fish that stuns its dinner.