Re: Why the heart of a dead fish can start beating if you spread NaCl on it?

The heart is remarkable in the body in that it has its own system to cause the heart muscle to contract. Unlike skeletal muscle that is innervated only by nerves that pass messages on from the brain, the heart can continue to contract or 'beat' when it is denervated or removed from the body. This is call myogenic rhythmicity (myo = muscle, genic = origin, rhythmicity = rhythm).

The electrical activity or action potentials that initiate each contraction originate in a small area of the heart called the sinoatrial node (the hearts pacemaker). Action potentials are caused by fluxes of ions in and out of heart cells; these fluxes cause changes in the membrane potential or voltage difference between the inside and outside of the cell. The ions involved are potassium (K+) and sodium Na+. These ions are found normally in the heart and all other cells of the body and regulate many electrical events in the muscles and nervous system. A sudden influx of Na+ into the heart cells (and a concurrent decrease in K+ inside the cells) is the basis of the action potential. This electrical signal tells to the muscle cells to contract. The sinoatrial node is particularly sensitive to changes in voltage - we say it has a low threshold.

So you can imagine that adding table salt or NaCl will increase the amount of Na+ available to the cardiac cells, especially the sinoatrial node. This will cause a sudden influx of Na+ into the cell, causeing an action potential that will conduct throughout the heart muscle and result in the heart contracting or beating.

I think the reason you only see it in fish heart is that you gut a fish so soon after its caught that it's internal organs are still healthy or alive. You just don't get the chance to see the hearts of other animals so soon after they have died.

Hopefully I've been of some help.
Good Luck with your fishing!