The following video shows the famous "dead frog legs twitching when you salt them" demonstration:
As you can see, the chef is preparing frog legs for dinner. He has skinned the frog legs and they are ready to be cooked. When he salts them, the muscles start twitching. The obvious question: why does that happen?
Because these are fresh frog legs, the cells inside them are all still intact. The biochemical machinery still functions. There is still a source of energy for the muscles in the form of unused ATP molecules stored in the cells. All that the muscles need is something to activate them and they can still contract and relax (until they run out of ATP or something else shuts down the biochemical machinery).
Galvani was the first scientist to demonstrate that electricity can activate a frog's leg:
Salt can do the same thing as electricity because salt contains sodium ions. Sodium (and potassium) ions are used in cells to create voltage differences, as described in How Nerves Work:
The nerve signal, or action potential, is a coordinated movement of sodium and potassium ions across the nerve cell membrane. Here's how it works: 1. As we discussed, the inside of the cell is slightly negatively charged (resting membrane potential of -70 to -80 mV). 2. A disturbance (mechanical, electrical, or sometimes chemical) causes a few sodium channels in a small portion of the membrane to open. 3. Sodium ions enter the cell through the open sodium channels. The positive charge that they carry makes the inside of the cell slightly less negative (depolarizes the cell). 4. When the depolarization reaches a certain threshold value, many more sodium channels in that area open. More sodium flows in and triggers an action potential. The inflow of sodium ions reverses the membrane potential in that area (making it positive inside and negative outside -- the electrical potential goes to about +40 mV inside)
In a living frog, it is a nerve signal from the brain that tells the muscle to contract. In this case, it is the sodium in the salt. Once the sodium delivers the signal, here's what happens next:
At a larger scale, there are millions of these tiny biochemical movements working together to make the muscle move:
For more info on muscles, myosin and actin see: How Muscles Work
Just to lighten things up a little, the Swedish Chef prepares frog legs: