Balloonist theory

Balloonist theory was a theory in early neuroscience that attempted to explain muscle movement by asserting that muscles contract by inflating with air or fluid.

The Greek physician Galen believed that muscles contracted due to a fluid flowing into them, and for 1500 years afterward, it was believed that nerves were hollow and that they carried fluid.

René Descartes, who was interested in hydraulics and used fluid pressure to explain various aspects of physiology such as the reflex arc, proposed that "animal spirits" flowed into muscle and were responsible for their contraction.

1n 1667, Thomas Willis proposed that muscles may expand by the reaction of animal spirits with vital spirits. He hypothesized that this reaction would produce air in a manner similar to the reaction that causes an explosion, causing muscles to swell and produce movement.

Debunking
Also in 1667, Jan Swammerdam, a Dutch anatomist famous for working with insects, made the first important blow to the balloonist theory. Swammerdam, who was the first to experiment on nerve-muscle preparations, showed that muscles do not increase in size when they contract. He separated a frog's leg from the frog, but left the sciatic nerve connecting them, and put the leg under water. Irritating the nerve caused the muscle to contract and the leg to twitch. Swammerdam noticed that the level of the water did not rise: no fluid or gas could be entering the leg. Further, he showed that irritation of the nerve alone was enough to provoke movement, an important step toward the current understanding of how nerves actually cause muscle contraction.

Balloonist theory took a second hit from Francis Glisson who performed an experiment in which a man flexed a muscle under water. The water level did not go up (in fact it went down slightly), further supporting the conclusion that no air or fluid could be entering the muscle.

The idea that muscle is inflated by air was further debunked when Giovanni Alfonso Borelli slit the muscle of an animal under water and watched to see if bubbles of air would rise to the surface; of course, none did.

The invention of the microscope allowed preparations of nerves to be viewed at high magnification, showing that they are not hollow.

In 1791 Luigi Galvani learned that frogs' muscles could be made to move by the application of electricity. This finding provided a basis for the current understanding that electrical energy (carried by ions), and not air or fluid, is the impetus behind muscle movement.