Galeno (Pérgamo, 129-Roma, c. 201/216) set up as an irrefutable theory, during centuries, the idea that nerves were hollow tubes, as blood vessels are, in wich a volatile content, of unknown nature, flowed.
This content was formed in the ventricles, and the term he used to refer to it was “spiritus animalis” (animal spirits).
The pneumatic-ventricular model, accepted for centuries, preached that muscular movements and the expression of emotions, were originated in the ventricles. Together with the medieval theory of the three ventricular cells, this was basically all the neurophysiological knowledge that had been condensed over the years.
This was the state of the field when René Descartes (La Haye en Touraine, 31 March, 1596 – Stockholm, Sweeden, 11 February, 1650), found himself trying to understand the mind. He tried to substantiate this neurophysiological doctrine, based on the mechanicist spirit that he adopted from his peer Galileo Galilei.
Descartes elaborated a complex theory about the physiology of the nervous system based on the agustinian dualism that separated the body (res extensa) and the spirit (res cogitans).
In this hypothesis, communication between will of mind and movement of the body was possible through the spirtus animalis, but in order to happen it was necessary that human soul had a biological and tangible origin. This corporeal location was the pineal gland.
In the middle of the XVIII century, the idea of the nerves as hollow tubes was still accepted, and there were numerous investigations that tried to unravel the nature of the substance that run across them.
Alexander Monro (19 September, 1697 – 10 July, 1767),
the founder of Edinburgh Medical School, experimented from different perspectives without finding a specific answer, and he concluded that it was improbable that any of the substances that were candidates to be the spiritus animalis were able to move fast into the narrow tubes that the nerves were considered to be.
Isaac Newton (25 de diciembre, 1642 – 20 de marzo, 1727) , backed up this idea, and he proposed that vibrations of the filaments that formed the nerves were the answer.
Electricity, at this point, was a hot topic, for scientists and the general public. It awoke curiosity in all the possible aspects. It was the protagonist in laboratories and popular fairs.
It didn’t take long to identify electricity as the “substance” that could flow through the nerves. At the beginning it was rejected because nerves didn’t seem to be insulated. In the case of a source of electricity in the organism, the lack of insulation of the conductors would makethe electricity propagate and wouldn’t follow the course of the nerves.
In 1780, Luis Galvani, made a serie of experiments that were proof enough to afirm that nerves and muscles needed electricity to work.
“We think it’s posible that the electrical fluid is the product of the work of the brain and that it comes from the blood, going through the nerves and running inside them”
In one experiment, he put a zinc band in the sciatic nerve of a frog and connected that with the muscle beside it with a silver clamp. When this circuit was closed and an electric discharge happened, the muscle always contracted.
In other experiment, he demonstrated that the frog’s leg also contracted when the muscle and the nerve were pierced by a hook made of bronze and the muscle was put in contact with an iron fence.
For Galvani, this two experiments not only proved that the muscle could be excited with electricity, but they also showed the fundamental nature of the relationship between the muscle and electricity. So he supported the idea that muscles had inner electricity. He introduced the term “animal electricity”.
Galvani developed a theory to explain his observations, it was very elaborated, and it went further than whan could be read from the experimental facts.
Here is where Alessandro Volta, an eager reader of Galvani, stepped in. He understood Galvani’s discoveries in a completely different way. For him, the connection between the two different metals was the only thing originating electricity, the muscles and nerves didn’t play an active part, the were just mere detectors, not generators. For Volta, animal electricity was a non sense.
This two interpretations seemed irreconcilable because it was not possible, at the time, to measure the electricity running through the frog’s tissue. So the two opposing ideas coexisted finding its own supporters and detractors each.
Carlo Matteuci, one of Galvani’s most renoun heir, improved the “frog galvanoscope” and he could determine precisely enough, the voltage in the frog’s leg.
He concluded by his measurments, that there was an electric current flowing from the interior to the exterior of all muscles. This idea was well accepted for his collegues at the time, but in the present is explained as injury potential.
Later, in 1843, Emil Du Bois-Reymond, described the current going through the nerves after an electrical stimuli. In 1849, he amplifies his discovery by adding chemical stimuli to the range of stimuli that can produce a current in an organism. It was stablished, then, that the nerves where not just passive, they were also implicated actively in an electromotor point of view. This made the idea of the hollow tubes fade away.
The idea continued its development and acquired shape. Nowadays, this contribution and several others, make us understand that, there is, in fact, electricity in the nervous system, and the action potential, later introduced, is actually the basic language that it uses.