In the first decades of the XXth century, relevant discoveries are organized forming a body of knowledge on how nervous cells work. This arised a lot of questions and set the framework for future investigations. The advance was unstoppable.

Neurons were known to be individual cells that do not touch. It was also a veridical fact, that they communicate via electricity, in what is called action potentials. The existence of a gap between them discontinues this signal, but when that happenes, a chemical substance is released, and later, received by a postsynaptic neuron, that generates the potential again.

All this, was a fine, yet, superficial characterization of the nervous system, more coherence was needed.

In this moment, scientists started, increasingly, to relate the brain to the processing and storage of information. Neurons transmitted this information, and the whole system was thought to be involved in emotions, memory and cognitive function.

Nevertheless, there was a gap between this assumptions and the empirical data available at the moment.
The neuron, its individuality, how it communicates, and the signals involved were outlined, but how this particular transmission of a signal, was related to the astonishing mental functions, was still a mistery.

A leap was necessary to jump from the function and connection of neurons, to the complex cognitive functions.

To tackle this problem, it seemed logical that it was necessary to put the individual neuron aside and focus on greater populations.

We have to go back to Ramón y Cajal, because he suspected that there were complex networks of neurons participating together to form a response. There was morphological variety among cells, and the similar ones group together. He tought, these group of cells might have a narrow relationship.

The neurophysiologist Charles Sherrington, was the first one to expound experimental evidence on a neuronal circuit.
He analyzed a reflex known since the middle of the XIXth century, the patellar reflex, and discovered a simple circuit underneath.

The speed of the answer after the force was applied in the rotula, was surprising. There were two interpretations of the phenomenon.

Sherrington explained that the information of the strike travelled to the spinal cord, and from there, a signal was sent to the muscle in order for the reflex to happen.

His detractors, said that the response was so quick, that it was impossible for a signal to travel to the medulla and then to the muscle. So, they understood that the reflex was the consequence of the mechanical tension applied in the strike.

Sherrington was aware that, in fact, the speed of the reflex was slower than the conduction of the nervous impulse, because this data had been supplied by Hermann Helmholtz. He knew it was actually possible for the signal to travel to the medulla, then back to the muscle, and make the muscular contraction happen as a consequence.

Sherrington had been in contact with Cajal’s work, immersing himself in his drawings, where the Spaniard had captured connections in the spinal cord, between sensory neurons and motor neurons.  

This two sources he was endorsed with, made him sure of the veracity of his theory, and he was determined to create an experiment to demonstrate what he so firmly believed. 

This experiment was simple, he sectioned portions of the spinal cord to interrupt the nerves that were going in and out of it, in both cases the reflex ceased.  This indicated that the process needed a way in signal and a way out between the spinal cord and the muscle.

Nowadays  we know that this circuit it’s composed by a single synapse between the receptors of the muscular bone that detect the tension in the muscle, and the motor neurons in the spinal cord that activate the contraction.

This discovery, might seem simple or obvious, but Sherrington was clever enough to provide it with the importance that it actually has. He was aware, that this was the first step of something big.



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