From the brain to the muscles: the journey of electrical messages in neurons

All this was established through a series of revolutionary advances unified in what came to be called the Neuron Doctrine, which in 1906 earned a Nobel Prize for Camillo Golgi and Santiago Ramón y Cajal.

For the first time, the workings of the nervous system were understood. (Previously, as exchanges among scientists concerning animal spirits make clear, the nervous system was thought to be a set of connected tubes branching from the brain throughout the body; for this reason, it was conceivable that they were filled with water or other fluids, and nothing else about them made sense.)

Ramón y Cajal and Golgi understood (again, amid many clashes and disagreements) that the nervous system was made up of individual cells, namely «neurons», capable of transmitting electrical signals from the brain to the nerves and muscles, and also in the opposite direction.

No one had previously realized that the nervous system was made up of cells, because neurons do not resemble ordinary cells at all, which almost always have the shape of a slightly flattened sphere. The neuron has three different parts: a cell body (similar to a normal cell), from which branches of various lengths extend in every direction. There are two types: dendrites, which are very short and carry incoming messages toward the cell body, and the axon, which can be up to a meter long and sends messages from the cell body to other neurons or to the muscles.

While some of the brain's 86 billion neurons are confined there, many extend down through the spinal cord until they reach the skin, heart, muscles, eyes, ears, nose, mouth, intestines and other organs—in other words, any structure in the body—to enable it to move, perceive and much more.

The so-called «sensory» neurons, through which the brain receives sensations and perceptions, are part of the «afferent system», which carries information about the outside world: images, sounds and smells, but also the fact that the body has suffered scratches and bruises. The «motor» neurons, which transmit the body's intention to move so that it can carry it out, form the «efferent system», which allows it to respond to sensations communicated by the afferent system.