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von Haller (1708–1777) stated a decade before the time of Franklin’s experiments in his
Dissertation on the Sensible and Irritable Parts of Animals
, “the nervous fluid must have 6 essential properties. 1) That it be highly moveable. 2) That not only must it be moveable, but that it must be capable of being set in motion only by the will and force of the soul and not by the help of the heart. 3) That it must be a fluid element, and very rapid in movement. 4) That it must be very subtle, so as not to be seen by a microscope. 5) That it must have a particular affinity for the nerves. 6) That it be colorless, odorless, tasteless, and without perceptible heat.”
    Haller was elucidating principles on which the agent of nervous action could be defined, and specifically, motor nerve action. Looking back, all these criteria fit nicely with electricity except number 2. The neuron undoubtedly conducts electricity, but what spurs it to action—other neurons in a billion-cell complex circuit? Neural conductance is rapid, can travel long distances, can cause muscular contractions and animal action, and it communicates sensory input to the brain. All cells have an electrical potential, but neural structure allows the current to travel long distances in an “all-or-nothing” manner. However, glial cells in the brain exhibit activity in more subtle and elegant mechanisms than neural electrical depolarization. Cajal did not have techniques available to understand glial function, and the Neuron Doctrine prevented glial research with new techniques only until recently. These cells have properties that are more likely to be initiators and masseuses of nervous action, the will of controllable ability.
References
     
    Abrams, R.
Electroconvulsive Therapy
, Fourth Edition. New York: Oxford University Press, 2002.
    Aminoff, M.J.
Electrodiagnosis in Clinical Neurology
, Fifth Edition. Philadelphia, PA: Elsevier, 2005.
    Andreassi, J.
Psychophysiology: Human Behavior and Physiological Response
, Third Edition. Hillsdale, NJ: Lawrence Erlbaum, 1995.
    Brazier, M.A.B.
A History of Neurophysiology in the Nineteenth Century
. New York: Raven Press, 1988.
    Galvani, L.
Commentary on the Effects of Electricity on Muscular Motion,
Introduction by Cohen, I.B. Norwalk, CT: Burndy Library, 1953.
    Müller, J.
Handbook of Human Physiology
. London: Baylor and Walton, 1842.
    Ochs, S.
A History of Nerve Functions: From Animal Spirits to Molecular Mechanisms
. New York: Cambridge University Press, 2004.
    Valenstein, E.
The War of the Soups and the Sparks
. New York: Columbia University Press, 2005.

4
Meet the astrocyte
     
    If you look at a tulip, you wouldn’t think it was an armadillo. Similarly, looking at a neuron, you wouldn’t think it was glia. But you might look at a whale and think it’s a fish, until you look at it closely and realize it has no gills and breathes through lungs. Then, you have a problem. Through genetic testing and excavations by paleontologists, we now know it likely originated as a land animal that took back to the sea and is related to hoofed animals like the horse. But before genetic testing and evolutionary biology, classification was based on appearance. This remains true for cellular classification.
    Glial cells include Schwann cells, Müller cells, epithelial cells, ependymal cells, oligodendricytes, tanycytes, microglia, and astrocytes—all function as differently from each other as they do from neurons.
    Students of scientific fields always claim that learning new terms in the field is like learning a new language. Just as the meaning of an English word can change over time (such as plane, buck, mouse, and gay), the meaning of a scientific label can change after its function is understood.
    As functions of cells are understood, the labels have remained. Glia are capable of signaling and communicating, but they will always be referred to as glia (glue). Virchow’s “glue” label sticks because it was originally