The Root of Thought by Andrew Koob Read Free Book Online

cochlea respond to sound vibrations. In our eyes, rods and cones are stimulated by photons and converted into electrical neural signals. Our skin has receptors for temperature and vibration. Our ever-changing nose is constantly turning over neurons and glia in response to the ever-changing nature of smell. The nose is especially interesting because in lower-life forms, the olfactory bulb is more prominent in size ratio to the rest of the brain.
    Motor neurons stimulate muscular action. But what is between? When someone tells you to use your noodle, what do you use?
    Although the term glia and its first drawings were likely contributed by Rudolph Virchow in 1846, Heinrich Müller (1820–1864)—no relation to Johannes, the great advisor—was the first to describe a retinal glia cell in 1851. The cells are known to modulate signaling in the retina. They can signal to themselves as well. Not knowing what to call them because they seem to be a completely different functional unit than neuronal ganglion cells extending out from the eye into the brain, researchers have classified them as glia and they are called Müller cells.
    Müller is often overlooked in favor of Virchow, although Virchow’s publication of his 1846 work didn’t happen until 1853, two years after Müller’s. And Müller’s drawings and description were much more intricate. In fact, Virchow got the best of Müller around this time professionally as well. Virchow sided with the revolutionaries in Berlin from 1848 to 1849, eventually losing his chair at the university due to his politics. The University of Würzburg pounced on the opportunity to have Virchow chair its department and chose him over the likely candidate Müller. Müller spent half a year in a sanitarium because of the slight.
    The next major glial cell was discovered in the peripheral nervous system by Theodore Schwann. Schwann was also a student of Johannes Müller. If neurons are the highways, then Schwann cells are the construction workers in the peripheral nervous system. The function of these cells is so different from Müller cells that it is amazing they are both called glia. They are about as related as Henrich and Johannes Müller. Schwann had no clue about the function of the cells, only that they were associated with axons in the periphery.
    The myelin on axons was not understood until the advent of the high-powered electron microscope in the mid-twentieth century. The electron microscope allowed researchers to see subcellular structures clearly for the first time. It was then that the wrapping twists of myelinating Schwann cells could be seen and the notion of axons secreting the myelin themselves debunked. Schwann cells myelinate the axons which extend out to the muscles so they are able quickly conduct electrical impulses. Schwann cells release the fatty myelin from their cell body and swirl it around a group of axons like a butcher wrapping up some sausages. Umami-flavored sausages.
    In the central nervous system, the construction workers that myelinate the axons are called the oligodendricytes. Cajal’s Argentine student Pío del Río Hortega first described this cell. Smaller than Schwann cells, oligodendrocytes abundantly reside in the white matter of the brain and wrap myelin around fibers.
    Other glial cells that line structures such as the ventricles and blood vesicles in the brain are called ependymal cells, endothelial cells, and tanycytes. These cells are the military defense of the brain. Ependymal cells seem to help cerebral spinal fluid flow with extending filipodia and lining the walls. Epithelial cells line blood vesicles and the retina. Epithelial cells have the important task of not allowing any contaminant from the blood to invade the brain. Tanycytes also separate the CSF and the blood with tight junctions.
    Throughout the brain, microglia (the smallest of all the glial cells) respond to injury and infection like the T cells and B cells in the blood. These cells