Connectome by Sebastian Seung

Author:Sebastian Seung
Language: eng
Format: mobi, epub
Publisher: Houghton Mifflin Harcourt
Published: 2013-02-17T07:43:21+00:00

Figure 48. Gray versus white matter of the cerebrum

The distinction between gray and white matter was known in antiquity, but their fundamental difference became clear only after the discovery of neurons. The outer gray matter is a mixture of all parts of neurons—cell bodies, dendrites, axons, and synapses—while the white matter contains only axons. In other words, the inner white matter is all “wires.”

Most white-matter axons come from neurons in the surrounding cerebral cortex. They belong to pyramidal neurons, which constitute about 80 percent of all cortical neurons. Earlier I mentioned that this neuron type has a cell body with a triangular or pyramidal shape, and an axon that travels a long distance from the cell body. Let’s refine the picture here. The apex of the pyramid points toward the exterior of the brain. The axon comes straight out of the base of the pyramid, perpendicular to the cortical sheet, and plunges into the white matter, as Figure 49 shows.

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