Brain Structure - Brain Theory from a Circuits and Systems Perspective - page 8

White Matter

White matter refers to myelinated axons, which are insulated with myelin to
transmit electrical impulses faster than what is possible in uninsulated dendrites.

Cerebrum

The cerebrum contains six layers of folded neural tissue also known as the cerebral
cortex, constituting most of the neural volume of the brain.

Cerebral Cortex

The cerebral cortex is composed of pyramidal and stellate (starlike) neurons.
Figure 1.3 illustrates six layers organized roughly as highly interconnected
columns. Through the six layers of gray matter extend hundreds of millions of
interconnected hairlike mini columns, each with about 100 neurons. Layer 4 is
thought to be for input. Layers 5 and 6 are thought to be for output. General
processing occurs in layers 1, 2, and 3. Possibly each mini column can process an
attribute such as a shape or a sound. One hundred mini columns form a macro
column, about the thickness of pencil lead.
Thousands of macro columns form 1 of the 50 or more Brodmann areas in each
hemisphere. Korbinian Brodmann was a pioneer in organizing the cerebral cortex
into distinct regions based on structural variations. He identified over 50 numbered
areas in 1909, and the system is still widely used, although others have identified
hundreds of additional structural variations.
The numbers add as follows. About 50 Brodmann areas might have, say, 10,000
macro columns; each macro column has perhaps 500 mini columns; each mini
column might have about 100 neurons. This represents 25 billion (25

10 9 )
neurons per hemisphere. Each neuron may have a great many logic gates. Some
undoubtedly are unused, ready to be turned to new purposes, and some are copies of
others for backup purposes, but even so, the amount of logic is astounding.
Gray matter is distributed roughly toward the outside as in Fig. 1.4 . White matter
forms the inner region and indeed connects the left brain to the right brain.

Corpus Callosum

The hemispheres are connected by the corpus callosum and the anterior commis-
sure. The corpus callosum, or “colossal commissure,” is a wide, flat bundle of