The Anatomy of the Brain

Growing out of the spinal cord like the crown of a tree out of its trunk, the brain has several major components (see diagram page 52). The limbic system, an area that surrounds the head of the brain stem and includes such structures as the amygdala, part of the thalamus, hypothalamus and hippocampus, regulates the emotions. The pituitary, which hangs down from the brain stem like an olive from the tree, produces the hormones that influence growth and development. The cerebellum, a fist-sized structure at the rear of the brain that controls movements and coordination, enables man to touch his nose with his finger or throw touchdown passes. But it is the cerebrum that distinguishes man from other animals. Fish have little or no cerebral tissue, nor do birds. Chimpanzees, man's closest animal relatives, have larger cerebrums than most other primates, but man's is the largest.

Consisting largely of gray matter and fissured like a lunar landscape, the cerebrum dominates the human brain, filling the dome of the skull. It also makes man what he is, for it contains the areas that control thought and consciousness, the quality that enables man to remember his past, understand the present and anticipate his future.

Divided down the middle like the two halves of a walnut, the cerebral hemispheres are anatomically separate, but are cross-wired so that each controls the opposite side of the body--the left monitoring the right side, the right regulating the left. One hemisphere--the left in most people--is dominant and contains the areas that are associated with speech and hearing and involved with analytical tasks such as solving an algebra problem. The other governs spatial perception, synthesis of ideas and aesthetic appreciation of art or music.

Normally these two hemispheres communicate with each other through a bundle of nerve fibers known as the corpus callosum. But if this connection is severed, their autonomy becomes evident.

"Split brain" patients lose few of the two-handed skills already learned; they do have great difficulty learning new tasks that require both hands.

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The brain is composed of two kinds of cells: neurons, or nerve cells, of which there are some 100 billion, and glia, which outnumber the neurons by a ratio of 10 to 1. Neurons, which are the functional units of the brain (glia, scientists believe, are largely "filler"), are connected to each other by means of long filaments, or dendrites, and form the body's nerve network. These cells receive sensory impulses, process the myriad bits of information pouring into the brain each moment, and transmit the brain's messages out to the various parts of the body, causing such reactions as the contracting and relaxing of muscles.

It has long been known that these messages are transmitted electrically. More recent research has shown that communication between the neurons is also chemical in nature. Neurons have bulbous endings called synapses. These secrete chemicals that cross the submicroscopic gaps between the individual cells, lock onto special sites on the dendrites of neighboring cells and cause these cells to release chemicals of their own. That action allows the passage of current from one cell to another.

The speed with which these cells can carry out their chemical transactions is, quite literally, mind-boggling. Manfred Eigen, 46, director of Germany's noted Max Planck Institute for Biophysical Chemistry in Gottingen, has found that some of the brain's chemical reactions take as little as one-millionth of a second. As many as 100,000 neurons may be involved in transmitting the information that results in as simple an action as stepping back to avoid being struck by an oncoming car. The entire process occurs in less than a second.

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