Just as nature combines basic building blocks called atoms, then molecules, and then cells to make organisms, so, too, it assembles basic mental structures to make our brains. In the course of eons, each of those structures has formed over and to some extent subsumed its evolutionary precursors. At the deepest level, at the top end of the spine, is a small bulb sometimes called the “fish brain.” It regulates basic body functions like pulse. Over that is the R-complex, which first evolved some sixty million years ago in reptiles. And above that is the more recent limbic system, or “mammal brain.” These deep structures feed raw mental energy—the instincts, drives, and feelings referred to by psychologists—up into the newer layers of the brain, to the cerebral hemispheres and their cortex.

Each of these structures lends different outlooks, abilities, and inclinations to the overall mix we call mentality. And to that we also have to add the often-contrary urges spurred by the release of various hormones. A picture emerges of mentality not as coherent but as a multitude of competing tendencies that we constantly balance against one another. Just as it is a key to ecology and so many other aspects of natural systems, that counterbalancing tendency is how most of the brain’s structures and processes function. In the introduction to their influential book, Parallel Distributed Processing: Explorations in the Microstructure of Cognition, David E. Rumelhart and James L. McClelland, founders of the San Diego group, write, “The currency of our systems is not symbols, but excitation and inhibition.”

All of those mental systems are dense with neurons, the elongated cells—some as long as three feet—that transmit electrochemical messages throughout the body. By far the densest concentration of neurons in the brain is found in the cerebral cortex, the thin grayish pink carpet of cells that overlays the hemispheres. In that eighth-inch-thick layer enfolding a structure no larger than a grapefruit, there are some thirty billion neurons. The words “density” and “complexity” fail spectacularly in trying to describe the actual density and complexity of the brain. If all the neurons in just one human brain were laid end to end, they would reach to the moon and back, and then to the moon again.

But the brain’s complexity only begins there. It’s further multiplied by the fantastic number of possible connections among its synapses: Each neuron is excited or inhibited by data it receives from other cells, and from chemical messengers in the bloodstream. It converts that information into signals that course through its elongated body at a rate of some one hundred pulses per second. At one end of the neuron there are thousands of dendrites, feathery tendrils that reach out to receive signals from other neurons. At its opposite end there is a corresponding number of axons, similar arrays that send off the neuron’s own signals. As they flash out through the axons, those signals are converted into molecules called neurotransmitters. These then leap from the axons’ sending synapses out across a gap to connect with the receiving synapses on nearby dendrites.

With these transmitters leaping among so many potential receiving synapses, the versatility of that interplay leverages the number of possible synaptic connections in the human brain to a figure quite literally beyond imagining. Some suggest it may exceed the number of atoms in the universe. What’s more, the system is in constant flux—its connections shifting and strengthening and weakening as it responds to changing signals from the world around it. Little wonder the attempt to equal all that with a linear processor failed.

Tags: axons, brain, cognition, complexity, cortex, dendrites, elongated cells, limbic system, neurobiology, neurons, neurotransmitters, R complex, synapses

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Posted on May 15th, 2006 @ 6:00 am