Neural Evolution

Cell death is a normal part of nervous system development. In some parts of the brain, more than half of the neurons generated during embryonic development die before a baby is born. To survive, a neuron requires a survival signal to turn off a built-in "suicide program." The suicide program within each cell is called programmed cell death or apoptosis . One neuron's loss is likely to be another neuron's gain, however. Neighboring neurons compete to fill the space left by a cell that has died due to apoptosis, and in some cases cell death actually stimulates the growth of new cells (Lasley, 2000).

How does the brain arrive at its adult form?

Evidently the brain arrives at its mature adult form by producing more nerve cells than it needs, then killing off the ineffective ones bit by bit while allowing successful neurons to grow. In children below the age of 7, new cells are added much faster than existing ones are killed. The number of neurons in the human brain peaks around age 7. After that, the total number of neurons decreases gradually for the remainder of a person's life. Remaining neurons can grow larger and more complex. Dendritic trees of individual neurons can continue to grow until a person is past the age of 90.


The number of neurons peaks in childhood, although individual neurons (such as the one highlighted in black here) may persist from infancy to old age, growing more complex even as the total number of neurons diminishes.

What role does NGF play in the competition among neurons?

How are the winning neurons selected? The answer involves neurotrophic (nerve-growing) factors. One important neurotrophic factor—the first discovered—is named "Nerve Growth Factor " (NGF). Actually there are many nerve growth factors; NGF is simply the first neurotrophin studied, and it is important. The nervous system has limited amounts of NGF. Neurons compete to get it. The ones that get some NGF grow while others shrivel and die. Apparently neurotrophins are channeled to neurons that make a useful contribution to the organism.

This process goes on throughout life, which is one reason the slogan "Use it or lose it" contains an enduring truth. Skills maintained through practice stay sharp even in old people, because the neurons are supported by neurotrophins and continue growing. Skills which are not practiced are lost, in part because the neurons which perform them are lost if they no longer contribute to mental activity.


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Copyright © 2007 Russ Dewey