Apoptosis (programmed cell death) and other reasons for elimination of neurons and axons.

Many of the neurons which migrate to the cortex die at an early stage of development. In certain animals many cells die when their task is done; a phenomenon known as programmed cell death, or apoptosis. In humans a far commoner cause is the competition hypothesis: if the axon of the cell does not make contact with the dendrites of the cell in its target area it will die. Then the complexity and number of these synapses is more essential to the function of the brain than the actual number of neurons. Examples are given of the effect of sensory deprivation on the survival of neurons. The withdrawal of neuronal growth factor, and subsequent loss of the axon or cell can be a factor in the fine tuning of neural circuits, and therefore in an improvement in learning and the development of skills. However, it may also account for the diminished plasticity of the nervous system, and its ability to adapt to injury. It has been postulated that these changes contribute to the development of personality. Factors which may be involved include differences in brain development between the sexes; temperamental traits which may have a genetic or prenatal origin; and the effects of sensory perception, including language. Surely there can be no doubt that the results of experience represent the fundamental mechanisms by which the organism adapts to its environment.