Rebuilding the Brain

For many years, it was thought that all of our brain cells (neurons) are produced before birth, or exceptionally, up to one or two years after birth, but then the process supposedly stopped. From then on, most brain scientists believed, brain circuits could only be modified by altering the synapses, or contacts between existing cells. In this view, the total complement of cells, anatomically organized by function and forming major pathways, constituted the brain’s “hardware.” The detailed nature of the relation between cells, the modifiable synapses, was the software.

Starting in the 1960s, evidence suggested that new neurons might be added beyond this early period and even in adulthood. This evidence became compelling in the 1980s, based on observations made while studying parts of the songbird brain involved in the acquisition and production of learned song. The original observations were met with skepticism, but the birdsong data proved compelling.

How does one prove that a neuron was born at a particular time? Each cell in the body contains DNA, which is responsible for producing the proteins that are necessary for a cell’s functioning. Though all cells in the body have the same DNA, only a subset of the genes encoded by the DNA is expressed in each cell type, thus accounting for the difference between, say, a skin cell, a liver cell, and a brain cell.

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