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Why are surplus nerve cells removed during brain development?

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During brain development, surplus nerve cells and synaptic connections are removed through a process known as programmed cell death (or apoptosis) and synaptic pruning. This process is essential for the organism to achieve its normal, mature form and functional organization.1

The brain initially generates an excess of neurons and forms a vast number of synaptic connections—far more than are present in the adult brain. This overproduction is necessary because the nervous system must establish a proper numerical balance between functionally related cell groups and refine its complex architecture.231

As the brain develops, connections that are not reinforced by sensory stimulation or environmental interaction weaken and are eventually eliminated, while those that are reinforced become stronger. This "pruning back" of excess synapses and the death of specific cells allow for the creation of efficient neural pathways. Ultimately, this genetically driven process ensures that the brain reaches maturity by fine-tuning its structure through the organism's ongoing interaction with its surroundings.312