synaptic pruning

Meaning: Early in life the brain forms far more synapses than it ends up keeping. Connections that are activated repeatedly are strengthened, for example through long-term potentiation, and connections that are rarely used are removed. The result is a more efficient, experience-tuned network. Pruning is fastest in early childhood and continues through adolescence. The prefrontal cortex is among the last regions to be refined, which is why it is linked to still-maturing judgment and impulse control in teenagers.

On the MCAT: Expect a passage on brain development, sensitive (critical) periods, or enriched versus deprived environments. You may be asked to predict that a skill or sensory input that is practiced keeps its synapses, while one that is absent during the sensitive period loses them (for example, reduced input from one eye leading to weaker connections for that eye). Graphs may show synapse density rising rapidly in infancy and then declining. Adolescent brain maturation questions may also use the idea.

Common confusion: Pruning removes synapses, not whole neurons. Programmed neuron death (apoptosis) is a separate process. It is also not damage or decline: having fewer synapses than a toddler does not mean worse function, because the remaining circuits are more efficient. Myelination, which speeds conduction, is a different developmental process that happens alongside it.