The Disordered Mind: What Unusual Brains Tell Us About Ourselves

by Eric R. Kandel

As we have seen, neurons send information outward along the axon, which forms synaptic connections with a target cell’s dendrites. Most of a pyramidal neuron’s synapses are located on small protrusions from the dendrites called dendritic spines. The number of dendritic spines on a neuron is a rough measure of the amount and richness of the information it receives. Dendritic spines begin to form on pyramidal neurons during the third trimester of pregnancy. From then through the first few years of life, the number of dendritic spines, and the number of synapses on them, expands rapidly. In fact, a three-year-old’s brain contains twice as many synapses as an adult’s brain. Beginning at about puberty, synaptic pruning removes the dendritic spines that the brain isn’t using, including spines that aren’t actually helping working memory. Synaptic pruning becomes particularly active during adolescence and early adulthood.