Synaptic pruning

Synaptic pruning is the process of synapse elimination or weakening.^[1] Though it occurs throughout the lifespan of a mammal, the most active period of synaptic pruning in the development of the nervous system occurs between early childhood and the onset of puberty in many mammals, including humans.^[2] Pruning starts near the time of birth and continues into the late-20s.^[3] During elimination of a synapse, the axon withdraws^[4] or dies off, and the dendrite decays and die off. Synaptic pruning was traditionally considered to be complete by the time of sexual maturation, but magnetic resonance imaging studies have discounted this idea.^[5]

The infant brain will increase in size by a factor of up to 5 by adulthood.^[6] Two factors contribute to this growth: the growth of synaptic connections between neurons and the myelination of nerve fibers. The total number of neurons, however, remains approximately the same, containing approximately 86 (± 8) billion neurons.^[7][8][6] After adolescence, the volume of the synaptic connections decreases again due to synaptic pruning.^[9]

Pruning is influenced by environmental factors. For instance, if the eyes are sewn shut in the critical period when synaptic pruning of the retina takes place, the lack of input of light will cause the synaptic connections required for vision to die off, resulting in blindness.^[1][10]