the process of eliminating or weakening synapses. Although it occurs throughout the life 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. Pruning begins around the time of birth and continues until the end of the 20s. During the elimination of the synapse, the axon retracts or dies, and the dendrite disintegrates and dies. Traditionally, synaptic pruning was thought to be complete by puberty, but magnetic resonance imaging studies have disproved this idea. By the time of adulthood, the baby’s brain increases in size up to 5 times. Two factors contribute to this growth: the growth of synaptic connections between neurons and the myelination of nerve fibers. However, the total number of neurons remains roughly the same at about 86 (± 8) billion neurons. After adolescence, the volume of synaptic connections decreases again due to synaptic pruning. The pruning process is influenced by environmental factors. For example, if the eyes are sutured during the critical period when synaptic retinal pruning occurs, the lack of light will lead to the death of synaptic connections necessary for vision, which will lead to blindness.