An infant’s brain is about ¼ the size of an adult’s. At birth, a newborn’s head accounts for roughly ¼ of an infant’s length. By age 3, a child’s brain will have reached at least 80% of its full size, according to Zero to Three.
The human brain and nervous system contain billions of neurons which form trillions of connections and pathways. How many and how organized these connections are influence a child’s life experience.
Neurons multiply rapidly and before birth, the number of neurons that an individual will have is at its highest. Throughout life, some new neurons may be produced but this is at a far less rapid rate, according to Zero to Three. Before birth, neurons migrate to various parts of the brain, but the connections of these neurons are incomplete and need to be established.
Early childhood experiences influence how a child’s brain becomes connected. Early sensory experiences create new synapses, or regions of the neurons where nerve impulses are sent and received. A child’s brain becomes specialized as active synapses are used and inactive ones are pruned.
These connections are built rapidly during the early years of a child’s life as the child has many new experiences. After early childhood, this process gradually slows down.
In the brain, layers of a protein and fatty substance, called myelin, insulate the sensory and motor neurons which make messages or impulses travel faster. This insulation process, called myelination, continues into adolescence and enables children to improve reaction times and coordination as they grow.