Development of the Brain
Brain development begins at conception, continues through adolescence, and extends beyond into adulthood.
In the Prenatal Period
Billions of neurons are formed in the brain. This brain cell formation begins in the human embryo after the basic brain structures are formed. By five weeks following fertilization, an embryo starts a phenomenal cell division in the forebrain region, which ultimately creates the cerebral hemispheres. Once formed, neurons must migrate to the correct location in the brain. Axons have specialized tips, or growth cones, that help neurons recognize the proper pathways of migration. Synapses begin forming. The fetus involves in actions such as kicking, turning, and sucking its thumb produces further stimulation of synapses. The great majority of synapses are formed after birth as the newborn starts to experience the environment. Myelination of neurons begins. Myelin is a white-colored fatty sheath surrounding axons and functions to promote impulse conduction. Myelination is the major cause of the increase in a child's brain size. At birth, the infant brain weighs 300-350 grams.
At birth
The newborn begins a rapid period of brain growth. Dramatic anatomical changes in the structure of brains occur during childhood and early adolescence. In the beginning, only reflexes needed for survival are completely myelinated; however, after birth the primary visual and auditory cortex neurons rapidly receive their myelination.
In childhood
Myelination continues through childhood. During the first year-and-a-half of life, the corticospinal motor tract receives its myelination enabling gross control over arms, torso, and legs. Neurons continue migrating into positions. Recent research challenged the assumption that neurons migrate to predetermined locations in the brain, and once there, perform only certain functions. Experiments with ferrets reconfiguring areas in their brains demonstrated it was possible for fully functioning visual pathways to develop in auditory parts of the brain. The visual abilities, arising from auditory portions of the ferrets' brains, developed in response to environmental input.
During Adolescence
The brain continues to change and mature during adolescence. Final myelination of the frontal lobes occurs in early adolescence. An adolescent's brain reaches the weight of an adult brain by about age fourteen due to myelin accumulation and branching of the dendrites. The adolescent's frontal lobes are increasingly active, and this ability enables the adolescent to consider several things in the mind while comparing or interrelating them. Most of the pruning takes place between ages ten and sixteen bringing the density of synapses to the adult level. The pruning process produces a more orderly circuit in all parts of the cortex based on repeated experiences of early and middle childhood. Synapse formation continues despite ongoing pruning. The remaining dendrites continue to branch, grow, and form new synapses in response to new experiences. Continued psychological and cognitive development in adolescence is due to this dendritic growth.
Into Adulthood
The brain continuously remodels itself - even in adulthood. Synapses continue to be formed in select areas of the brain, but the growth of new neurons is actually limited. Important forebrain regions, such as the hippocampus, continue to receive new nerve cells into adulthood. Lifelong enrichment experiences are important as it is these experiences that cause the dendrites to continuously branch, grow, and form new synaptic connections.