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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. In fact, between the fifth and twentieth week of gestational life, an estimated 50,000 to 100,000 new brain cells are generated each second.

Once formed, neurons must migrate to the correct location in the brain. How neurons navigate to predetermined locations is the subject of intense research. Axons have specialized tips, or growth cones, that researchers believe help neurons recognize the proper pathways of migration.

Synapses begin forming. Prenatal conditions including temperature, pressure, and even a beginning hearing ability influence formation of synapses in the fetus. The fetus itself 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 begins in the spinal cord at three months gestation and continues its development in a head-to-toe direction. Myelination begins in the brain at six months gestation, following a similar head-to-toe direction, and continues into puberty and adulthood.

Myelination is the major cause of the increase in a child's brain size.

At birth, the infant brain weighs 300-350 grams (2/3 to ? pound).
In the first four years of life, the brain increases to 80% of the adult weight of 1200-1500 grams (2.6 - 3.3 pounds).
At birth

The newborn begins a rapid period of brain growth. Recent research challenged the prevailing belief that maximum brain growth occurs in the first three years of life. Using high-resolution three-dimensional magnetic resonance imaging (MRI) scans from normal children, aged 3 to 15 years, researchers documented dramatic anatomical changes in the structure of brains during childhood and early adolescence. These findings provided proof that significant brain growth continues at developmental periods beyond the first three years of life.

Few nerve centers are myelinated at birth. 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. 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. Thus, the research provided some evidence that experience also influences how the brain functions.

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 dendritic branching. At this time the potential for contribution to insight, judgment, inhibition, reasoning, and social conscience are possible. 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.

The density of synapses declines during adolescence due to selective pruning of redundant or unused connections. 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 circuitry 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. The prefrontal cortex responsible for reasoning and judgment continues to develop.

Into Adulthood

The brain continuously remodels itself-even into adulthood.

Synapses continue to be formed in select areas of the brain but growth of new neurons is limited. Prevailing knowledge that the adult brain does not produce new neurons is currently being challenged. Recent research has suggested that important forebrain regions, such as the hippocampus (involved in establishing memory for facts and relationships), continue to receive new nerve cells into adulthood in humans.

Lifelong enrichment experiences are important. These experiences continue to cause dendrites to branch, grow, and form new synaptic connections. Brain development continues in adults who regularly exercise their brains with new and varied experiences. Even in adulthood, the brain is continuously remodeling itself.

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