How the Brain Develops

by Child Welfare Information Gateway

What we have learned about the process of brain development has helped us understand more about the influence of genetics and environment on our total development - the "nature versus nurture" debate. It appears that genetics predispose us to develop in certain ways. But our interactions with our environment have a significant impact on how our predispositions will be expressed; these interactions organize our brain's development and, therefore, shape the person we become.

Forming the Structure

The raw material of the brain is the nerve cell, called the neuron. When babies are born, they have almost all of the neurons they will ever have, more than 100 billion of them. Although there is research that indicates some neurons are developed after birth and well into adulthood, the neurons babies have at birth are primarily what they have to work with as they develop into children, adolescents, and adults.

During fetal development, the neurons that are created migrate to form the various parts of the brain. While the basic structure is intact at birth, much of the brain's growth occurs during the first few years after birth. This process of growth, or development, occurs sequentially from the "bottom up".

The first areas of the brain to fully develop are the brainstem and midbrain; they govern the bodily functions necessary for life, called the autonomic functions. The last regions of the brain to fully develop are the limbic system, involved in regulating emotions, and the cortex, involved in abstract thought. Each region manages its assigned functions through complex processes, often using chemical messengers (such as neurotransmitters and hormones) to help transmit information to other parts of the brain and body.

As the brain develops, it grows larger and more dense. By the age of 3, a baby's brain has reached almost 90 percent of its adult size. The growth in each region of the brain largely depends on receiving stimulation, which spurs activity in that region. This stimulation provides the foundation for learning.

Prenatal Exposure to Alcohol and Other Drugs

Exposure to alcohol and other drugs in utero can disrupt and significantly impair the way a baby's brain is formed.

Studies have shown that exposure to alcohol or other drugs, especially early in pregnancy, can alter the development of the cortex, reduce the number of neurons that are created, and affect the way in which chemical messengers are used. Although not all children who are exposed develop neurobiological problems, many do. These problems include difficulties with attention, memory, problem-solving, and abstract thinking. Many children born with Fetal Alcohol Syndrome are mentally retarded.

Organizing the Structure

Brain development, or learning, is actually the process of creating, strengthening, and discarding connections among the neurons; these connections are called synapses. Synapses organize the brain by forming neuronal pathways that connect the parts of the brain governing everything we do-from breathing and sleeping to thinking and feeling. This is the essence of post-natal development, because at birth, very few synapses have been formed. The synapses present at birth are primarily those that govern our bodily functions such as heart rate, breathing, eating, and sleeping. Almost all other functions are developed as babies grow up into children and adults.

The development of synapses occurs at an astounding rate during children's early years. By the time children are 3, their brains have approximately 1,000 trillion synapses, many more than they will ever need. Some of these synapses are strengthened and remain intact, but many are discarded. By the time children have reached adolescence, about half of their synapses have been discarded, leaving about 500 trillion, the number they will have for most of the rest of their lives.

Plasticity - The Influence of Environment

Researchers use the term plasticity to describe the way the brain creates, strengthens, and discards synapses and neuronal pathways in response to the environment. The brain's "plasticity" is the reason that environment plays a vital role in brain development.

The early over-production of synapses appears to be the result of evolution that has led our brains to expect certain experiences. Our brains prepare us for these experiences by forming the pathways needed to respond to those experiences. For example, our brains are "wired" to respond to the sound of speech; this is how we learn to talk. But these early synapses are weak; we must be repeatedly exposed to the expected experiences within a certain time period to activate and strengthen them. If this does not happen, the pathways developed in anticipation of those experiences may be discarded, and the development of the related functions will not occur as expected. This is often referred to as the "use it or lose it" principle.

In addition to strengthening or discarding existing synapses, researchers theorize that some synapses may be newly developed in response to unique environmental conditions. It is through these processes of creating, strengthening, and discarding synapses that our brains adapt each of us to our unique environment.

The ability to adapt to our environment is a part of normal development. Children growing up in cold climates or rural farms or large sibling groups learn how to function in those environments. But regardless of the general environment, all children need stimulation and nurturance for healthy development. If these are lacking-if a child's caretakers are indifferent or hostile-the child's brain development may be impaired. Because the brain adapts to its environment, it will adapt to a negative environment just as readily as it will adapt to a positive environment.

About the Author

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Formerly the National Clearinghouse on Child Abuse and Neglect Information and the National Adoption Information Clearinghouse, Child Welfare Information Gateway provides access to information and resources to help protect children and strengthen families. A service of the Children's Bureau, Administration for Children and Families, U.S. Department of Health and Human Services.