What we have learned about the process of brain development has helped us understand more about the roles both genetics and the environment play in our development.
It appears that genetics predisposes us to develop in certain ways. But our experiences, including our interactions with other people, have a significant impact on how our predispositions are expressed. In fact, research now shows that many capacities thought to be fixed at birth are actually dependent on a sequence of experiences combined with heredity. Both factors are essential for optimum development of the human brain.
The newborn brain
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 research 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, neurons are created and migrate to form the various parts of the brain. As neurons migrate, they also differentiate, so they begin to “specialize” in response to chemical signals. This process of development occurs sequentially from the “bottom up,” that is, from the more primitive sections of the brain to the more sophisticated sections.
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. At birth, these lower portions of the nervous system are very well developed, whereas the higher regions (the limbic system and cerebral cortex) are still rather primitive.
Newborns’ brains allow babies to do many things, including breathe, eat, sleep, see, hear, smell, make noise, feel sensations, and recognize the people close to them. But the majority of brain growth and development takes place after birth, especially in the higher brain regions involved in regulating emotions, language, and abstract thought. (See illustration at right.)
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.
The growing baby’s brain
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 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 postnatal brain 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.
The development of synapses occurs at an astounding rate during children’s early years, in response to the young child’s experiences. At its peak, the cerebral cortex of a healthy toddler may create 2 million synapses per second. By the time children are three year old, 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 gradually discarded. This process of synapse elimination — or pruning — is a normal part of development. By the time children reach adolescence, about half of their synapses have been discarded, leaving the number they will have for most of the rest of their lives. Brain development continues throughout the lifespan. This allows us to continue to learn, remember, and adapt to new circumstances.
Another important process that takes place in the developing brain is myelination. Myelin is the white fatty tissue that insulates mature brain cells by forming a sheath, thus ensuring clear transmission across synapses. Young children process information slowly because their brain cells lack the myelin necessary for fast, clear nerve impulse transmission.
Like other neuronal growth processes, myelination begins in the primary motor and sensory areas (the brain stem and cortex) and gradually progresses to the higher-order regions that control thought, memories, and feelings. Also, like other neuronal growth processes, a child’s experiences affect the rate and growth of myelination, which continues into young adulthood.
By the age of three, 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.