As one of the partners in the dance between nature and nurture, genetic influences play a major role in the growth of the brain over the first years of life. There is evidence that many of the skills a baby develops in the first years (such as sitting and remembering) occur as a result of physical changes in the structure of the brain. These genetically programmed influences start in utero and continue throughout a person’s life.

The Birth of Your Baby’s Brain
When your baby is born, her brain will have just about all the nerve cells – or neurons – she will ever have. Development of these neurons begins in the first days after conception when just a few cells come together to form a hollow tube. This tube will grow, according to a genetic blueprint, to form the complete brain. Genes determine where and when the neurons will form and they control the basic circuitry of the brain. They also, of course, endow each child with the genetic qualities of her parents, including factors relating to how, and how well, the brain will process certain kinds of information.

Basic Brain Architecture
Neurons are the workhorses of the brain; they are the cells that process information. It has been estimated that from the time your baby’s brain begins to form, it will generate more than 250,000 new neurons per minute, working toward its final total of approximately 100 billion at the time of birth. Neurons come in many shapes and sizes, reflecting the different functions they will ultimately perform. Scientists have now identified approximately 25 different kinds of neurons.

As soon as a neuron has arrived at its ultimate destination, several important events occur:
• Axons sprout: Axons are cables that reach out to other neurons. Information is passed along them by an electric current created by a chemical reaction (also known as “firing”).
• Dendrites grow: Dendrites are the tip of the neuron that make the final connection. They look like tiny branches of a tree, but the branches never touch each other.
• Information flows: The gaps between the dendrites of two neurons are called synapses; it is here that chemicals called neurotransmitters flow. These chemicals pass along information, enabling one neuron to tell another what to do. The way the neurons, dendrites and synapses fit together is significant because the organization of the synapses is what ultimately determines how information will flow through the brain. It is early experiences that determine synaptic organization.

The Timing of Brain Growth
Neuroscientists believe that many of the skills your baby develops during the first year, such as grasping a rattle, sitting up or smiling, are related to changes in the anatomy of the brain. They also believe that the emergence of some of these skills is related to the time at which different areas of the brain mature. As a general rule, the areas of the brain responsible for bodily functions, such as breathing and heartbeat, mature very early. Areas responsible for higher levels of functioning, such as language and memory for specific events, don’t develop until later.

There are three key developments occurring in the brain, each related to a baby’s inherent nature, that prepare her to acquire new skills:

1. At certain times, huge numbers of new synaptic connections are being formed. These synaptic bursts occur at least twelve different times throughout a person’s life, with over half of them occurring during the first year. These bursts occur at the same time as some of the advances in your baby’s skills. For example:
• At three to four weeks, your baby suddenly has greater control over her reflex actions.
• At around eight months, she develops greatly improved hand-eye coordination.

2. Myelination (or insulation) of the various parts of the brain happens at different times. Myelin is a fatty sheath that surrounds and insulates many of the neurons in the brain, enabling them to:
• Process information faster.
• Focus on specific responsibilities.
• Become more efficient.
• Fire shortly after having just fired.

3. Finally, we know that energy usage in different parts of the brain changes over time. An infant’s brain uses enormous amounts of energy. Harry Chugani, a Pediatric Neurobiologist at Wayne State University, found that at different points during the first year, the brain uses its energy in very different ways:
• Prior to three month, much of your baby’s behavior is based on reflexes. At this time, the majority of energy is used by subcortical areas, such as the brain stem, the area responsible for controlling reflexes.
• After the third month, your baby begins to reach for everything in sight. At this time, activity around the thalamus, the area responsible for reaching movements, is increasing.
• Around eight months, your child is beginning to demonstrate higher level intellectual functioning (such as a knowledge of the permanence of objects). At this time, activity in the prefrontal cortex, the area responsible for higher level processes, is increasing.

It’s tough to ignore all these coincidences of timing and maturation. Most neuroscientists now believe that the advances in the capacity of the brain not only happen at the same time as the advances in a baby’s skills, but seem to be a trigger for these skills.