Neural Development in Infancy
A newborn’s head is disproportionately large compared with his or her body. At birth, the newborn's brain is 25 percent of her estimated adult weight; compare that to her body, which is 5% of her estimated adult weight. The skull must be bigger to house that “all powerful” brain. As your baby grows, her brain will continue to develop.
Neurons are the basic building blocks or cells of the central nervous system. It is estimated that by adulthood, most humans have about 100 billion neurons. Most neurons are produced before a child is born. At two months old, a child will have billions of neurons. Neurons are the basic brain cells and are interconnected through a complex system of fibers of dendrites and axons. Dendrites receive messages from other neurons and are important in processing brain messages. Axons serve as the terminal ends of neurons allowing for the release and reuptake of neurotransmitters.
Neurons are located in different areas of your baby's brain; the majority of them in an area called the cortex, also known as the neocortex. In their well-known textbook, Fundamentals of Human Neuropsychology, Bryan Kolb and Ian Whishaw highlight that 70 percent of neurons exist on the six outer layers of the brain, and approximately 80 percent of brain material resides within the neocortex. Neurons compose brain structures, many of which are integral to life sustaining functions in all humans regardless of age, including the regulation of heartbeat, breathing, temperature regulation and sleeping.
How Neurons Communicate
Neurons communicate with one another through chemicals called neurotransmitters. About 100 different neurotransmitters have been identified. These chemicals communicate among various neurons and directly contribute to your infant’s mood and behavior. The production or reduction of neurotransmitters results from electrical and chemical changes within the interior and exterior membranes of the neuron. The rate at which neurotransmitters proliferate or atrophy (die), vary across an individual's lifespan.
Serotonin, one of many neurotransmitters, has been identified as intrinsic to mood stability. Low levels of serotonin have been correlated with increased depression and anxiety in infants and adults. In a 2006 article titled, "Low Levels Of Neurotransmitter Serotonin May Perpetuate Child Abuse Across Generations," and published through the University of Chicago, William Harms suggests that infants who exhibit low levels of serotonin often grew to become abusive adults. These results suggest a biological variable involved in the development of aggressive behaviors.
The Neocortex and Other Regions of the Brain
The neocortex is responsible for higher order processes such as thinking, processing information and sensations, feelings and motivations. The frontal portion of the neocortex, the orbital frontal cortex, which is responsible for executive functions such as planning and control, is immature in a newborn. Some current studies suggest full maturation does not occur until an individual reaches her mid 20's.
The human brain also contains the auditory cortex and the visual cortex. In an area called the somatosensory cortex, a type of body map exists as well. Your newborn’s auditory cortex, a region of the brain responsible for the processing of sound, is highly active and functional. This is because your baby was developing her eavesdropping skills during the fetal period. She will know who you are when you walk in the room because of the sound of your voice.
Interestingly, your baby’s visual senses are the least developed at birth. Researchers have found that the brain region labeled the “fusiform face area” is responsible for your baby’s ability to recognize faces. According to Mark Johnson's 2005 publication "Developmental Neuroscience, Psychophysiology and Genetics," these abilities mature as your infant matures. By the time your baby is about six months old, he should be able to recognize your face immediately.
Transient Exuberance and Pruning
Although the infant brain has billions of neurons, their proportion of dendrites, those important branch like structures that process neural messages, are less abundant than in the brain of a school-age child. During the first few months, dendrites and synapses proliferate. The proliferation of neuronal growth during the first two years of life is called transient exuberance. Unused or misconnected dendrites are removed or atrophy naturally in a process called pruning. You and your baby will experience this throughout your lifetimes, but transient exuberance occurs most rapidly during infancy and in the first two years of life.