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. The brain starts forming prenatally, about three weeks after conception. Before birth, the brain produces trillions more neurons and “synapses” (connections between the brain cells) than it needs. During the first years of life, the brain undergoes a series of extraordinary changes.
As the neurons mature, more and more synapses are made. At birth, the number of synapses per neuron is 2,500, but by age two or three, it’s about 15,000 per neuron. The brain eliminates connections that are seldom or never used, which is a normal part of brain development.
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 (Perry, 2002). 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 (Perry, 20002). 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. 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 (Perry, 2000a).
Lellah answered the question on August 4, 2021 at 11:17