The formation of neurons begins very early in the human embryo. By five weeks after conception, the cells in the developing brain begin dividing rapidly to form the 100 billion or so neurons that an infant’s brain has at birth. Once formed, neurons begin to migrate to the correct location in the brain, and some synapses begin forming. (Neurons are specialized cells that process information. Synapse is an area between neurons that information is transmitted between them.)
Prenatal conditions — including temperature, pressure and fetal movements such as kicking— stimulate the development of synapses. Myelination of neurons also begins prenatally, starting with the neurons of the spinal cord and brain stem. (Myelin sheath (myelination) is a protective cover that surrounds nerve fibres that extend from neurons to facilitate speedier and more efficient communication between neurons.)
At birth, the infant brain weighs 2/3 to 3/4 pounds (300-350 grams). Connections in the parts of the brain that control basic survival and reflexes are already well developed, and myelination in those areas is nearly complete. The newborn begins a rapid period of brain growth. Synaptogenesis continues very rapidly, with neurons in all parts of the brain making trillions of connections. Myelination continues in most parts of the brain. The neurons controlling hearing and vision rapidly become myelinated.
An average of 1,000,000 synapses per second are formed in the brain of the fetus and infant in the first two years of life. No wonder they are so tired and need so much help.