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The Growth of the Brain in Man
Initially, the human embryo is made up of only a few primitive cells, which develop into all the vital organs, including the brain. As the embryo's brain grows, cells called neurons migrate to various locations in what will eventually become the brain. This migration is guided by glial cells, which direct the neurons to their ultimate location. Later, other glial cells provide physical structure for the brain and may help send neural message. When they reach their destinations, the cells begin to specialize. They secrete chemicals that allow them to cluster and organize around specific brain function. The neurons continue to grow, forming thin fibres, called axons, send electrical messages to neighbouring neurons. The messages are received by other fibres, dendrites don't actually touch. They are separated by a tiny gap, called the synapse. How a message gets across the gap, and which dendrite receives it, determines whether you laugh or cry, whisper or shout.
The human brain grows remarkably fast before birth. The brain of a newborn baby contains about 100 billion neurons. This means that, from its beginnings in the early embryo, the brain grows an average of 250,000 nerve cells every minute during pregnancy. Events before birth affect every aspect of brain function for the remainder of life.
Early development if the brain follows a similar pattern in all mammals, but in humans the process takes longer. By three weeks after conception, the human embryo is about 2.5mm long, and a long hollow tube, the neural tube, runs along its length. This will become the spinal cord, and a bulge at one end of the tube will become the brain. A week, later, the three major brain regions - hindbrain, midbrain and forebrain- have begun to form from the neural tube.
Halfway through a human pregnancy, about 15 to 20 weeks after conception, cells of the cerebral cortex, produced by cells that line a hollow in the forebrain, are increasing by the millions every hour. They must then undertake a complex migration to their eventual locations. The location determines a neuron's behavior and function; neurons with similar characteristic often lie close to one another. Once they reached their final destinations, the neurons get bigger. Axons lengthen and dendrites branch out from the cell body.
With the brain growth so fast and complex, things can sometimes go wrong. But brain growth is also adaptable, so that many errors get corrected. Still, experts suspect that many human disorders and abnormalities (for example, some kinds of learning disabilities and mental illness) may result when the brain cells fail to connect as they should during pregnancy. A mother's use of illegal drugs like cocaine, during pregnancy can disturb the normal links between the nerve cells.
A neuron will not survive unless its axon reaches a target to synapse with. The target cell provides small proteins, called trophic factors, that are essential if the neuron is to survive. Between half and three0wuarters of neurons fail to forge connections and die.
By six months of pregnancy, the fetal brain is functioning, as it will after birth. The fetus itself is an active participant in this process, employing its brain to make its muscles work. It kicks, turns and even sucks its thumb. Each action causes axons and dendrites to branch, creating new synapses in the developing brain, and these new structures help the fetus acquire additional skills.
By seven months, a fetus's electrical brain waves can be measured through its mother's abdomen. Toward month nine, neurons increase more slowly, and then stop increasing. From then on, humans lose the ability to grow new neurons. But the existing neurons go on getting bigger, and their axons and dendrites get more complicated, so the young human brain's rapid growth continues.
By nine months the infant brain is so enormous that the human birth canal cannot accommodate any more growth. As a result, humans are born at a much earlier stage of development, and with many fewer skills, than other primates. This makes them much more dependent on adult care. The brains of newborn chimpanzee weigh more than half the weight of the adult chimpanzee brain. A human newborn's brain weighs less than a quarter of its adult weight. By contrast with other animals, the human brain does most of its growing after birth, which is why human infancy and childhood are so long.
This growth takes place along with massive elimination of synapses, a process known as pruning.. The adult rhesus monkey brain possesses only half the number of axons it had as a fetus. Humans are believed to lose at least ten times as many axons and synapses as monkeys do, in a pruning process that goes on at least to the age of 12. Pruning is essential for normal development. Because it is guided and shaped by the unique experiences each individual animal undergoes, the pruning process permits mammals to adapt flexibly to a variety of environments, and make possible the emergence of new behavioral traits and new skills. Pruning also eliminates incorrect neuron connections, helping to ensure that the brain functions well.
The human brain increases in size until about the age of 18. But even after growth ceases, the human brain continues to increase in complexity. Responding to experience, the neurons of adults alter their shape, their neurotransmitters, their synapses, and their second messengers, and all these changes in turn alter the way the brain functions. Even though the brain cannot grow new neurons, its ability to remodel its connections allow the human brain to recover some function after injury. In general, however, brain injury often results in permanent deficits.
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