Human brain development

The brain is a magnificent organ. It is the reason humans spend 9 months in utero – to give the brain time to develop sufficiently. Human infants could even spend more time in the womb but due to birth canal size restraints, 9 months of development and head growth is all that mothers can handle. The human brain at birth has an over-abundance of neurons. Within the first 2 years of life, the brain prunes back the number of neurons as they are unneeded. Even in adulthood the number of neurons in the Central Nervous System (brain and spinal cord) is astounding – estimated at 100 billion! The number of connections between neurons – composed of dendrites, axons, and synapses – is estimated at 100 trillion.

At birth, few areas of the brain are well-myelinated. Myelin is a largely lipid-based substance (part of a type of glial cell; glial cells serve in mainly supportive roles to neurons) that wraps around the axons of neurons, like insulation around electrical wires, which increases the speed of transmission of action potentials – electrical impulses that travel down the axon when the signal is outgoing (or down the dendrites if the action potential is incoming). Myelination of the brain is not complete until into a person’s third decade of life, with the frontal lobes being myelinated last. The frontal lobes provide a lot of the oversight and control of the brain – decision making, language, and problem solving – so this slowness to myelinate in part explains children’s and adolescents’ often less-than-ideal reasoning (not that adults have wonderful reasoning all the time but adults often are more likely to think things through and be able to reason with complexity about situations and ideas).

So, the brain is such a complex and marvelous organ that it is a wonder that it develops so well most of the time.

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