Caution: BRAIN Under Construction
From a single cell, the human brain grows into the most complex object in the cosmos. As it grows, the brain is sculpted from the raw material of its cells, called neurons, and human experience.
WIRING THE MIND
A baby is born with all the brain cells he or she will ever have, but with relatively few connections--called synapses--between those cells.
The connections are forged by the growing child’s experience with the surrounding world. If neurons are used, they are integrated into the brain’s living circuitry. Unstimulated, the neurons die.
In the first years of life, a child generates up to 15,000 connections to each one of his 100 billion brain cells.
By age 2, the number of synapses reaches adult levels and surpasses them between ages 4 and 10. A child’s brain has twice as many neurons and twice as many connections between them and is twice as energetic as an adult brain.
A QUESTION OF TIMING
Chemicals promoting neural growth are released in waves. That means layers of tissue in different parts of the brain mature in sequence.
The timing of those chemical waves may determine when the brain is especially receptive to acquiring crucial abilities and some forms of learning such as language, sensory motor skills, math and music.
NEURAL MIGRATION
As neurons multiply, they travel outward to form the layers of the cortex, sliding up elongated glial cells.
But these young neurons are not fully functional when they start this journey. Only through contact with various other cells on the way, the stimulation of the senses, and through the action of various genes triggered by these encounters do brain cells take on their ultimate form, location and function.
SYNAPSE
The junction of an axon of one neuron and a dendrite from another is called a synapse.
The passing of a signal across this gap is mediated by neurotransmitter chemicals such as serotonin, dopamine and the endorphins.
Synapses develop in response to the stimulus of learning and experience.
ADULT BRAIN
By about age 16, the brain assumes its adult form.
The number of synapses falls dramatically as experience prunes unused neural connections.
Still, the adult brain retains a surprising degree of flexibility, and experience continues to mold the physical structure of the brain well into old age, researchers now believe.
AGING BRAIN
After age 50, some neurons start to die and neural synapses can atrophy.
But exercise, diet and mental stimulation help keep mental function at its peak.
The solitary lifestyles common among elderly Americans may worsen the mental effects of aging, researchers suspect, because if the brain is not stimulated, it may not retain a sufficient reserve of brain cells to withstand neurodegenerative diseases like Alzheimer’s.
NEURONS
Neurons are the special nerve cells that make up the central nervous system and the brain. A neuron may both receive and send out electrical signals to neighboring neurons.
A single axon conveys electrical signals to other neurons, and numerous hair-like structures called dendrites receive incoming signals.
THE BRAIN AT WORK
* Learning and memory are partly handled by structures, in green, such as the thalamus, hippocampus, amygdala and basal forebrain. Information, concepts and memories are stored in the furrowed cerebral cortex.
* Matching ideas to words and constructing sentences are handled in several areas, including areas for verb mediation, in blue; noun mediation, in purple; and word formation and sentence implementation, in red.
* Reactions to pressure and stress are in part the job of a brain system, in green, that includes the hypothalamus, cingulate gyrus, the pituitary gland and the amygdala. When activated, the system also indirectly affects the secretion of cortisol by the adrenal glands.
* Feelings are largely controlled by the brain’s limbic system, which includes, in green, the cingulate cortex, thalamus and hippocampus, as well as the fornix, amygdala and mamillary body.
Source: UC San Diego, UCLA, Caltech, Scientific American, Salk Institute
