Glial cells serve as “nannies” to the neurons. They transport nutrients and oxygen to them and remove debris from them, keeping neurons healthy and alive. Each human brain has over 100 billion neurons: the brain’s “gray matter,” which is composed of neuron cell bodies. Glial cells, however, far outnumber neurons; there are 10 to 50 times more “nannies” than neurons in the human brain.
Neuroscientists are fond of saying, “Neurons that fire together, wire together” and “Neurons not in sync, do not link.” Dendrites form tree-like extensions that put a neuron in touch with as many as 200,000 of its neighbors, resulting in what we call new thinking and learning. When the brain learns, new dendrites grow. Early brain theorists believed that with each new memory, a new neuron grew. Today, we know that newly learned information is encoded as new dendrites sprout to connect neurons to specific sites, producing a new pathway that represents the experience.
In order for us to move, feel, and think, neurons relay messages to one another, using both electricity and chemistry. Once incoming stimuli reach a threshold point, a 270 mph electrical impulse “fires” down the axon. Once the electrical impulse reaches the end of the axon, a tiny pocket of chemicals bursts, sending neurotransmitters (the “chemical couriers”) across the synapse, the microscopic space between neurons. As neurotransmitters cross the synaptic gap they lock into receptor sites on the postsynaptic neuron and convey their chemical message only if their molecular properties fit the precise configuration of the receptor sites on the postsynaptic neuron. Over one quadrillion synaptic connections can be established inside the human brain.
To optimize message transmission, myelin, a fatty substance, coats the long axonal region of a neuron, speeding up signaling and insulating the axon from extraneous electrical or chemical impulses. A breakdown in myelin exposes the axon to misdirected electrical impulses. When diverted to unintended neurons, extraneous impulses can have devastating mental and physical consequences. Multiple sclerosis is caused by progressive degeneration of myelin.
Different regions of the brain become heavily myelinated during preprogrammed sensitive periods, which opens up windows of opportunity for developing specific skills or competencies. After a region is myelinated, a performance permanence sets in. Language-learning is one example. Every brain begins life with the capacity to learn any of the 6,000 languages spoken on Earth. When a child consistently hears the regular sounds (phonemes) in a given language, neural connections are created in the auditory cortex. The “window” for language-learning closes with the onset of puberty. Afterward, learning a new language will be more difficult and will typically be accompanied by a noticeable accent.
Pruning the Garden of the Brain
Synaptic proliferation is the prenatal overproduction of synapses that gives a young brain its incredible adaptability. We are born with many more connections than our adult brains will use. This neural insurance policy guarantees that infants born in San Francisco, Shanghai, or Soweto can flourish with equal ease. In the first two decades of life, the human brain “prunes” away connections in a dynamic self-reorganization that operates by the “use it or lose it” principle.
There is an old story about a man who walked from his farmhouse to his barn every day. After following the same path day in and day out, it wore into a groove. Eventually, the old man could walk to the barn blindfolded, since the deep channel would steer him directly where he was going. Neural pathways in the brain follow a similar pattern: They are strengthened with repeated use, while neglected networks become unreliable and eventually are pruned away.
Pruning helps the brain protect itself from devoting precious resources to useless networks and inefficient overwiring. Apoptosis (programmed cell death) eliminates unneeded neurons, just as roads that are seldom traveled fall into disrepair and eventually are closed down for good. Unused skills suffer a similar fate: what we call “forgetting.” (While memory failures are generally due to degraded neural networks, accelerated memory loss is associated with stress, aging, or acute brain damage.) Decreased use of skills reduces the nourishment of their networks, diminishing memory and performance.
In the absence of nearby land, some tadpoles will arrest the natural process of metamorphosis into frogs, because environmental conditions suggest that such a change is by no means beneficial to survival. Instead, those tadpoles remain swimmers. It is an apt metaphor for the developing brain.
Mother Nature offers a trade-off: instinct or flexibility. Those species whose behavior is dominated by instinct — e.g. reptiles, fish, amphibians, and insects — have brains that leave little room for neuroplasticity but are highly efficient. As a result, they are less adaptable. Human brains, on the other hand, were shaped by evolutionary pressures that rewarded adaptability. One example of our flexibility is the way our brains accommodate stimuli in multiple patterns and formats, but still accept them as the same object (See Chart 3: The Letter A).
Early Brain Growth
Neurogenesis is the rapid production of brain cells in utero, when neurons are produced at the incredible rate of 250,000 to one million per minute. The rapid growth of the young brain system begins 18 days after fertilization. The brain develops quickly through first-hand experiences. Computer simulations and early-learning videos are no substitute for the real world. A mere picture of an orange shortchanges the learner, who cannot directly experience its smell, texture, taste, and mass. Learners create meaning from what they do in their world, not from exposure to its representations.
While genetics and prenatal influences may calibrate the brain at birth, it is largely dependent on subsequent experiences to determine its capacities and deficiencies. Author Joseph Epstein stated, “We are what we read.” Neuroscientists would assert, “We are what we experience.” Neural circuits are constantly reorganized and rerouted, based on the quantity, quality, and timing of our experiences. This has profound implications for what we should do in every home and school.
The stimulation young children receive from early interactions determines how their brains develop in the crucial postnatal period, when experiences have a decisive impact on the brain’s architecture and later capabilities. Brain cells create connections each time we integrate something new. Whether we are learning to crawl or dance, these experiences create brain pathways that capture what we know and who we are.
In its early years, the brain goes on a connectivity binge. The immature brain quickly links hundreds of millions of neurons together, forming efficient brain circuits (See Chart 4: Neural Connections). During these stages, children make learning look easy. By adding, removing, or changing the strength of the connections among neurons, linking cells together or eliminating brain cells from existing neural pathways, neuronal activations change, making specific new learning possible. The word specific must be underscored here. All learning must be specific and transferable if it is to have any currency.
Creating new neural pathways is physically exhausting. The infant brain requires near-constant feeding to keep up with the energy consumption necessary for early brain development. Infants tax their energies when they are learning how to walk, talk, think, speak, and remember, along with familiarizing themselves with all of the people, places, and objects in their environment. Toddlers must also learn the complexities of language, and must master critical cultural and socialization skills. All of these are the minimum challenges that must be successfully and simultaneously met for adaptation to the environment. Synaptic connectivity maxes out during the second year of life. At its peak level, each neuron averages 15,000 connections. That number occurs in the early years of child development, when a toddler’s brain consumes 225 percent of the energy of an adult brain.
