Let's get started. Pictures of the brain are all around us in newspapers, magazines, and on television. Therefore, you probably already have a general idea of what the brain looks like. We're going to probe deeply into the structure of the brain so that you will know precisely why we want you to practice certain strategies and techniques and exactly what is happening as you reinforce your mental agility. Let's start with some of the vocabulary and imagery needed to talk about and visualize brain structure.
neuron—nerve cell in the brain
The neuron is the basis of the brain. It is estimated that the brain contains 100 billion neurons, which is roughly equivalent to the number of stars in the Milky Way galaxy.2 Each neuron is a nerve cell composed of a center, named the nucleus, with long, slender axons and short, bristly dendrites.
axon—messenger of the brain dendrite—message receiver in the brain
Neurons communicate with each other by passing nerve impulses down the axon of one neuron. At the end of each axon is a minute gap called the synapse synapse—gap between axon and dendrites
As the nerve impulse reaches the end of the axon, a chemical, called a neurotransmitter, is released into the synapse. On the other side of the synapse is a dendrite of another cell that acts as the receiver for the signal.
neurotransmitter— a chemical released by the axon to send and store messages across a synapse
In Figure 2-1, you can see the path the message follows from one neuron to another. This message travels from one
Figure 2-1 Connections between brain cells
Figure 2-1 Connections between brain cells neuron out its axon, is transmitted over the synapse, and travels up one of the dendrites of another neuron on its way to its destination. The message may be passed from neuron to neuron until it reaches the final destination. On the other hand, the neurotransmitters released into the synapse may alert multiple neurons at the same time. These message pathways are called connections.
connections—message pathways in the brain
Also present in the spaces between the neurons are trillions of glial cells, from the Greek word for glue. These cells support, sustain, and safeguard the neurons and may have a communications system of their own.
glial—glue-like cells that form a support for the neurons
The neurotransmitter performs a critical function in the creation of these communications connections. Each time a message is transmitted between cells, the chemical that is released into the synapse physically changes the receptor dendrite. Bristly spikes build on the dendrite as a result of the interaction of the chemical neurotransmitter and the dendrite. It is the repetition of this chemical reaction along the message route that strengthens the connection. You will learn in Chapter 4, "Develop a Brawny Brain," how to use rehearsal to make a memory stronger. (Visit the Web site www.mentalagility.com to view an animation of this phenomenon.)
The basis for these communications connections already can be identified approximately 10 weeks after conception when the brain is merely one-half inch long.3 Neurons begin pushing their way from the neural tube deep within the interior of the fetus's brain to the outside margins, with newer neurons pushing past older neurons. Neurons travel along glial fibers much like traveling down a highway toward their final destination with the assistance of proteins and chemical cues.4 By five months into the pregnancy, the fetus's brain is now two inches long, and the migration of neurons to the outer layers of the brain is almost complete.5
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