Tag Archives: neurons

Neuron Development

Neurons grow like wildfire before birth! The developing brain generates between 50 and 100 thousand new cells per second from the fifth through twentieth weeks of gestation. These brain cells migrate to different locations in the brain and begin to differentiate, although many more neurons than are needed are produced.

Eventually about 75 percent of these cells will die in the normal development of the brain and nervous system. This process – programmed cell death – is known as apoptosis. Of those cell connections present at birth, some 40 percent will be pruned, just like weeding a garden to dispose of what is not needed.

Birth to Age 3
It might surprise you to know that a three year old has about twice as many connections as an adult, but who has more “intelligence”? From birth through about age three there are vast numbers of connections and collections being recorded in the brain, but there is no understanding or organizing of this wiring. The brain does not yet have the ability to distinguish what is important.

Ages 3 to 12
From age three through twelve the brain begins to realize, as a result of collecting too much, that it has recorded events in more than one place, resulting in dense sets of synapses. At this point it begins to prune the excessive synapses in an attempt to get organized and eliminate what is not necessary.

Ages 12 to 19
The teenage years consist of more aggressive pruning as the brain begins to specialize and build an identity. While the brain does a ton of learning in the early years, the bulk of learning takes place after the frenzy of forming synapses has stabilized and the duplicated synapses have been pruned.

Adulthood
Adulthood ushers in a bit of a pruning plateau, where some connections are diminished and others are enhanced. These are the years of using the brain as life experiences fine-tune the neural connections!

For more on brain development, check out J. Madeleine Nash’s article, Fertile Minds, in TIME.

Neurons

As noted previously, the Cerebellum takes up just ten percent of the brain’s mass but contains about half of the brain’s neurons. That is a huge amount of processing power contained in a relatively small portion of the brain; kind of a heady responsibility for the Cerebellum!

One possible reason that the Cerebellum contains this large quantity of neurons is that the Cerebellum does a lot of communicating with many areas of the brain in its diverse role as coordinator of muscle movement, maintainer of bodily equilibrium, handler of cognitive patterns such as speaking, automator of certain repetitive tasks, and responder to novelty. Whew, a busy schedule, to say the least! Since the neurons are the pathways that let all of this communication take place, it makes perfect sense to have so many in the Cerebellum.

There are about one hundred billion neurons in the brain. Essentially they all deal with the same thing – facilitating communication within the brain by sending impulses between neurons and thus throughout the brain. This sending of impulses is assisted by neurotransmitters.

neuron-chat-21.jpgSo how do neurons communicate with each other? First it helps to have an idea of what a neuron looks like. Think of the nucleus, which is surrounded by the cell body (called the soma), as a circle in the center of an octopus. The dendrites of the nucleus are like the tentacles of the octopus, extending out from all around the cell. Imagine one dendrite, called the axon, stretched out longer than the rest and at the other end of this axon are many axon terminals that look like spindly fingers. And now imagine that there are billions of these neurons within the brain.

A typical neuron “chat” takes place extremely fast. The dendrites extend from the perimeter of the cell body, with the nucleus in the center. The cell either fires or not, and the fired message shoots down the axon to the pre-synaptic terminal, which is the end of the neuron and the end of the axon. The pre-synaptic terminal is right next to, but does not touch, the dendrites of the next neuron. Simplified, the dendrites receive signals, and the axon and its terminals send signals.

At the end of each axon terminal are neurotransmitters. The neurotransmitters float across the synapse, which is the space between neurons, and communicate with the next neuron. Each neuron synapses onto many different neurons. On any given neuron some synapses will tell it NOT to fire while other synapses will tell it TO fire. The sum total of all the synapses’ influences will determine whether the neuron fires or not.

For a more detailed description of this process along with a wonderful set of Flash movies that clearly animate and visually explain neurons firing, please visit The Consortium on Mind/Brain Science Instruction’s article Neurons, synapses, and neurotransmission: An introduction and scroll to the bottom of the screen for the Flash animations.