The Amazing Human Brain and Human Development

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Lesson 3: The Brain's Building Blocks
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The Brain's Building Blocks

Lesson 2 taught us about the major sections of the brain and what their primary functions are. Lesson 3 is going to take you deep into the structural units of the brain, to the neurons and cells.

If you are feeling even slightly confused about some information, don't hesitate to go back and reread the previous lessons, or log on to the Message Board and ask about it.

Parts in the Trillions

The brain is an amazingly complex organ. Indeed, it is the most complex biological organ in the known universe. It is composed of trillions of "moving parts" -- the cells of the nervous system.

The basic structural units of the human brain are cells. The brain is made of two major types of specialized cells, neurons and glial cells. Neurons, as you may recall from your high school biology class, are cells that specialize in receiving and transmitting signals.

Each cell, whether neuronal or glial, possesses a cell wall. The cell wall is a membrane that separates the inside (intracellular) components of the cell from the outside (extracellular) environment of the cell. It is inside of these that we store our genetic material -- genes. In each cell, at any given moment, only small portions of our genes are being activated and producing proteins, while the rest remains unexpressed in that cell. Amazingly, each of the 100 billion neurons and one trillion glial cells of the brain contain the exact same genetic material, the same genes. And more amazing, each of these cells expresses a unique pattern of gene activation that is a reflection of the cell's history and current environment.

This is one of the miracles of biology. Each cell will produce those genetic products that it needs to do its specific "job." And, in ways we have yet to fully understand, billions of cells can coordinate, orchestrate, communicate, and work together to create the most complex of biological machines from these billions of "moving parts" -- a human being.

Neurons are cells specialized to receive, store, and transmit information. The business of neurons is communication. All neurons have special structural features that allow neurons to "communicate" -- to receive, process, store, and send "information" that comes from their outside (extra-cellular) world.

Specialized structural and biochemical properties allow neurons to receive a stream of chemical signals from other neurons, process these incoming "messages," change their chemical interior in response to these signals (and thereby, store important information), and then transmit the summed signals to other neurons. Chains of neurons engaged in continuous dialogue and communication create the functional systems that allow the brain to mediate and control a host of remarkable activities.

There are hundreds of "types" of neurons. They can either be classified by unique structural properties (i.e., how they look) or by unique functional properties (i.e., what they do). Neurons that are directly involved in the transduction of physical or chemical signals from sense organs are called sensory neurons. Motor neurons end directly on muscles or glands. Interneurons, not surprisingly, interconnect other neurons.

In some areas of the brain, neurons are densely packed, while in others they are relatively distant. Most neurons form their connections with neurons that are physically adjacent. Other neurons send axons to neurons in distant areas of the brain. These are called extrinsic neurons. Extrinsic neurons tend to form groups or clusters called nuclei. A single neuron or group of neurons can send simultaneous signals to many areas. These nuclei play important roles in the orchestrating and coordination of communication.

The neuron is one of two types of specialized cells that make up the human brain. Image courtesy of Bruce D. Perry, M.D., Ph.D.
The neuron is one of two types of specialized cells that make up the human brain. Image courtesy of Bruce D. Perry, M.D., Ph.D.


What's an Axon Anyway?

The axon is a tiny fibrous extension of the neuron. An axon looks like a tail on the neuronal cell. It is the axon's job to facilitate the neurochemical transmissions that allow communication from neuron to neuron. Axons are extremely thin but can be incredibly long. Some axons reach all the way from the spinal cord to the feet.

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