This is the 2007 version. Click here for the 2017 chapter 02 table of contents.

Neurons

To understand how the brain works, even on the simplest level, we must understand the building block out of which the brain is constructed: the individual nerve cell or neuron. Chemicals found in and around neurons are known to be involved in pleasure, pain, excitement, depression, sex, hunger, and the effects of drugs. But that is only the beginning. Neurons are ultimately involved in everything the brain does.

Components of the Neuron

What are the three basic parts of a neuron?

The first type of neuron studied was the motor neuron (or motoneuron), which stimulates muscle fibers to produce movement. Because it was the first type studied, the motor neuron became the prototypical textbook neuron, used to illustrate the three basic parts of neurons: dendrites, cell body, and axon.

The textbook neuron above was based on the first type of neuron discovered: the motor neuron (a.k.a. motoneuron).

Dendrites are tubes composed of the cell's outer layer or membrane. They stretch out from the cell body and divide into many small tubes (many more than the textbook neuron shows, and in all directions). The result is at least as complex as the branches of a tree. The dendritic structure as a whole is called the dendritic tree (the Greek word dendros means tree).

What happens to a successful neuron?

A successful neuron—one that makes a valuable contribution to the nervous system—will generally grow because it will be allocated more of a chemical called a nerve growth factor. After humans pass the age of seven, the number of neurons in the nervous system declines, but the complexity continues to increase, because the dendritic trees continue to grow. An adult human has fewer nerve cells with each passing year, but the remaining neurons can become more complex. Dendritic growth continues in healthy people until their 90s.

What is an axon?

Leading away from the cell body is a specialized fiber called the axon, a thin tube specialized for carrying messages to other cells. Axons can stretch over long distances in the body. The longest axon in the human body stretches from the base of the spine to a muscle in the big toe. An axon on one of these motor neurons can be over a meter long. In the classic 1930s model of the neuron (which proved to be oversimplified in many respects) axons were the output structures of neurons, carrying signals to synapses (areas of near-contact with other cells) where communication between neurons takes place.

Axons are like dendrites in several respects. Like dendrites, axons are made out of cell membrane stretched into long tubes. Like dendrites, the axons may branch into a tree-shaped structures. In the case of axons, they are called axonal arborizations (the tree metaphor again, this time from the Latin arbor rather than Greek). In fact, dendritic trees and axonal arborizations look much the same and are difficult to distinguish under a microscope.

What is the point of the professor's demonstration with a ball of string?

Axons—the structures that carry nerve impulses—are extremely thin, compared to their potential length. One professor illustrated this by bringing a ball of string into class. She said, "Suppose you were making a scale model of a neuron. The string represents the axon. If the thickness of the string represents the thickness of the axon, how long should the string be, in our scale model?"

The professor started pulling armlengths of string off the ball, then stopped and revealed the truth: "Actually, if it is long axon cell, you would have to unravel about two miles of string." Appropriate gasps of amazement came from the audience of attentive students...

Well, perhaps nobody gasped, but it is amazing that axons can be so very thin compared to their length. An axon propagates messages down its entire length, without loss of strength, at speeds from 2-200 mph (3-320 kph). Thus axons communicate rapidly, with great precision, over long distances in the body. This is the only way the body can communicate rapidly over long distances. Hormones and other influences are much slower.


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