44

key concepts

44.1

Neurons and Glia Are Unique Cells of Nervous Systems

44.2

Neurons Generate and Transmit Electric Signals

44.3

Neurons Communicate with Other Cells

44.4

Neurons and Glia Form Information-Processing Circuits

Balancing the Brain

Your brain enables you to learn the material in this chapter—to read the words, understand the illustrations, and store information to allow you to answer questions. You must study to master this chapter, but if you do you will learn how the brain receives and processes information.

Imagine what it would be like if you could not learn any of this material—no matter how much you poured over the book. This is the situation faced by individuals with a condition known as Down syndrome, which affects 1 out of every 700 children born in the United States.

Individuals with Down syndrome are born with three copies of most of chromosome 21, one of the smallest of the human chromosomes (containing about 225 confirmed genes). Having an extra copy of these genes causes numerous developmental and functional problems, including learning disability. Can we understand the cause of this disability and perhaps find a way to remedy it?

One productive way of investigating the causes and possible treatments for a human disease or deficit is to develop an animal model. Through genetic engineering, researchers created a “Down syndrome mouse” that has most of the same genes triplicated as those in humans with Down syndrome. These mice cannot perform in mouse memory tests as normal mice do. The mouse Down syndrome model revealed that excess inhibition in the brain causes the learning disability. What is meant by “excess inhibition in the brain”? We tend to think mostly about the excitatory actions of the brain, likening the brain to a puppet master who pulls the strings that activate the muscles and organs of the body. In fact the brain is more like an orchestra conductor, making some sections louder, some softer, speeding up, slowing down. The brain must constantly maintain a delicate balance of excitation and inhibition, acting on some signals and ignoring others. In the brains of Down syndrome model mice there is consistently too much inhibition. When inhibition is experimentally reduced with certain drugs, the mice are able to learn. Research like this is a first step toward bringing potential therapies for humans into clinical trials in the hope of improving the cognitive functions of individuals with Down syndrome.

Can learning be restored in mouse models of Down syndrome?