Daily RC Article 42

Neurogenesis


Paragraph 1

A major tenet of the neurosciences has been that all neurons (nerve cells) in the brains of vertebrate animals are formed early in development.

An adult vertebrate, it was believed, must make do with a fixed number of neurons: those lost through disease or injury are not replaced, and adult learning takes place not through generation of new cells but through modification of connections among existing ones.

Paragraph 2

However, new evidence for neurogenesis (the birth of new neurons) has come from the study of canary song. Young canaries and other songbirds learn to sing much as humans learn to speak, by imitating models provided by their elders.

Several weeks after birth, a young bird produces its first rudimentary attempts at singing; over the next few months the song becomes more structured and stable, reaching a fully developed state by the time the bird approaches its first breeding season.

But this repertoire of song is not permanently learned. After each breeding season, during late summer and fall, the bird loses mastery of its developed “vocabulary,” and its song becomes as unstable as that of a juvenile bird.

During the following winter and spring, however, the canary acquires new songs, and by the next breeding season it has developed an entirely new repertoire.

Paragraph 3

Recent neurological research into this learning and relearning process has shown that the two most important regions of the canary’s brain related to the learning of songs actually vary in size at different times of the year.

In the spring, when the bird’s song is highly developed and uniform, the regions are roughly twice as large as they are in the fall. Further experiments tracing individual nerve cells within these regions have shown that the number of neurons drops by about 38 percent after the breeding season, but by the following breeding season, new ones have been generated to replace them.

A possible explanation for this continual replacement of nerve cells may have to do with the canary’s relatively long life span and the requirements of flight. Its brain would have to be substantially larger and heavier than might be feasible for flying if it had to carry all the brain cells needed to process and retain all the information gathered over a lifetime.

Paragraph 4

Although the idea of neurogenesis in the adult mammalian brain is still not generally accepted, these findings might help uncover a mechanism that would enable the human brain to repair itself through neurogenesis.

Whether such replacement of neurons would disrupt complex learning processes or long-term memory is not known, but songbird research challenges scientists to identify the genes or hormones that orchestrate neurogenesis in the young human brain and to learn how to activate them in the adult brain.

Topic and Scope:

Neurogenesis (the birth of new neurons); specifically, whether traditional thinking about vertebrate neurogenesis needs modification in light of research into how the process seems to work in canary brains.

Purpose and Main Idea:

To suggest that current thinking about vertebrate (especially human) neurogenesis needs to be modified in light of research into canary brains. The author’s main idea is that the results of this research both defy the existing theory about vertebrate neurogenesis and open up the possibility that human brains may have the capability to regenerate themselves.

Paragraph Structure:

Paragraph1 outlines the traditional theory that vertebrate neurogenesis basically doesn’t exist—that an adult vertebrate’s brain cannot create new nerve cells. The Contrast Keyword “However” signals that the passage is about to provide some evidence to the contrary and, predictably, Paragraphs 2 and 3 go on to discuss in detail the behavioral and neurological evidence (from canaries) that throws the accuracy of the traditional theory into question.

In Paragraph4, the author addresses the implications of these research findings for humans. The Contrast Keyword “Although” serves to distinguish the general scientific community’s skeptical response from the author’s much more optimistic stance that human brains may be able to regenerate themselves.

The Big Picture:

  • A good grasp of a passage doesn’t mean assimilating all of its details (you can look them up if you have to). It means understanding what the author’s doing in the text—in this case, understanding that the author’s taking issue with the traditional theory of vertebrate neurogenesis (and its implications for the human brain) by providing evidence that contradicts it.
  • While the author’s specific main idea isn’t entirely clear until you’ve read through the whole passage, topic, scope, and purpose are all revealed early. That makes this passage an ideal place to begin working in the RC section, even if you’re a science-ophobe. Topic, scope, and purpose, after all, are the three things you need to grab onto as quickly as possible in order to get a passage under control.
  • If you suffer from “science anxiety,” this passage should have come as a pleasant surprise to you. In practice, even the most jargon-filled and complex science passage won’t prove very difficult if you apply the Bodhee Prep techniques for critical reading.

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