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Study Guide
Biologists often take the comparative approach to answer research questions. The comparative approach uses the organism that is best suited to answer a particular question. For example, the first experiments to determine how a nerve impulse is sent were done on the giant axon of the squid. The axon is the long skinny part of a nerve. Because it is so skinny, it is difficult to put an electrode in it and record. The giant axon of the squid is terrific because it is big enough to make these kinds of recordings.
Parts of a Nerve Cell
Credit: Massimo Scanziani and Frédéric Pouille, UCSD
The giant axon of the squid allowed researchers to determine what exactly is going on within a nerve cell when a squid decides to make a quick escape. The researchers were able, for the first time, to measure the electrical impulse - or action potential - that occurs when the nerve is activated, such as when a signal from the squid's brain is sent to the muscles that propel it away from danger This research resulted in a Nobel Prize in 1963.
Studying the squid's escape response provided important information regarding what is happening inside a nerve cell when a decision is made. However, it is a simple either/or response - the squid either propels itself out of danger, or decides it is not in danger and stays put. Of course, researchers want to know what happens when an animal has to choose between multiple options. To do this, Professor William Kristan studies leeches, which have to choose between three options when faced with possible danger: to stay put, to crawl or to swim.
Leeches
Credit: William Kristan, UCSD
Close-up of a Leech
Credit: William Kristan, UCSD
Leeches may make you feel squeamish, but they are little miracle workers in medicine. They are used to help keep circulation going in people's toes, fingers or limbs that have been reattached after being severed in an accident. They are also quite useful for the comparative approach in biology. They have relatively large nerve cells, and it is possible to record electrical impulses from their cells. Also, they can crawl like inch-worms, or swim like they are doing "the wave." So researchers want to know how they decide to use one form of locomotion or the other.
You can see leeches crawling and swimming in the following video segments (in Quicktime):
Professor Kristan and his colleagues can record from nerve cells when the leeches are exhibiting these behaviors, and therefore can determine what is happening inside the nervous system when leeches are deciding what to do. They have learned that individual nerve cells are not specialized for one behavior or another. Instead, it is what combinations of nerve cells that are active that determines what behavior the leech will engage in.
To learn more about how leeches decide what to do, watch the following video segment:
Leech Behavior (in Realplayer)
Just as with the leech, the parts of the brains of humans are not just specialized to do one thing. Instead, different combinations of areas of the brain play a role in different behaviors. Making decisions may engage the parts of the brain involved in emotions, memory and rational thought. Recently, researchers at UCSD found a specific nerve pathway that seems to be involved in making rapid decisions. The findings may shed light on how movement is controlled, and also what happens in a range of disorders including addiction, Tourette's syndrome, stuttering and Attention Deficit Hyperactivity Disorder.
For more information on how humans make decisions, visit National Public Radio's Science Friday.
The comparative biology research that has examined decision-making in invertebrates, such as leeches, is revealing important information about the basic mechanisms that control behavior. For example, researchers have learned how different networks of cells switch on certain behaviors. Tracking behaviors down to groups of nerve cells is more challenging in the human brain with its billions of cells. However, brain scans and behavioral studies are also shedding light on human decision-making. Therefore, through these different types of studies, researchers are putting together the pieces of a complex puzzle.
