New Chapt 4 voltage clamp(1)

New Chapt 4 voltage clamp(1) - Chapter 4 Hodgkin and Huxley...

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33 Chapter 4 Hodgkin and Huxley and the Squid Giant Axon: The Discovery of How Ions Generate Action Potentials In the previous chapter, you were introduced to the action potential together with the ways that the voltage gated ion Na + and K+ channels operate to generate it. But how did scientists discover ion channels and how did they discover the types of gates that Na + and K + channels have and the time scales upon which they operate? The answers came from the extraordinary studies of Alan Hodgkin and Andrew Huxley, two of the greatest neuroscientists of the last century. In 1952, Hodgkin and Huxley wrote a series of papers that described the experiments they conducted that were aimed at determining the laws that govern the movement of ions in a nerve cell during an action potential. These were among the greatest experiments ever conducted in neuroscience and were the first to explain how ion channels work, which ions produced which effects on the membrane potential, and provide the basis for our understanding of how action potentials are generated and how they propagate down axons. Their insights form the bedrock of modern neuroscience. For this achievement, they were awarded the Nobel Prize in Medicine or Physiology in 1963. The Hodgkin and Huxley experiments were conducted on a most unusual animal, the squid, and used a newly developed electronic method called the voltage clamp. The reason they used the squid for their experiments is because the squid has giant axons that are used for escape (Fig. 1). To give you an idea how large these axons are, a typical axon in your body has a diameter of about 2 ! m (1.0 ! m is 1/1,000,000 of a meter) whereas the diameter of a giant axon is 800-1000 ! m, almost a full millimeter. In biology, bigger is always better because something larger is easier to see and far easier to manipulate than a smaller version. The squid giant axon beautifully illustrates the advantages of being large; indeed, it is so large that it can be seen with the naked eye and it can be cut out of the squid and placed in a dish filled with seawater. Wires, to record membrane potentials or to pass current, can easily be inserted down its interior, thereby allowing Hodgkin and Huxley, for the first time in history, to record a resting potential. The wire also allowed them to stimulate the axon and record the changes in membrane potential and ion currents that occurred during an action potential. As we shall see, the features they observed and manipulated were critical for understanding the ionic basis of the action potential, and could not have been done on any mammal because the axons in mammals are much too small. Hodgkin and Huxley used a new technique called voltage clamp to evaluate the ion currents that generate an action potential. The rationale for using voltage clamp and how it is accomplished is explained in a later section.
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34 Fig. 1. Left panel shows a squid and its nervous system. The giant stellate ganglion is where the fibers
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This note was uploaded on 09/19/2011 for the course BIO 365R taught by Professor Draper during the Spring '08 term at University of Texas at Austin.

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New Chapt 4 voltage clamp(1) - Chapter 4 Hodgkin and Huxley...

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