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LAB 20 - Recording Action Potentials in Giant Axons of Lumbriculus Variegatus


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These worms are a freshwater annelid of the class Oligochaeta. Many annelids have medial and lateral giant axons that run the length of their ventral nerve cord. Their large size means fast conduction which is necessary for the rapid escape responses that many worms are capable of. This lab has been set up by the new physiologist at Humboldt State University, Dr. Bruce O'Gara. The purpose of this lab was to use the MacLab A/D recorders to visualize action potentials in individual axons.

Procedure and Equipment

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The oblong piece of plexiglass in the picture above acts as a sort of race track around which the worm will travel. We had to guide our worm over the two pairs of electrodes which can also be seen in the picture above, and then get him to stay there while we probed at him in order to generate an action potential. We were equipped with two tools for maneuvering our worm: a rubber band mounted on an applicator stick for moving the worm, and a hair, also mounted on an applicator stick for mechanically stimulating touch receptors in the skin of the worm. Sensory neurons carry the touch information to the giant axons which shoot action potentials down the length of the worm to activate motor neurons which supply shortening muscles in each segment. The net result is that the worm displays the quick escape response which protects it from fish. These are aquatic worms which dig into the substrates of ponds and streams and poke their heads above to capture food. Any slight touch stimulus will cause the worms to withdraw back into the substrate. During this escape response the action potentials travel down the giant axons quickly. Due to the size of the action potentials they can be recorded across the body wall of these slender worms by the electrodes of the grid.

High Gain Preamplifier

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The action potentials are conveyed to a high gain preamplifier, which in turn sends the signal to the input of the MacLab units. Note in the photo that the input is also sent to a small speaker so that a click is heard when the action potentials are produced. A scope software program was used with the MacLabs which makes them function as digital oscilloscopes.

MacLab Unit

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Here is a picture of the data on the MacLab screen after performing this procedure on our worm.