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Experiment: Measuring the Speed of the Worm's Giant Nerve

Revision as of 07:56, 12 October 2012 by Kyle (Talk | contribs)
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Background

--write up currently in progress--

We previously discussed how to record the conduction velocity from the earthworm fiber system. As we said before, the earthworm contains three large axons that run its length, the “medial giant” fiber and the two “lateral giant” fibers.

PIC OF WORM LGN/MGN GENERAL

The medial giant fiber (MGN) transmits information about the front of the worm (the part closest to the clitellum or anterior), and the lateral giant fibers (LGN) transmits information from the skin cells of the rear or posterior end of the worm. The medial giant fiber, at 0.07 mm in diameter, is slightly wider than the lateral giant fiber (0.05 mm in diameter) (Kladt et. al 2010)

PIC OF LGN AND MGN SAGITTAL CUT

Both the MGN and LGN play an important role in the helping the worm’s sensory system to communicate(Drewes et. al 1984 & 1990) with its muscular system. The MGN and LGN also correspond to either anterior stimuli or posterior stimuli. There is a shortening or lack of sensory involvement of the MGN as you get closer to the posterior end of the worm and the same for the LGN as you approach the anterior. Having such shortening in both ends of the worm begs an interesting question: Does the worm have different conduction velocities in its LGN and MGN?

PIC OF SHORTENING

There are two ways conduction velocity can be increased in the animal. Once is called “saltatory conduction” which is done when action potentials “jump” between spaces called “nodes of ranvier” These nodes are spaces between the myelin sheath. Myelin covers the axons of many nerves in your body, and it allows neurons to stay small yet still have fast conduction velocities. The actual way myelin allows saltatory conduction to occur is quite complicated, and Neuroscientists are still learning how to explain it easily

PIC OF NODE OF RAVIENER

As you increase the diameter of the axon, you decrease the longitudinal resistance, which in turn increases the space constant. If you double the space constant, you double the conduction velocity, and the space constant increases as a square of the diameter. Thus, the MGN is 1.4 times greater in size than the LGN, and thus should be 1.18 times faster.

MATH...............

Let’s go ahead and measure this. But how! We have already shown how to record from the LGN, here is how you record from the LGN. We can use our newfound knowledge of Statistics (hyper link to stats) to see if the conduction velocities you mesure are significantly different?

Procedure