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					                                                                             Neuro 405
                                                                Crawdad Lab 2 Questions

                    Lab 2- Crayfish Motor Nerve Recording
-Spontaneous Activity- drum roll, please…There are 6 axons in nerve 3. As you may
have noticed, at least one axon doesn’t show much/any activity until the tail fan has been
stimulated. I’ll show a picture of the nerve in cross section in lab, in case you are
dubious!

1. Why do the action potentials in your recordings differ in amplitude? What does the
amplitude predict about the axon’s conduction velocity and why? Why would it not be
this easy to distinguish action potential amplitudes of single neurons in a typical
vertebrate nerve like the sciatic nerve?
     APs differ in amplitude due to differences in axon diameter. Larger axons have a
        larger surface area for more AP current to flow through many more channels, thus
        giving a larger extracellular potential. Differences in AP shape come partly from
        different positions of axons relative to the electrode (this is why some APs appear
        triphasic or W-shaped, and others are biphasic) and partly form different
        conduction velocities.
     Large AP amplitude implies a larger-diameter axon, which conducts faster
        because of lower internal resistance (larger wires); remember the space/length
        constant?
     There are many thousands more axons in a typical vertebrate nerve such as the
        sciatic nerve. This means that the APs tend to sum in an extracellular recording
        and become indistinguishable. Individual APs are still all-or-none, but appear in a
        recording as a graded compound action potential. Additionally, vertebrate axons
        do not typically vary in size to the same degree that invertebrate axons do.

3. What factors limit the conduction velocity of an axon? Describe the two major ways
in which conduction velocity has been maximized in animals in the course of evolution.
Speculate on some experimental ways of changing these limiting factors and thus testing
their importance (imagine that you have a long large-diameter axon to work with).
     The time () and space/length () constants limit conduction velocity.
     Invertebrates generally use large-diameter axons to reduce Ri and increase .
        Vertebrates have myelin to increase Rm and increase . Larger diameter does
        increase Cm, which slows voltage changes, but the decrease in Ri due to volume
        increase is greater than the increase in Cm due to surface area increase. Myelin
        increases Rm and decreases Cm. Some animals have increased axon diameter and
        added myelin to speed conductance.
     Experimental ways to change the space constant include decreasing Ri by
        inserting a wire down the middle of a giant axon, increasing Ro with paraffin oil,
        and decreasing Ro by laying an axon on metal (Aidley, 1998). These all change
        conduction velocity.
4. If other lab groups found different numbers of action potential sizes and concluded
that there were more or fewer axons than you did, how do you account for the
discrepancy?
     More spike classes are seen if the signal-to-noise ratio is higher.
     Some axons could die during the course of an experiment if the nerve is stretched.

				
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posted:12/1/2011
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