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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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