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
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
Large AP amplitude implies a larger-diameter axon, which conducts faster
because of lower internal resistance (larger wires); remember the space/length
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
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
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.