At some point you may have thought: do all neurons have the same conduction velocity,
or do certain neurons have different conduction velocities? And if so, why?
In this experiment, you'll record the conduction velocity of the earthworm's lateral and medial
giant fibers.
Both fibers run the length of the worm, but each fiber serves a different purpose in the
earthworm's nervous system.
Let's get started!
In this experiment, I'll be using a 10% alcohol solution to anesthetize the earthworm. However,
if you do not have alcohol available, seltzer water, which is pure carbon dioxide can be
used as an alternative instead.
Set your stopwatch for about 4 minutes, then place the worm inside the anesthetic bath...
Now that approximately 4 minutes have passed, take the earthworm out of the anesthetic bath
and place it in the water bath.
The water bath washes excess alcohol off the worm.
This experiment is in many ways just like the first earthworm experiment.
Place the worm dorsal-side up, so the dark side of the worm faces up towards you.
Also, make sure that the posterior -or back- end of the worm is closer to the left edge.
Start by inserting the electrodes into the worm, slightly off the center line.
Place electrode 1 closer to the posterior end of the worm, and electrode 2 half an inch
further up the worm.
Place the final ground electrode near the anterior -or front- end of the worm.
Now place the earthworm into the Faraday Cage with the worm's posterior end facing towards
you.
Once the SpikerBox is turned on and Audacity is recording, you'll start recording from
the posterior-or back- end of the worm.
Now, let's listen for the sound of some glorious, beautiful spikes.
When you touch the posterior end of the worm you hear some great-?spikes. Once you hear
those great-? spikes, you'll want to turn off the SpikerBox, but you MUST NOT stop Audacity.
Allow it to continue recording without interruption.
Now that you've turned off the SpikerBox, remove the worm from the Faraday Cage and
take out electrodes 1, and 2, and the ground.
Next, spin the worm around, 180 º, and make sure the dorsal, or dark side of the worm
continues to face up towards you.
Finally, you'll replace the 3 electrodes: electrode 1 will be at the anterior end of
the worm, and the ground electrode will be at the worm's posterior end.
Now place the earthworm, with it's anterior side facing you, back into the Faraday Cage,
and turn the SpikerBox back on.
Don't forget, you do NOT want to stop your Audacity recording during the experiment.
That sounds like great neural activity to me!
Once you hear some good spikes, go ahead and turn off the SpikerBox and stop the Audacity
recording.
Return the earthworm to its box and clean your electrodes.
Next, you'll be taking a closer look at the data you just recorded.
Here's the Audacity recording we just took. The first group of spikes -highlighted- are
your posterior lateral giant fiber recordings.
The lateral fiber receives sensory information from the posterior end of the worm.
This middle flat-line area is where you turned off the SpikerBox.
This was done to create an easily recognizable area that separate your posterior and anterior
data.
This last group of spikes -highlighted- are your anterior medial giant fiber recordings.
The medial fiber receives sensory information from the anterior end of the worm.
Now you want to zoom in on one of the posterior spikes in the first group, so we can figure
out the conduction velocity.
Continue to zoom in; you want to be able to see each spike in great detail so you can
see which spike belongs to electrode 1 and electrode 2.
Now you can see both channels. The first spike is electrode 1, and the second spike
is electrode 2.
Conduction velocity is measured in meters per second, or meters over seconds, so you
want distance and time.
Time is the milliseconds it took to go from electrode 1 to electrode 2.
You essentially want to find out how many milliseconds elapsed in the highlighted area.
That is, how many milliseconds was it between electrode 1's dip and electrode 2's dip.
An easy way to find this is go down to the selection start area at the bottom of Audacity
and make sure you have the "length" button checked.
Then click on the drop-down menu and make sure you have "hours, minutes, seconds + milliseconds"
selected.
Next, you want to click on the on the bottom of electrode 1's dip, and drag the mouse
to the bottom of electrode 2's dip, so that you highlight the area in between the two.
Once you've done that, look back down at the bottom of Audacity, and right below where
you previously clicked the "length" button, you'll see 0.002 seconds, which equals 2 milliseconds.
2 milliseconds is the time between when the spike passed electrode 1 and electrode 2.
Now let's go find the time for one of the anterior recordings.
Remember, conduction velocity is a measurement of distance over time.
We already have our distance -this was how far apart electrode 1 and 2 were, which was
0.5 inches.
We just got the time for the posterior spike, now we're going to go get it for the anterior
spikes so that we can compare the two.
And once we highlight the area, we see that there's a time of...1 millisecond.
But, are we really happy with a measurement of one millisecond? Can we get a more accurate
measurement?
What if we try to zoom in on each dip and get a precise time for each electrode?
We can then subtract electrode 1's time from electrode 2's time for a more precise
measurement.
Go ahead and zoom in on the first electrode...
...and click at the very bottom of the dip...
...and look at the very top, and you can find an exact time of 34.9084 seconds...
...and now you're going to go ahead and do that for the second electrode...
...and you get a time of 34.9089 seconds for electrode 2...
...and then if you go ahead and subtract electrode 1's time from electrode 2' s time, you wind
up with 0.0005 seconds, or half a millisecond.
And this is a big difference from what we originally got, which was 1 millisecond with
the other method.
While half a millisecond may not seem like a big deal, the impact it has on the final
result will be huge.
Remember, the purpose of this experiment was to compare the conduction velocity from the
posterior and anterior end of the earthworm.
You want to see if there is a difference or if the are actually the same.
Because conduction velocity is measured in distance over time, you need to collect both
of those measurements.
Distance is easy, because it is how far apart the electrodes are placed in the worm.
Time is a bit trickier, and requires you to find the millisecond difference between the
two electrodes.
Here are the final results!
Remember when we were looking at the anterior spike, and at first we had measured a time
difference of 0.001 second, or 1 millisecond?
If we substitute 0.001 seconds, for the anterior's, 0.0005 seconds, we'll get a conduction velocity
of about 12.5 meters per second.
This is a huge difference, and it really highlights how the methods you choose to analyze your
data can have large impacts on the final results.
From these two measurements, it seems there may be a difference in conduction velocity
between the medial and lateral giant fibers.
Do you think it has to do with the axon diameter size difference?
Or perhaps with the difference with the amount of myelin wrapping the axons?
We would love to hear what you think, and what measurements you got!
Thank you for watching, and until next time! Have fun discovering and learning.