You have probably wondered, how fast are those action potentials traveling down a neuron anyway?
Well, here we present how to measure conduction veolcity in neurons, using a 2-channel spikerbox,
an earthworm, a faraday cage, a laptop, a ruler, and some balsa wood
You can find earthworms in your own yard after a rainstorm,
or you can buy them at a pet store, a sporting goods store or even a gas
station depending on where you live.
Now, earthworms can handle cold temperatures quite well, so we have to use a chemical means to
anesthetize them. I create a ten percent alcohol solution by mixing ten mL
of 80 proof vodka
with thirty mL of tap water,
and I take one of the healthy worms and put him in the solution...
and I wait about seven minutes for the anesthesia to take hold. It's important
not to wait too long or else you will pickle?? the earthworm. I then place the earthworm
in tap water to remove the alcohol solution and then place the earthworm in a piece of balsa wood.
You should become familiar with the anatomy of the earthworm.
Make note of the clitellum, which is the fleshy,
fatty? part near the anterior end or mouth of the earthworm.
Yu'll use this as a reference point.
Now you need to make two measurements to measure conduction velocity
and we hereby announce our new 2-channel SpikerBox! The 2-channel SpikerBox
comes with three electrodes: a ground I am placing near the anterior end of the
earthworm; channel 2 which I am placing near the center of the earthworm; and channel 1
which I am placing near the posterior end of the worm.
The silly?? needle electrodes are placed directly into the earthworm, slightly
off the center line.
Don't worry, the earthworm can recover quite well from this experiment and he
will be returned to his soil after we are done.
Now, the the earthworm's neurons are actually myelinated, so this makes the electrical
signals we're recording much smaller than we're used to with our cockroach experiments
In addition,
the earthworm is terrific antenna for electrical noise so we have to
enclose the earthworm in a metal mesh called a Faraday Cage.
Once we have this though,
we plug the 2-channel SpikerBox into our laptop and fire up Audacity.
Open up the preferences, select Audio I/O and in the device output select
built-in output. On the device recording input select built-in input or
line-in, and make sure to select 2 channels -that is stereo mode. Also select
hardware and software play-through so you can hear what you are recording.
Hit record and you should see two channels of information streaming across
your screen.
Maximize the display of the two channels and also zoom-in on the y-axis so you
can see the signals recording.
Why no spikes? That's strange...
Welcome to Sparse Coding,
when a single spike means a lot! Let's take a closer look.
What you are hearing are one or two spikes elicited when I touch the posterior end
with a toothpick.
Now, since I'm brushing the posterior end the spikes caused then travel down the
length of the earthworm past the two electrodes. Let's take a closer look.
I'm opening up the recording I just made
and again maximizing the window and zooming in on the y-axis so I
can see the spikes clearly.
You can see about 15 spikes in the recording. Let's listen again.
Now, to measure whether there is any time difference or lag
between the spikes on electrode 1, or top channel, and the spikes on electrode 2 or
bottom channel,
we have to zoom in at much smaller timescales as the spikes can occur quite
fast. And as you see, as I move in on very small timescales, I see there is a
difference between the two channels. I can measure this time difference
between the two dips??
and this comes out to about 4.6 milliseconds.
Taking into account the separation between the two electrodes
at 0.03 meters this comes up with the conduction velocity of
6.5 meters per second
or 14 miles per hour
I can do this analysis for every single spike in the recording.
I'm going to zoom out of my recording here and pick a different spike to do my analysis.
Focusing back in on a very tight timescale, I see again the lag between
the top and bottom channels, and again,
I can measure the difference between the dip on the first channel, and the dip on the second channel
and I come up with a time difference of 4.6 milliseconds,
exactly the same as the previous spike.
That's interesting: do you think it changes for different spikes in the recording?
why? Or why not?
Now, this 6.5 meters per second conduction velocity is actually on the low
end for what we would expected in an earthworm, which would normally be
between 10 and 20 meters per second. Do you think the anesthesia
had anything to do with it? Do you think there was any error in the accuracy
of my measurement with the ruler, or the time? We'd love to see what you measure
and what you come up with.
Backyard Brains, Neuroscience for Everyone!