Background

One of the major discoveries of 20th century neuroscience was the principle of somatotopy, that spatial arrangements of the body are similarly organized in the nervous system. For example, you have touch receptors in your fingers and palm that synapse with neurons that go to specific parts of the spinal cord:

That then synapse with specific parts of the brain on up to the neocortex (wrinkly part of the brain).

Procedure

We can view the first stages of somatotopy using our cockroach leg & the SpikerBox. The leg of the cockroach is covered with about 20 barbs that are sensitive to motion.

In this recording arrangement, let’s make the electrodes a little bit more selective (you will learn about this more in experiment 7). Place the electrodes in the leg like this.

Using a fine-tipped toothpick, try to touch each barb as you are listening to the spikes. If you are lucky, you should be able to find a barb, that, when touched with a toothpick, causes vigorous changes in the spiking activity. If you went further “up the path” of the nervous system to the cockroach ganglia, you would observe the same thing.

Now, move your recording electrode to a different place in the leg.

Can you find a new barb that is responsive? Can you find any sort of organization? Try this with a couple legs. You might even observe different firing characteristics based on placement.

Note, there is also a problem with this experiment. What happens as you keep reinserting the needle? Do you think this will affect your observation of somatotopy?