It’s summer in Kansas, and although the weather has been mild and rainy all year, it has recently turned into the hot, humid days that I’ve come to expect since moving here five years ago.
As part of our effort to escape the sun, my son and I decided to do an experiment today on the the nervous system. It was a spontaneous idea based on something I recall Mr. Wizard doing years ago. In case you’re too young to be familiar with Mr. Wizard, he was something like a cross between Bill Nye and a crotchety old man chasing the neighborhood kids off his lawn.
In this experiment we wanted to ask whether all areas of the body were uniformly innervated. The way Mr. Wizard, and we, went about asking this was to make some assumptions and fashion a device to examine sensitivity based on those assumptions.
The reason I bring up the assumptions is that it is always important to recognize what you are taking for granted. This way, when you get your results, you can frame your conclusions within the scope of the assumptions you make (it will be obvious what I mean when we get to this point below).
1. Sensory nerves are what allow us to feel touch (pain / pressure in this case) on our skin.
2. We receive input from our sensory nerves from different parts of our bodies and we can both feel the pain and distinguish where the pain is coming from (i.e. you can tell the difference between pain in your finger from pain in your foot).
3. We distinguish these pains due to signals coming from different nerves (or possibly clusters)
4. We can activate these pain sensing nerves with the prick of a needle.
Given all the above, we devised a gadget to provide the nerve stimulation and get a measure of how far apart two stimuli are from one another. This was done by fastening one needle to the end of a ruler and another to a paper sleeve that can move along the ruler, thereby adjusting the distance between the two needles.
This experiment reminds me of the time we made our own laser-assisted surveying tool to determine how tall a tree was and whether it might be in danger of crushing our house when I cut it down.
Then we gathered some experimental data by using this device to stab one another (not too hard) with the two needles at the same time and measure the distance (in mm) between the two needles. we recorded the minimum distances that we could feel two pin pricks. Closer than these distances, the pain felt like a single pin.
As a second variable in the experiment, my son and I are very different in height. Because of this, we chose not to average the data between us, but to just have two measures (one for him and one for me) coming from each test point.
Here is a graphic representation of our data:
Minimum distance (in mm) that each subject could feel two pin pricks rather than one.
We propose that different areas of the body are differently sensitive. This followed a fairly predictable pattern with the fingers and tongue being the most sensitive and the arm and back being the least sensitive. Although we don’t have sufficient data to be certain, it appears that Harry (a smaller child) was more sensitive throughout his body that Jack (an adult). Further, as distances got smaller, error in our apparatus made it impossible to get accurate results for the finger and tongue. Assuming that differences in sensitivity are determined by the degree of innervation and accurately provide a measure of innervation, then the arm and back are populated by many fewer nerves than the fingers and tongue.