Afferents to Atoms: Your Experience of Touch | By Alex Cojocaru

What are you touching right now? Probably a set of clothes, a pair of shoes, perhaps a computer mouse. Maybe one or two other things.

Before we can explore the thought-provoking ideas I want to touch on, here’s a bit of high school review. Tactile sensation is as familiar to us as any of the other traditional senses, and with good reason. It’s effective for obtaining information about your environment, picking up and manipulating objects, and if you include pain and temperature, it’s also a valuable mechanism of protection. Like too many other physiological marvels, tactile sensation and perception are easy to take for granted. But what does it really mean to touch something?

From a physiological point of view, your experience of touch – let’s say in the hand – could begin with a distortion of the skin on your fingertips. Pretty much any disturbance you can nominate activates a unique receptor or group of receptors. It can be a stretch, poke, pressure, vibration, or other occurrence that activates a corresponding receptor. In fact, even your limb position has a special class of receptor devoted to its detection, called proprioceptors. Try it right now: without looking down, visualize the position and angle of every joint in your body.

A handful of the amazing receptors constantly feeding your brain information

After a receptor is activated, the neuron connecting to it depolarizes (or “fires”) and the signal shoots up into your spinal cord, whereupon it takes one of several vertical paths to your brain. In your brain the signal is propagated by one or more neurons and ultimately gets projected onto your brain’s sensory cortex.

But hold on.

What causes that distortion of the skin, anyway? It happens when you, for example, press your fingertip against some object such that your skin and the object can’t occupy the same space, right? That means that 1) one of them changes shape until 2) the forces equalize and you can’t keep pushing into the object. But why can’t your finger and the object occupy the same space? After all, recall that matter is mostly empty space. If the nucleus of an atom were 2 centimetres across, the nearest electron would be about 100 meters away. Do the math and you’ll see that only about 1/1012 (one trillionth) of the volume to the nearest electron is actually occupied by mass. That’s 0.0000000001%, much less than a thousandth of a pixel on your whole monitor.

And the electrons are basically delocalized fluff around the comparatively massive nucleus. Matter is mostly empty space.


Pictured: something that’s WAY off scale

The reason you can’t push right through something with your finger is because the electron clouds surrounding every atom clash once the atoms get within a certain distance of one another. Electrons are negative, and negative charges repel other negative charges. This is called the electromagnetic force and it can be observed when you push two magnets into one another with the poles that repel. If they’re strong enough magnets, you cannot get them to touch no matter how hard you push.

Cow magnets: strong enough to keep dumb quadrupeds alive

You might have heard that gravity is the weakest of the four fundamental forces of our universe. It’s hard to believe when gravity is the one force you experience most often. But keep in mind that the gravitational field you observe is the result of 6×1021 (six sextillion) metric tonnes worth of planet beneath your feet. And by rubbing a balloon against your hair for a few seconds, you can easily overcome it with electrostatic forces as you attract and lift a feather. The bottom line is that no matter is touching when you see two objects touching. No matter is contacting other matter when you grasp a doorknob in your hand and feel it turn. None of your atoms are touching any of your other atoms. Ever.

Remember the last time you fell to the ground. Maybe you slipped on ice or lost your footing while climbing over a barbed wire fence for some reason. It probably hurt. Your brain told you that because events in your spinal cord indicated injury. That signal came from ascending neurons in your feet or rear as you hit the ground. Those neurons were activated when some of your tissues were squished and distorted by one another or by the pavement beneath you. Irrespective of where you fall from – your own height or the cruising altitude of a 747 – the electromagnetic force won’t let you pass through the ground unharmed. It’s committed to fighting the gravitational force at all costs and will keep you (or what used to be you) above ground, no matter what.

So keep this in mind next time you stub your toe or bump your head. Imagine the pathway of the pain signal and remember that ultimately…

The electrons did it.

Alex Cojocaru is a fourth-year student at the University of Alberta, entering his first year in Medicine.

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  • anon physicist

    We don’t fall through things because of the EM force alone. While the EM force is important to make up the crystalline structure of the solid, it is not the repulsion of electrons that keeps you from falling through. The stability of the matter (your inability to fall through) is derived from the application of Fermi statistics and the Pauli exclusion principle – quantum mechanical effects (Lieb, 1976).

    Long story short, these quantum mechanical effects put a lower bound on the ground state of the solid in question. This ground state is what we mean by solid. Pretty cool, right?

    As for what is written in the article, I don’t know if it should be corrected since it is beside the point of the article… But it’s not technically correct. Nice try though :)!

    • Alex

      You’re getting a bit too technical. While I always shoot for maximum accuracy in what I write, describing these phenomena in such detail is beyond me. Nor would I really want to do it since it would interfere with the point I’m trying to make.

      I’m glad you didn’t comment on my math though.

      • anon physicist

        You may have misunderstood. I was not elaborating on a point in your piece. In this case, I was explaining why

        “The reason you can’t push right through something with your finger is because the electron clouds surrounding every atom clash once the atoms get within a certain distance of one another. Electrons are negative, and negative charges repel other negative charges. This is called the electromagnetic force…”

        is simply wrong. It’s not even correct at a surface level for casual readers.

        • Alex Cojocaru

          How is that wrong? Electrostatic attraction and repulsion are physical phenomena you learn about in high school. They’re very basic and have been observed countless times, like in Millikan’s oil drop experiment (attraction) or when Rutherford shot gold nuclei with alpha particles (repulsion).

          I’m not equipped to invoke exotic high level principles like the ones you cited, so it’s fine if I missed a piece of the puzzle because this isn’t meant to be an in-depth discussion.

          Besides, I checked with another physicist and he assured me my description was correct (but incomplete, as you pointed out).

  • Eric D

    Really clear and neat approach! I would’ve love to see an approach like this to the other senses – hearing would be especially neat I think due to the similarities.

  • Franklin

    I’d like to see an article like this written by a group of people in various sciences just to see what comes out of it

  • Jayne

    Again, another masterpiece by Dr Cojo… This actually makes a lot of sense to me, which is surprising because science like this usually confuses me… Great work for creating an article that those who are not scholars can understand. Thank you!!! Amazing work!!

  • Ege

    I like turtles

  • Stephanie

    Fun and concise article! Great emphasis on some of the more trippy aspects of science.