Emma Dauster | October 12th, 2020

You’re walking into a room and Bang! You hit your elbow on a table. You exclaim “Ouch” and grab your elbow. Why did you just feel the need to touch your elbow? You just experienced pain from a touch to the elbow, so why would your next response be to touch it more? And why did it actually make you feel better?

Figure 1. Something inside you tells you to grab your elbow after you bang it on a table. source: pxfuel

Pain is conveyed to your brain through nociceptors, or pain receptors, that are present under your skin and in your muscles. They are a sub-classification of receptors sensitive to touch, also known as somatosensory receptors. Touch signals come from all over the body, but you have way more receptors on your hands, feet, and mouth than anywhere else [1]. These regions are super sensitive to touch and their representation takes up way more of your literal brain space than touch signals from the rest of your body.

*Fun at-home science experiment* Ask your roommate/parent/sibling to close their eyes. Take a pencil and lightly touch their fingertip. Move 1cm down the finger and touch the same fingertip. Keep moving 1cm away until they can tell that you’re touching a different location on their finger. Now try the same thing on the back of their arm where the triceps are. Spoiler alert: you can probably go farther away from the original touch before a difference is noticed on the triceps than on the finger because there are way more touch receptors on the finger.

But let’s get back to your hurt elbow. First there’s a sharp pain. Then after one or two seconds, a dull throbbing pain usually comes to your attention. This first and second wave of pain are due to two different fibers that bring information from the part of the body that you just hit to your brain [2,3]. The first sharp pain comes from a myelinated (insulated) fiber, so it sends the pain signal to your brain faster than the unmyelinated second dull pain fiber. See this post for more information on the effects of myelination on speed of information relays. The fast pain signal can travel up to 30 meters per second, while the second wave of pain travels at 1 meter per second [1].

So how does that bump to the elbow travel all the way to your brain? Well, the receptors for pain are just under your skin, but the cell bodies attached to those receptors are actually located in your spinal cord. Imagine a cell sitting in the spinal cord and reaching out their arm (fiber) to access parts of the body like just under the skin on your elbow that are more likely to experience disruptions due to external stimuli like hitting a table. The pain signal travels from your elbow to your spinal cord. The same cell that reached an arm or two to the elbow usually reaches another arm (axon) all the way up the spinal cord and into a part of the brain called the thalamus. The thalamus processes sensory information from all five senses (although olfactory information goes through the thalamus indirectly).

Figure 2. This is a representation of what your spinal cord looks like in your body. Source: Bruce Blaus, Wikimedia commons

The intrigue comes at the level of the spinal cord. Some of the cells that receive this pain information in your spinal cord can also receive information about non-painful stimuli in your environment. They are called wide-dynamic-range neurons (yes, there are neurons in the body, not just the brain) and get information about a breeze on your skin or a light touch that shouldn’t hurt. This might be a reason that you grab your elbow after you bang it on the table. The touch signal goes to the same neuron that the pain signal goes to so it could be distracting your brain from pain. By stimulating the second wave of pain, you can suppress the first wave and have a smaller pain response [4].

Figure 3. This is a drawing of a cross section of the spinal cord in figure 2. Here, you can see the axons going up or down the spinal cord. Source: Villiger, Emil, Piersol, George A. (1912) Brain and spinal cord; a manual for the study of the morphology and fibre-tracts of the central nervous system

Next time you injure yourself, you can distract your brain from the pain in two ways:

1.      Touch the part of the body that just got hurt

2.      Think about how cool your neuronal system is and marvel at the inner workings of your body.

References:

1.      Kandel E, Schwartz JH, Jessell T (1981) Principles of Neural Science.

2.      Fields H (1987) Pain. New York: McGraw-Hill.

3.      Perl ER (2007) Ideas about pain, a historical view. Nat Rev Neurosci 8:71-80.

4.      Melzak R, Wall PD (1965) Pain mechanisms: a new theory. Science 150:971-9.