How does the outer layer of skin cells on my finger detect when I am touching an object?
Category: Biology Published: November 23, 2015
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and Associate Professor of Physics at West Texas A&M University
The outer layer of skin cells on your finger does not detect anything. The outer layer of your skin contains cells that are dead. In fact, the outermost 25 to 30 cell layers of your skin consist of dead cells that do nothing beyond providing a physical barrier that keeps water in and chemicals out. Furthermore, all regular skin cells (keratinocytes), whether alive or dead, don't detect physical sensations since they are not designed to do this. Physical sensations that are experienced when touching an object are detected by special receptors that sit in lower layers of the skin. Such sensations include pressure, temperature, vibration, and skin stretching. Therefore, in order for you to detect a physical effect, it must first pass through all the layers of dead skin. For instance, in order for you to feel that a frying pan is hot, heat from the pan must travel through the outer layers of dead skin before it can reach the thermoreceptors nestled in the lower layers of the skin. The thermoreceptors then detect the heat and in response send an electrical signal along your nerves to your brain.
If you lightly, directly touch a solid object that is at the same temperature as your finger, the physical sensation that you detect is pressure. It is therefore the sensation of pressure that lets you feel the presence of the object. For instance, close your eyes and slowly move your finger towards the wall. At some point, you can feel that your finger tip is touching the wall. But how exactly does it do this?
When your finger gets very close to the wall, the atoms that make up the outermost skin cells of your finger start to significantly overlap with the atoms of the wall. When this starts happening, the Pauli exclusion principle of quantum theory (which states that no two electrons can be in the exact same state at the same place at the same time) leads to a repulsive electromagnetic force. This repulsive force pushes back on the atoms that make up the outer layer of your skin. This repulsive force is strong enough that it halts the forward movement of the outer layer of skin. This layer therefore gets in the way of the lower layers of skin that are still trying to move towards the wall along with the rest of your hand. In this way, pressure is transmitted through the outer dead layers of skin cells to the inner layers where the pressure can be detected. As long as you are using your muscles to push your finger toward the wall and as long as the wall is pushing back without breaking, all the layers of skin on the tip of your finger are sandwiched between these two forces and continue to experience increased pressure. Therefore, certain nerve endings in your skin called mechanoreceptors will continue to detect the pressure and continue to send the corresponding signals to your brain, creating the ongoing perception of touching the wall.