Why your nervous system wrinkles your fingers

Photo credit: flickr/Sharron Drummond
Photo credit: Flickr/Sharron Drummond

 

You come back home really tired after a long day at work and you decide to take that loooong bath you’ve been waiting for. You stay in the water for some time and suddenly you realize your fingers are wrinkling. Oh boy, that is telling you that you’ve been there for a while and reminds you all the stuff you still have to do! But… does the body do this to let us know that we’ve been in a bath for too long? What is the real reason behind this process? And how is it really happening?

It is commonly assumed that finger wrinkling is the result of water passing passively into the outer layer of the skin and making it swell up. However, although everybody seems to believe that explanation, that is actually quite wrong. Researchers have known since the 30s that skin wrinkling does not occur when there is nerve damage in the fingers indicating that is not a passive process but an active one that is controlled by the nerves 1. Even more, this knowledge has led to the implementation of the Wrinkling Test, a medical test that checks wrinkling of patients to assess their possible peripheral nerve damage 23. More recent studies have shown that finger wrinkling is controlled by the sympathetic nervous system, a division of the peripheral nervous system that also controls other involuntary body functions such as breathing or digesting. However, it is still unclear how the whole system works, and more importantly, why it is there.

The way this system works is not perfectly clear, but we have some clues. First, our hands have to sense the aquatic environment in a distinctive way (we don’t have wrinkling in our arms, for example). The palm of the hand possesses an incredible amount of sweat glands, around 370 per cm2. Each of those glands has a small duct to reach the surface of the skin and it is believed that when those are underwater, some amount of water can go through the duct and reach the gland inside the hand. This process is thought to be sensed by the sympathetic nervous system that in turn reacts by reducing blood flow through the vessels of the finger. Now blood vessels have restricted blood, which reduces their volume, and this situation causes the skin to shrink inward, forming the wrinkles we observe after taking a bath 4.

The physiology behind this process is quite amazing and involves a coordinated action between the nervous system and the circulatory system. So the obvious question is: Why is this process acquired during evolution? What is the advantage of finger wrinkling?

Some researchers have started to study this phenomenon. The first approach was theoretical and was proposed by Mark Changizi and colleges in 2011 5. They propose that wet-induced wrinkles have been selected to enhance grip in wet conditions, i.e. one should be able to pick up wet objects better after finger wrinkling. Changizi proposes that the wrinkles in fingers work like rain treads on tires. They create channels that allow water to drain away as you press your fingertips on to wet surfaces, which would allow your fingers to make better contact giving you a better grip.

Figure 1. Smooth tires such as the racing tire (left) provide the best grip in dry conditions. However, in wet conditions rain treads (right) are better. The hypothesis proposed by Changizi et al suggests that, although smooth fingertips provide the best grip in dry conditions, fingertips wrinkle in wet conditions for better grip, akin to rain treads.
Figure 1. Smooth tires such as the racing tire (left) provide the best grip in dry conditions. However, in wet conditions rain treads (right) are better. The hypothesis proposed by Changizi et al suggests that, although smooth fingertips provide the best grip in dry conditions, fingertips wrinkle in wet conditions for better grip, akin to rain treads.

A few years later, another study aimed to experimentally test the prediction that handling of submerged objects is more efficient with wrinkled fingers than without. To do so, they recruited participants and asked them to put their hands in warm water for 30min or keep them dry. After that, the participants had to move dry or wet (submerged) glass marbles of different sizes from one container to another 6.

Fig2
Figure 2. Experimental setup to test if finger wrinkling gives an advantage for handling wet objects. Participants have to transfer dry or submerged glass marbles of different sizes from one container to the next. Finger wrinkling is induced before the experiment by submerging participants hands in warm water for 30min. A significant reduction in time for handling is observed if finger wrinkling has been previously induced (right graph). | Credit: Changizi et al (2011)

After analyzing the data, the researchers discovered that the time taken to transfer wet glass marbles was reduced if participants had had their fingers in water before, suggesting that Changizi’s hypothesis was right and finger wrinkling helps handling wet objects, which would agree with the idea that this process could be evolutionarily favorable and could have been selected over the years. However, there is some controversy since the sample size of the study was quite small (20 participants only). In order to reproduce the results, a different group tried to do the same experiments in a bigger cohort (40 participants). Unfortunately, the results were not reproducible and this second lab was not able to conclude that finger wrinkling was really an advantage for handling wet objects 7 so right now there is a bit of uncertainty about the real reason of this process.

Anyhow, the scientific community still thinks that there must be a reason behind this wrinkling process. Why the nervous system should control water-induced finger wrinkling? The answer is: we don’t really know so we need to do more research!

References

  1. O’Riain S (1973) New and simple test of nerve function in hand. Br Med J 3: 615–616. doi:10.1136/bmj.3.5881.615
  2. Vasudevan TM, van Rij AM, Nukada H, Taylor PK (2000) Skin wrinkling for the assessment of sympathetic function in the limbs. Aust N Z J Surg 70: 57–59. doi:10.1046/j.14401622.2000.01744.x
  3. Tindall, A., Dawood, R., & Povlsen, B. (2006). Case of the month: The skin wrinkle test: a simple nerve injury test for paediatric and uncooperative patients. Emergency medicine journal, 23(11), 883-886. doi:10.1136/emj.2005.031377
  4. Wilder‐Smith, E. P., & Chow, A. (2003). Water‐immersion wrinkling is due to vasoconstriction. Muscle & nerve, 27(3), 307-311. doi:10.1002/mus.10323
  5. Changizi, M., Weber, R., Kotecha, R., & Palazzo, J. (2011). Are wet-induced wrinkled fingers primate rain treads?. Brain, behavior and evolution, 77(4), 286-290. doi:10.1159/000328223
  6. Kareklas, K., Nettle, D., & Smulders, T. V. (2013). Water-induced finger wrinkles improve handling of wet objects. Biology letters, 9(2), 20120999. doi: 0.1098/rsbl.2012.0999
  7. Haseleu, J., Omerbašić, D., Frenzel, H., Gross, M., & Lewin, G. R. (2014). Water-induced finger wrinkles do not affect touch acuity or dexterity in handling wet objects. PloS one, 9(1), e84949. doi:10.1371/journal.pone.0084949

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