Itch, Scratch, and the Origins of Social Touch
Your skin is essentially a body envelope. It keeps your vital fluids in and protects your heart, lungs, and other viscera from bacteria and dehydration. It?s also the primary medium through which we bond with other people.
This bonding comes in a variety of flavors. We can hold hands, high-five, kiss, cuddle, have sex. But also link arms, play with each others hair, and?sometimes to the point of annoyance?pop each other?s pimples. If there?s an objective fact to be stated about the nature of this universe, it?s that back scratching is the best of these practices.
Back scratching belongs to a subset of bonding behaviors called allogrooming. Allogrooming (allo meaning ?other? and grooming meaning ?grooming?) didn?t originate for bonding, though. Instead, it evolved as a way to satiate each others grooming needs. Only later did it develop this ability to fortify social bonds.
This miraculous transition was bumpy. It grew from itchy pimples, ugly parasites, and dastardly dead skin. From these cruel curses emerged the shiny and delectable practice it is today. This is the story of that transition.
It Started With An Itch
Certain places on the body are difficult to reach. In primates, these are the head, neck, and back. Because these places are difficult to reach, we would need the help of friends, loved ones, or nearby sticks to deal with them. Friends and loved ones were probably more helpful.
These places would itch for a number of reasons. Lice, for instance, evolved to avoid host grooming and thus focus themselves to these difficult to reach locations. They began an arms race, if you will, between the places we could reach them and the location of their ?microhabitat? on the body. Those that survived moved far from reach. Those that didn?t were promptly plucked.
Today we?re not so plagued by lice. We have the acquisition of fire, clothing, and shelter to thank for this. These powerful accoutrements rendered fur more of a flea-ridden nuisance than temperature-regulating asset. Those who dedicated their physiological resources to other things, then, were better off. And so, our fur grew thin, fair, and (mostly) lice-free in response.
Lice and other bugs make us itch by biting into the skin. Proteins from their saliva penetrate our dermal tissue where they?re recognized as foreign. This triggers an immune response which causes, among other things, the secretion of pruritogens. These pruritogens make the affected region itch.
The most potent and well-studied pruritogen is histamine. It?s produced from cells within the dermal tissue?called mast cells?during an immune response. This is the primary pruritogen secreted in response to lice bites. But it?s also secreted for more maddening things like acne.
Acne itches once it becomes inflamed. This inflammation occurs after a trapped bacterium (Propionibacterium acnes) secretes proinflammatory signals to surrounding cells. These signals induce the secretion of histamine, which in turn makes the region itch. Those irksome pimples embedded within the sixth cervical dermatome of your back, then, are the friendly neighborhood products of histamine.
Once pruritogens are secreted they bind to receptors in the skin that send an ?itch? signal to the brain. This signal ultimately lands in the somatosensory cortex, a sliver of cortex in the frontal lobes that contains a map of our body. They call this map the sensory homunculus.
The sensory homunculus has tissue dedicated to every region of the skin (although to some places more than others). When an itch signal is sent to this map, it will tell us the approximate location of the itch. With this knowledge in hand, we can find someone to implement the scratch.
How Scratch Works
Scratching ameliorates the frustration of itch in one of two ways. The first is by removing itchy things from the skin. A lice bites into your back, say, which induces the secretion of histamine. This histamine then makes the bite itch. Now filled with hatred and frustration, you can find a friend to scratch the bug, free you from your plight, and be on his or her merry way.
But scratching also inhibits itch through pain. This relief is only temporary, though, so don?t get too excited.
Two explanations are offered for this relationship. The first is that certain pathways in the spine respond to both pain and itch. When pain is present, these pathways suppress itch. The other explanation suggests that pain inhibits itch after it reaches the brain. This makes sense once you consider the different degrees of myelination between the two pathways.
Myelination is the process wherein axons (the structures that make up these pathways) are wrapped with fatty sheathing. This sheathing insulates the axon, allowing its electrochemical signal to travel faster to its destination. The common analogy is that it?s like insulation wrapped around wire: the more insulation, the faster the signal.
Pathways in the spine that give us sensation are distinguished by the thickness of their myelination. As a rule of thumb, the more myelination, the more important the pathway is for survival. Here?s a list of different pathways, the thickness of their myelination, and the function they serve in the body:
- A-alpha. Thickest. Proprioception. This lets us know that our left arm is to our left and our back is to our back???it tells us where our bodies are in physical space.
- A-beta. Second thickest. Discriminatory touch. These let us play Halloween games where we?re told spaghetti is squished brain and hardboiled eggs are eyeballs. They also let us read braille.
- A-delta. Thinnest. Pain and temperature. These remind us that we shouldn?t submerge our hands in boiling hot water.
- C-fibers. Unmyelinated. Itch. So we can find bug bites annoying.
If you didn?t notice, pain pathways have myelination. Itch pathways don?t. While this distinction isn?t always true (some pain pathways are unmyelinated), it?s true for the most part. This suggests that, in the grand scheme, it?s more important to feel pain than itch. When you scratch, then, the pain signal should reach the brain first, dominate, and suppress itch for another day.
Pain does this for all sorts of physiological processes. Insulin secretion, for instance, is silenced by pain. Pain activates the sympathetic nervous system, which inhibits beta cells in the pancreas (where insulin comes from). This keeps more glucose in the blood so it can be used by the muscles to fight or flee. If pain will do this for insulin, it will do it for itch.
This type of scratch is only a temporary relief, though, and wouldn?t yield maximum dividends for your itchy back. It would be more like a favor than anything. But this favor is in part what elevated back scratching from the slums of mere hygienic grooming to the zenith of all social pleasures.
For your sanity, however, I want to point out that scratch doesn?t always help. When we?re bitten by a mosquito or spider, for instance, scratching will just spread the invading proteins around. This will cause a greater number of pruritogens to be released and, consequently, a lot more itch. Since you?re not actually removing the itcher from the skin (e.g., the lice), you?re just making the problem worse. As a remedy, I would suggest cold. Cold inhibits itch through a mechanism not mentioned here.
Beyond Mere Grooming
Allogrooming evolved to do more than just alleviate itchy pimples or parasitic infestation, though. Most of us would welcome it anytime, anywhere?regardless of itch. This is because the rubbing, scratching, and kneading of our heads, necks, and backs has become a type of social touch.
Social touch is a type of physical contact that deepens a relationship with someone you know. It?s that list of behaviors I described at the beginning: kissing, cuddling, sex, high-fives. You?ll touch the arm of a friend, say, or makeout with your boyfriend. These types of touch bring us closer together.
Such bonding is advantageous for us primates. In baboons, those you groom are more likely to come to your defense in the event of a fight. They?re also more likely to protect your baby (if you?re a nursing baboon female) from infanticidal alpha males. In humans, it does something similar.
Social bonds are instrumental to human health and well-being. The more abundant and robust your bonds with other people, the happier you?ll be. These connections are forged in part by the lubricant of social touch.
Evolution has a way of turning advantageous practices like these into pleasurable commodities. The most famous example is what happened with our taste for sugary and salty foods.
In our grassland savanna habitat, foods high in sugar and salt were hard to come by. Our diets consisted mainly of roots, berries, grass, and the occasional small game animal. Over time, this paucity of calorically rich and cerebrally necessary food (neurons use a lot of sodium) led us to develop an affinity for sugar and salt. Evolution made foods with their taste pleasurable. Now we?re stuck in a world with Doritos and ice cream.
A similar thing happened with social touch. It represented something advantageous in the environment?social bonding?that made us more likely to survive. As with our taste for sugary and salty foods, evolution made it pleasurable. It?s not as fattening, though.
There are a few different ways through which this happened. The first is through specialized pathways in the spine.
CT afferents are a newly discovered set of pathways (like those for itch, pain, or proprioception) responsible for transmitting ?pleasant touch.? Like itch pathways, they are unmyelinated and travel slowly to the brain. Researchers often activate these pathways by stroking the skin with a soft, blunt object. People describe these strokes as pleasurable.
These afferents are one reason we like to have our backs rubbed or our hair played with. They send a signal to the brain telling us we?re getting pleasant touch.
Oxytocin is another mediator of such touch. In nonhuman primates, cerebral injections of oxytocin will increase the amount of time dedicated to allogrooming. In humans, there is a greater amount among those in romantic relationships than those who are single. Oxytocin plays some role in the formation and upkeep of social bonds.
Back scratching, the best of all social touch (objective fact), is different. It evolved from itchy places that were difficult to reach. People would scratch these places to relieve us of our discord and, eventually, to strengthen their relationship with us. You scratch my back, I?ll scratch yours?now let?s fall in love. Since these practices increased our chances of survival, evolution made them pleasurable.
Ultimately, back scratching is like chocolate. You could have it any time of day. You can never have enough. You only get it from your girlfriend when she?s not pissed. But it?s also a way of social bonding. So if you want to make some friends, I suggest you go out there and scratch some backs.
Other Important Articles
- If you want to know more about how we lost our fur, read here.
- This article gives a nice summary of oxytocin and its role in bonding.
- Itch relief gives you pleasure. This article describes how.
- Here is some more information on CT afferents and their role in social touch.
- This paper provides some background information on the different spinal pathways.
- This paper will tell you all you would ever want to know about itch.