The Science of Touch
In recent years, a flurry of studies has documented some incredible emotional and physical health benefits that come from touch. This research suggests that touch is truly fundamental to human communication, bonding, and health.
The largest organ of the human body is the skin. It assumes the role of protector and protects our fragile frames and sensitive organs. It also gives us a sense of touch.
Nerve endings in the skin respond to different types of human touch, and these nerve endings can sense physical pain and discomfort, as well as the soothing vibrations and pressure transmitted by the soothing hands of others.
The benefits of human touch go far beyond the realm of massage and the relief of physical misery. It has been proven that a hug instead of a handshake – and a kiss on the forehead instead of a smile – can psychologically improve mood and create a sense of overall well-being.
The Skin
The Science of Touch
Our skin acts as the protective barrier between our internal body systems and the outside world. The ability to perceive touch sensations gives our brain a wealth of information about the environment around us, such as temperature, pain, and pressure.
Without our sense of touch, it would be very difficult to move around in this world. We wouldn’t feel our feet falling to the ground when we walked, we wouldn’t feel when something sharp cuts us, and we wouldn’t feel the warm sun on our skin.
It’s amazing how much information we receive about the world through our sense of touch, and while we still don’t know all the ins and outs of how the skin perceives touch, what we do know is interesting.
Anatomy Of The Skin
The Science of Touch
The skin is made up of several layers. The top layer is the epidermis and is the layer of skin that you can see. In Latin, the prefix “epi-” means “on” or “over”. Thus, the epidermis is the layer on top of the dermis (the dermis is the second layer of skin).
The epidermis is made of dead skin cells, is waterproof and serves as a protective wrap for the underlying layers of skin and the rest of the body.
It contains melanin, which protects against the sun’s harmful rays and gives the skin its color. When you’re out in the sun, the melanin builds up to increase its protective properties, which also darkens the skin.
The epidermis also contains highly sensitive cells called touch receptors, which provide the brain with a variety of information about the environment the body is in.
The second layer of skin is the dermis. The dermis contains hair follicles, sweat glands, sebaceous glands (oil) glands, blood vessels, nerve endings, and a variety of tactile receptors.
Its primary function is to support and sustain the epidermis by distributing nutrients to it and replacing the skin cells shed from the top layer of the epidermis.
New cells are formed at the junction between the dermis and the epidermis, and they slowly push their way to the surface of the skin so that they can replace the dead skin cells that are shed.
Oil and sweat glands eliminate waste produced at the level of the dermis of the skin by opening their pores on the surface of the epidermis and releasing the waste.
The bottom layer is the subcutaneous tissue that consists of fat and connective tissue. The fat layer acts as an insulator and helps regulate body temperature.
It also acts as a cushion to protect underlying tissue from damage when you bump into something. The connective tissue ensures that the skin is attached to the muscles and tendons underneath.
Somatosensory System: The Ability To Sense Touch
The Science of Touch
Our sense of touch is controlled by a vast network of nerve endings and touch receptors in the skin known as the somatosensory system. This system is responsible for all the sensations we feel – cold, hot, smooth, rough, pressure, tickling, itching, pain, tremors, and more.
Within the somatosensory system, there are four main types of receptors: mechanoreceptors, thermoreceptors, pain receptors, and proprioceptors. Before delving further into these specialized receptors, it is important to understand how they adapt to a change in stimulus (anything that touches the skin and causes sensations such as hot, cold, pressure, tickling, etc.).
A touch receptor is rapidly adapting if it responds very quickly to a change in stimulus. Basically, this means that it can sense right away when the skin touches an object and when it stops touching that object.
However, rapidly adapting receptors cannot sense the continuation and duration of a stimulus touching the skin (how long the skin touches an object). These receptors best sense vibrations that occur on or in the skin.
A touch receptor is considered to adapt slowly if it doesn’t respond very quickly to a change in stimulus. These receptors are very good at sensing the continuous pressure of an object touching or indenting the skin but are not very good at detecting when the stimulus started or ended.
1. Mechanoreceptors
These receptors perceive sensations such as pressure, vibration, and texture. There are four known types of mechanoreceptors whose sole function is to detect indentations and vibrations of the skin: Merkel's discs, Meissner's corpuscles, Ruffini's corpuscles, and Pacinian's corpuscles.
The most sensitive mechanoreceptors, Merkel's discs and Meissner's corpuscles, are found in the upper layers of the dermis and epidermis and are generally found in non-hairy skin such as the palms, lips, tongue, soles, fingertips, eyelids and face.
Merkel's discs adapt slowly to receptors, and Meissner's corpuscles adapt quickly to receptors, so your skin can sense both when you touch something and how long the object touches the skin
Your brain receives a huge amount of information about the texture of objects through your fingertips because the ridges that make up your fingerprints are full of these sensitive mechanoreceptors.
Deeper in the dermis and along joints, tendons and muscles are Ruffini's corpuscles and Pacin's corpuscles. These mechanoreceptors can sense sensations such as vibrations traveling along bones and tendons, rotational movement of limbs, and stretching of the skin. This helps you immensely in doing physical activities like walking and playing with the ball.
2. Thermoreceptors
As their name suggests, these receptors perceive sensations related to the temperature of objects that the skin feels. They are found in the dermis layer of the skin. There are two basic categories of thermoreceptors: hot and cold receptors.
Cold receptors begin to sense cold sensations when the skin's surface drops below 35°C. They are most stimulated when the skin's surface is 25°C and are no longer stimulated when the skin's surface drops below 5°C. This is why your feet or hands go numb when submerged in ice-cold water for long periods of time.
Warm receptors begin to sense warm sensations when the skin's surface rises above 30°C and are most stimulated at 45°C. But above 45°C, pain receptors take over to prevent damage to the skin and underlying tissues.
Thermoreceptors are found all over the body, but cold receptors are found in greater density than heat receptors. The highest concentration of thermoreceptors is found in the face and ears (which is why your nose and ears always get colder faster than the rest of your body on a chilly winter's day).
3. Pain receptors
Pain receptors: The scientific term is nocireceptor. "Noci-" means "harmful" or "hurt" in Latin, which is a good indication that these receptors detect pain or stimuli that can or cause damage to the skin and other tissues of the body.
There are more than three million pain receptors throughout the body, found in skin, muscles, bones, blood vessels, and some organs. They can detect pain caused by mechanical stimuli (cutting or scraping), thermal stimuli (burns), or chemical stimuli (venom from an insect sting).
These receptors cause a sensation of sharp pain to encourage you to quickly distance yourself from a harmful stimulus such as a broken piece of glass or a hot stove stopper. They also have receptors that cause a dull ache in an area that has been injured, to encourage you not to use or touch that limb or body part until the damaged area has healed.
While it's never fun to activate these receptors that cause pain, they play an important role in protecting the body from serious injury or damage by sending these early warning signals to the brain.
4. Proprioceptors
In Latin, the word "proprius" means "one's own" and is used in the name of these receptors because they perceive the position of the different parts of the body in relation to each other and the environment.
Proprioceptors are found in tendons, muscles, and joint capsules. This location in the body allows these special cells to detect changes in muscle length and muscle tone. Without proprioceptors, we would not be able to do fundamental things like feeding or clothing ourselves.
While many receptors have specific functions to help us perceive different sensations of touch, almost never only one type is active at a time.
When you drink from a freshly opened can of carbonated beverage, your hand can perceive many different sensations just by holding it.
Thermoreceptors sense that the can is much colder than the surrounding air, while the mechanoreceptors in your fingers sense the smoothness of the can and the little fluttering sensations inside the can caused by the carbon dioxide bubbles rising to the surface of the soda.
Mechanoreceptors located deeper in your hand can sense that your hand is extending around the can, that pressure is being applied to hold the can, and that your hand is grasping the can.
Proprioceptors also feel the extending hand and how the hand and fingers hold the can relative to each other and the rest of the body.
Even with all of this going on, your somatosensory system is likely to send even more information to the brain than what has just been described.
Nerve Signals: Understanding Everything
The Science of Touch
Of course, none of the sensations felt by the somatosensory system would make any difference if these sensations couldn’t reach the brain. The body’s nervous system takes on this important task.
Neurons (which are specialized nerve cells that are the smallest unit of the nervous system) receive and transmit messages with other neurons so that messages can be sent to and from the brain. This allows the brain to communicate with the body.
When your hand touches an object, the mechanoreceptors in the skin are activated and start a chain of events by indicating to the nearest neuron that they have touched something.
This neuron then transmits this message to the next neuron which is passed on to the next neuron and so it continues until the message is sent to the brain.
Now the brain can process what your hand has touched and send messages back to your hand through the same pathway to let the hand know if the brain wants more information about the object, it is touching or if the hand should stop touching.
Oxytocin
The Science of Touch
One of the hormones released when touched is oxytocin. This chemical messenger promotes a sense of calm and security in both infants and adults.
Indeed, touch on the receiving end has been shown to function as a highly effective stress reliever for people of all ages. Recipients may experience specific physical manifestations of stress reduction, such as decreased blood pressure and decreased heart rate.
These bodily reactions are likely related to an increase in bonding hormones and a reduction in stress hormones. Exactly the opposite of a chemical messenger such as oxytocin, these stress hormones are produced by the adrenal glands in response to unpleasant situations. Lowering these levels through touch has the added benefit of improving immune function.
Touch can also affect mental and emotional health by reducing depression and anxiety and increasing positive feelings of belonging, well-being, and self-worth.
Oxytocin is one of the two main hormones secreted by a part of the brain called the posterior pituitary gland (the other is called vasopressin and is involved in water regulation, although it is also known to curl the toes of male voles).
Hormones can have three main types of action: autocrine, paracrine, and endocrine.
Autocrine refers to chemicals released from a cell that act on that same cell (such as negative feedback, where a cell releases a chemical that hits receptors on the same cell and stops further release of the chemical).
Paracrine refers to chemicals released from a cell that interact with other cells nearby (such as neurotransmitters).
And endocrine (which you’ve probably heard of) refers to chemicals that are released from one type of cell and have effects on other types of cells or tissues (such as insulin, which is released from the pancreas and has effects everywhere else, or like the hormones you usually associate with teenagers).
Oxytocin, along with vasopressin, is secreted by an area in the brain known as the posterior pituitary gland.
The posterior pituitary gland is the one that goes further to the back of the head. Oxytocin is not made there, it is made above the pituitary gland, in the brain called the hypothalamus, and the cells in the hypothalamus extend all the way to the posterior pituitary gland, allowing oxytocin to be made in one place and released in another.
Thus, oxytocin is produced, and when it is stimulated properly, the brain releases it. Oxytocin has an endocrine effect throughout the body.
It plays a very big role in sexual arousal and orgasm in both sexes. In women, oxytocin is very important in stimulating uterine contractions before birth, so much so that oxytocin is given to induce labor, and medications that counteract oxytocin are used to suppress it if labor is premature.
Oxytocin doesn’t just act on the body; It also has some big effects on the brain. Some studies have shown that oxytocin has strong effects on trust and generosity, making it an important chemical in human social interaction.
In addition to these major effects, there may be roles for oxytocin in autism, in depression (especially in women), and of course, in things like social bonding.
How Emotions & Feelings Affect Our Lives
The Science of Touch
In this YouTube video, David J. Linden discusses the importance of touch and what it means for us as humans. He explains that touch is not only a physical experience but is also strongly connected to our emotions. From a young age, touch plays a huge role in our development, and if we don’t get it, serious problems can arise.
David tells of children in orphanages who were barely touched, which led to growth problems, emotional disturbances and even physical complaints. But he also emphasizes that just 30 minutes a day of loving touch could completely change their situation, provided it happened in the first two years of life.
He also discusses how touch affects adults. Doctors who touch their patients are perceived as more caring, and even basketball teams that touch each other more often win more games. Touch helps people work better together, whether that’s in relationships, in families, or at work.
Touch is made possible by different sensors in our skin that perceive something different, such as pressure, temperature, pain, and texture. These sensors cause us to experience touch differently on different parts of our body. In addition, David explains that emotions are always connected to touch: you can feel pain without emotion, or experience pain but not know exactly where it comes from.
David concludes by noting that our senses do not always accurately represent reality. Our brains process the information that comes in, mix it with our emotions and expectations, and then present it as “the truth.”
David J. Linden, Ph.D., is Professor of Neuroscience at Johns Hopkins University School of Medicine. His lab has spent years researching how memories are stored at the cellular level and how functions recover after brain injury, among many other topics.
He has a keen interest in science communication and was editor-in-chief of the Journal of Neurophysiology for many years. Linden is also the author of three bestselling books on the biology of behavior for a wide audience: The Accidental Mind (2007) and The Compass of Pleasure (2011), which have been translated into 19 languages.
His most recent book, Touch: The Science of Hand, Heart and Mind (2015), was recently published by Viking Press in the U.S. and Canada.
David J. Linden offers interesting talks on how touch works and how important it is in our daily lives!
