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Human Body & Anatomy
In This Issue:
Our skin acts as the protective barrier between our internal body systems and the outside world. Its ability to perceive touch sensations gives our brains a wealth of information about the environment around us, such as temperature, pain, and pressure. Without our sense of touch, it would be very hard to get around in this world! We wouldn't feel our feet hitting the floor when we walked, we wouldn't sense when something sharp cut us, and we wouldn't feel the warm sun on our skin. It is truly amazing how much information we receive about the world through our sense of touch, and although we still don't know all the ins and outs of how the skin perceives touch, what we do know is interesting.
The skin is composed of several layers. The very top layer is the epidermis and is the layer of skin you can see. In Latin, the prefix "epi-" means "upon" or "over." So the epidermis is the layer upon the dermis (the dermis is the second layer of skin). Made of dead skin cells, the epidermis is waterproof and serves as a protective wrap for the underlying skin layers and the rest of the body. It contains melanin, which protects against the sun's harmful rays and also gives skin its color. When you are in the sun, the melanin builds up to increase its protective properties, which also causes the skin to darken. The epidermis also contains very sensitive cells called touch receptors that give the brain 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 (oil) glands, blood vessels, nerve endings, and a variety of touch receptors. Its primary function is to sustain and support the epidermis by diffusing nutrients to it and replacing the skin cells that are shed off the upper layer of the epidermis. New cells are formed at the junction between the dermis and epidermis, and they slowly push their way towards 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 dermis level of the skin by opening their pores at the surface of the epidermis and releasing the waste.
The bottom layer is the subcutaneous tissue which is composed of fat and connective tissue. The layer of fat 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 things. The connective tissue keeps the skin attached to the muscles and tendons underneath.
Somatosensory System: The Ability To Sense Touch
Our sense of touch is controlled by a huge 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, tickle, itch, pain, vibrations, and more. Within the somatosensory system, there are four main types of receptors: mechanoreceptors, thermoreceptors, pain receptors, and proprioceptors.
Before we dig 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, tickle, etc). A touch receptor is considered rapidly adapting if it responds to a change in stimulus very quickly. Basically this means that it can sense right away when the skin is touching an object and when it stops touching that object. However, rapidly adapting receptors can't sense the continuation and duration of a stimulus touching the skin (how long the skin is touching an object). These receptors best sense vibrations occurring on or within the skin. A touch receptor is considered slowly adapting if it does not respond to a change in stimulus very quickly. These receptors are very good at sensing the continuous pressure of an object touching or indenting the skin but are not very good at sensing when the stimulus started or ended.
While many receptors have specific functions to help us perceive different touch sensations, almost never are just one type active at any one time. When drinking from a freshly opened can of soda, your hand can perceive many different sensations just by holding it. Thermoreceptors are sensing that the can is much colder than the surrounding air, while the mechanoreceptors in your fingers are feeling the smoothness of the can and the small 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 stretching around the can, that pressure is being exerted to hold the can, and that your hand is grasping the can. Proprioceptors are also sensing the hand stretching as well as how the hand and fingers are holding the can in relation to each other and the rest of the body. Even with all this going on, your somatosensory system is probably sending even more information to the brain than what was just described.
Nerve Signals: Making Sense of It All
Of course, none of the sensations felt by the somatosensory system would make any difference if these sensations could not reach the brain. The nervous system of the body takes up 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 they start a chain of events by signaling to the nearest neuron that they touched something. This neuron then transmits this message to the next neuron which gets passed on to the next neuron and on it goes until the message is sent to the brain. Now the brain can process what your hand touched and send messages back to your hand via this 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 it.
Is the Glass of Water Hot or Cold?
With this experiment, test your skin's ability to perceive whether an object is hot or cold.
Your brain just received confusing messages from your hands about what the temperature of the third glass was. The hand originally holding the hot glass told you the third glass was cold, whereas the hand originally holding the cold glass told you the third glass was hot. But they were both touching the same glass. How can this be?
You received these confusing messages because our skin does not perceive the exact temperature of an object. Instead, your skin can sense the difference in temperature of a new object in comparison to the temperature of an object the skin was already used to ("relative temperature"). This is why entering a body of water, such as a pool or lake, seems really cold at first (your body was used to the warmer air) but then gradually "warms up" after being in the water for a while (your body adjusts to the temperature of the water).
Is your skin equally sensitive all over your body? Try this experiment to find out more about how well your skin perceives touch.
The ability to distinguish between one point or two points of sensation depends on how dense mechanoreceptors are in the area of the skin being touched. You most likely found that certain areas of your body are much more sensitive to touch than other areas. Highly sensitive areas such as the fingertips and tongue can have as many as 100 pressure receptors in one cubic centimeter. Less sensitive areas, such as your back, can have as few as 10 pressure receptors in one cubic centimeter. Because of this, areas such as your back are much less responsive to touch and can gather less information about what is touching it than your fingertips can.
Science in the News
When a person loses an arm or a leg in an accident, they not only lose that limb, they also lose all the touch sensations that were received through that limb. Many amputees (people who have lost a limb) receive prosthetic (artificial) limbs to help regain some of the function and mobility they had before they lost the limb. However, conventional prosthetic limbs couldn't let the person feel when he grabbed hold of an object or when her foot touched the ground, making it a struggle for most amputees to successfully use a prosthetic limb for everyday purposes.
But scientists are making leaps and bounds in helping people regain mobility and sensation after losing a limb. The goal is to someday give amputees prosthetic limbs that will allow them to use only nerve signals from the brain to control the prosthetic limb, and have the limb send touch information back to the brain to make control over the limb a little easier. That someday is getting closer.
Claudia Mitchell, a former US marine who lost her arm in a motorcycle accident, is one of the most recent people to receive such an advanced prosthetic limb. Her new limb has been connected to the motor and sensory nerves in her chest that once controlled her real limb and now control her new arm. While it is still limited in what it can feel and do, this new limb allows Claudia to feel touch and heat, and she can perform conventional tasks (such as folding laundry and cutting up vegetables) four times faster than she could with a regular prosthetic limb.
To read more about Claudia and her bionic arm, click here.
Learn more about the skin's structure and function. Be sure to check out the "Structure of the skin" and "What does the skin do?" links to see animated clips on how the skin renews itself and senses temperature changes.
For more information on how well different areas of your body perceive touch sensations, take a look at this animated version of a homunculus, which is a map of the body drawn in proportion to how much information the brain receives about touch from the different parts of the body.