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We can perceive sensation of pain if we are poked on any part of the skin. Does this mean there is a nerve ending at every point on the skin?
Does this also mean that each of these nerves go all the way to the brain as single strands so that the brain can distinguish between the touch at any two distinct locations?
The Human Body: Skin Biology and Structure
Organs are groups of tissues that work together to achieve a special purpose. Organs keep the body functioning and maintain life. Your skin is considered an organ, and its purpose is to protect the body from outside contaminants such as germs and pollution. The skin also helps keep your body temperature consistent, receives and processes sensory information, and stores fat, water, and vitamin D.
Most researchers agree that the skin is the largest organ. Skin covers the entire body, making up approximately 16 percent of the body’s total mass. Recently, some scientists have changed their thinking about the classification of organs and have shifted to calling the interstitium the largest organ in the body. This would make the skin the second-largest organ in the body. The interstitium contains fluid-filled spaces in flexible connective tissues, and this network exists beneath the skin, around arteries and veins, and in the linings of the lungs, digestive tract, and urinary system. Not all scientists agree about the classification of the interstitium as an organ, however.
The three layers of the skin, beginning with the outside layer, are the epidermis, the dermis, and the subcutaneous or hypodermis tissues. The epidermis doesn’t contain any blood vessels, so it relies on the dermis to deliver nutrients and get rid of waste. The epidermis is made of skin cells that are held together by lipids. The epidermis is the physical and biological barrier that prevents allergens and irritants from penetrating into the body. The epidermis also prevents the loss of too much fluid. The dermis is thicker than the epidermis and lies immediately under it. The dermis supports the epidermis, further protecting, cushioning, nourishing, and helping to heal wounds when they occur. The dermis contains nerve endings and hair roots, and this layer is also where sweat glands and blood vessels are. Below the dermis lies the subcutaneous layer that is made mostly of fat. This is where the main support and structure for the skin comes from, and this layer also helps insulate the body from cold. The subcutaneous layer has both nerves and blood vessels in it.
The skin is a fascinating organ. There are six main types of skin, ranging from fair to dark, and melanin is the pigment in the skin that determines skin’s color. Skin color is mostly determined by a person’s genetics. The skin cells produce melanin with exposure to the sun, so the more skin is exposed to the sun, the more melanin the skin contains.
Sebaceous glands are present around hair follicles, and the scalp and face have the highest concentration of sebaceous glands. In fact, the face may have up to 900 sebaceous glands in every square centimeter of skin. At puberty, the sebaceous glands begin producing more sebum, which tends to cause acne. Sometimes, people also have health conditions that make sebaceous glands more active. If oily skin is a problem, it may help to see a physician. Some medications are available by prescription that can help lower sebum production.
As a person ages, their skin starts showing signs of aging, including sagging, wrinkles, and dark spots. This happens naturally, but it can happen earlier for people who have overexposed their skin to ultraviolet radiation on a regular basis. The dermis layer contains special proteins that help keep the skin supple and smooth. With aging, the body produces lesser amounts of these proteins, which reduces elasticity of the skin. The amount of fat in the hypodermis also shrinks with aging, which contributes to saggy skin.
Peripheral Nerve Injury of the Upper Extremity
Evaluate cutaneous sensation over the radial dorsal portion of the hand.
Begin the motor function analysis by asking the patient to extend the elbow, evaluating triceps muscle function. Make sure gravity is eliminated because patients with radial nerve palsy who have retained elbow flexors can mimic elbow extension. If the elbow is flexed and then allowed to return to a normal position with the patient sitting upright, the elbow will appear to be extended, although this has been a passive return to extension, not an active one.
Examine ability of the patient to extend at the MCP joint (extensor digitorum communis). It is not as useful to test the IP joint extension as intrinsic muscles of the hand innervated by the ulnar and median nerves contribute to IP joint function.
Test wrist extension ability and strength. Patients with an isolated PIN injury may still have some ability to extend the wrist due to the function of the ECRL and brevis.
Assess the ability to supinate the forearm.
Each part of the skin is supplied by a huge, dense network of nerve fibers. Sensory nerve endings recognize practically all types of stimuli, which result in the tactile sensations of heat, cold, pressure, vibration, pain and itch. Because of these versatile functions, the skin is considered the largest sensory organ.
The skin produces endocannabinoid molecules such as anandamide (AEA) and 2-AG. These endocannabinoids are constantly released in specific amounts, depending on the “healthy need” of the organ, resulting in the skin’s cannabinoid tone. Endocannabinoid molecules are synthesized by several cell types in the epidermis, hair follicles and sebaceous glands.
The skin’s cannabinoid tone constantly affects all compartments of the skin, as endocannabinoids act on various cell types and contribute to their healthy physiological function.
Keratinocytes are the most abundant type of skin cell found in the epidermis and account for around 90-95% of the epidermal cells.
They produce and store a protein called keratin, a structural protein that makes skin, hair, and nails tough and waterproof. The main function of the keratinocytes is to form a strong barrier against pathogens, UV radiation, and harmful chemicals, while also minimizing the loss of water and heat from the body.
Keratinocytes originate from stem cells in the deepest layer of the epidermis (the basal layer) and are pushed up through the layers of the epidermis as new cells are produced. As they migrate upwards, keratinocytes differentiate and undergo structural and functional changes.
The stratum basal (or basal layer) is where keratinocytes are produced by mitosis. Cells in this layer of the epidermis may also be referred to as basal cells. As new cells are continually produced, older cells are pushed up into the next layer of the epidermis the stratum spinosum.
In the stratum spinosum (or squamous cell layer), keratinocytes take on a spiky appearance and are known as spinous cells or prickle cells. The main function of this epidermal layer is to maintain the strength and flexibility of the skin.
Next, the keratinocytes migrate to the stratum granulosum. Cells in this layer are highly keratinized and have a granular appearance. As they move closer to the surface of the skin, keratinocytes begin to flatten and dry out.
By the time keratinocytes enter the stratum lucidum (AKA the clear layer), they have flattened and died, thanks to their increasing distance from the nutrient-rich blood supply of the stratum basal. The stratum corneum (the outermost layer of the epidermis) is composed of 10 – 30 layers of dead keratinocytes that are constantly shed from the skin. Keratinocytes of the stratum corneum may also be referred to as corneocytes.
Melanocytes are another major type of skin cell and comprise 5-10% of skin cells in the basal layer of the epidermis.
The main function of melanocytes is to produce melanin, which is the pigment that gives skin and hair its color. Melanin protects skin cells against harmful UV radiation and is produced as a response to sun exposure. In cases of continuous sun exposure, melanin will accumulate in the skin and cause it to become darker i.e., a ‘suntan’ develops.
Langerhans cells are immune cells of the epidermis and play an essential role in protecting the skin against pathogens. They are found throughout the epidermis but are most concentrated in the stratum spinosum.
Langerhans cells are antigen-presenting cells and, upon encountering a foreign pathogen, will engulf and digest it into protein fragments. Some of these fragments are displayed on the surface of the Langerhans cell as part of its MHCI complex and are presented to naïve T cells in the lymph nodes. The T cells are activated to launch an adaptive immune response, and effector T cells are deployed to find and destroy the invading pathogen.
Merkel cells are found in the basal layer of the epidermis and are especially concentrated in the palms, finger pads, feet, and undersides of the toes. They are positioned very close to sensory nerve endings and are thought to function as touch-sensitive cells. Merkel cells allow us to perceive sensory information (such as touch, pressure, and texture) from our external environment.
Why Do Human Fingers Have Nails?
One of the reasons that humans were able to develop into such dextrous and talented creatures, and able to perform fine motor movements like operating on a human brain or performing sleight of hand close-up magic, was the development of our hands, which are some of the most flexible and highly adapted parts of the body, with three major nerves controlling all muscle activity, and 27 different bones. This composition allows virtuosic piano players to wow the world, writers like myself to type 120 words per minute, and brilliant painters to apply nuance and beauty to a photorealistic portrait.
As mentioned above, there are three main nerves in the hand&mdashthe median, ulnar and radial nerves&mdashbut each of those nerves is just a main highway, from which there are thousands of branches and smaller divisions, providing the incredible sensitivity present in our hands and fingertips. There are large nerve terminals located along and off the main nerve, each of which may have hundreds of different nerve endings emanating from them. In fact, the human fingertip has the greatest density of nerve endings anywhere in the body, approximately 2,500 per square centimeter! Some of those nerve cell receptors are also highly specialized, increasing their sensitivity even further. Hands and fingers are out primary tools to explore our sense of touch, so it makes sense that such sensitivity and complexity would develop there.
Now, the fingertips may be the densest region of nerve endings, but there is another key structure on the finger that have helped primates and humans for millions of years&mdashfingernails. Nails are essentially flattened forms of claws, which is what most other mammals possess. Claws were extremely useful for other mammals, for defensive purposes and for locomotion, particularly climbing up trees. However, as primates began to adapt and change, their needs similarly shifted.
Making your way along thinner branches and grasping fruits, nuts and flowers required digits that could grip, and as the ends of the fingers flattened into fingertips, the claws similarly flattened into nails. The interesting thing, however, is that researchers are still uncertain of whether nails adapted to the flattening of the fingers in order to support the structure, or whether they are simply a leftover of claws as they shrank and receded.
The nervous system is divided into two camps. There is the central nervous system (CNS) which is your brain and your spinal cord. Then there is the peripheral nervous system (PNS), which is everything else.
In the central nervous system (the brain and spinal cord), if you injure that - as far as we know - it's permanent. The nerve cells may die or they may not die but they certainly don't reconnect with where they should connect. That stops signals getting through which is why you get problems of paralysis or loss of sensation, depending on where the damage is. That's why a stroke is so disabling.
In the skin the nerve cells there seem to be able to survive injury. They also seem to be able to re-grow to their targets so they go back to where they connected to in the first place. So if it was a muscle they were supposed to be supplying, they'll reconnect with the muscle. If it was a patch of skin they can branch out and re-supply the skin so you do get sensation back.
But nerves grow quite slowly, probably a couple of millimetres a day. So if you've got a big injury the length of your arm it can take a few weeks before the nerves can get back to your arm. The sensation may not be absolutely perfect because some nerve cells might die but you should get coverage of the skin back afterwards.
What happens when you break or interrupt a nerve is that the actual cell inside is just one massive long cell. The distal bit (the bit downstream of the cut site) will degenerate. It retracts and forms this little lump bulb. This then grows back along the original path of the nerve so it uses the original pathway of the nerve as a guide, rather like a motorway cone. It uses the cones and lays down a new road surface, which is the nerve, and it gets back to where it was supposed to attach. The distal site it was supposed to attach to switches on various markers so it can recognise it and off it goes!
Touching a nerve: How every hair in skin feels touch and how it all gets to the brain
Neuroscientists at the Johns Hopkins University School of Medicine have discovered how the sense of touch is wired in the skin and nervous system. The new findings, published Dec. 22 in Cell, open new doors for understanding how the brain collects and processes information from hairy skin.
"You can deflect a single hair on your arm and feel it, but how can you tell the difference between a raindrop, a light breeze or a poke of a stick?" says David Ginty, Ph.D., professor of neuroscience at Johns Hopkins. "Touch is not yes or no it's very rich, and now we're starting to understand how all those inputs are processed."
Ginty and his colleagues study how the nervous system develops and is wired. In trying to understand how touch-responsive nerve cells develop, they set out to build new tools that enable them to look at individual nerve cells. According to Ginty, there are more than 20 broad classes of so-called mechanosensory nerve cells in the skin -- of which only six account for light touch -- that sense everything from temperature to pain. But until now, the only way to tell one cell from another was to take electrical recordings as each type of cell generates a different current based on what it senses.
The team first genetically engineered mice to make a fluorescent protein in one type of nerve cell -- called the C-type low-threshold mechanosensory receptor or C-LTMR. C-LTMR cells stretch from the spinal cord to the skin, and those cells containing fluorescent protein could be seen in their entirety under a microscope. The team found that each C-LTMR cell branched to send projections to as many as 30 different hair follicles.
Mice have three different types of hair: a thick, long guard hair that accounts for only about 1 percent of total hairs on the body a shorter hair called the awl/auchene that constitutes about 23 percent of body hair and a fine hair called the zigzag that makes up 76 percent of body hair. The team found that most of the C-LTMR cell endings -- about 80 percent -- associate with zigzag hair follicles, the rest with the awl/auchene and none with the guard hair follicles.
The researchers then similarly marked two other types of touch nerve cells and found that each hair type has a different and specific set of nerve endings associated with it. "This makes every hair a unique mechanosensory organ," says Ginty. Moreover, with their new marking tools, they found that each hair type is evenly spaced and patterned throughout the skin.
The team then wondered how all the input from these individual hairs is collected and sent to the brain. Using a different dying technique, the researchers were able to stain the other end of the cell, in the spinal cord. They found that the nerves connecting each patch of skin containing one guard hair and other associated smaller hairs line up in columns in the spinal cord -- neighboring columns correspond to neighboring patches of skin. They estimate that there are about 3,000 to 5,000 columns in the spinal cord, with each column accounting for 100 to 150 hair follicles.
So how does the brain interpret what each hair follicle experiences? "How this happens is remarkable and we're fairly clueless about it," says Ginty. But he suspects that the organization of the columns is key to how all the various inputs are processed before a message goes to the brain. And while people are not as hairy as mice, Ginty believes that many of the same structures are shared. This study and the new cell-marking tools they developed, he says, open a lot of doors for new research in understanding touch and other senses.
This study was funded by the National institutes of Health, the Johns Hopkins NINDS core imaging facility and the Howard Hughes Medical Institute.
Authors on the paper are Lishi Li, Michael Rutlin, Victoria Abraira, Wenqin Luo and David Ginty of Johns Hopkins Colleen Cassidy and C. Jeffery Woodbury of University of Wyoming Laura Kus, Shiaoching Gong and Nathaniel Heintz of the Howard Hughes Medical Institute and The Rockefeller University and Michael Jankowski and H. Richard Koerber of University of Pittsburgh School of Medicine.
Research illuminates 'touchy' subject: Sensory nerve endings
By solving a long standing scientific mystery, the common saying "you just hit a nerve" might need to be updated to "you just hit a Merkel cell," jokes Jianguo Gu, PhD, a pain researcher at the University of Cincinnati (UC).
That's because Gu and his research colleagues have proved that Merkel cells -- which contact many sensory nerve endings in the skin -- are the initial sites for sensing touch.
"Scientists have spent over a century trying to understand the function of this specialized skin cell and now we are the first to know &hellip we've proved the Merkel cell to be a primary point of tactile detection," Gu, principal investigator and a professor in UC's department of anesthesiology, says of their research study published in the April 15 edition of Cell, a leading scientific journal.
Of all the five senses, touch, Gu says, has been the least understood by science -- especially in relation to the Merkel cell, discovered by Friedrich Sigmund Merkel in 1875.
"It's been a great debate because for over two centuries nobody really knew what function this cell had," Gu says, adding that while some scientists -- including him -- suspected that the Merkel cell was related to touch because of the high abundance of these cells in the ridges of fingertips, the lips and other touch sensitive spots throughout the body others dismissed the cell as not related to sensing touch at all.
To prove their hypothesis that Merkel cells were indeed the very foundation of touch, Gu's team -- which included UC postgraduate fellow Ryo Ikeda, PhD -- studied Merkel cells in rat whisker hair follicles , because the hair follicles are functionally similar to human fingertips and have high abundance of Merkel cells. What they found was that the cells immediately fired up in response to gentle touch of whiskers.
"There was a marked response in Merkel cells the recording trace 'spiked'. With non-Merkel cells you don't get anything," says Ikeda.
What they also found, and of equal importance, both say, was that gentle touch makes Merkel cells to fire "action potentials" and this mechano-electrical transduction was through a receptor/ion channel called the Piezo2.
"The implications here are profound," Gu says, pointing to the clinical applications of treating and preventing disease states that affect touch such as diabetes and fibromyalgia and pathological conditions such as peripheral neuropathy. Abnormal touch sensation, he says, can also be a side effect of many medical treatments such as with chemotherapy.
The discovery also has relevance to those who are blind and rely on touch to navigate a sighted world.
"This is a paradigm shift in the entire field," Gu says, pointing to touch as also indispensable for environmental exploration, tactile discrimination and other tasks in life such as modern social interaction.
"Think of the cellphone. You can hardly fit into social life without good touch sensation."
Touch is one of the five major sensory channels by which humans sample and experience their environment. The word "touch" describes the sensory experience resulting from gentle contact of the skin with the environment, including air moving over the skin and hairs. The sense of touch is so exquisitely sensitive that the brain can consciously experience the activity of a single neuron supplying the skin. Touch sensation not only informs one about the near environment but plays an essential role in guiding fine movements basic to such skills as playing musical instruments, reading Braille, typing on a computer keyboard, or performing surgery.
Touch (mechanoreception) is distinguished from pain (nociception) and temperature perception (thermoreception). Pain is sensed by free nerve endings, mostly located in the skin, bones, and joint capsules, and around blood vessels. Two broad categories of painful sensations, fast pricking pain versus slow aching or burning pain, are carried to the spine by two different types of sensory neurons. Thermoreceptors are located immediately below the skin, with warmth receptors more numerous than cool receptors. They are most sensitive not to absolute level of temperature, but to rapid change in temperature, and quickly become quieter once the temperature has stabilized at a new level.
Detection of touch stimuli begins with mechanical deformation of several types of specialized touch receptors, distributed unevenly over the body surface. Nerve fiber endings in the skin may be free, "naked" endings (for light touch) or more commonly are associated with other, cooperating cells. Thus nerve endings that wrap around hair follicles are activated by hair movement other nerve endings adhere closely to specialized accessory cells or have tiny cellular capsules. The latter include pacinian corpuscles for vibration, and Meissner corpuscles (abundant in sensitive, hairless skin of the fingertips) for light touch. Ruffini corpuscles and Merkel disks respond to pressure or to stretch of the skin with signals that continue as long as a stimulus is applied.
When any of these touch-sensitive nerve endings are mechanically deformed, electrical signals (action potentials) are transmitted along the axons of sensory nerve cells. These signals pass rapidly to the spinal cord and brainstem to activate a second set of neurons. As these secondary touch cells relay information up the brainstem, their axons cross the body's midline, so that the touch information they carry activates neurons in the thalamus on
All of the touch information transmitted from the various receptor types in a given body area is combined in the cerebral cortex . It provides sophisticated analysis of the total pattern of nerve signals so that one can instantly (and consciously) judge the texture, force, location, and movement of the skin stimulus with great precision.
Touch sensitivity varies in different body regions because of differential density of distribution of the specific nerve endings. Areas such as the fingertips and lips (glabrous skin) are richly endowed with nerve endings and are very sensitive. Hairy skin has fewer endings and different kinds, and so produces a different sensory experience skin of the trunk and back, with a low density of touch receptors, is less sensitive to touch than skin elsewhere.
Touch receptors branch out at their ends, and a single neuron may receive input from a region of the skin several centimeters in diameter, called its receptor field. Receptor fields in the lips may be as small as 2 to 3 millimeters (.78 to .118 inches), while in much of the rest of the body they are 4 to 7 centimeters (1.5 to 2.7 inches).