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88-399: Unmyelinated nerve fibers for itches and pain both originate in the skin . Information for them is conveyed centrally in two distinct systems that both use the same nerve bundle and spinothalamic tract . Most commonly, an itch is felt in one place. If it is felt all over the body, then it is called generalized itch or generalized pruritus . Generalized itch is infrequently a symptom of

176-495: A blood-nerve barrier similar to the blood–brain barrier . Molecules are thereby prevented from crossing the blood into the endoneurial fluid. During the development of nerve edema from nerve irritation (or injury), the amount of endoneurial fluid may increase at the site of irritation. This increase in fluid can be visualized using magnetic resonance neurography , and thus MR neurography can identify nerve irritation and/or injury. Nerves are categorized into three groups based on

264-523: A flare reaction that involves no histamine. Therefore, it is possible that pruritoceptive nerve fibres have different classes of fibres, which is unclear in current research. Histology and skin layers . Studies have been done to show that itch receptors are found only on the top two skin layers, the epidermis and the epidermal/ dermal transition layers. Shelley and Arthur verified the depth by injecting individual itch powder ( Mucuna pruriens ) spicules and noting that maximal sensitivity occurred at

352-448: A stratum germinativum and stratum corneum , but the other intermediate layers found in humans are not always distinguishable. Hair is a distinctive feature of mammalian skin, while feathers are (at least among living species) similarly unique to birds . Birds and reptiles have relatively few skin glands , although there may be a few structures for specific purposes, such as pheromone -secreting cells in some reptiles , or

440-411: A Mauthner cell are so powerful that a single action potential gives rise to a major behavioral response: within milliseconds the fish curves its body into a C-shape , then straightens, thereby propelling itself rapidly forward. Functionally this is a fast escape response, triggered most easily by a strong sound wave or pressure wave impinging on the lateral line organ of the fish. Mauthner cells are not

528-578: A command neuron has, however, become controversial, because of studies showing that some neurons that initially appeared to fit the description were really only capable of evoking a response in a limited set of circumstances. In organisms of radial symmetry , nerve nets serve for the nervous system. There is no brain or centralised head region, and instead there are interconnected neurons spread out in nerve nets. These are found in Cnidaria , Ctenophora and Echinodermata . Herophilos (335–280 BC) described

616-514: A complete A-fiber conduction block , it is significantly diminished. Overall, itch sensation is mediated by A-delta and C nociceptors located in the uppermost layer of the skin. Gene expression . Using single-cell mRNA sequencing, clusters of genes expressed in itch-related tissues were identified, e.g. NP1-3, transmitting itch information; where NP3 expresses neuropeptides Nppb and Sst as well as genes involved in inflammatory itch ( Il31ra , Osmr and Crystrl2 ). The histamine receptor gene Hrh1

704-419: A cylindrical shape. When the ducts mature and fill with fluid, the base of the ducts become swollen due to the pressure from the inside. This causes the epidermal layer to form a pit like opening on the surface of the duct in which the inner fluid will be secreted in an upwards fashion. The intercalary region of granular glands is more developed and mature in comparison with mucous glands. This region resides as

792-463: A different functionality for amphibians than granular. Mucous glands cover the entire surface area of the amphibian body and specialize in keeping the body lubricated. There are many other functions of the mucous glands such as controlling the pH, thermoregulation, adhesive properties to the environment, anti-predator behaviors (slimy to the grasp), chemical communication, even anti-bacterial/viral properties for protection against pathogens. The ducts of

880-460: A modified intercalary region (depending on the function of the glands), yet the majority share the same structure. The alveolar or mucous glands are much more simple and only consist of an epithelium layer as well as connective tissue which forms a cover over the gland. This gland lacks a tunica propria and appears to have delicate and intricate fibers which pass over the gland's muscle and epithelial layers. The epidermis of birds and reptiles

968-417: A numerous individual mucus -secreting skin cells that aid in insulation and protection, but may also have poison glands , photophores , or cells that produce a more watery, serous fluid. In amphibians , the mucous cells are gathered together to form sac-like glands . Most living amphibians also possess granular glands in the skin, that secrete irritating or toxic compounds. Although melanin

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1056-429: A painful or pruritic inflammatory condition not only activate pruriceptors but also cause acute sensitization of the nociceptors. In addition, expression of neuro growth factors (NGF) can cause structural changes in nociceptors , such as sprouting. NGF is high in injured or inflamed tissue. Increased NGF is also found in atopic dermatitis , a hereditary and non-contagious skin disease with chronic inflammation . NGF

1144-455: A public lecture on itching. The sensation of pain can also be induced in a similar fashion, often by listening to a description of an injury, or viewing an injury itself. There is little detailed data on central activation for contagious itching, but it is hypothesized that a human mirror neuron system exists in which one imitates certain motor actions when they view others performing the same action. A similar hypothesis has been used to explain

1232-436: A regular pattern. Sonic hedgehog-expressing epidermal cells induce the condensation of cells in the mesoderm . The clusters of mesodermal cells signal back to the epidermis to form the appropriate structure for that position. BMP signals from the epidermis inhibit the formation of placodes in nearby ectoderm. It is believed that the mesoderm defines the pattern. The epidermis instructs the mesodermal cells to condense and then

1320-400: A ring of cells surrounding the basal portion of the duct which are argued to have an ectodermal muscular nature due to their influence over the lumen (space inside the tube) of the duct with dilation and constriction functions during secretions. The cells are found radially around the duct and provide a distinct attachment site for muscle fibers around the gland's body. The gland alveolus is

1408-430: A sac that is divided into three specific regions/layers. The outer layer or tunica fibrosa is composed of densely packed connective-tissue which connects with fibers from the spongy intermediate layer where elastic fibers, as well as nerves, reside. The nerves send signals to the muscles as well as the epithelial layers. Lastly, the epithelium or tunica propria encloses the gland. Mucous glands are non-venomous and offer

1496-438: A sensation of burning pain rather than itch, which would be induced in normal healthy patients. This shows that there is spinal hypersensitivity to C-fiber input in chronic pain. A variety of over-the-counter and prescription anti-itch drugs are available. Some plant products have been found to be effective anti-pruritics, others not. Non-chemical remedies include cooling, warming, soft stimulation. Topical antipruritics in

1584-444: A serious underlying condition, such as cholestatic liver disease. If the sensation of itching persists for six weeks or longer, then it is called chronic itch or chronic pruritus . Chronic idiopathic pruritus or Chronic Pruritus of Unknown Origin is a form of itch that persists for longer than six weeks, and for which no clear cause can be identified . Pain and itch have very different behavioral response patterns. Pain elicits

1672-416: A thin sheet of fibers called the basement membrane , which is made through the action of both tissues . The basement membrane controls the traffic of the cells and molecules between the dermis and epidermis but also serves, through the binding of a variety of cytokines and growth factors , as a reservoir for their controlled release during physiological remodeling or repair processes. The dermis

1760-480: A way to relieve oneself by reducing the annoying itch sensation. However, there are hedonic aspects to scratching, as one would find noxious scratching highly pleasurable. This can be problematic with chronic itch patients, such as ones with atopic dermatitis , who may scratch affected spots until they no longer produce a pleasant or painful sensation, instead of when the itch sensation disappears. It has been hypothesized that motivational aspects of scratching include

1848-466: A withdrawal reflex, which leads to retraction and therefore a reaction trying to protect an endangered part of the body. Itch in contrast creates a scratch reflex , which draws one to the affected skin site. Itch generates stimulus of a foreign object underneath or upon the skin and also the urge to remove it. For example, responding to a local itch sensation is an effective way to remove insects from one's skin. Scratching has traditionally been regarded as

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1936-411: Is a special type of identified neuron, defined as a neuron that is capable of driving a specific behavior all by itself. Such neurons appear most commonly in the fast escape systems of various species—the squid giant axon and squid giant synapse , used for pioneering experiments in neurophysiology because of their enormous size, both participate in the fast escape circuit of the squid. The concept of

2024-524: Is called identified if it has properties that distinguish it from every other neuron in the same animal—properties such as location, neurotransmitter, gene expression pattern, and connectivity—and if every individual organism belonging to the same species has exactly one neuron with the same set of properties. In vertebrate nervous systems, very few neurons are "identified" in this sense. Researchers believe humans have none—but in simpler nervous systems, some or all neurons may be thus unique. In vertebrates,

2112-438: Is closer to that of mammals , with a layer of dead keratin-filled cells at the surface, to help reduce water loss. A similar pattern is also seen in some of the more terrestrial amphibians such as toads . In these animals, there is no clear differentiation of the epidermis into distinct layers, as occurs in humans , with the change in cell type being relatively gradual. The mammalian epidermis always possesses at least

2200-407: Is found in the skin of many species, in the reptiles , the amphibians , and fish , the epidermis is often relatively colorless. Instead, the color of the skin is largely due to chromatophores in the dermis , which, in addition to melanin, may contain guanine or carotenoid pigments . Many species, such as chameleons and flounders may be able to change the color of their skin by adjusting

2288-436: Is further subdivided into the sympathetic and the parasympathetic nervous systems. The sympathetic nervous system is activated in cases of emergencies to mobilize energy, while the parasympathetic nervous system is activated when organisms are in a relaxed state. The enteric nervous system functions to control the gastrointestinal system. Both autonomic and enteric nervous systems function involuntarily. Nerves that exit from

2376-525: Is itch induced centrally but with no neural damage, is mostly associated with increased accumulation of exogenous opioids and possibly synthetic opioids. Itch is also associated with some symptoms of psychiatric disorders such as tactile hallucinations , delusions of parasitosis , or obsessive-compulsive disorders (as in OCD -related neurotic scratching). Inflammatory mediators—such as bradykinin , serotonin (5-HT) and prostaglandins —released during

2464-442: Is known to up-regulate neuropeptides, especially substance P . Substance P has been found to have an important role in inducing pain; however, there is no confirmation that substance P directly causes acute sensitization. Instead, substance P may contribute to itch by increasing neuronal sensitization and may affect release of mast cells , which contain many granules rich in histamine, during long-term interaction. Noxious input to

2552-563: Is maintained as a stem cell layer through an autocrine signal, TGF alpha , and through paracrine signaling from FGF7 ( keratinocyte growth factor ) produced by the dermis below the basal cells. In mice, over-expression of these factors leads to an overproduction of granular cells and thick skin. Hair and feathers are formed in a regular pattern and it is believed to be the result of a reaction-diffusion system. This reaction-diffusion system combines an activator, Sonic hedgehog , with an inhibitor, BMP4 or BMP2, to form clusters of cells in

2640-445: Is perceived as more painful around a focused area of inflammation (punctuate hyperalgesia). Touch-evoked hyperalgesia requires continuous firing of primary afferent nociceptors, and punctuate hyperalgesia does not require continuous firing which means it can persist for hours after a trauma and can be stronger than normally experienced. In addition, it was found that patients with neuropathic pain, histamine ionophoresis resulted in

2728-491: Is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis provides tensile strength and elasticity to the skin through an extracellular matrix composed of collagen fibrils , microfibrils , and elastic fibers , embedded in hyaluronan and proteoglycans . Skin proteoglycans are varied and have very specific locations. For example, hyaluronan , versican and decorin are present throughout

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2816-432: Is the part of an animal that coordinates its actions by transmitting signals to and from different parts of its body. In vertebrates it consists of two main parts, the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain , including the brainstem , and spinal cord . The PNS consists mainly of nerves, which are enclosed bundles of the long fibers or axons , that connect

2904-472: Is thought to alleviate itch by acting on the IL-4 receptor on sensory neurons. The effectiveness of therapeutic options for people who are terminally ill with malignant cancer is not known. Approximately 280 million people globally, 4% of the population, have difficulty with itchiness. This is comparable to the 2–3% of the population who have psoriasis . In 1660, German physician Samuel Hafenreffer introduced

2992-434: Is wrapped in a layer of connective tissue called the perineurium . Finally, the entire nerve is wrapped in a layer of connective tissue called the epineurium . Nerve cells (often called neurons) are further classified as sensory , motor , or mixed nerves . In the central nervous system , the analogous structures are known as nerve tracts . Each nerve is covered on the outside by a dense sheath of connective tissue ,

3080-475: The axons to peripheral organs or, in the case of sensory nerves , from the periphery back to the central nervous system . Each axon, within the nerve, is an extension of an individual neuron , along with other supportive cells such as some Schwann cells that coat the axons in myelin . Within a nerve, each axon is surrounded by a layer of connective tissue called the endoneurium . The axons are bundled together into groups called fascicles , and each fascicle

3168-524: The basal cell layer or the innermost layer of the epidermis. Surgical removal of those skin layers removed the ability for a patient to perceive itch. Itch is never felt in muscle or joints, which strongly suggests that deep tissue probably does not contain itch signaling apparatuses. Sensitivity to pruritic stimuli is evenly distributed across the skin and has a clear spot distribution with similar density to that of pain. The different substances that elicit itch upon intracutaneous injection (injection within

3256-429: The dermis provide nourishment and waste removal from its own cells as well as for the epidermis . Dermis and subcutaneous tissues are thought to contain germinative cells involved in formation of horns, osteoderm, and other extra-skeletal apparatus in mammals. The dermis is tightly connected to the epidermis through a basement membrane and is structurally divided into two areas: a superficial area adjacent to

3344-490: The epineurium . Beneath this is a layer of fat cells, the perineurium , which forms a complete sleeve around a bundle of axons. Perineurial septae extend into the nerve and subdivide it into several bundles of fibres. Surrounding each such fibre is the endoneurium . This forms an unbroken tube from the surface of the spinal cord to the level where the axon synapses with its muscle fibres, or ends in sensory receptors . The endoneurium consists of an inner sleeve of material called

3432-428: The glycocalyx and an outer, delicate, meshwork of collagen fibres. Nerves are bundled and often travel along with blood vessels , since the neurons of a nerve have fairly high energy requirements. Within the endoneurium, the individual nerve fibres are surrounded by a low-protein liquid called endoneurial fluid . This acts in a similar way to the cerebrospinal fluid in the central nervous system and constitutes

3520-402: The skull , these scales are lost in tetrapods , although many reptiles do have scales of a different kind, as do pangolins . Cartilaginous fish have numerous tooth-like denticles embedded in their skin, in place of true scales . Sweat glands and sebaceous glands are both unique to mammals , but other types of skin gland are found in other vertebrates . Fish typically have

3608-410: The spinal cord is known to produce central sensitization, which consists of allodynia , exaggeration of pain, and punctuate hyperalgesia , extreme sensitivity to pain. Two types of mechanical hyperalgesia can occur: 1) touch that is normally painless in the uninjured surroundings of a cut or tear can trigger painful sensations (touch-evoked hyperalgesia), and 2) a slightly painful pin prick stimulation

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3696-403: The uropygial gland of most birds. Cutaneous structures arise from the epidermis and include a variety of features such as hair, feathers, claws and nails. During embryogenesis, the epidermis splits into two layers: the periderm (which is lost) and the basal layer . The basal layer is a stem cell layer and through asymmetrical divisions, becomes the source of skin cells throughout life. It

3784-529: The CNS to all remaining body parts. Nerves that transmit signals from the CNS are called motor or efferent nerves, while those nerves that transmit information from the body to the CNS are called sensory or afferent . Spinal nerves serve both functions and are called mixed nerves. The PNS is divided into three separate subsystems, the somatic , autonomic , and enteric nervous systems. Somatic nerves mediate voluntary movement. The autonomic nervous system

3872-529: The GRP neurons activate GRPR neurons to promote itch Neuropathic itch can originate at any point along the afferent pathway as a result of damage of the nervous system . They could include diseases or disorders in the central nervous system or peripheral nervous system . Examples of neuropathic itch in origin are notalgia paresthetica, brachioradial pruritus , brain tumors , multiple sclerosis , peripheral neuropathy , and nerve irritation . Neurogenic itch, which

3960-518: The actual site of damage, a phenomenon called referred pain . Referred pain can happen when the damage causes altered signalling to other areas. Neurologists usually diagnose disorders of nerves by a physical examination , including the testing of reflexes , walking and other directed movements, muscle weakness , proprioception , and the sense of touch . This initial exam can be followed with tests such as nerve conduction study , electromyography (EMG), and computed tomography (CT). A neuron

4048-400: The alveolar gland (sac). Structurally, the duct is derived via keratinocytes and passes through to the surface of the epidermal or outer skin layer thus allowing external secretions of the body. The gland alveolus is a sac-shaped structure that is found on the bottom or base region of the granular gland. The cells in this sac specialize in secretion. Between the alveolar gland and the duct is

4136-401: The amphibians. They are located in clusters differing in concentration depending on amphibian taxa. The toxins can be fatal to most vertebrates or have no effect against others. These glands are alveolar meaning they structurally have little sacs in which venom is produced and held before it is secreted upon defensive behaviors. Structurally, the ducts of the granular gland initially maintain

4224-428: The best known identified neurons are the gigantic Mauthner cells of fish. Every fish has two Mauthner cells, located in the bottom part of the brainstem, one on the left side and one on the right. Each Mauthner cell has an axon that crosses over, innervating (stimulating) neurons at the same brain level and then travelling down through the spinal cord, making numerous connections as it goes. The synapses generated by

4312-538: The blood vessels surrounding the nerve all cause nerve damage , which can vary in severity. Multiple sclerosis is a disease associated with extensive nerve damage. It occurs when the macrophages of an individual's own immune system damage the myelin sheaths that insulate the axon of the nerve. A pinched nerve occurs when pressure is placed on a nerve, usually from swelling due to an injury, or pregnancy and can result in pain , weakness, numbness or paralysis, an example being CTS. Symptoms can be felt in areas far from

4400-429: The body. Microorganisms like Staphylococcus epidermidis colonize the skin surface. The density of skin flora depends on region of the skin. The disinfected skin surface gets recolonized from bacteria residing in the deeper areas of the hair follicle , gut and urogenital openings. The epidermis of fish and of most amphibians consists entirely of live cells , with only minimal quantities of keratin in

4488-433: The cause of contagious yawning . Studies done in the last decade have shown that itch can be inhibited by many other forms of painful stimuli, such as noxious heat, physical rubbing/scratching, noxious chemicals, and electric shock . Itch can originate in the peripheral nervous system ( dermal or neuropathic ) or in the central nervous system (neuropathic, neurogenic , or psychogenic ). Itch originating in

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4576-414: The cells of the superficial layer. It is generally permeable, and in the case of many amphibians , may actually be a major respiratory organ. The dermis of bony fish typically contains relatively little of the connective tissue found in tetrapods . Instead, in most species, it is largely replaced by solid, protective bony scales . Apart from some particularly large dermal bones that form parts of

4664-503: The common traditional phrases "my poor nerves", " high-strung ", and " nervous breakdown ". Skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other animal coverings , such as the arthropod exoskeleton , have different developmental origin , structure and chemical composition . The adjective cutaneous means "of

4752-553: The cranium are called cranial nerves while those exiting from the spinal cord are called spinal nerves . Cancer can spread by invading the spaces around nerves. This is particularly common in head and neck cancer , prostate cancer and colorectal cancer . Nerves can be damaged by physical injury as well as conditions like carpal tunnel syndrome (CTS) and repetitive strain injury . Autoimmune diseases such as Guillain–Barré syndrome , neurodegenerative diseases , polyneuropathy , infection, neuritis , diabetes , or failure of

4840-404: The definition of pruritus (itch). Nerve fibers A nerve is an enclosed, cable-like bundle of nerve fibers (called axons ) in the peripheral nervous system . Nerves have historically been considered the basic units of the peripheral nervous system. A nerve provides a common pathway for the electrochemical nerve impulses called action potentials that are transmitted along each of

4928-416: The dermis and epidermis extracellular matrix , whereas biglycan and perlecan are only found in the epidermis. It harbors many mechanoreceptors (nerve endings) that provide the sense of touch and heat through nociceptors and thermoreceptors . It also contains the hair follicles , sweat glands , sebaceous glands , apocrine glands , lymphatic vessels and blood vessels . The blood vessels in

5016-403: The direction that signals are conducted: Nerves can be categorized into two groups based on where they connect to the central nervous system: Specific terms are used to describe nerves and their actions. A nerve that supplies information to the brain from an area of the body, or controls an action of the body is said to innervate that section of the body or organ. Other terms relate to whether

5104-506: The environment and is the first line of defense from external factors. For example, the skin plays a key role in protecting the body against pathogens and excessive water loss. Its other functions are insulation , temperature regulation , sensation, and the production of vitamin D folates. Severely damaged skin may heal by forming scar tissue . This is sometimes discoloured and depigmented. The thickness of skin also varies from location to location on an organism. In humans, for example,

5192-408: The epidermis, called the papillary region , and a deep thicker area known as the reticular region . The papillary region is composed of loose areolar connective tissue . This is named for its fingerlike projections called papillae that extend toward the epidermis . The papillae provide the dermis with a "bumpy" surface that interdigitates with the epidermis, strengthening the connection between

5280-676: The existence of devices such as the back scratcher . Often, however, scratching only offers temporary relief and can intensify itching, even causing further damage to the skin, dubbed the "itch-scratch cycle". The mainstay of therapy for dry skin is maintaining adequate skin moisture and topical emollients . No studies have been conducted to investigate the effectiveness of emollient creams, cooling lotions, topical corticosteroids, topical antidepressants, systemic antihistamines, systemic antidepressants, systemic anticonvulsants, and phototherapy on chronic pruritus of unknown origin. However, there are clinical trials currently underway with dupilumab which

5368-479: The following functions: Skin is a soft tissue and exhibits key mechanical behaviors of these tissues. The most pronounced feature is the J-curve stress strain response, in which a region of large strain and minimal stress exists and corresponds to the microstructural straightening and reorientation of collagen fibrils. In some cases the intact skin is prestreched, like wetsuits around the diver's body, and in other cases

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5456-404: The form of creams and sprays are often available over-the-counter . Oral anti-itch drugs also exist and are usually prescription drugs . The active ingredients usually belong to the following classes: Phototherapy is helpful for severe itching, especially if caused by chronic kidney disease . The common type of light used is UVB . Sometimes scratching relieves isolated itches, hence

5544-418: The formation of an extracellular matrix and provide mechanical strength to the skin. Keratinocytes from the stratum corneum are eventually shed from the surface ( desquamation ). The epidermis contains no blood vessels , and cells in the deepest layers are nourished by diffusion from blood capillaries extending to the upper layers of the dermis . The epidermis and dermis are separated by

5632-461: The frontal brain areas of reward and decision making. These aspects might therefore contribute to the compulsive nature of itch and scratching. Events of " contagious itch" are very common occurrences. Even a discussion on the topic of itch can give one the desire to scratch. Itch is likely to be more than a localized phenomenon in the place one scratches. Results from a study showed that itching and scratching were induced purely by visual stimuli in

5720-518: The functions of the optic nerve in sight and the oculomotor nerve in eye movement. Analysis of the nerves in the cranium enabled him to differentiate between blood vessels and nerves ( Ancient Greek : νεῦρον (neûron) "string, plant fiber, nerve"). Modern research has not confirmed William Cullen 's 1785 hypothesis associating mental states with physical nerves, although popular or lay medicine may still invoke "nerves" in diagnosing or blaming any sort of psychological worry or hesitancy, as in

5808-487: The growth processes finds the regeneration tube, it begins to grow rapidly towards its original destination guided the entire time by the regeneration tube. Nerve regeneration is very slow and can take up to several months to complete. While this process does repair some nerves, there will still be some functional deficit as the repairs are not perfect. A nerve conveys information in the form of electrochemical impulses (as nerve impulses known as action potentials ) carried by

5896-404: The individual neurons that make up the nerve. These impulses are extremely fast, with some myelinated neurons conducting at speeds up to 120 m/s. The impulses travel from one neuron to another by crossing a synapse , where the message is converted from electrical to chemical and then back to electrical. Nerves can be categorized into two groups based on function: The nervous system

5984-418: The insulation the skin provides but can also serve as a secondary sexual characteristic or as camouflage . On some animals, the skin is very hard and thick and can be processed to create leather . Reptiles and most fish have hard protective scales on their skin for protection, and birds have hard feathers , all made of tough beta-keratins . Amphibian skin is not a strong barrier, especially regarding

6072-459: The intact skin is under compression. Small circular holes punched on the skin may widen or close into ellipses, or shrink and remain circular, depending on preexisting stresses. Tissue homeostasis generally declines with age, in part because stem /progenitor cells fail to self-renew or differentiate . Skin aging is caused in part by TGF-β by blocking the conversion of dermal fibroblasts into fat cells which provide support. Common changes in

6160-420: The intercalary system which can be summed up as a transitional region connecting the duct to the grand alveolar beneath the epidermal skin layer. In general, granular glands are larger in size than the mucous glands, which are greater in number. Granular glands can be identified as venomous and often differ in the type of toxin as well as the concentrations of secretions across various orders and species within

6248-689: The major cells , constituting 95% of the epidermis , while Merkel cells , melanocytes and Langerhans cells are also present. The epidermis can be further subdivided into the following strata or layers (beginning with the outermost layer): Keratinocytes in the stratum basale proliferate through mitosis and the daughter cells move up the strata changing shape and composition as they undergo multiple stages of cell differentiation to eventually become anucleated. During that process, keratinocytes will become highly organized, forming cellular junctions ( desmosomes ) between each other and secreting keratin proteins and lipids which contribute to

6336-435: The mesoderm instructs the epidermis of what structure to make through a series of reciprocal inductions. Transplantation experiments involving frog and newt epidermis indicated that the mesodermal signals are conserved between species but the epidermal response is species-specific meaning that the mesoderm instructs the epidermis of its position and the epidermis uses this information to make a specific structure. Skin performs

6424-413: The mucous gland appear as cylindrical vertical tubes that break through the epidermal layer to the surface of the skin. The cells lining the inside of the ducts are oriented with their longitudinal axis forming 90-degree angles surrounding the duct in a helical fashion. Intercalary cells react identically to those of granular glands but on a smaller scale. Among the amphibians, there are taxa which contain

6512-401: The nerve affects the same side ("ipsilateral") or opposite side ("contralateral") of the body, to the part of the brain that supplies it. Nerve growth normally ends in adolescence, but can be re-stimulated with a molecular mechanism known as " Notch signaling ". If the axons of a neuron are damaged, as long as the cell body of the neuron is not damaged, the axons can regenerate and remake

6600-491: The oldest known skin, fossilized about 289 million years ago, and possibly the skin from an ancient reptile. The word skin originally only referred to dressed and tanned animal hide and the usual word for human skin was hide. Skin is a borrowing from Old Norse skinn "animal hide, fur", ultimately from the Proto-Indo-European root *sek-, meaning "to cut" (probably a reference to the fact that in those times animal hide

6688-441: The only identified neurons in fish—there are about 20 more types, including pairs of "Mauthner cell analogs" in each spinal segmental nucleus. Although a Mauthner cell is capable of bringing about an escape response all by itself, in the context of ordinary behavior other types of cells usually contribute to shaping the amplitude and direction of the response. Mauthner cells have been described as command neurons . A command neuron

6776-411: The passage of chemicals via skin, and is often subject to osmosis and diffusive forces. For example, a frog sitting in an anesthetic solution would be sedated quickly as the chemical diffuses through its skin. Amphibian skin plays key roles in everyday survival and their ability to exploit a wide range of habitats and ecological conditions. On 11 January 2024, biologists reported the discovery of

6864-422: The pruriceptive primary afferent has been activated, the signal is transmitted from the skin into the spinal dorsal horn. In this area, a number of interneurons will either be inhibited or activated to promote activation of projection neurons, mediating the pruriceptive signal to the brain. The GRP-GRPR interneuron system has been found to be important for mediating both histaminergic and non-histaminergic itch, where

6952-420: The relative size of their chromatophores . Amphibians possess two types of glands , mucous and granular (serous). Both of these glands are part of the integument and thus considered cutaneous . Mucous and granular glands are both divided into three different sections which all connect to structure the gland as a whole. The three individual parts of the gland are the duct, the intercalary region, and lastly

7040-581: The reticular region are the roots of the hair , sweat glands , sebaceous glands , receptors , nails , and blood vessels . The subcutaneous tissue (also hypodermis) is not part of the skin, and lies below the dermis . Its purpose is to attach the skin to underlying bone and muscle as well as supplying it with blood vessels and nerves . It consists of loose connective tissue and elastin . The main cell types are fibroblasts , macrophages and adipocytes (the subcutaneous tissue contains 50% of body fat ). Fat serves as padding and insulation for

7128-657: The skin is known as pruritoceptive , and can be induced by a variety of stimuli, including mechanical, chemical, thermal, and electrical stimulation, or infection. The primary afferent neurons responsible for histamine -induced itch are unmyelinated C-fibres . Nociceptors . Two major classes of human C-fibre nociceptors exist: mechano-responsive nociceptors and mechano-insensitive nociceptors. Mechano-responsive nociceptors have been shown in studies to respond to mostly pain, and mechano-insensitive receptors respond mostly to itch induced by histamine. However, it does not explain mechanically induced itch or itch produced without

7216-422: The skin located under the eyes and around the eyelids is the thinnest skin on the body at 0.5 mm thick and is one of the first areas to show signs of aging such as "crows feet" and wrinkles. The skin on the palms and the soles of the feet is the thickest skin on the body at 4 mm thick. The speed and quality of wound healing in skin is promoted by estrogen . Fur is dense hair. Primarily, fur augments

7304-760: The skin" (from Latin cutis 'skin'). In mammals , the skin is an organ of the integumentary system made up of multiple layers of ectodermal tissue and guards the underlying muscles , bones , ligaments , and internal organs . Skin of a different nature exists in amphibians , reptiles , and birds . Skin (including cutaneous and subcutaneous tissues) plays crucial roles in formation, structure, and function of extraskeletal apparatus such as horns of bovids (e.g., cattle) and rhinos, cervids' antlers, giraffids' ossicones, armadillos' osteoderm, and os penis / os clitoris . All mammals have some hair on their skin, even marine mammals like whales , dolphins , and porpoises that appear to be hairless. The skin interfaces with

7392-452: The skin) elicit only pain when injected subcutaneously (beneath the skin). Itch is often classified as that which is histamine mediated (histaminergic) and nonhistaminergic. Itch is readily abolished in skin areas treated with nociceptor excitotoxin capsaicin but remains unchanged in skin areas rendered touch insensitive by pretreatment with anti-inflammatory saponins . Although experimentally induced itch can still be perceived under

7480-404: The synaptic connections with neurons with the help of guidepost cells . This is also referred to as neuroregeneration . The nerve begins the process by destroying the nerve distal to the site of injury allowing Schwann cells, basal lamina, and the neurilemma near the injury to begin producing a regeneration tube. Nerve growth factors are produced causing many nerve sprouts to bud. When one of

7568-418: The two layers of skin. The reticular region lies deep in the papillary region and is usually much thicker. It is composed of dense irregular connective tissue and receives its name from the dense concentration of collagenous , elastic , and reticular fibers that weave throughout it. These protein fibers give the dermis its properties of strength , extensibility , and elasticity . Also located within

7656-453: Was commonly cut off to be used as garment). Mammalian skin is composed of two primary layers: The epidermis is composed of the outermost layers of the skin. It forms a protective barrier over the body's surface, responsible for keeping water in the body and preventing pathogens from entering, and is a stratified squamous epithelium , composed of proliferating basal and differentiated suprabasal keratinocytes . Keratinocytes are

7744-743: Was found in NP2 and NP3, suggesting that histaminergic itch is transmitted by both these pruriceptive sub clusters. Infection . Staphylococcus aureus , a bacterial pathogen associated with itchy skin diseases, directly activates pruriceptor sensory neurons to drive itch. Skin exposure to S. aureus causes robust itch and scratch-induced damage. This reaction is mediated by S. aureus serine protease V8 which cleaves proteinase-activated receptor 1 ( PAR1 ) on mouse and human sensory neurons. Targeting PAR1 through genetic deficiency, small interfering RNA ( siRNA ) knockdown, or pharmacological blockade decreases itch and skin damage caused by V8 and S. aureus exposure. After

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