As a surface organ, the skin (cutis) forms the outer boundary between the human organism and the outside world. It is our largest sensory organ andhas a number of different receptors and nerve endings with which it receives pressure, touch, temperature and pain stimuli.
In addition to these skin sensory organs, the hair, nails and skin glands are also counted as skin appendages.
The skin is made up of the epidermis , dermis (also known as the corium) and subcutis .
The epidermis is the uppermost, keratinizing squamous epithelial layer. It is multi-layered. On the surface, it closes off to the outside with the horny layer (the so-called stratum corneum). The stratum corneum prevents the penetration of pathogenic germs such as bacteria, viruses and fungi. It is particularly thick in mechanically stressed areas such as the sole of the foot, heel or palm, which is why these areas are also referred to as the cornea.
Beneath the horny layer lies the germ layer. New cells are continuously formed in this layer (regeneration). These cells migrate outwards to the surface where they gradually keratinize until they are shed as horny scales.
The pigment-forming cells - the so-called melanocytes - are located in the deeper cell layers of the epidermis. These determine the skin type with the melanin they produce, a black-brown pigment. Langerhans cells are also found in the epidermis. These are important for the skin's immune response.
The dermis is made up of a network of tensile, collagen and elastic fibers. The upper layers contain so-called papillae, which extend into the epidermis and thus interlock the epidermis and dermis.
The majority of the skin's sensory organs are located in the dermis. These include the Meissner corpuscles, which are responsible for the sense of touch and the sensation of touch, as well as free nerve endings for the sensation of pain. While the epidermis has no blood vessels, the dermis contains many fine capillaries, which are particularly involved in regulating the heat balance.
The subcutis consists of loose connective tissue rich in fatty tissue (subcutaneous fatty tissue). This varies greatly from person to person and region to region and is primarily used to store fat. The subcutis also connects the skin with the body fascia (connective tissue sheath) that covers the muscles.
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Hairs are tensile and at the same time flexible horny threads. They are located in an epidermal pocket with which they sink into the subcutis. The hair shaft shows a swelling (hair bulb) in the lower area, which sits on a connective tissue hair papilla (growth zone).
There are also melanocytes at this point, which produce pigment and store it in the hair. Hair follicle and sebaceous glands are located on the sides of the hair shaft. These secrete an oily secretion that gives the hair suppleness and protects the hair and skin from germs (bacteria).
In addition to the sebaceous glands, the skin also has sweat and scent glands. The sweat glands, which consist of a cluster of glands, are located in the transition area between the dermis and subcutis. They have a long duct that leads to the surface of the skin.
The sweat glands form an acidic secretion with the sweat, which inhibits the growth of bacteria on the surface of the skin (= acid mantle). Sweat consists mainly of salts. However, metabolic waste products such as urea, uric acid, ammonia and metabolized drugs are also excreted with the sweat.
The sweat glands are also important for temperature regulation . The scent glands are located in the skin of the armpits and in the scalp and pubic area. These are present as large clusters and secrete an alkaline secretion during puberty.
There are different receptors in the skin for the perception of stimuli:
The stimuli received by the mechanoreceptors, thermoreceptors and nociceptors are converted into so-called action potentials and transmitted to the nerve fibers. The stimuli from a strip of skin always reach their respective segment of the spinal cord.
A large part of surface sensitivity such as touch, pressure, tickle and vibration perception is transmitted via the mechanoreceptors. The mechanoreceptors are located in different layers of the skin and have different structures. The mechanoreceptors include
The thermoreceptors are located as free nerve endings in the skin and inside the body. Their density is much lower than that of the mechanoreceptors. They are divided into cold and warm receptors. They help to detect and regulate the body's temperature.
As free nerve endings, nociceptors are responsible for the sensation of pain. Surface pain in particular is transmitted via the skin. In general, all environmental influences that lead to tissue damage can trigger a pain stimulus. These primarily include mechanical stimuli (e.g. cuts), but also high temperatures (destruction of body protein from 45 °C) and chemical substances (e.g. acids).
As an exchange and protective surface between the body and the environment, the skin has a number of functions. These include
Sensory function: Thermal and mechanical stimuli as well as pain are perceived through the skin via a series of receptors.
Protective function: The skin provides passive protection for the body against potentially damaging environmental influences. These include, in particular, heat, cold, impact, pressure, friction and radiation. It also provides active protection through its involvement in the immune defense system.
Storage organ: The skin stores fatty tissue and is therefore an important energy store.
Heat balance: The skin ensures heat balance by constricting or dilating its skin vessels. It is therefore involved in the regulation of body temperature and circulation. When the outside temperature is high, the fine vessels dilate and the skin, which is supplied with more blood, warms up.
This releases heat into the cooler outside air. At colder temperatures, on the other hand, the fine vessels constrict. Less heat is released into the outside air and heat loss is reduced.
Excretory function: Water, salt and metabolic waste products are excreted through the skin via the release of various glandular secretions such as sebum and sweat. At the same time, the skin is involved in regulating the water and salt balance.
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Diseases of the skin (dermatoses) are treated by a dermatologist . The most common diseases include
Neurodermatitis © Gina Sanders / Fotolia
