Skin

The skin • Structure and classification • Sensory functions - skin sensory receptors • Experimental results • Functions of skin

Structure and classification • Body’s largest organ; covers area of 1.5 to 2.0 m2; accounts for about 15% of the total body weight; • Consist of 2 layers: epidermis (top) stratified, and dermis (deep) connective tissue layer; • Below the skin is the hypodermis (another connective tissue layer, it is not a technical part of the skin)

Structure and classification

Structure and classification • Most of the skin is 1-2mm thick. although eyelids(0.5mm) and 6mm between the shoulder blades. variation is due to the thickness of the dermis; • Classification of the skin is based on the epidermis alone especially the surface layer of dead cells (stratum corneum): -thick:

-thin:

palm, soles, fingers, and toes; has sweat glands but no hair follicles, nor oil glands (sebaceous); rest of the body; has hair follicles, sebaceous glands and sweat glands;

• Hairless (Glabrous) skin has an epidermal layer of about 1.5 mm in thickness and a dermis of about 3 mm • Hairy skin has an epidermal layer of 0.07 mm in thickness and a dermis of about 1-2 mm;

Epidermis • Consist of a surface comprised of dead cells packed with keratin (a tough protein); • Does not contain blood vessels. but it contains sparse nerve endings for touch and pain (But most sensations are due to the dermis); • Cell types: – Keratinocytes - produce keratin – Melanocytes - produce melanin – Dendritic (Langerhans) cells - immune function cells

Epidermis thick dead layer of keratinocytes appears only in thick skin; with flat keratinocytes keratinocytes and dendritic cells composed of layers of keratinocytes, pushing the dead ones atop, and live ones stay below. and dendritic (langerhans) cells (produced in the bone marrow and surface to the epidermis to protect us from patogens)

deepest layer with 3 cell types: living keratinocytes (metabolism, produce keratin), melanocytes (block UV and give skin colour), and Tactile cells (Merkel)

Epidermis

Dermis • Composed mainly of collagen, but also contains elastic reticular fibers, blood vessels, sweat glands, sebaceous glands, hair follicles, nail roots, sensory nerve endings and muscular tissue (facial expressions are due to the skeletal muscle connection to the dermal collagen fibers to produce smile, frown, eyebrow movement…) • The boundaries of dermis vs epidermis is not strict, it’s like cardboard ridges that merge together. In sensitive areas, the dermis is more extended and pushed to the surface allowing blood vessels and nerve endings to reach closer to the surface (look at hand, front vs back) • Functions: Pressure detection; metabolism (duplication of cells)…

Dermis

Hypodermis • A sub dermal layer of adipose tissue or otherwise called subcutaneous fat; which is made up of loose, fibrous tissue, rich in blood vessels, lymphatic vessels and nerves; • The base of hair follicles and the coiled tubes of sweat glands may also project down into the hypodermis; • This is the layer that pads the body, serves as an energy reservoir and provides thermal insulation, (it is differently distributed in females vs males)

Hypodermis

Sensory functions • The skin is our most extensive sense organ. It is equipped with a variety of nerve endings that react to heat, cold, touch, texture, pressure, vibration, and tissue injury (pain); • The sensory receptors are specially abundant on the face, palms, fingers, soles, nipples and genitals. there are relatively few on the back, and joint areas; • Some receptors are naked dendrites that penetrate into the epidermis, and most others are limited to the dermis and hypodermis, where specialized connective tissues give the nerve cells more selective sensitivity to particular stimuli

Skin sensory receptors The skin contains different types of receptors. The mechanoreceptors on the skin can be classified into several types:

STRUCTURE

LOCATION

FUNCTION

Free nerve endings

Widespread, Epidermis, dermis

Pain, temperature

Merkel’s (tactile) disks

Epidermis (basale)

Light touch, texture, edge, shape

hair follicle receptors

Dermis, around hair follicle

Detect motion of hair

Meissner’s (tactile) corpuscles

Dermis (papillae of fingertips, palms, lips, eyelids)

Light touch, texture, vibration (2040 Hz range )

Ruffini endings

Dermis, hypodermis

Responds to pressure on skin

Pacinian (Lamellated) corpuscles

Dermis, hypodermis

Deep touch, Pressure, vibration (150300 Hz )

Skin sensory receptors •

Hairy skin has different mechanoreceptors than hairless (glabrous) skin.

•

Hairy: - hair follicle receptors - tactile disk - pacinian corpuscles - ruffini endings

•

Non-hairy (glabrous): - merkel’s disks - meissner’s corpuscles - pacinian corpuscles - ruffini endings

Skin sensory receptors • The skin also has four kinds of thermal receptors: warmth, cool, heat and cold. the last two are regarded as nociceptors because they also mediate the sensation of pain; • Some nociceptors are also regarded as polymodal nociceptors since they respond to different types of stimuli (eg. some thermal receptors also mediate the stimuli of itch and/or pain).

Receptor properties • • • •

Adaptation Frequency Selectivity Change in Receptive fields Neuro Plasticity

Adaptation • The receptors in the skin have different rates of adaptation, by adaptation we imply that the response to steady stimulation decreases gradually after an onset of steady stimulation; • In the skin there are Rapid Adapting receptors (RA), those that respond best to rapid changes in the deformation of the skin, and Slow Adapting receptors (SA), those to respond to slow deformations as well as fast stimuli; • Hence SA respond to the displacement of the skin, and RA responds to the rate of displacement of the skin (velocity).

Frequency Selectivity • Different receptors respond to different frequencies of vibrations. The different receptors in the skin also have this characteristic • For example, the Pacinian corpuscle responds best to sinusoidal vibrations within a narrow frequency range, whereas the hair follicles are more sensitive to a wider range of vibrations.

Change in receptive fields • The medium that conducts the physical stimulus can affect the receptive field (the range of the stimulus that a receptor can detect) of a receptor; this happens because the stimulus can be spread to other adjacent receptors. • For example: only a selected area of the skin will feel the effect of a light pin like object on it. but once there is more pressure imposed on the object, then the skin deformation broadens to cover other receptors, hence responding to the stimuli. – in other words, the receptive field of the skin will become broader when the stimulus intensity is increased.

Neuro Plasticity • It has been shown that the function of the somatosensory system can change as a result of external circumstances such as deprivation of input; • In other words neuro plasticity refers to the ability of the skin to become more or less sensitive to stimulation depending on the environment it has been exposed to.

Experimental results • Temporal differences - Studies have been made to determine the limits of human perception of the temporal aspects of tactile stimuli and the results have been compared to other senses. - If two tactile stimuli are separated by about 5 ms or more, they are perceived as separate events. If the separation is less, they are perceived as one (For hearing the threshold is about 0.01 ms and for vision it is about 25 ms) • Spatial differences: - One of the most important measures used in those studies is the two-point limen; If the distance between the points of stimuli is less than the two-point limen, the subject perceives the two points as one.

Spatial resolution Two-point discrimination is defined as the minimum distance between two stimuli that can be perceived as separate stimuli.

Functions of skin • Body temperature regulation • Protection by keratinized stratified squamous epithelium • Sensory reception • Excretion • Synthesis of Vitamin D • Blood and moisture storage

Conclusion

References • Survey of Studies on Tactile Senses, http://www.media.mit.edu/resenv/classes/MAS965/readings/pohja96su rvey.pdf • Our Sense of Touch, http://faculty.washington.edu/chudler/twopt.html • MIT Touch Lab Research, http://touchlab.mit.edu/oldresearch/index.html • Tactile/Kinesthetic Learning, http://volcano.und.nodak.edu/vwdocs/msh/llc/is/tkl.html • Skin, http://www.cytochemistry.net/microanatomy/skin/skin_and_mammary_ glands.htm • The skin, http://faculty.washington.edu/chudler/receptor.html • Skin (Integument) and Tongue, http://www.meddean.luc.edu/lumen/MedEd/Histo/frames/Histo13.html • http:// www.fpnotebook.com/ DermSkinAnatomy.jpg