Skin

The skin

• Structure and classification

• Sensory functions - skin sensory receptors

• Experimental results

Structure and classification

• Body’s largest organ; covers area of 1.5 to 2.0

m

; 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

• 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: palm, soles, fingers, and toes;

has sweat glands but no hair follicles, nor oil glands (sebaceous);

-thin: 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

Epidermis

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)

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)…

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

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

Skin sensory receptors

The skin contains different types of receptors. The mechanoreceptors on the skin can be classified into several types: Detect motion of hair Dermis, around hair follicle hair follicle receptors Pain, temperature Widespread, Epidermis, dermis Free nerve endings Deep touch, Pressure, vibration (150-300 Hz ) Dermis, hypodermis Pacinian (Lamellated) corpuscles Responds to pressure on skin Dermis, hypodermis Ruffini endings Light touch, texture, vibration (20-40 Hz range ) Dermis (papillae of fingertips, palms, lips, eyelids) Meissner’s (tactile) corpuscles Light touch, texture, edge, shape Epidermis (basale) Merkel’s (tactile) disks FUNCTION LOCATION STRUCTURE

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

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