Human Skin: Structure, Layers, and Functions, Schemes and Mind Maps of Anatomy

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SENSORY SYSTEM

INTRODUCTION

The sensory nerves reaching the central nervous system (CNS) have their beginnings in various peripheral structure's such as

1. Skin
2. Muscles
3. Joints
4. Special organs like eye and ear Structures concerned in the production of sensations are :
5. An end organ or a sensory receptor
6. Afferent nerve fibers
7. Thalamus
8. Cerebral cortex

SKIN 

Skin is the largest organ of the body



It is not uniformly thick



At some places it is thick and at some places it is thin

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The average thickness of the skin is about 1 to 2mm



In the sole of the foot, palm of the hand and in the

interscapular region ,it is considerably thick, measuring

about 5mm

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Skin is thin over eyelids and penis, measuring about

0.5mm only

Types of skin  (^) Thick or glabrous skin : in this type of skin, epidermis is very thick with a thick layer of stratum corneum. It is found in palms of hands and soles of feet has no hairs  Thin or hairy skin : in this type epidermis is very thin. It contains hair and is found in all parts of the body except palms and soles The skin has two layers  A superficial layer, the epidermis: made up of stratified squamous epithelium  (^) A deep layer: made up of connective tissues

Epidermis

 (^) Epidermis is the outer layer of skin  (^) It is formed by stratified squamous epithelium  (^) Important feature of epidermis is that, it does not have blood vessels  (^) Nutrition is provided to the epidermis by the capillaries of dermis  (^) Epidermis is formed by five layers

1. Stratum corneum
2. Stratum lucidum
3. Stratum granulosum
4. Stratum spinosum
5. Stratum germinativum/basale

Stratum corneum  (^) It Is also known as horny layer  (^) It is the outer most layer and consist of dead cells, which are called corneocytes  (^) These cells lose their nucleus due to pressure and become dead cells  (^) The cytoplasm is flattened with fibrous protein known as keratin  (^) They are held together by a layer of lipid, which makes the layer water proof  (^) Apart from this, these cells also contain phospholipids and glycogen

Stratum lucidum

Stratum lucidum is made up of flattened epithelial cells

Many cells have degenerated nucleus and some cells, do not have nucleus

As this cells exhibit shiny character, the layer look like a homogenous translucent zone

So, this layer is called stratum lucidum (lucid means clear)

Stratum granulosum 

Stratum granulosum is a thin layer

with two to five rows of flattened

rhomboid cells



Cytoplasm contains granules of a

protein called keratohyalin

Stratum germinativum  (^) It is the deepest, basal layer of epidermis  (^) It is made up of a single layer of columnar cells that rest on basal lamina  (^) These cells undergo mitotic division and gives off cells called keratinocytes, which form the more superficial layer of the epidermis  (^) Other types of cells present in this layer include;

1. Melanocytes that are scattered between the keratinocytes. Melanocytes produce the pigment called melanin. The colour of skin depend upon melanin
2. Langerhans cells are responsible for protective immune reactions following microbial infection of skin From this layer, some projections called rete ridges extend down up to dermis. These provide anchoring and nutritional function

DERMIS  (^) Dermis is inner layer of skin  (^) It is a connective tissue layer, made up of dense and stout collagen fibers, fibroblasts and histiocytes  (^) Collagen fibers exhibit elastic property and are capable of storing or holding water  (^) Collagen fibers contain the enzyme collagenase, which is responsible for wound healing  (^) Made up of two layers

1. Superficial papillary layer: it projects into the epidermis. It contains blood vessels, lymphatics and nerve fibers. This layer also has some pigment containing cells known as chromatophores

Functions of skin 1.Protective function  (^) Skin forms the covering of all the organs of the body and protects these organs from,  (^) Bacteria and toxic substances, mechanical blow and ultraviolet rays Protection from bacteria and toxic substances Skin covers and protects the organs from having direct contact with external environment Thus it prevents bacterial infection Lysozyme secreted in skin destroys the bacteria. Stratum corneum of epidermis is responsible for the protective function of skin. This layer also offers resistance against toxic chemicals like acids and alkalis

 Protection from mechanical blow Skin is not tightly placed over the underlying organs or tissues. It is somewhat loose and moves over the underlying subcutaneous tissues. So, the mechanical impact of any blow to skin is not transmitted to underlying tissues Protection from ultraviolet rays Exposure to sunlight or any other source of ultraviolet rays increase the production of melanin pigment in skin. Melanin absorbs ultraviolet rays. At this same time the thickness of stratum corneum increase's. This layer of epidermis also absorbs the UV rays

Storage function  Skin stores fat, water, chloride and sugar  It can also store blood by the dilation of the cutaneous blood vessels

Synthetic function  Vitamin D3 Is synthesised in skin by the action of uv rays from sunlight on cholesterol