Skin and touch

• Examples of skin diseases

   Rosacea Acne Eczema Cancers of the skin (Melanoma) Psoriasis Viral, bacterial and fungal infections of the skin Allergic contact dermatitis Viral warts
• Skin (different layers )

  
• Skin:is the organ acts as a protective barrier that limits the migration ofmicrobes and chemicals into the body.

   Additionally, it plays an integral role in thermoregulation as it participatesin evaporation in hyperthermic environments.

  neurons in the skin detect sensory input that helps with-interacting with the environment
• Protection – biological, chemical, physical the skin contains an intricate network of immune cellsresident to the tissue, crucial for host defence as wellas tissue homeostasis

  Thermoregulation – blood vessels, sweat

  Sensory – temperature, touch, pressure, pain

  Vitamin D synthesis

  Excretion of waste products: Sweating eliminatesexcess water and salts, as well as a small amount ofurea
• Skin is the largest body organ that covers and protects the external surface of the body.
• Three main layers of skin

  Epidermis - most superficial

  Dermis - deep layer

  Hypodermis - deepest layer with loose connective and adipose tissue
• Function of the skin

  Protection, thermoregulation, detecting sensory stimuli
• the skin is the largest organ in thehuman body
• Pyknosis is a characteristic feature of apoptosis (and necrosis) in which the nuclear material condenses.
• Key facts about the Epidermis

  
• The epidermis is made up a variety of cell types. Cell that contain keratin are known as keratinocytes. Keratinocytes are the most abundant cells in the epidermis, responsible for producing keratin, a protein that provides structural integrity to the skin.
• Cells in epidermis :Melanocytes synthesize and store melanin, which not only contributes to the color of the skin and hair, but also provides protection against ultraviolet radiation. They appear as clear cells in the basal layer with large, round, euchromatic nuclei.
• cells in epidermis: Merkel cells are clear, ovoid and may occur singly or in clusters in the stratum basale. Merkel cells may act as a sensory mechanoreceptors and are also thought to function as neuroendocrine sensory receptors.
• Layers of epidermis

  
• Dermis contains collagen and elastic fibres

  Both the collagen and elastic network are held together by a mixture of glycoproteins, bound water and glycosaminoglycans.
• Dermis :

  
• The dermis rests on a layer of loose connective tissue known as the hypodermis.
• The fascia reduces the friction between the dermis and deeper musculature, while the adipose tissue participates in thermoregulatory mechanisms as well as disperses forces generated from direct impact.
• Hair is made of keratin and is anchored into the skin by hair follicle. The base of the hair follicle is formed by a root (or bulb), where live cells divide and keratinize to form the shaft (the visible part of the hair).

  
• The hair follicle originates from a peg of epidermal cells that grows down into the underlying dermis, where it forms a hair cone over a piece of dermis called the follicle papilla. The papilla provides the blood and nerve supply for the growing hair. Each hair follicle has a sebaceous gland and an arrector pili muscle associated with it. The role of this muscle is to make hair "stand up" on the skin. The functions of the hair include protection and touch sensing
• Skin inflammation

  
• Wound healing

  
• Peripheral mechanoreceptors in the skin and organs detect displacement, velocity, and acceleration, providing information about stimuli location, duration, and intensity. They are crucial for social interactions. Mechanoreceptors act like dendrites for sensory neurons, distorting their structure to create action potentials relayed to the spinal cord's dorsal horn and then to the thalamus.
• Free nerve endings are the most copious mechanoreceptors found in the epidermis, made up of branched termini of sensory fibres. They typically have minimal or no Schwann cells around their fibres. Free nerve endings are unspecialized, and can function as nocireceptors (respond to pain), mechanoreceptors (respond to displacement), or thermoreceptors (respond to temperature).
• Adaptation is the receptor's tendency to respond less to sustained stimuli. Receptors vary in adaptation rate: slowly adapting receptors are suited to perceiving constant stimuli like pressure, while rapidly adapting receptors are for detecting movement or changes like vibrations or quick touches.
• Slowly adapting receptor: Merkel cells, found in the stratum basale of sensitive skin areas like fingertips, are tightly connected to nerve endings, forming Merkel nerve endings. These endings, called Merkel's discs, are specialized for fine touch due to their small receptive fields.

  • Merkel cells are common in the tips of the fingers.
  • Their small receptive fields make them well suited for responding to fine touch.
• Slowly adapting skin receptors: Ruffini endings

  Ruffini endings or corpuscle (bulbous corpuscle) are small, spindle-shaped, slowly adapting receptors found throughout the dermis, subcutaneous tissue, and some connective tissues.

  Ruffini ending is made up of a single, branching sensory fibre in a thin capsule surrounded by collagen fibres. The inelastic properties of collagen cause the compression of the nerve fibres to be maintained as long as the tension is applied to the capsule. Nerve impulses from these receptors are perceived as stretching.
• Rapidly adaptin:

  Hair follicle receptors

  Some free nerve endings surround hair follicles,referred to as hair follicle receptors and they sense fine movements of hair on the skin. It is thought that bending the hair distorts the receptor ending wrapped around the hair follicle and generates a potential.
• Rapidly adapting skin receptors:

  Meissner's corpuscles are found in the dermal papillae of hairless skin. They are characterized by encapsulated nerve endings sensitive to light pressure. they are thought to be responsible for our ability to manipulate fine objects with precision. For example, they are are found in the finger pads.n corpuscles (lamellar corpuscles)
• Skin receptors vary in distribution across hairy and glabrous skin. While free nerve endings are ubiquitous, Ruffini endings, Pacinian corpuscles, and Merkel nerve endings are found in both skin types. Meissner's corpuscles, sensitive to light touch, are exclusive to glabrous skin in areas like fingertips and eyelids. Hair follicle receptors, associated with hair follicles, are limited to hairy skin.
• Skin as pharmacological target

  
• In diseases like psoriasis, stressors such as infection or trauma trigger T-lymphocyte activation in the epidermis, usually found in the basal and spinous layers. This activation is in response to cytokine release from keratinocytes, which also stimulates Langerhans' cells, leading to T-cell proliferation. Excess cytokines upregulate keratinocyte cell cycle, causing the epidermis to exfoliate every 4 days instead of the normal 26-day cycle.
• Albinism is an autosomal recessive disorder characterized by a lack of pigment due to abnormalities in the tyrosinase enzyme. This absence of pigment affects the skin, eyes, and visual acuity, leading to photophobia. Additionally, it increases the risk of skin cancer development.
• When body temperature rises, blood flow to the skin increases, facilitating heat dissipation. Langerhans cells, unique macrophages in the epidermis, initiate immune responses against skin-penetrating pathogens. Thickest skin: feet; thinnest: eyelids.
• Sebaceous glands secrete oil into hair follicles, These glands are responsible for producing sebum, which helps to moisturize and protect the skin and hair. Keratinocytes, abundant in the epidermis, produce keratin for structural support, immunity, and wound healing. Langerhans cells, melanocytes, adipocytes, and Merkel cells are also present but less abundant than keratinocytes.
• Vitamin D3 production takes place in the epidermis, under the influence of UVB, this is the primary source of vitamin D for most people!

  Excretion of waste products also takes place in the skin - small quantities are secreted with sweat.
• Epidermis

  The outermost layer of the skin
• Layers of the epidermis from deepest to outermost

  • Stratum basale (basal layer)
  • Stratum spinosum (prickle layer)
  • Stratum granulosum (granular layer)
  • Stratum lucidum (clear layer)
  • Stratum corneum(cornified layer)
• The epidermis, the outermost layer of the skin, consists of five layers, from deepest to outermost:Stratum basale (basal layer): acts as stem cell region,This layer is closest to the dermis and contains actively dividing cells responsible for re-new the cells in the epidermis.
• Stratum spinosum (spinous layer):(it’s post mitotic) It provides strength and flexibility to the skin due to its interconnected cells, .Stratum granulosum (granular layer): Here, cells begin to flatten and lose their nuclei, while keratinization (the process of converting cells into tough, insoluble keratin) begins.