Sensory Receptors

Sensory receptors: Sensory receptors are divided into 2 types. I. Sensory receptors which are surrounded by a capsule are called encapsulated sensory receptors, Examples: 1- Meissner’s corpuscles, which present in dermal papillae and because they are superficial in location they are responsible for light touch reception, more prominent in fingertips, palms and soles . Note: with age the Meissner’s corpuscle decline in number that’s why our ability for light touch with age is reduced 2- Pacinian corpuscles,(onion in shape) which present in reticular layer of dermis (deep layer of dermis )or sometimes can reach the hypodermal at the dermal hypodermal junction , from their deep location we can tell they are responsible of transmission of deep pressure. 2|Page 3- Ruffini’s corpuscles, fusiform in shape, which present in reticular layer of dermis, respond to stretch (tension) or twisting in the skin ((torsional forces)  If they are not surrounded by a capsule they are called unencapsulated sensory receptors or nerve endings(naked) (Examples: Merkel’s disk, free nerve endings, root hair plexus) Merkel disc:  In the stratum basal  Expanded nerve endings( merkel cells forming disc )  Important for light touch reception and sensing an object texture Free nerve endings  In papillary dermis 3|Page  Special (for temperature, pain, itching or tactile sensation (sensation produced by pressure receptors in the skin ,example: touch of silk or a smooth surface ) Root hair plexuses  At the end of hair follicle, you find root hair plexus Sensory neuron surrounding(wrapping around) the basal part of hair follicle which detects the movement of the hair(gets activated) Pacinian corpuscle Part in the middle is sensory neuron and tissue surrounding it is connective tissue 4|Page Skin appendages: accessory structures of the skin, the dermis and epidermis and all the accessory structures form the integumentary system and we can find inside the skin appendages like the hair (hair shaft)and the hair follicles(where the hair is found ), the nails (which are keratinized plates of dead cells, extensions of skin epidermis ), sebaceous glands that open into the hair follicle, we also talked about eccrine and apocrine sweat glands. HAIR It is an elongated keratinized structure of dead cells that is found inside the hair follicle (pocket shaped invagination or canal ). The hair fiber itself is divided into two parts: 1.the part that is embedded inside the skin is called  root of the hair 2.tha part that is visible (above the level of the skin,visible part ) is called  shaft of the hair The epidermis of the skin invaginates deep inside the dermis and forms a pocket where you find the hair inside, this is called hair follicle. Hair bulb: the most basal swollen area of the hair follicle. 5|Page Types of hair:  Lanugo: foetal hair , Even during foetal life a very fine hair grows all the surface of the foetal body and falls after a few days after birth and is mainly concentrated on head  Down hair: light coloured hair of child ,very thin hair  Terminal adult hair: thicker and darker hair that begins to grow at puberty and starts to grow on places in our body that never grew before What do we have surrounding the hair follicle? The dermis, we call it dermal sheath Structure of the hair follicle -Inner root sheathmost inner part, terminates at the level of the opening of the sebaceous gland duct Why inner root sheath didn’t continuous to the surface of epithelium because we need space or canal in order for the sebum to be discharged and lubricate the skin and the hair. Inner root sheath is composed of three layers (not required) -Outer root sheathis continuous with the skin epidermis, in particular it is continuous with stratum Basale and stratum Spinosum of the epidermis. Surrounding inner root sheath . -Glassy membrane (thickened basement membrane) also it is continuous with the basement membrane of skin epidermis. It is called glassy since it is thick and it appears glassy (transparent) under the light microscope -all Surrounded by dermal sheath (connective tissue sheath ) Note :The most basal part of hair follicle is swollen and rounded and called hair bulb 6|Page At the base of the hair bulb, the dermis invaginates inside the hair bulb and forms a dermal papilla (hair papilla to differentiate it from dermal papilla of the dermis ), since it’s similar to the dermal papilla of the epidermis). The dermal hair papilla contains tiny blood vessels to nourish the hair cells. Stratum Basale: if we follow the stratum Basale layer, it goes inside the dermal papilla and lines it, it forms a layer of cells called the matrix, we previously mentioned that we have mitotic activity inside the stratum Basale so these matrix cells undergo mitosis and give up cells that become keratinized as we go up to form the hair shaft. The hair shaft formation process is similar to the keratinization of the skin, only the matrix cells undergo mitosis. They divide and form something similar to Spinosum layer and granulosum and they accumulate more and more keratin. The difference between the keratin of the hair and the keratin of the stratum Corneum is that the keratin of stratum Corneum of the skin is soft while the keratin of the hair is hard and it is highly compacted to form the hair shaft. Note in between these matrix cells we have melanocytes that give color to our hair fibres 7|Page These matrix cells divide and form the hair shaft and the inner root sheath. Sebaceous gland : simple (one duct)branched acinar gland and the duct of this gland opens into hair follicle exactly above the level of inner root sheath ,secretion of sebum by holocrine mode of secretion(whole cells explode in order to release sebum ,remember in thick skin we said we don’t have hair and sebaceous glands on palms and soles ,why? To be able to grasp things and walk and run barefoot And that’s why these get dried the most thus all the hand lotions Secretion of sebaceous gland is stimulated by oestrogen and testosterone , this takes place around puberty time we start producing these hormones and our skin starts to be oily Since cells secrete sebum by holocrine mode of secretion these ducts could be blocked by the sebum secreted by these cells specially during puberty time because we have excessive secretion from these glands and sebum will collect bellow level of epidermis producing black heads if duct still open to surface If an infection takes place by certain type of bacteria produces acne 8|Page Arrector pili muscle: (Pilli in Latin is hair and processes of removing hair is called depilatory )Arrector because it causes erection to the hair, pili: hairs. It is a smooth muscle which means it’s involuntary, it is connected to the hair follicle from one side and to the papillary layer of the dermis from the other side. It is supplied by sympathetic nervous system-autonomic nervous system. The contraction of this muscle will cause hair to stand up (perpendicularly to the skin) and this will close the pore which will reduce the heat loss from surface of our skin, this condition is known as goose bumps (when you are cold or when you are frightened) under the stimulation of the sympathetic nervous system. Role of this muscle in humans not important , It is however more prominent in animals it reduces the heat loss, in animals covered with fur or hair, the erect hairs trap air to create a layer of insulation. The erect hairs also make the animal appear larger in response to fear. Structure of the hair shaft; it consists of 3 layers of hard protein (keratin): The inner most layer  medulla Medulla have large vacuolated moderately keratinized cells 9|Page Intermediate layer ,Cortex have heavily keratinized and densely packed cells and contains an accumulation of melanin pigment Outer layer flat squamous cells filled with very hard keratin, called the cuticle (when the hair is damaged with split ends ‫ مقصف‬those cells are desquamated from their place) Between the matrix cells of hair follicle there are melanocytes that produce the colour (same concept as the epidermis) Hair Growth Cycle Is all your hair growing at the same time? For example, the hair follicles on your scalp are they growing at the same rate? No.We have hair growth cyle divided into phases The first phase is called anagen: in this phase, the matrix cells of the dermal papilla undergo mitosis and give new cells to form the hair root and the hair shaft which gets longer and longer, and more keratinized , we call this the growth state, it lasts from 3-6 years. Some people have difficulty growing their hair beyond a certain length because they have a short active phase of growth. On the other hand, people with very long hair have a long active phase of growth. The hair on the arms, legs, eyelashes, and eyebrows have a very short active growth phase, explaining why they are so much shorter than scalp hair The second phase is called catagen: it is a transitional state, very short phase, lasts for 1-2 weeks, 10 days, the dermal papilla disintegrates; it separates from the hair bulb and once you separate the blood vessels from the hair bulb what will happen? The hair follicle shrinks 10 | P a g e After this period, the dermal papilla is completely detached from the hair bulb and we call this phase the telogen phase it is a resting phase; nothing happens. In this case shedding of hair is possible because it is no longer attached to the dermal papilla it lasts from 5 to 6 weeks. If hairs enter the resting phase too early, excess shedding of the hair can occur After 6 weeks, we go back to anagen; formation of new dermal papilla within the hair bulb and new formation of hair takes place. 85% of hair follicles are in anagen phase, 15% at telogen phase which means if we have around 150,000 hair follicles in our scalp then it is normal to shed up to 100 hairs per day. Hair shedding increases during stress Note: we do not generate new hair follicles anytime during our lives. We have about 5 million hair follicles at the time of birth Note: what forms hair shaft is only dead cells thus anucleate so to extract DNA out of hair you have to take hair root and bulb Sweat glands Revision of modes of secretion : 1. Merocrine: typical exocytosis from cell 2. Apocrine: part of apical cell membrane and apical cytoplasm is lost during secretory process 3. Holocrine: example of this is sebaceous glands ,cells accumulate more and more secretory products until cell rupture and secretory process takes place We have about 3 million sweat glands distributed on our skin. There are two types. 11 | P a g e - Eccrine (merocrine)are distributed all over our body and the ducts of these glands open into the surface, they produce true sweat and are the most numerous of the sweat glands, and are abundant on the palms of the hands, soles of the feet, and forehead; their secretion is clear watery secretion with electrolytes and it is filtration from the blood plasma also it’s Hypotonic (lesser salt concentration) - What is the main function of sweating? To cool the body temperature by evaporation - Apocrine: they are in restricted places like the axilla or the genitalia. In human, their role is not really understood well .. but they are important in sexual attraction between animals The secretion of apocrine sweat glands is modified that’s why they are called modified sweat glands ,it is a viscous material rich in proteins and lipids and this is called pheromones 12 | P a g e The secretion of apocrine sweat glands is associated with the presence of the hair follicle. Can we find apocrine sweat glands in thick skin? No, but we can find the eccrine in the palms and the soles and that is why we have smelly feet sometimes, not because sweat has odour but because it is exposed to bacterial decomposition. For example, your feet sweat into your shoes all day so they get wet and bacteria start to grow. The bacteria continue to breed once you've taken your shoes off, especially if you put them in a dark cupboard. Then, when you put your shoes back on the next day, even if you've just had a shower, putting your feet into still wet shoes creates the perfect conditions for the bacteria to thrive – warm, dark and moist  thus you need to dry your shoe to prevent growth of bacteria. The structure of the sweat gland: simple coiled tubular gland, the secretory part is lightly stained while the duct is darkly stained. The duct morphology concerning the eccrine and the apocrine is the same histological features : simple coiled tubular gland that’s why when you take histological section ,only rounded profiles(light colored part representing secretory portion and the dark part represent the duct of gland)The difference between the two types lies in the secretory portion; in the apocrine they are very large (lumen is very wide. 13 | P a g e The Nail Last accessory structure of skin Dead cells(keratinized cells ) Hard plate of keratin, it is not colored because there is no activity for the melanocytes.(thus translucent) Nail is divided to 3 parts 1-free edge; which is the part we cut. 2-body of the nail; it appears pinkish because under the nail we have nail bed and this is very important clinically as an indicator of distal tissue vascularization of the patient. When you press on the nail you can see the vascularization through the nail. 3-root of the nail; embedded deep inside the skin and covered by a fold called eponychium or cuticle. Another fold is the hyponychium (under the free edge) and it is where the dirt accumulates. Nail bed: under the body of the nail, layer of the epidermis (stratum Basale and stratum Spinosum) and because layer of epidermis is thin they show pink color of vascularization 4-lunula: white semi-circular area 5-eponichium(cuticle):old of skin covering root of nail that is embedded deep in the skin 14 | P a g e 6-hyponichium: fold of skin where dirt accumulates The nail matrix is the layer of cells at the base of the nail. It consists of rapidly dividing skin cells that soon fill with the protein keratin. When matrix cells undergo mitosis, they push the nail plates over the nail beds and that’s how the nails grow. Last 4 minutes of online video lecture is practical histologic sections, study from eLearning 15 | P a g e