Special Sense Organs Lecture PDF

Special sense organs Dr. Manal F. Yaya 2024/2025 Intended learning outcomes 1. Classify the types of the receptors, and distinguish between different sensation according to type of stimulus and their location 2. Understand the histological structure of different receptors 3. Describe...

Special sense organs Dr. Manal F. Yaya 2024/2025 Intended learning outcomes 1. Classify the types of the receptors, and distinguish between different sensation according to type of stimulus and their location 2. Understand the histological structure of different receptors 3. Describe the ultrastructure and the function of three layers of the eye and their medical and functional correlation 4. Identify the histological structure of the conjunctiva, eyelid, and lacrimal glands. 5. Explore the structure of the three parts of the ear. 6. Understand the morphological features of the bony and membranous labyrinth of the ear 7. Discuss the microscopic structure of the sensory receptor in the ear and correlate it with medical applications. Introduction The sense organs are formed of sensory units called receptors, that convey information about the external world to the C.N.S. Peripheral nerve terminals are of two structural types: 1) Terminals of dendrites, called sensory endings or receptors, which recognize various stimuli and transmit this sensory input to the central nervous system 2) Terminals of axons-transmit impulses from central nervous system to skeletal and smooth muscles (motor endings) or to glands (secretory endings). Sensory receptors are classified according to functions into: Somatic and visceral receptor system (superficial and deep sensation). Proprioceptor system (detection of body position in space). Chemoreceptor system (taste and smell). Photoreceptor system (vision). Audio receptor system (hearing). Sensory receptors are classified according to their distribution in the body 1-Exteroceptors: located near the body surface, They are specialized to recognize stimuli from external environment They include: A) General Somatic afferent: these receptor sensitive to temperature, touch, pressure and pain. B) Special Somatic afferent: specialized for perceiving light (sense of vision) and sound (sense of hearing). C) Special visceral afferent: smell and taste stimuli are perceived by nerve endings in the viscera of respiratory and digestive system respectively. 2.Proprioceptors: They are specialized receptors found in joint capsules, tendons, and intrafusal fibers within muscle. These regarded as general somatic afferent receptors that gives information about body position and movement. Another receptors located within the inner ear , related to vestibular (balance) mechanism (awareness of motion) 3-Interoceptors : are specialized receptors that receive sensory information from within organs of the body. They are called general visceral afferent Specialized peripheral receptors The dendritic endings of certain sensory receptors, located in various regions of the body are specialized to receive particular stimuli, these receptors are classified into: I. Mechanoreceptors: ❑Responding to touch, stretch, vibration, and pressure ❑ They include: A- Nonencapsulated mechanoreceptors: 1. Free nerve endings 2. Merkel's disks B- Encapsulated mechanoreceptors: 1. Meissner's corpuscles 2. Pacinian corpuscles 3. Ruffini's corpuscles 4. Krause’s end bulbs 5. Muscle spindles 6. Golgi tendon organs II. Thermoreceptors (responding to cold and wormth) : cold receptors are derived from naked nerve endings of myelinated fibers. Warmth receptors are naked endings of non-mylinated fibers III. Nociceptors (responding to pain): are naked endings of myelinated nerve fibers that branch freely in dermis before entering the epidermis They are three groups: 1. Receptors that respond to mechanical stress and damage 2. Receptors that respond to extreme cold and heat 3. Receptors that respond to chemical compound such as bradykinin, histamine an serotonin  Mechanoreceptors I) Unencapsulated receptors: 1-Free nerve endings. Sites: The epidermis of the skin, and epithelium of the cornea and face. Structure: formed of unmyelinated sensory nerve fibers that branch in the C.T. (dermis). No Schwan cells No connective tissue capsule Function: Touch, Pain,& Temperature 2-Merkel's disc. Sites: -This is present in the deep layers of the epidermis(stratum Basale) of non hairy skin Structure: -expanded unmyelinated nerve terminal Specialized epithelial cells Merkel cells Function: Touch sensation. II) Encapsulated receptors: 1-Meissner's corpuscles Function: They are specialized for tactile discrimination, detect light touch. Site: Located in the dermal papillae of the glabrous ( non-hairy) portion of the fingers and palms of the hands, eyelids, lips, tongue, nipples, and skin of foot and forearm Structure: They are elliptical structures oriented perpendicular to the epidermis in the dermal papillae. Each corpuscle is formed of: 1. Three to four nerve terminals coursing spirally 2. They are enfolded by horizontally arranged (stacks) Schwann cells. 3. Connective tissue capsule (fibroblasts and collagen fibers) surrounding the corpuscles. L/M Photo of Meissner's corpuscles  II) Encapsulated receptors: 2-Pacinian corpuscle Sites: It is found in the dermis and hypodermis of the skin especially that of palms, soles and tips of fingers,in mesenteries and periosteum,breast. Functions: 1-Detection of vibration and posture of the body. 2-Deep pressure sensation 3-Traction sensation. Pacinian corpuscle Structure: The corpuscle is oval in shape. In histological sections, it resembles sliced onion consisting of The myelinated nerve fiber enters one pole of the corpuscle, loses its myelin sheath, and then loses its neurolemmal sheath. and terminates by an expanded end 60 layers of modified fibroblasts separated by fluid-filled space 30 layer of less dense modified fibroblast Connective tissue capsule 3-Ruffini's corpuscles Structure: They are large receptors Composed of 1. Branched nonmyelinated terminals 2. Collagen fibers between terminals and surrounded by 4 to 5 layers of modified fibroblasts. 3. The connective tissue capsule (thin) which is anchored at each end, increasing their sensibility to stretching and pressure in the skin and in the joint capsules. Sites: in the dermis of the skin (hairy & non-hairy), nail beds, periodontal ligaments, and joint capsules Function: sensitive to stretch, pressure, and tensile forces. 4- Krause’s end bulbs Are spherical, encapsulated nerve endings Structure: 1. The myelinated fiber penetrates the corpuscle and breaks up into numerous non-myelinated terminals which end by bulbous expansion Site: located in the , joint, conjunctiva, peritoneum, genital regions, and oral and nasal cavities (subendothelium) Function : ???? sensitive to cold  Proprioceptor system 1-Muscle spindle: Site: Between skeletal muscle fibers. Structure: Fusiform structure that consists of: ❑ Capsule: Connective tissue. ❑ Extrafusal muscle fibers outside the capsule (skeletal muscle fibers). ❑ Intrafusal muscle fibers) inside the capsule , are two types: 1.Nuclear bag fibers 2.Nuclear chain fibers: -Function: Sensitive to muscle stretch and reflexly controls the muscle tone, movement and posture. Muscle spindle 2-Tendon spindles: Site: In tendons and joints, near the insertion sites of muscle fibers. Structure: They are similar to muscle spindles, but the intrafusal fibers are collagen fibers. - A connective tissue sheath encapsulates several large bundles of collagen fibers that are continuous with the collagen fibers that make up the myotendinous junction. Sensory nerves penetrate the connective tissue capsule. -Function: contribute to proprioception by detecting tensional differences in tendons  Chemoreceptor system 1-Taste: Site: Taste is sensation perceived by taste buds receptors located principally on the surface of the tongue. Lingual taste buds are embedded within the stratified of the circumvallate, foliate and fungiform papillea. Structure: Taste buds are intraepithelial sensory organs , composed of 60 to 80 spindle shaped cells, is an oval structure, and is distinctly paler than the epithelium surrounding it. - The narrow end of the taste bud, located at the free surface of the epithelium, projects into an opening, the taste pore. Four types of cells constitute the taste bud: 1. Basal cells 2. Dark cells 3. Light cells, 4. Intermediate cells. Each of these cell types has long, slender microvilli that protrude from the taste pore. These microvilli are called taste hairs. -Undifferentiated basal cells are responsible for the replacement of all the cell types.  2-Smell (Olfaction) Site: The olfactory chemoreceptors are located in the olfactory epithelium , In the roof of the nasal cavity Structure: The olfactory epithelium is composed of three types of cells: Olfactory Sustentacular basal cells. The underlying lamina propria houses serous fluid– secreting Bowman’s glands, a rich vascular plexus, and collections of axons that arise from the olfactory cells of the olfactory epithelium. Eyes (The Photoreceptor System)  Eyes (The Photoreceptor System) The eye is the highly specialized organ of photoreception The eye has the ability to analyze the form, intensity, and color of light reflected from objects, providing the sense of vision. The eyes are located in protective bony structures of the skull, the orbits. which also contain adipose cushions. Layers of the eye - The eye ball is composed of three tunics: fibrous, vascular, and neural tunics. 1- External (fibrous) layer: - It is supportive which maintains an eye’s overall shape and formed of: a) The sclera, dense opaque connective tissue (posterior 5/6) b) The cornea , the anterior transparent connective tissue (anterior 1/6) Layers of the eye 2- Middle (vascular) layer or uveal tract: - It consists of 3 continuous structures; a) The choroid is extremely vascular and highly pigmented. b) The ciliary body is the intermediate part. c) The iris is the most anterior part. It consists of a pigmented disk with a central opening called the pupil which rests on the lens. 3- Inner (nervous) layer or retina: This consists of: a) Outer pigment epithelium. b) The photosensitive retina Proper: An inner sensory layer, the retina , which communicates with the cerebrum through the posterior optic nerve The eye contains two-fluid filled cavities The anterior chamber: fills the space between the cornea, and the iris. The posterior chamber: between the iris, ciliary processes, and lens. Interconnected at the pupil, both chambers contain clear fluid called (aqueous humor)  Aqueous humor Aqueous humor is secreted by the ciliary body, and circulated through the pupil to drain into a canal at the angle of the anterior chamber, the canal of Schlemm. The aqueous humor Is a source of nutrients for the non-vascular lens and cornea The pressure of aqueous humor maintains the shape of the cornea. Vitreous body The posterior vitreous chamber , surrounded by the retina, lies behind the lens and its zonular fibers and contains a large gelatinous mass of transparent connective tissue called the vitreous body. It consists of water (99%), collagen fibers, and hyaluronic acid The vitreous body supports the lens and retina from within  THE EXTERNAL (FIBROUS) LAYER: a) The sclera: The sclera is a tough, opaque white fibrous coat that covers the posterior (5/6 ) of the eye ball. Function: protects the more delicate internal structures and provides sites for muscle insertion - It is made up of tough dense connective tissue that consists of: Flat collagen bundles in various directions and Few fibroblasts. Externally, it is covered by Tenon's capsule separated from it by the loose Tenon's space. The tendons of the extraocular muscles are attached to the outer surface of the sclera. Internally, it is separated from the choroid by suprachoroidal lamina a thin layer of loose connective tissue. b) The cornea: ▪ It is the colorless transparent anterior (1/6 )of the fibrous layer. ▪ The five layers of the cornea seen in a transverse section are the following: 1. Corneal epithelium 2. Bowman’s membrane (anterior basement membrane) 3. Corneal stroma 4. Descemet’s membrane (posterior basement membrane) 5. Corneal endothelium Layers of cornea 1- Epithelium: It is stratified squamous non- keratinized. Five or six cells thick about 10% of corneal thickness The basal cells have a high proliferative capacity important for renewal and repair of the corneal surface The flattened surface cells have microvilli protruding into a protective tear film of lipid, glycoprotein, and water. As another protective adaptation, the corneal epithelium has rich sensory nerve supply that trigger the blinking reflex 2- Bowman's membrane: The basement membrane of this epithelium It consists of collagen fibers. It acts as a protective barrier against mechanical injuries and bacterial invasion. If destroyed it does not regenerate, but it heals by irregular fibrous tissue causing corneal opacities.  3- Stroma (substantia propria): It is the thickest layer of the cornea (90%). It is normally non vascular. Components: 1. Sixty layers of parallel collagen bundles aligned at approximately right angles to each other and extending almost the full diameter of the cornea. The uniform orthogonal array of collagen fibrils contributes to the transparency of this avascular tissue. 2. Flattened fibroblast-like cells called keratocytes with cytoplasmic extensions Between collagen lamellae 3. Ground substance contains proteoglycans such as lumican, with keratan sulfate and chondroitin sulfate, which help maintain the precise organization and spacing of the collagen fibrils 4- Descemet's membrane(posterior basement membrane) Thick homogenous structure composed of fine collagen fibrils bound the posterior surface of the stroma 5- Endothelium: Simple squamous epithelium The corneal endothelium provides for metabolic exchange between the cornea and aqueous humor It is the most metabolically active cells of the cornea. The Na+/K+ ATPase pumps in the basolateral membranes of these cells are largely responsible for regulating the proper hydration state of the corneal stroma to provide maximal transparency and optimal light refraction 1.The relative dehydrated state. 2.The regular arrangement of the thin collagen fibrils. 3.The absence of blood vessels. 1.By diffusion from adjacent blood vessels in the corneoscleral junction. 2.From the aqueous humor of the anterior chamber. 3.It also gets O2 from air on the outer surface Medical application The shape or curvature of the cornea can be changed surgically , by laser - assisted in situ keratomileusis (LASiK) surgery to correct: ❑ Myopia (near-sightedness) ❑ Hyperopia (far-sightedness) ❑Astigmatism (irregular curvature of the cornea). ❑Corneal grafts (transplants) The corneal epithelium is displaced as a flap and the stroma reshaped by an excimer laser which vaporizes collagen and keratocytes in a highly controlled manner with no damage to adjacent cells or ECM. After reshaping the stroma, the epithelial flap is repositioned and a relatively rapid regenerative response reestablishes normal corneal physiology.  Limbus (corneoscleral junction) : A transitional area where the transparent cornea merges with the opaque sclera At corneoscleral junction the following changes appear 1. Bowman’s membrane ends and the surface epithelium becomes more stratified as the conjunctiva that covers the anterior part of the sclera (and lines the eyelids). Epithelial stem cells located at the limbus surface give rise to rapidly dividing progenitor cells, which then move centripetally into the corneal epithelium. The stroma becomes vascular and less well-organized at the limbus, as the collagen bundles merge with those of the sclera. Descemet’s membrane and its simple endothelium are replaced with a system of irregular endothelium-lined channels called the trabecular meshwork  These trabecular meshwork penetrate the stroma at the corneoscleral junction and allow slow, continuous drainage of from the anterior chamber. This aqueous humor moves from these channels into the adjacent larger space of the scleral venous sinus, or canal of Schlemm which encircles the eye. From this sinus aqueous humor drains into small blood vessels (veins) of the sclera  MIDDLE (VASCULAR) LAYER ( Uveal tract) - It consists of 3 continuous structures; a) The choroid is extremely vascular and highly pigmented. b) The ciliary body is the intermediate part. c) The iris is the most anterior part. It consists of a pigmented disk with a central opening called the pupil which rests on the lens  Choroid Choroid Located in the posterior two-thirds of the eye The choroid is a dark brown vascular sheet only 0.25 mm thick posteriorly and 0.1 mm thick anteriorly. It lies between the sclera and retina It consists of loose, well-vascularized connective tissue and contains numerous melanocytes These form a characteristic black layer in the choroid and prevent light from entering the eye except through the pupil.  It is maid of two layers : The inner choroido-capillary lamina: has a rich microvasculature important for nutrition of the outer retinal layers. Bruch membrane: a thin extracellular sheet, is composed of collagen and elastic fibers surrounding the adjacent microvasculature and basal lamina of the retina’s pigmented layer  Five different layers are identified in Bruch’s membrane: The basal lamina of the endothelial cells of the chorio-capillary layer A layer of collagen fibers approximately 0.5 m thick A layer of elastic fibers approximately 2 m thick A second layer of collagen fibers (thus forming a “sandwich” around the intervening elastic tissue layer) The basal lamina of the retinal epithelial cells Ciliary Body The ciliary body, the anterior expansion of the choroid that encircles the lens, lies posterior to the limbus. Most of the ciliary body rests on the sclera. Important structures associated with the ciliary body include the following: 1. Ciliary muscle makes up most of the ciliary body’s stroma and consists of three groups of smooth muscle fibers. Contraction of these muscles affects the shape of the lens and is important in visual accommodation 2. Ciliary process: Are a radially arranged series of about 75 ridges extending from the inner highly vascular region of the ciliary body. These provide a large surface area covered by a double layer of low columnar epithelial cells, called ciliary epithelium. The epithelial cells directly covering the stroma contain much melanin and correspond to the anterior projection of the pigmented retina epithelium. The surface layer of cells lacks melanin and is contiguous with the sensory layer of the retina. Acqeuos humor circulation: Cells of this dual epithelium have extensive basolateral folds with Na+/K+-ATPase activity and are specialized for secretion of aqueous humor. Fluid from the stromal microvasculature moves across this epithelium as aqueous humor, with an inorganic ion composition similar to that of plasma but almost no protein. Aqueous humor is secreted by ciliary processes into the posterior chamber, flows through the pupil into the anterior chamber, and drains at the angle formed by the cornea and the iris into the channels of the trabecular meshwork and the scleral venous sinus, from which it enters venules of the sclera 3. The ciliary zonule is a system of many radially oriented fibers composed largely of fibrillin The fibers extend from grooves between the ciliary processes and attach to the surface of the lens holding that structure in place  3- The iris The iris is the most anterior part of the vascular coat of the eyeball which covers part of the lens, leaving a round central pupil. Structure: 1. The anterior surface of the iris, exposed to aqueous humor in the anterior chamber, consists of a dense layer of fibroblasts and melanocytes with no epithelial covering. 2. The stroma consists of loose connective tissue with melanocytes and few microvasculature. 3. The posterior surface of the iris has a two-layered epithelium continuous with that covering the ciliary processes, but very heavily filled with melanin (blocks all light from entering the eye except that passing through the pupil). THE IRIS ❑Melanocytes of the iris stroma provide the color of one’s eyes: Few lightly pigmented cells in the stroma, the iris color blue Melanin increase in the stroma, the iris color changes through various shades of green, gray, and brown. Individuals with albinism have almost no pigment and the pink color of their irises is due to the reflection of light from the blood vessels of the stroma. ❑ Muscle of iris: Dilator pupillae muscle: extend radially Sphincter pupillae muscle: form a circular bundle near the pupil Both have sympathetic and parasympathetic innervation, respectively, for enlarging and constricting the pupil  Medical application Aqueous humor is produced continuously. If its drainage from the anterior chamber is impeded, typically by obstruction of the trabecular meshwork or scleral venous sinus, intraocular pressure can increase, causing the condition called glaucoma. Untreated glaucoma can cause pressing of the vitreous body against the retina, affecting visual function and possibly leading to neuropathy in that tissue.  Lens The lens is a transparent biconvex structure suspended immediately behind the iris, which focuses light on the retina the lens is a unique avascular tissue and is highly elastic. The lens has three principal components: 1. lens capsule: composed of proteoglycans and type IV collagen surrounds the lens and provides the place of attachment for the fibers of the ciliary zonule. 2. A subcapsular lens epithelium a single layer of cuboidal cells present only on the anterior surface of the lens Near the equator of the lens, the epithelial cells divide to provide new cells, which differentiate as lens fibers. This process allows for growth of the lens and continues at a slow, decreasing rate near the equator of the lens throughout adult life. 3. Lens fibers : Are highly elongated, terminally differentiated cells that appear as thin, flattened structures. The cytoplasm becomes filled with a group of proteins called crystallins, and the organelles and nuclei undergo autophagy. Lens fibers are packed tightly together and form a perfectly transparent tissue highly specialized for light refraction The lens is held in place by fibers of the ciliary zonule, which extend from the lens capsule to the ciliary body Visual accommodation Is the continuous changes in the shape of the lens keep images focused on the retina. Together the ciliary zonule and ciliary muscles, this structure allows the process of visual accommodation (a) The lens flattens for distant vision when the ciliary muscles are relaxed and the shape of the ciliary body holds the ciliary zonule taut. (b) To see closer objects, the ciliary muscle fibers contract, changing the shape of the ciliary body, relaxing tension on the ciliary zonule, and allowing the lens to assume the more rounded shape.. Medical application Presbyopia: In the fourth decade of life normally causes the lenses to lose elasticity and their ability to undergo accommodation Is corrected by wearing glasses with convex lenses (reading glasses) Cataract Denaturation of crystallins commonly begins to occur in lens fibers, making them less transparent (opaque), which lead to impairment of vision. Causes: 1. excessive exposure to ultraviolet light or other radiation 2. Trauma 3. secondary effects in diseases such as diabetes mellitus and hypertension. Treatment: excision of opaque lens and replacement with a corrective lens.  Vitreous Body The vitreous body occupies the large vitreous chamber behind the lens It consists of transparent, gel like connective tissue that is 99% water (vitreous humor), with collagen fibrils and hyaluronate, contained within an external lamina called the vitreous membrane. The only cells in the vitreous body are a small mesenchymal population near the membrane called hyalocytes, which synthesize the hyaluronate and collagen, and a few macrophages INNER (NERVOUS) LAYER(retina) The retina, the inner most tunic of the eye It has two layers: The outer pigmented layer is a simple cuboidal epithelium attached to Bruch’s membrane and the choroidocapillary lamina of the choroid. This heavily pigmented layer forms the other part of the dual epithelium covering the ciliary body and posterior iris. The inner retinal region, the neural layer, is thick and stratified with various neurons and photoreceptors. A potential space exists between the two layers of the retina. The two layers may be separated mechanically in the preparation of histologic specimens. Separation of the layers, “retinal detachment”  According to function neural retina has two regions that differ in function are recognized: The non-photosensitive region (non visual part), located anterior to the ora serrata, lines the inner aspect of the ciliary body and the posterior surface of the iris The photosensitive region (optic part) lines the inner surface of the eye posterior to the ora serrata except where it is pierced by the optic nerve. The site where the optic nerve joins the retina is called the optic disc or optic papilla. The fovea centralis is a shallow depression located about 2.5 mm lateral to the optic disc. It is the area of greatest visual acuity Retina Pigmented Epithelium It consists of cuboidal or low columnar cells with basal nuclei. The cells have well-developed junctional complexes on lateral membrane, gap junctions is the major component of the blood retinal barrier. The basal membranes attached tightly to Bruch's membranes, and have numerous invaginations associated with mitochondria (ion transporting) The apical membrane of the cells extend processes that surround the tips of the photoreceptors (rods and cones). The cytoplasm of these extensions contains: 1. Abundant SER (vitamin "A" transport to the photoreceptors) 2. Numerous melanin granules (for absorption of light after stimulation of photoreceptors). 3. Numerous phagocytic vacuoles and secondary lysosomes,and peroxisomes (representing various stages in phagocytosis and digestion of the tips of the outer segments of rods and cones) Retinal pigmented epithelium  Neural Retina It is a complex structure composed of nine layers 1. Rods and cones (mainly outer segment) 2. External limiting membrane: It is not truly a membrane, but junctional complexes between Muller's cells (the major supportive glial cells of the retina), and the cell bodies of the rods and cones 3. The outer nuclear layer (ONL) Contains nuclear regions of the rods & cones. - The nuclei of the cones are adjacent to the external limiting membrane. - The nuclei of the rods occupy several layers next to that of cones except at the fovea centralis which contains only cone cells 4- The outer plexiform layer (OPL) It is a synaptic layer between: I. – The synaptic regions of the rods and cones. II. – The dendrites of the bipolar neurons III. – The processes of horizontal cells (association neurons 5- The inner nuclear layer (INL) contains the cell bodies of: I. Bipolar neurons. II. Horizontal cells that lie in the outer part of this layer. III. Amacrine cells (interconnect bipolar neurons & ganglion cells) IV. Muller cells: supportive neuroglial cells that extend from the external to internal limiting membranes (whole thickness of retina). 6-The inner plexiform layer: It is a synaptic layer between: I. The axons of bipolar cells. II. Dendrites of ganglion cells. III. Processes of amacrine cells 7- Ganglion cell layer: It contains I. Multipolar ganglion cells II. Branches of central retinal artery 8- Nerve fiber layer: It is composed of unmyelinated axons of ganglion cells which run parallel to the inner surface of the retina then pass through the optic disk and form the optic nerve which is myelinated. 9- Internal limiting membrane: It is formed of the terminal expansions of Muller's cells ,that cover the collagenous membrane of the vitreous body. Light must traverse eight (8) layers before arriving at the rods and cones outer segments to initiate a sensory nerve impulse that passes in the reverse direction to the ganglion cells, to the optic nerve. The region at which axons of ganglion cells come together to form the optic nerve is devoid of photoreceptors and known as the blind spot( the papilla of optic nerve, or optic nerve disk). The bipolar and ganglion cells accumulate in the periphery of fovea centralis and the center consists only of cone cells. The fovea is the area with the greatest visual acuity.  Rods and Cones The rod and cone cells, named for the shape of their outer segments The structure of rod and cone cells composed of outer and inner segments, a nuclear region, and a synaptic region The cones respond best to bright light (photopic vision) They are responsible for sharp vision and for the discrimination of color. Rods can respond to poor light (scotopic vision) and specially to movement across the field of vision.  Rod Cells The human retina has on average 92 million rod cells Rod cells are thin, elongated cells, composed of: 1- The outer segment is a modified primary cilium, photosensitive and shaped like a short rod Composed of flattened membranous discs stacked like coins and surrounded by the plasma membrane Proteins on the cytoplasmic surface of each disc include abundant rhodopsin (or visual purple) which is bleached by light and initiates the visual stimulus 2- The inner segment contains glycogen, mitochondria, and polyribosomes for the cell’s biosynthetic activity The inner segment synthesizes the proteins of the membranes of the flattened disks which are added to the outer segment disks at their basal region. These disks gradually migrate to the cell apex where they are phagocytosed by the cells of the pigment epithelium Between this outer segment and the cell’s inner segment is a constriction, the connecting stalk, which is part of the modified primary cilium arising from a basal body. 3- The nuclear region: it is a broad area with nucleus. 4- The synaptic region: club shaped expanded presynaptic terminal that contains synaptic vesicles and mitochondria  Cone Cells The human retina has on average 4.6 million cone cells Responsible for color vision in bright light. Cones contain a variety of the cone photopigment called iodopsin, its maximum sensitivity is in the red, green, or blue region of the visible spectrum. There are three morphologically similar classes of cones, each containing one type of the visual pigment iodopsin (or photopsin). By mixing neural input produced by these visual pigments, cones produce a color image.  The structure of the cone cells is similar to that of rods, but they differ from the rods in the following: 1. The outer segment is short and conical in shape 2. The stacked membranous discs are stay as continuous invaginations of the plasma membrane along one side 3. The newly synthesized iodopsins and other membrane proteins are distributed uniformly throughout the cone outer segment and not only to the recent disks in the basal region. 4. Discs in cones are shed much less frequently than in rods Rods and Cones Rods and Cones  Rods Cones Rods can respond to poor light The cones respond best to (scotopic vision) and specially to bright light (photopic vision). movement across the field of They are responsible for vision sharp vision and for the discrimination of colour. Include abundant rhodopsin (or visual purple) which is bleached Cones contain of the cone photopigment called by light and initiates the visual iodopsin, its maximum stimulus sensitivity is in the red, green, The human retina has on or blue region of the visible average 92 million rod cells spectrum. The human retina has on Rod cells are thin, elongated average 4.6 million cone cells cells. The outer segment is short and conical in shape Accessory structures of the eye  The eye lid Are flexible structures that protect the eyes. Histologically, the eye lid consists of the following layers from front backwards : A) The skin is loose and elastic, lacks fat, and has only very small hair follicles The eye lashes are rows of long and stiff hairs projecting from the lid margin. Associated with the follicles of eyelashes are sebaceous glands (The glands of Zeis) and modified apocrine sweat glands (the gland of Moll) B) Muscle :striated fascicles of the orbicularis oculi and levator palpebrae muscles. C) The trasus is a dense fibroelastic plate that containing the Meibomian glands which are modified sebaceous glands that secrete oily secretion (sebum)delivered by ducts which open on the lid margin , the secretion form a surface layer on the tear film, reducing its rate of evaporation, and help lubricate the ocular surface. D) Palpebral conjunctiva: covers the internal surface of eye lid. Eye lid The lacrimal glands oProduce fluid continuously for the tear film that moistens and lubricates the cornea and conjunctiva and supplies O2 to the corneal epithelial cells. o The main lacrimal glands are located in the upper temporal portion of the orbit oThe lacrimal gland is a tubuloalveolar gland and composed of columnar cells which contains secretory granules of the serous type. o The lacrimal glands are surrounded by well-developed myoepithelial cells o The main glands drain through individual excretory ducts into the superior fornix, the conjunctiva-lined recess between the eyelids and the eye oThe secretion of the gland passes down into the lacrimal canaliculi which join to form a common canaliculus just before opening into the lacrimal sac, and finally draining into the nasal cavity. oThe canaliculi are lined by stratified squamous epithelium, but the more distal sac and duct are lined by pseudostratified ciliated epithelium like that of the nasal cavity Histology of lacrimal glands  Refences: Junqueira’s : Basic Histology, Text and Atlas, 15th edition. Gartner and Hiatt: Color Textbook of Histology, 3rd ed. Philadelphia, W.B. Saunders, 2007 Ross, Michael H. Histology: a text and atlas: with correlated cell and molecular bioloy/Michael H. Ross, Wojciech Pawlina.—6th ed.