Vision 3&4 PDF - Histology - CNS Module

Dr. Omar Nasr Histology CNS Module Lens - It is an elastic transparent biconvex disc. - The lens is avascular. - Histologically, it is formed of: a. Lens capsule:  It is a thin elastic transparent B.M that refracts light....

Dr. Omar Nasr Histology CNS Module Lens - It is an elastic transparent biconvex disc. - The lens is avascular. - Histologically, it is formed of: a. Lens capsule:  It is a thin elastic transparent B.M that refracts light.  Formed of collagen type IV & glycoprotein.  It covers the whole lens but thickened ant. to support the lens epithelium. b. Sub capsular epithelium:  It is single layer of cuboidal epithelium located under the capsule and covers the ant. and lateral surface of the lens. c. Lens fibers:  They are long hexagonal cells forming the bulk of the lens.  They are concentrically arranged and perpendicular to the lens surface.  The cells of the subcapsular epithelium give rise to these cells which lose their nuclei and organelles. These cells become filled with crystallins which are lens proteins increasing the refractive index of the lens fibers.  The lens grows during life by addition of new fibers to the periphery of the lens.  The lens is suspended in its place by suspensory ligament that connects the lens to the ciliary body Dr. Omar Nasr Histology CNS Module Causes of lens transparency 1. It is a vascular structure 2. No nerve supply 3. Lens fibers are uniformly arranged 4. Various lens constituents have almost the refractive indices 5. Active metabolic process of lens epithelium which maintains the chemical composition of the lens and transparency. Functions of the lens 1. One of the refractive media of the eye not fixed). 2. Accommodation for near vision. 3. It protects the retina from harmful ultraviolet rays. Dr. Omar Nasr Histology CNS Module Structures related to the Eye 1. The Conjunctiva: It is a thin transparent mucous membrane that covers the sclera (bulbar conjunctiva) then reflected to line the eye lid (palpebral conjunctiva). The conjunctiva is formed of stratified columnar epithelium that contains goblet cells lying on a basal lamina and lamina propria (loose CT). At the corneoscleral junction, the conjunctiva continues as corneal epithelium which is devoid of goblet cells. Function: secretion of goblet cells become a part of the tear film. 2. Eye lid: Histological structure: 1. Thin skin devoid of fat and has fine hair. The lid margin carries 3-4 rows of eye lashes (lid margin) in between them there are sebaceous glands called Zeis glands and apocrine sweat glands called Moll’s glands. 2. Orbicularis Oculi: they are striated muscles that close the eye. 3. Tarsal plate: It is a thick plate of dense collagenous C.T that contains a modified sebaceous gland called meibomian gland. 4. Palpebral conjunctiva: stratified columnar epithelium and lamina propria. Dr. Omar Nasr Histology CNS Module Meibomian gland They are about 20:25 modified sebaceous glands, their ducts open in the margin of the eye lid among the eye lashes. They secrete oily secretion that prevents the edges of the opposing eye lid from sticking together, this oily secretion forms a layer over the tear film to slow the rate of evaporation. N.B Inflammation of Zeis or Moll’s glands leads to a Sty. Inflammation of meibomian glands and obstruction of its duct by secretion and granulation tissue leads to Chalazion. Dr. Omar Nasr Histology CNS Module Lacrimal system The lacrimal gland: - Situated in the anterio-superior part of the orbit. - Secretes the tears, behind the upper eye lid and flow over the eye, wetting and lubricating anterior surface of the eye. - Serous compound tubulo-alveolar gland. - Myoepithelial cells surround its secretory acini. The lacrimal canaliculi: - Carry the lacrimal fluid away from the surface of the eye. - They are lined by stratified squamous epithelium The lacrimal sac: - dilated portion of the duct system. - It is lined by pseudostratified ciliated columnar epithelium The nasolacrimal duct: - Delivers the lacrimal fluid to the nasal cavity. - It is also lined by pseudostratified ciliated columnar epithelium. Dr. Omar Nasr Histology CNS Module The structure of Inner Eye Retina (Inner nervous layer) The retina contains three basic cell types: 1. Neurons (Photoreceptors, nerve cells). 2. Neuroglial supporting cells. 3. Pigment epithelial cells. Histologically, the retina is formed of ten layers: 1. The pigmented epithelium: o L/M: - It is composed of a single layer of cubical epithelial cells with rounded basal nuclei resting on Bruch’s membrane - The basal region firmly attached to Bruch’s membrane, the apical surface is close to the rod and cone - Very rich in melanin pigments o E/M: - The cells are rich in rER, sER, lysosomes, mitochondria, apical melanin granules. - The cells have microvilli that interdigitate with the rods and cones. - Desmosomes and tight junction are present on the lateral cell membranes. o Functions: a. Formation of blood retinal barrier. b. Esterifying vitamin A in sER. c. Phagocytosis of old discs shed by rods. d. Absorption of excess light after it has passed through and stimulated the photoreceptors, thus preventing reflections which impair focus. e. Shares in formation of Bruch’s membrane. f. Synthesis of melanin granules at its apex. Dr. Omar Nasr Histology CNS Module N.B Retinal detachment occurs between pigmented epithelial layer and other nine layers of retina as this layer is firmly adherent to Bruch’s membrane. 2. The rods and cones layer: o This layer is made of dendritic processes of the rod and cone cells. o These processes are photoreceptors. 3. The outer limiting membrane: o It is formed of a junctional complex between Muller’s cells (neuroglia) and the photoreceptors (rod & cone). 4. The outer nuclear layer: o It contains nuclei and cell bodies of rod and cone cells (1st order neurons). 5. The outer reticular (plexiform) layer: o This layer is formed of synapse between axons of rod and cone cells with dendrites of bipolar cells and short horizontal cells. o N.B: - Usually, two or more rod cells synapse with one bipolar cell and horizontal cell. - The cone nucleus is large, in close contact with outer limiting membrane. - Each cone cell synapse with one bipolar cell. 6. The inner nuclear layer: o It contains the cell bodies (nuclei) of four cells: a. The bipolar nerve cells (2nd order neuron), they make synapse with either the rod or cone cells. b. Horizontal cells: present in the outer part of this layer. c. Amacrine cells: present in the inner part of this layer. d. Muller’s cells: - Long supportive and nutritive glial cells. - The bodies of these cells are present in the inner nuclear layer. - They have long cytoplasmic processes, the outer processes extend to outer limiting membrane where they form junctional complex with rods and cones, their inner processes terminate by tight junction between them forming the inner limiting membrane. Dr. Omar Nasr Histology CNS Module 7. The inner reticular (plexiform) layer: o This layer contains the synapse between the axons of bipolar nerve cells and dendrites of ganglion cells. Also, amacrine cells synapse with them. 8. The ganglion cell layer: o It contains large ganglion nerve cells (3rd order neurons). 9. The optic nerve fibers layer: o This layer contains the unmyelinated axons of the ganglion cells which have no neurolemmal sheath (transparent, can’t regenerate). o The axons bend at right angle and come out as optic nerve which pass to the brain. Dr. Omar Nasr Histology CNS Module 10. Inner limiting membrane: o It is formed by a tight junction between the end processes of Muller’s cells and their basal lamina. Dr. Omar Nasr Histology CNS Module Optic disc (blind spot) The optic disc is the exit site of optic nerve, entrance of central artery & vein to the retina. Located medial and slightly above the posterior pole (macula lutea). This area has no photoreceptors and so called blind spot. Macula lutea (yellow spot) It is a yellow pigmented depressed spot (zone) in the retina opposite to the posterior pole of the eye lateral to optic disc. The center of this depression is called fovea centralis. This area is devoid of blood vessels and rods receptors. Fovea centralis: It is the area of the best (maximal) visual acuity as: 1. Cons are the only photoreceptors. So, light rays fall directly on the cones, producing the area of greatest visual acuity and sharpest color discrimination. 2. In fovea each cone is connected to bipolar cell  connected to one ganglion cell  form single optic nerve 3. Pigment is well developed in fovea which absorbs excessive light  sharp vision. N.B Retinal supporting cells: Muller’s cells. Nerve cells of the retina: 1. Bipolar nerve cells: 2. Ganglion cells: 3. Horizontal, amacrine and interplexiform cells: are neurons that act as gates to modulate the passage of impulses from photoreceptors to ganglion cells. Dr. Omar Nasr Histology CNS Module Rod and cone cells (Photoreceptors) These are the 1st order neuron in the visual pathway. They oriented parallel to one another but perpendicular to the retina. each receptor is formed of 4 functional layers: 1. Outer segment 2. Inner segment 3. Nucleus 4. Synaptic body. Dr. Omar Nasr Histology CNS Module Photoreceptor potential & signal transmission in retina of 1) photochemical changes (bleaching) The photochemical substances are two: Rhodopsin: present in rods & formed of 11 cis retina & protein(scotopsin) Iodopsin: present in cones & formed of 11 cis retina & protein (Photopsin 3 types)  1) photochemical changes (bleaching) A) On exposure to light light is absorbed by the photo pigment 1) Rhodopsinbathorhodopsinlumirhodopsin metarhodopsin Imetarhodopsin II (activated rhodopsin) which excites the electrical responses in retina Light 2) 11 cis retinal (curved) all trans retinal (straight) cannot combine with opsin). (complete bleaching)  1) photochemical changes (bleaching) 2) On dark Reformation of rhodopsin Retinal isomerase enzyme All trans retinal 11 cis retinal which can join opsin rhodopsin  2) Genesis of electrical response in retina A) During rest (dark condition) 1) The inner segment of the photoreceptor contains active Na+ pumb (from in to out) (–ve) intracellular potential. 2) The outer segment contain Na+ channels and & c Gmp which maintains the channel in open state (pass Na from out to in). 3) So, Na+ pumped by the inner, returns again by the outer one, so, during rest the photoreception are depolarized & their resting membrane potential is low (-40mV) entry of Ca++release of chemical transmitter(↑↑glutamate)inhibition of bibolar cells no action potential in ganglion cells. The flow of Na in & out of the cells is called dark current.  2) Genesis of electrical response in retina B) On light exposure 1) Activation of rhodopsin metarhodopsin 11activation of cGMP phosphodiestrase enzyme ↓↓c GMPClosure of Na+ channels in the outer segment intracellular potential become more -ve hyperpolarization (-70mV) ↓↓release of synaptic transmitter(glutamate)stimulation of bipolar cellsdepolarization & action potential in ganglion cells signals in the optic nerve.  Light adaptation Dark adaptation Definition Retinal adaptation ↓↓ retinal sensitivity to light & ↑↑ ↑↑retinal sensitivity to light &↓ retinal threshold when a person ↓retinal threshold when a person shifts from a dim lighted place to shifts from a bright lighted place bright lighted place to dim lighted place mechanism Break down of photo pigment in Regeneration of photo pigment rods & cones in rods & cones changes Constriction of the pupil dilatation of the pupil Break down of photo pigment in Regeneration of photo pigment rods & cones in rods & cones ↓↓retinal sensitivity to light ↑↑retinal sensitivity to light ↓↓Signal intensity in retinal ↑↑Signal intensity in retinal neuron neuron. The yellow green part is the most The bue green part is the most luminous luminous Visual acuity Increase visual acuity Decrease visual acuity