Adnexal Ocular Structures and Tear Film - Eye Physiology

Content  Ocular surface structures (Orbit-Conjunctiva)  Cornea  Aqueous  Lens  Vitreous  Retina  Neurophysiology  Visual perceptions 02/18/2025 1 Ocular Adnexal Structures (Orbit- Conjunctiva) Session objectives Explain the main functions of orbital...

Content  Ocular surface structures (Orbit-Conjunctiva)  Cornea  Aqueous  Lens  Vitreous  Retina  Neurophysiology  Visual perceptions 02/18/2025 1 Ocular Adnexal Structures (Orbit- Conjunctiva) Session objectives Explain the main functions of orbital bones Identify the components of eyelids Figure out the major function of eye lid Describe different types of eyelid glands List the compositions of tear film Define the functions of tear film Describe the functions of conjunctiva 02/18/2025 2 Orbit The bony socket(cavity) houses the eyeball and extra ocular muscles Has four walls and constitute of seven bones Four sided , pyramidal shape , 4 margins Contains orbital soft tissues:(periorbita), globe, extra ocular muscles, nerves, vessels, lacrimal gland, apertures( foramen, fissures (inf & sup),canal and nasolacrimal duct Give protection to the eye and surrounding structures (adnexa) and insulation 02/18/2025 3 02/18/2025 4 Cont’d 02/18/2025 5 Orbital Septum A membranous sheet that acts as the anterior boundary of the orbit Extends from the orbital rims (periosteum) to the eyelids Perforated by the vessels and nerves which pass from the orbital cavity to the face (scalp) Protects the orbit from potential pathogens (limit the infection to the anterior orbit) Has a cushioning effect for the orbital fat. With age it may weaken and as a result orbital fat may herniate forward 02/18/2025 6 Cont’d 02/18/2025 7 Eye Lid and Eye brow 02/18/2025 8 Eye brow The arch of hairs growing along the ridge formed by the supraorbital arch of the frontal bone Protect the eye, communication (winking) and facial expression (Surprise, fright and pleasure, deep thought, anger, puzzlements) 02/18/2025 9 Eye lid A movable fold of thin skin over the eye, consisting of loose connective tissue containing a thin plate of fibrous tissue lined with mucous membrane Developed from surface ectoderm -2nd month gestation Differentiation – begins about 3-4th month of gestation Separated completely at 7th month gestation The margin is separated with a grey line into anterior and posterior 02/18/2025 10 Cont’d Anterior lamella- skin ,muscle and associated glands Posterior lamella- tarsal plate, conjunctiva and associated glands (meibomian glands) Shutters protecting the eye from injuries and excessive light Main function is to spread the tear film over the cornea and conjunctiva Assist the lacrimal pumping mechanism via orbicularis oculi muscle Helps in the secretion and draining of tear 02/18/2025 11 Cont’d 02/18/2025 12 Functions of the eye lid 1. Opening Upper lid elevators Levator Palpera Superiors (LPS) Frontalis muscle Muller muscle Lower lid retractors Inferior rectus muscle Inferior palpebral muscle 2. Closings Orbicularis Oculi Muscles (OOM) innervated by----CN VII Orbital part of OOM- Forceful closures 02/18/2025 13 3. Blinking Voluntary and winking – performed by LPS and OOM Involuntary A. Spontaneous- without external stimulus (Average - 15blinks/minute) B. Reflex-response to a stimulus Tactile – touch Optic-bright light , dazzle, menace Auditory-noise ,sudden presence of near object Clinical significance……..? 02/18/2025 14 Cont’d  Bell’s phenomena on closure of the eyelids, the eyeball is rotated upward and outward  This is a protective mechanism NOT present in 10% of otherwise healthy persons, and therefore its absence is not necessarily a sign of disease 02/18/2025 15 During eye lid movements In openings; Upper eye lid moves against gravity Both the lower and upper begins in phase (same rate) Lower lid more slower than upper eye lid due to lack of direct muscular pull Upper eye lid moves vertically upward while lower eye lid laterally in a horizontal directions Two levators behave as yoke muscles –herring’s law 02/18/2025 16 Cont’d In closings; Upper lid dawn ward vertically while lower lid moves medially (horizontally) Rate of movement is similar Gravity no role in closing upper eye lid Reciprocal innervations (LPS and OOM)-Sherrington's law 02/18/2025 17 Eye lashes Known as hair follicles, protect the eye from direct sunlight, dust, perspiration and foreign bodies They grow in a double row along the border located in two rows both upper and lower lids The first line of defense with eyebrows Grow from hair follicles surrounded by a nerve plexus Life span of each cilia= 3 to 5 months and replacements -10wks 02/18/2025 18 Eye Lid Glands A. Glands of Zies and Moll Located in the anterior lamella at the base of each cilia (eye lashes) Modified sebaceous and sweat glands, secrete sebum that keeps the lash supple Clinical. Sign? 02/18/2025 19 B. Meibomian Glands (MG) Located in the posterior portion of the eye lid (tarsal plate) Developed from down growth of basal cells in the posterior margins of lids Their secretion (oily secretion) form the superficial part of the pre corneal tear film (lipid Layer) that helps to prevent evaporation of the tears Arranged in a single row vertically to each other 20-30 in each lid (25 upper, and 20 in lower) Produce oily secretion through their openings (mebibomian gland orifices) 02/18/2025 20 Regulation of meibum secretion A. Neural regulation Meibomian glands are innervated richly by sensory, sympathetic, and parasympathetic nerves B. Hormonal regulation Meibomian glands have androgen and estrogen receptors Meibomian gland secretion is influenced by lipid synthesis, which is regulated by circulating androgen and estrogen levels Androgens appear to stimulate lipid synthesis and secretion by meibomian glands 02/18/2025 21 C. Blinking Meibomian secretion occurs on blinking due to contraction of the muscle of Riolan Increased blink rate and force might increase the volume of secreted meibum 02/18/2025 22 Cont’d 02/18/2025 23 C. Accessory lacrimal glands of Wolfring- Present along the upper border of the superior tarsus and along the lower border of the inferior tarsus About 2-5 in the upper lid and 2-3 in the lower lid Krause –in conjunctiva fornix Used for producing aqueous layer of tear film(2nd layer) 02/18/2025 24 Cont’d 02/18/2025 25 Lacrimal Gland (LG) Found superiorly in the anterolateral superior orbit A lobulated tubuloacinar gland, multiple acini drain into progressively larger tubules which drain into the superolateral fornix 02/18/2025 26 Lacrimal Gland Secretion Forms most of the aqueous component of the tear film consists of: 1. Fluid and electrolytes The acinar secretory cells produce a primary secretion that is similar to plasma This is modified by the epithelial cells lining the ductless which secrete additional K + and Cl− At low flow rates, this is hypertonic to plasma; at high flow rates, it is isotonic 02/18/2025 27 Cont’d 2. Proteins Constitutive proteins secreted by the lacrimal gland 3. Metabolic pump Acinar cell secretion is maintained by basolateral Na + / K + ATPase pump activity Intracellular Na + is depleted This encourages entry of Na+, K+, and Cl− via a co- transporter This causes a net movement of Cl− across the cell into the lumen of the acinus Paracellular Na+ and water movement result in secretion of fluid 02/18/2025 28 Cont’d 02/18/2025 29 Control of Lacrimal Gland Secretion The lacrimal gland is innervated by parasympathetic and sympathetic fibers 1. Parasympathetic innervation Secretion is controlled by parasympathetic fibers from the lacrimal nucleus of the pons Parasympathetic signaling uses neurotransmitters such as Acetyl choline (Ach); stimulate cholinergic receptors on lacrimal secretory cells This largely controls the water, electrolyte, and protein content of the secretion 02/18/2025 30 2. Sympathetic innervation Have a minor role in controlling lacrimal gland secretion These use noradrenalin to stimulate α- and β-adrenergic receptors on lacrimal secretory cells Stimulation results in constriction of local blood vessels and contraction of myoepithelial cells 02/18/2025 31 Tear film Highly ordered fluid layer lining the cornea and bulbar and palpebral conjunctiva Has a total volume of 7–10 μL, 70–90 % reside in the upper and lower tear menisci Menisci are curvilinear collections of tears that line the ocular surface immediately adjacent to the lid margins Can be stored in the upper and lower conjunctival cul- de-sacs (fornices) Normal tear volume turnover occurs every 5–7min 02/18/2025 32 Structure of Tear Film From superficial to deep: A. Lipid layer (0.1 μm) B. Aqueous layer (7 μm) C. Mucous layer (3–30 μm) D. Glycocalyx (0.01–0.02 μm) 02/18/2025 33  A. Lipid layer (0.1 μm) Consists of hydrocarbons, sterol esters, waxy esters, triglycerides, free cholesterol, free fatty acids, polar lipids and proteins Primarily secreted from meibomian glands with additional contributions from the glands of Moll and Zeiss Emitted as a liquid spreading over the aqueous on blinking Polar lipids form the inner surface of the lipid layer, with their charged side facing aqueous Nonpolar lipids spread over the polar lipids 02/18/2025 34 Functions of the lipid layer  Inhibits evaporation of underlying aqueous  Maintains tear film stability  Prevents contamination with skin lipids (which can destabilize the aqueous)  Prevents tears spilling over the eyelid. This occurs because the skin’s sebum has mostly nonpolar lipids and tends to repel meibum which has a greater proportion of polar lipids 02/18/2025 35 B. Aqueous Layer (7 um) Origin Around 95 % is from lacrimal gland secretion; 5 % from the accessory glands of Krause and Wolfring Composition The aqueous contains solutes essential for epithelial integrity Contains nutrients and waste products important in corneal and conjunctival metabolism Regulation of tear pH and osmolarity is essential for optimal epithelial cell function and survival 02/18/2025 36 Tear pH Tear pH lowest on awakening due to overnight build up of acid by-products On eye opening, it rapidly corrects due to loss of CO2 Stable through the day due to buffering systems Tear osmolarity Lower during closure overnight due to reduced evaporative loss During the day, tear osmolarity stabilizes like pH Protein constituents (out lined in a table) 02/18/2025 37 Tear film aqueous layer proteins Protein class Examples Function Antibacterial Secretory Binds and opsonizes agents immunoglobulin A foreign antigen Laysozyme Damages bacterial cell walls Lactoferrin Damages bacterial cell walls Wetting agents Lipocalin Promotes surface wettability, allowing the tear film to spread uniformly over the corneal and conjunctival surfaces Growth factor Lacritin Promotes epithelial renewal 02/18/2025 38 C. Mucus Layer and Glycocalyx 1. Composition The mucus layer consists of:  Mucins (glycoproteins) secreted by conjunctival goblet cells  Water and electrolytes secreted by conjunctival goblet and non- goblet epithelial cells High molecular weight proteins with many carbohydrate side groups Maintain a high water content and confer a viscous texture to mucous The glycocalyx is a membrane-bound network of mucins attached to the apical microvilli of corneal and conjunctival epithelial cells 02/18/2025 39 Cont’d 02/18/2025 40 2. Storage and secretion Mucin is stored in large secretory granules at apical surface of goblet cells Neuronal control of secretion allows mucin release in response to surface irritation or microtrauma Goblet cells are not directly innervated: cholinergic neurotransmitters provide the predominant goblet cell stimulation Water and electrolytes are secreted across all conjunctival cells using basolateral Na + / K + ATPase pump activity, with water being transported transcellularly by aquaporins This can be stimulated by noradrenergic or purinergic mechanisms 02/18/2025 41 Functions of Mucus Layer and Glycocalyx I) Mucin Enhances lubrication, allowing the palpebral and bulbar conjunctiva to slide over each other with minimal trauma during blinking or eye movements Protects the epithelial surface; it spreads rapidly to heal defects and cover foreign bodies Acts as reservoir for immunoglobulin  Promotes surface wettability by overcoming corneal epithelial hydrophobicity II) The glycocalyx The glycocalyx renders the ocular surface polar and thus wettable 02/18/2025 42 Functions of Tear Film The tear film has four main functions: 1) Optical 2) Mechanical, 3) Nutritional, and 4) Defensive 02/18/2025 43 Functions of the Tear Film 1. Optical The tear film provides a smooth, regular optical surface for refraction, filling corneal irregularities The air-tear film interface is the most powerful refractive surface of the eye 2. Mechanical The tear film adheres to the bulbar and palpebral conjunctiva ensuring well- lubricated surfaces Blinking flushes debris and exfoliated cells from the ocular surface out through the tear duct 02/18/2025 44 Cont’d 3. Nutritional Oxygen dissolves in the tear film from air, supplying the avascular cornea Nutrients (e.g., glucose) pass from the conjunctival vessels to the cornea via the tear film 4. Defensive First line of defense against ocular pathogens Contains antibacterial constituents (e.g., secretory immunoglobulin A (sIgA) lysozyme, lactoferrin) and has a low pH to maintain an antibacterial environment 02/18/2025 45 Types of Tear Secretions 1. Basal Tear secretion (Normal tear production rate is 1– 2 μl/min) 2. Reflex Tear (Normal tear production rate is >100 μl/min) 3. Psychic tear (Read more……..) 02/18/2025 46 Physical and chemical properties of tears Water-98.2% Thicknes= (4-8 um) Solid -1.8% Volume =7ul (4- 13) Na+ -142meq/l Rate of secretions=1.2ul/min K+-15-29meq/l Refractive index=1.357 Glucose -3-10mg/100ml Osmolarity=298- 308 MOSM/L Amino acids -8mg/100ml PH=7.4(7.3-7.7) Total proteins -0.6-2mg/100ml Turn over rate =18% per minute 02/18/2025 47 Tear Drainage System Elimination of the tear film occurs mainly via the lacrimal drainage system, while some is lost through evaporation and conjunctival absorption Drainage begins at the punctum, a 0.3 mm opening located on the medial eyelid margin The External Surface Tears are conducted along the menisci then into the canaliculated via capillary attraction Lid movement contributes to tear movement, both across the eye and toward the puncta 02/18/2025 48 Lacrimal Pump Mechanisms When the orbicularis contracts (blinking), the punctum is drawn nasally and the canaliculus compressed, forcing tears into lacrimal sac 02/18/2025 49  Conjunctiva 02/18/2025 50 Conjunctiva A transparent mucous membrane lining the inner surface of the eyelids and the surfaces of the globe as far as the limbus (Major part of the ocular surface) Joins the corneal epithelium to form the ocular surface Has a densely lymphatic supply The lymphatic drainage is to preauricular and submandibular lymph nodes Anatomically classified into three:  Palpebral  Bulbar  Forniceal 02/18/2025 51 Cont’d The bulbar conjunctiva lines the sclera; the palpebral conjunctiva lines the eyelid inner surface The two join at a superior or inferior conjunctival recess (fornix) On the palpebral side, the conjunctiva is firmly adherent to the tarsal plate The conjunctiva consists of a surface epithelium and an underlying substantia propria 02/18/2025 52 Cont’d 1. Epithelium A 2- to 3-cell layer non-keratinized cuboidal stratified epithelium At the limbus, the conjunctival epithelium blends with the corneal epithelium, and the loose vascular stroma of bulbar conjunctiva changes to the avascular Bowman’s layer 2. Substantia propria Highly vascular and contains immune cells (mast cells, plasma cells, neutrophils, and lymphocytes) The suspensory apparatus that forms the fornix is found underlying the palpebral conjunctiva attached to the recti muscles 02/18/2025 53 Conjunctival Tear Film Contribution 1. Glycocalyx A network of membrane bounds mucins that projects from the apical surface of the conjunctival and corneal epithelial cells 2. Mucous layer Conjunctival goblet cells produce soluble mucins that form the mucous layer of the tear film Goblet cells are unevenly distributed over the conjunctiva Non-goblet epithelial cells (especially in Henle’s crypts) secrete mucus, electrolytes and water 02/18/2025 54 3. Aqueous layer The accessory glands of Krause and Wolfring open onto the conjunctival surface The glands of Krause are mostly forniceal; 20 in the superior and 6–8 in the inferior fornix The glands of Wolfring are in the tarsal conjunctiva of the upper and occasionally lower lid 02/18/2025 55 4. Ocular surface stem cells Conjunctival stem cells are located at the limbus and dispersed throughout the conjunctiva Epithelial stem cells in the corneal limbus provide the main source for mitotic activity to replenish the conjunctival and corneal epithelium after injury In addition, there may be stem cells located in the basal layer of corneal epithelium These cells can do every thing for the corneal (ocular surface) integrity. They are the defense army of the cornea. 02/18/2025 56 Functions of Conjunctiva Provision of mucus for the tear film Protection of the ocular surface by barrier function Defense against pathogens as an element of the mucosa-associated lymphoid tissue Provision of limbal stem cells to maintain and heal the corneal and conjunctival epithelia 02/18/2025 57  Thank you! 02/18/2025 58