Unit 1- Ocular Physiology | 2nd Semester Bachelor of Optometry

Protective Mechanisms in the Eye: Eyelids and Lacrimation, Description of the Globe

Introduction

The human eye, often referred to as the window to the soul, is one of the most intricate and sensitive organs in the human body. Its proper functioning is essential for vision, and therefore, nature has provided it with several protective mechanisms. These include anatomical barriers like the eyelids, physiological processes like lacrimation (tear production), and structural support from the globe (eyeball) itself. These systems work together to shield the eye from external injuries, infections, desiccation, and other environmental hazards.

I. Eyelids: Anatomy and Protective Function

1. Structure of Eyelids

Structure of Eyelid

The eyelids (palpebrae) are movable folds of skin that protect the anterior surface of the eye. There are two eyelids – upper and lower. Each eyelid is made up of five layers:

Skin: The thinnest skin in the body, highly mobile and flexible.
Subcutaneous Tissue: Contains loose connective tissue and fat.
Muscular Layer: Includes the orbicularis oculi (responsible for closing the eyelid) and the levator palpebrae superioris (elevates upper lid).
Tarsal Plate: A dense connective tissue plate that provides rigidity and contains Meibomian glands.
Conjunctiva: A thin mucous membrane that lines the inner surface of the eyelids and reflects onto the eyeball.

2. Functions of Eyelids

Protection from Injury: Eyelids act as mechanical barriers against dust, bright light, and foreign objects.
Lubrication: With every blink, the eyelids help spread the tear film evenly across the cornea, preventing dryness.
Cleaning Mechanism: Blinking helps in sweeping away debris and expired tears via the lacrimal drainage system.
Light Control: Reflexive closure in response to intense light protects the retina from photic injury.

3. Reflex Actions of Eyelids

Eyelids participate in various reflexes that enhance ocular protection:

Blink Reflex: Involuntary blinking protects the eye from drying and minor irritants.
Corneal Reflex: Triggered when the cornea is touched or irritated; it causes immediate closure of the eyelids.
Threat Reflex: Activated when an object suddenly approaches the eye, leading to a rapid blink or eyelid closure.

II. Lacrimation: The Tear Film and Tear Production

1. Anatomy of the Lacrimal Apparatus

Anatomy of Lacrimal Apparatus

The lacrimal system is composed of the lacrimal gland and the lacrimal drainage apparatus.

Lacrimal Gland: Located in the superolateral part of the orbit. It is divided into orbital and palpebral lobes.
Accessory Lacrimal Glands: Include glands of Krause and Wolfring. These contribute to basal tear secretion.
Lacrimal Drainage System: Includes puncta, canaliculi, lacrimal sac, and nasolacrimal duct which empties into the inferior meatus of the nose.

2. Composition of Tear Film

Microscopic Layer of Tear Film

The tear film is made up of three distinct layers:

Lipid Layer: Outermost layer produced by Meibomian glands; prevents evaporation of tears.
Aqueous Layer: Middle layer secreted by the lacrimal gland; contains water, enzymes (like lysozyme), salts, and antibodies.
Mucin Layer: Innermost layer secreted by goblet cells in conjunctiva; helps spread the aqueous layer evenly across the cornea.

3. Functions of Lacrimation

Lubrication: Prevents dryness by moistening the conjunctiva and cornea.
Nutrient Supply: Provides oxygen and nutrients to the avascular cornea.
Antibacterial Action: Enzymes like lysozyme and lactoferrin provide antimicrobial protection.
Debris Removal: Helps wash away foreign particles and epithelial debris.
Emotional Expression: Tear secretion increases in response to emotional stimuli (crying).

4. Regulation of Tear Secretion

Tear production is controlled by the autonomic nervous system:

Parasympathetic Stimulation: Major contributor to tear secretion via facial nerve (VII cranial nerve).
Sympathetic Stimulation: Modulates the quality of tears (especially lipid layer).

III. Description of the Globe (Eyeball)

1. Overview

Anatomy of Eyeball

The globe or eyeball is a spherical structure that houses the optical and neural apparatus required for vision. Its average diameter is about 24 mm in adults. The globe is divided into three tunics or coats:

Fibrous Tunic: Outermost layer composed of sclera and cornea.
Vascular Tunic (Uveal Tract): Includes iris, ciliary body, and choroid.
Neural Tunic: Comprises the retina, which contains photoreceptor cells.

2. Fibrous Tunic

Cornea: Transparent, avascular structure responsible for major refraction of light.
Sclera: Opaque, white portion providing structural strength and attachment for extraocular muscles.

3. Vascular Tunic

Iris: Colored part of the eye that controls pupil size and regulates light entry.
Ciliary Body: Produces aqueous humor and controls lens shape via zonular fibers.
Choroid: Highly vascular layer that supplies nutrients to the outer retina.

4. Neural Tunic (Retina)

The retina is the innermost layer that contains photoreceptor cells (rods and cones) responsible for converting light into neural signals. It also includes several layers of neurons and supporting cells.

5. Chambers and Contents of the Globe

Anterior Chamber: Between cornea and iris; filled with aqueous humor.
Posterior Chamber: Between iris and lens; also contains aqueous humor.
Vitreous Chamber: Large space behind the lens filled with vitreous humor (gel-like substance).

6. Intraocular Pressure (IOP)

IOP is the pressure exerted by fluids (mainly aqueous humor) inside the eye. It is crucial for maintaining the globe’s shape and normal functioning. Normal IOP ranges between 10–21 mmHg.

IV. Integration of Protective Mechanisms

The eyelids and lacrimal system work in harmony to protect the ocular globe. When a threat is detected:

The eyelids reflexively close to protect the cornea and inner eye.
The lacrimal system flushes out any irritants via tear production.
Ocular surface immunity acts via antimicrobial tear components to neutralize pathogens.

V. Clinical Relevance

1. Disorders of Eyelids

Ptosis: Drooping of the upper eyelid due to levator muscle dysfunction.
Blepharitis: Inflammation of the eyelid margin causing redness and irritation.
Entropion/Ectropion: Inward or outward turning of eyelid leading to exposure and corneal damage.

2. Disorders of Lacrimation

Dry Eye Syndrome: Inadequate tear production or poor tear quality leading to irritation and visual disturbances.
Dacryocystitis: Infection of the lacrimal sac, often due to blockage of the nasolacrimal duct.

3. Globe-Related Disorders

Glaucoma: Raised intraocular pressure damaging the optic nerve.
Uveitis: Inflammation of the uveal tract that can threaten vision if untreated.

Conclusion

The eye is a remarkably complex organ that depends on multiple protective mechanisms to preserve its function and structure. The eyelids and the lacrimal system provide mechanical and biochemical defenses, while the structure of the globe ensures proper accommodation and vision processing. A thorough understanding of these systems is essential for optometry students to evaluate, diagnose, and manage ocular conditions effectively.

Extrinsic Eye Muscles, Their Actions and Control of Their Movements

Introduction

Eye movements are vital for proper vision, allowing us to track objects, shift gaze, and maintain stable visual perception. These movements are controlled by six extrinsic (extraocular) muscles that are attached to the sclera of the eyeball. These muscles are responsible for moving the eye in various directions within the orbit. The control of these muscles involves complex neural mechanisms through cranial nerves and brainstem centers, ensuring coordination between both eyes (binocular vision) and rapid responses to stimuli.

I. Overview of Extrinsic Eye Muscles

Extra-Ocular Muscles

The six extrinsic muscles of the eye are:

Superior Rectus (SR)
Inferior Rectus (IR)
Medial Rectus (MR)
Lateral Rectus (LR)
Superior Oblique (SO)
Inferior Oblique (IO)

All of these muscles (except Inferior Oblique) originate from the annulus of Zinn, a fibrous ring at the apex of the orbit. They insert on the sclera of the eyeball. The extraocular muscles are arranged in a way that allows smooth, controlled, and precise eye movements.

II. Actions of the Extraocular Muscles

Fig. Depicting the Actions of EOMs

1. Rectus Muscles

Medial Rectus (MR): Moves the eye inward (adduction).
Lateral Rectus (LR): Moves the eye outward (abduction).
Superior Rectus (SR): Elevates the eye, also causes adduction and intorsion.
Inferior Rectus (IR): Depresses the eye, also causes adduction and extorsion.

2. Oblique Muscles

Superior Oblique (SO): Intorsion (inward rotation), depression, and abduction of the eyeball.
Inferior Oblique (IO): Extorsion (outward rotation), elevation, and abduction of the eyeball.

3. Primary, Secondary, and Tertiary Actions

Each muscle can have multiple actions depending on the position of the eye:

Muscle	Primary Action	Secondary Action	Tertiary Action
Medial Rectus	Adduction	–	–
Lateral Rectus	Abduction	–	–
Superior Rectus	Elevation	Intorsion	Adduction
Inferior Rectus	Depression	Extorsion	Adduction
Superior Oblique	Intorsion	Depression	Abduction
Inferior Oblique	Extorsion	Elevation	Abduction

III. Innervation of the Extraocular Muscles

The extrinsic muscles are innervated by three cranial nerves:

Oculomotor Nerve (CN III): Innervates SR, IR, MR, and IO.
Trochlear Nerve (CN IV): Innervates SO (Superior Oblique).
Abducens Nerve (CN VI): Innervates LR (Lateral Rectus).

An easy way to remember this is: LR6 SO4, all the rest by 3 (Lateral Rectus by CN VI, Superior Oblique by CN IV, and the rest by CN III).

IV. Types of Eye Movements

1. Ductions

Adduction: Movement of the eye toward the nose.
Abduction: Movement of the eye away from the nose.
Elevation (Supraduction): Movement of the eye upward.
Depression (Infraduction): Movement of the eye downward.
Intorsion: Rotation of the top of the eye toward the nose.
Extorsion: Rotation of the top of the eye away from the nose.

2. Versions

Simultaneous and symmetric movement of both eyes in the same direction.

Dextroversion: Both eyes move to the right.
Levoversion: Both eyes move to the left.
Sursumversion: Both eyes move upward.
Deorsumversion: Both eyes move downward.
Dextrocycloversion / Levocycloversion: Rotatory movements in cyclo directions.

3. Vergence Movements

Disjunctive movements in opposite directions:

Convergence: Eyes move toward each other (for near objects).
Divergence: Eyes move away from each other (for distant objects).

4. Saccadic and Pursuit Movements

Saccades: Rapid, jerky movements to shift gaze from one point to another.
Smooth Pursuit: Slow, tracking movements following a moving object.

5. Vestibulo-Ocular Reflex (VOR)

A reflex that stabilizes vision during head movement by producing an eye movement in the opposite direction of head motion.

V. Control Centers for Eye Movement

1. Brainstem Centers

Several brainstem nuclei coordinate extraocular muscle actions:

Oculomotor Nucleus: Controls SR, IR, MR, IO, and levator palpebrae superioris.
Trochlear Nucleus: Controls SO.
Abducens Nucleus: Controls LR and communicates with MR via medial longitudinal fasciculus (MLF).

2. Supranuclear Control

Higher centers that plan and initiate eye movements:

Frontal Eye Field (FEF): Controls voluntary saccadic movements.
Parietal Eye Field: Involved in tracking and attention-related movements.
Superior Colliculus: Integrates visual stimuli with motor commands.
Cerebellum: Coordinates smooth pursuit and stabilizes gaze.

VI. Clinical Considerations

1. Nerve Palsies

CN III Palsy: Causes ptosis, eye turned "down and out", and dilated pupil.
CN IV Palsy: Vertical diplopia, worsens on looking down and inward.
CN VI Palsy: Inability to abduct the affected eye, causing horizontal diplopia.

2. Strabismus (Squint)

A misalignment of the eyes due to muscle imbalance. Can be:

Esotropia: Eye turns inward.
Exotropia: Eye turns outward.
Hypertropia: Eye turns upward.
Hypotropia: Eye turns downward.

3. Internuclear Ophthalmoplegia (INO)

A lesion in the medial longitudinal fasciculus (MLF), leading to defective horizontal gaze and disconjugate eye movements.

4. Nystagmus

Involuntary, rhythmic oscillation of the eyes due to abnormal muscle control or neurological dysfunction.

VII. Diagnostic Testing of Extraocular Movements

1. Cardinal Positions of Gaze

Six positions used to test the function of each extraocular muscle individually by asking the patient to follow a target.

2. Cover Test

Used to detect phorias and tropias (latent and manifest squint). Helpful in diagnosing muscle imbalances.

3. Hess Charting

A graphical test that maps the function of each muscle in binocular vision conditions, useful in diagnosing paresis or restrictions.

4. Forced Duction Test

Used to differentiate between a paretic and a mechanically restricted eye muscle.

Conclusion

The extraocular muscles are essential for precise and coordinated eye movements. Their functions allow us to explore our environment, maintain binocular vision, and stabilize gaze. Any defect in the muscles, their nerve supply, or control centers can lead to significant visual disturbances. Therefore, understanding their anatomy, physiology, and control is critical for students of optometry and eye care professionals in diagnosing and treating ocular motility disorders.

Coats of the Eyeball

Introduction

The eyeball is a complex, spherical sensory organ that is responsible for the sense of vision. Anatomically and functionally, the eye is organized into three concentric layers or coats: the outer fibrous coat, the middle vascular coat (uvea), and the inner nervous coat (retina). Each coat has a distinct structure and function that contributes to the overall physiology of vision. Understanding these coats is essential in studying ocular anatomy, physiology, and pathology, as many diseases originate or affect specific layers of the eye.

I. Overview of the Three Coats

Coats of Eyeball

The three coats of the eyeball from outermost to innermost are:

Fibrous Coat (Outer Layer) – Consists of the cornea and sclera.
Vascular Coat or Uveal Tract (Middle Layer) – Composed of the iris, ciliary body, and choroid.
Nervous Coat (Inner Layer) – The retina, which houses photoreceptor cells.

II. Fibrous Coat (Outer Layer)

1. Cornea

The cornea is the transparent, anterior part of the fibrous coat. It is avascular and convex in shape. It forms about 1/6th of the outer coat.

Structure of Cornea (5 Layers)

Epithelium: Outermost layer of non-keratinized stratified squamous cells, providing a smooth optical surface and barrier to infection.
Bowman’s Layer: Acellular collagen layer that adds structural support.
Stroma: Makes up 90% of corneal thickness; composed of regularly arranged collagen fibers for transparency.
Descemet’s Membrane: Basement membrane of the endothelium; regenerates if damaged.
Endothelium: Innermost single layer of hexagonal cells; responsible for fluid and ion transport to maintain corneal clarity.

Functions of the Cornea

Major refractive surface of the eye (~70% of total refractive power).
Protects intraocular contents from external insults.
Allows transmission of light to the retina.

2. Sclera

The sclera forms the posterior 5/6th of the fibrous coat. It is white, opaque, and dense connective tissue.

Features of the Sclera

Provides structural strength and maintains the shape of the eyeball.
Serves as an attachment for extraocular muscles.
Contains perforations for passage of optic nerve (at the lamina cribrosa), blood vessels, and nerves.

Important Parts of the Sclera

Limbus: Junction between cornea and sclera.
Lamina Cribrosa: Sieve-like area through which the optic nerve fibers exit the eye.

III. Vascular Coat (Middle Layer / Uveal Tract)

The vascular coat is responsible for nourishing the eye, regulating light entry, and producing aqueous humor. It includes:

1. Iris

The iris is the colored part of the eye located in front of the lens. It regulates the amount of light entering the eye by controlling pupil size.

Muscles of the Iris

Sphincter Pupillae: Circular muscle, constricts pupil (parasympathetic control).
Dilator Pupillae: Radial muscle, dilates pupil (sympathetic control).

Functions

Controls pupil size.
Prevents light scattering inside the eye.
Contributes to eye color (pigmentation).

2. Ciliary Body

Located behind the iris, it connects the iris to the choroid. It includes:

Ciliary Muscle: Controls accommodation by changing lens shape.
Ciliary Processes: Secretes aqueous humor into the posterior chamber.

Functions

Accommodation for near vision.
Aqueous humor production and maintenance of intraocular pressure.
Suspends the lens through zonules (suspensory ligaments).

3. Choroid

The choroid is a highly vascular and pigmented layer located between the sclera and retina. It extends from the ora serrata to the optic disc.

Functions

Provides oxygen and nutrients to the outer retina.
Absorbs stray light to prevent internal reflection and image distortion.
Helps regulate the thermal environment of the eye.

IV. Nervous Coat (Inner Layer / Retina)

The retina is the light-sensitive inner coat of the eye responsible for converting light into neural signals. It lines the posterior two-thirds of the eyeball.

Layers of Retina (10 Layers from Outer to Inner)

Retinal Pigment Epithelium (RPE)
Photoreceptor Layer (rods and cones)
External Limiting Membrane
Outer Nuclear Layer
Outer Plexiform Layer
Inner Nuclear Layer
Inner Plexiform Layer
Ganglion Cell Layer
Nerve Fiber Layer
Internal Limiting Membrane

Photoreceptors

Rods: Night vision (scotopic), found mostly in the peripheral retina.
Cones: Daylight and color vision (photopic), concentrated in the macula and fovea.

Specialized Areas of Retina

Macula: Area for detailed central vision.
Fovea Centralis: Center of the macula with the highest visual acuity.
Optic Disc: Blind spot where the optic nerve exits; no photoreceptors.

Functions of Retina

Captures light and initiates visual transduction.
Processes visual signals before they reach the brain.
Provides color, contrast, and motion perception.

V. Functional Integration of the Three Coats

Though each layer has distinct anatomy and physiology, they function together to enable clear and protected vision:

The fibrous coat provides rigidity and initial refraction.
The vascular coat maintains internal homeostasis and nourishes ocular tissues.
The nervous coat transforms light energy into nerve impulses that are sent to the brain.

VI. Clinical Correlation: Disorders Related to Eye Coats

1. Fibrous Coat Disorders

Corneal Ulcer: Infection or trauma leading to stromal loss and scarring.
Corneal Edema: Swelling due to endothelial dysfunction.
Scleritis: Inflammation of the sclera, often painful and associated with systemic autoimmune diseases.

2. Vascular Coat Disorders

Uveitis: Inflammation of the iris, ciliary body, or choroid.
Choroidal Neovascularization: Abnormal vessel growth under the retina, common in age-related macular degeneration.
Glaucoma: Damage to optic nerve due to increased intraocular pressure often related to aqueous humor dynamics.

3. Nervous Coat Disorders

Retinitis Pigmentosa: Degeneration of photoreceptors causing night blindness.
Diabetic Retinopathy: Vascular damage in retina due to chronic hyperglycemia.
Macular Degeneration: Age-related damage to the macula leading to central vision loss.

Conclusion

The three coats of the eyeball are intricately designed to support and sustain the function of vision. The fibrous layer forms a protective shell and refractive surface; the vascular coat regulates nutrition, intraocular fluid dynamics, and pupil function; and the neural coat is the actual light-processing unit. A clear understanding of these layers provides a strong foundation for diagnosing and managing ocular diseases effectively. For optometry students, mastering the coats of the eye is an essential step in becoming competent eye care professionals.

Cornea – Structure and Physiology

Introduction

The cornea is the transparent, avascular, dome-shaped anterior part of the eye that serves as the main refractive surface. Though its anatomical structure is well-known, the physiology of the cornea is equally crucial in maintaining its transparency, optical function, and immune protection. Corneal physiology encompasses its nourishment, metabolism, oxygenation, hydration balance, barrier function, transparency mechanisms, innervation, and wound healing. A thorough understanding of corneal physiology is vital for recognizing and managing corneal disorders effectively.

I. General Features of the Cornea

Diameter: ~11.5 mm vertically and 12 mm horizontally.
Thickness: ~550 microns centrally and ~700 microns peripherally.
Refractive Power: About 43 diopters (accounts for 2/3 of total eye’s refractive power).
Avascular: Nutrients and oxygen are derived from tear film, aqueous humor, and limbal vasculature.
Highly innervated: Contains 300–600 times more nerve endings than the skin.

II. Layers of the Cornea

Microscopic Layer of Cornea

From anterior to posterior, the cornea has six distinct layers:

Epithelium
Bowman’s Layer
Stroma
Dua’s Layer (recently discovered)
Descemet’s Membrane
Endothelium

Each layer has a specific physiological role in maintaining corneal transparency and function.

III. Physiology of the Corneal Epithelium

The corneal epithelium consists of 5–7 layers of non-keratinized squamous epithelial cells. It is the most superficial layer and serves as a barrier and regulator of fluid transport.

1. Barrier Function

The tight junctions in the surface epithelial cells prevent pathogens and toxins from entering the corneal stroma.
It resists the penetration of fluorescein dye, bacteria, and drugs.

2. Tear Film Interaction

The epithelium maintains surface wettability, crucial for tear film adhesion.
Mucin produced by conjunctival goblet cells and membrane-bound mucins (MUC1, MUC4) help in tear spreading.

3. Epithelial Turnover

Complete epithelial renewal occurs every 7–10 days.
Basal cells proliferate and differentiate as they migrate upward, eventually being shed into the tear film.

4. Sensory Innervation

Extremely rich sensory innervation via branches of the ophthalmic division of the trigeminal nerve.
Supports reflex tearing and blinking, essential for protection and homeostasis.

IV. Physiology of the Corneal Stroma

The stroma comprises ~90% of corneal thickness and consists of parallel collagen lamellae, keratocytes, and ground substance.

The Schematic of Corneal Transparency

1. Transparency Mechanism

The uniform arrangement of collagen fibrils with precise spacing (less than the wavelength of light) leads to destructive interference of scattered light, allowing transparency.
Keratocytes and extracellular matrix regulate hydration and maintain collagen alignment.

2. Hydration Balance

The stroma must remain at 78% water content for optimal transparency. Both the epithelium and endothelium regulate stromal hydration:

Swelling pressure and imbibition pressure are balanced by endothelial pump action.
Any disruption causes corneal edema, leading to haziness.

V. Physiology of Dua’s Layer

This recently discovered layer lies between the posterior stroma and Descemet's membrane. It is acellular and made of strong collagen fibers.

Plays a role in posterior corneal strength and surgical planes in corneal transplantation.

VI. Physiology of Descemet’s Membrane

It is a thick basement membrane secreted by the endothelium. It is elastic, regenerates throughout life, and serves as a strong structural support.

It offers resistance against trauma and infections spreading from the anterior chamber to the stroma.

VII. Physiology of the Corneal Endothelium

This is a single layer of hexagonal cells that do not regenerate. Its physiological role is crucial in maintaining deturgescence (relative dehydration) of the cornea.

1. Endothelial Pump Function

Na+/K+ ATPase pumps create ionic gradients, facilitating water transport from stroma to aqueous humor.
This active pumping mechanism prevents stromal edema and maintains corneal clarity.

2. Barrier Function

Though not as tight as epithelial junctions, endothelial cells limit bulk fluid flow.

3. Aging and Cell Loss

Endothelial cells decrease with age (from ~4000 cells/mm² at birth to ~2000 cells/mm² in the elderly).
Once below 500 cells/mm², corneal decompensation and edema may occur.

VIII. Corneal Transparency – Physiological Basis

Absence of blood vessels and pigmentation.
Regular collagen fibril arrangement in the stroma.
Non-myelinated nerves.
Constant hydration at 78% maintained by endothelial pump and epithelial barrier.
Absence of cellular organelles in central optical zone.

IX. Oxygen and Nutrient Supply

1. Sources of Oxygen

Open Eye: Atmospheric oxygen dissolved in tear film (main source).
Closed Eye: Palpebral conjunctival vessels and limbal vessels.
Hypoxia: Wearing tight contact lenses reduces oxygen diffusion, leading to corneal swelling (hypoxic edema).

2. Nutrients

Glucose is derived from aqueous humor and is metabolized by epithelial and endothelial cells for energy.
Amino acids and vitamins also enter via diffusion from aqueous humor.

X. Metabolism of the Cornea

1. Aerobic and Anaerobic Glycolysis

Majority of glucose is metabolized anaerobically (~85%).
Only a small percentage undergoes aerobic respiration in mitochondria of epithelium and endothelium.

2. Pentose Phosphate Pathway

Important in producing NADPH for lipid synthesis and antioxidant defense (especially in epithelial cells).

3. Lactate Production

Lactate accumulates as a by-product of anaerobic metabolism and is transported out by endothelial pumps.

XI. Innervation of the Cornea

The cornea is among the most richly innervated tissues in the human body. Nerves are derived from the ophthalmic division of the trigeminal nerve.

1. Types of Sensory Nerve Endings

Mechanoreceptors: Detect touch and pressure.
Polymodal Nociceptors: Respond to mechanical, chemical, and thermal stimuli.
Cold Receptors: Detect changes in temperature and dryness.

2. Functions

Trigger reflex blinking and lacrimation.
Modulate epithelial healing and trophic maintenance.

XII. Corneal Wound Healing

1. Epithelial Healing

Involves migration, proliferation, and differentiation of basal cells.
Healing is rapid, usually within 24–48 hours.

2. Stromal Healing

Involves keratocyte activation and collagen remodeling.
Improper healing can lead to scarring and loss of transparency.

3. Endothelial Response

Limited mitotic potential. Healing occurs by cell enlargement and spreading.

XIII. Immune Privilege of the Cornea

Lack of blood vessels prevents immune cell infiltration.
Expression of immunosuppressive factors (e.g., Fas ligand) limits inflammation.
Helps in graft survival after corneal transplant.

XIV. Contact Lens Physiology and Cornea

Wearing contact lenses alters the normal physiology of the cornea:

Reduces oxygen supply leading to hypoxia.
Alters tear film dynamics and epithelial cell metabolism.
Long-term use may lead to neovascularization or corneal warpage.

Conclusion

The physiology of the cornea is a finely balanced system that ensures its transparency, protection, nourishment, and optical performance. From its avascular structure and highly specialized endothelium to its precise metabolic and pumping functions, every aspect of the cornea is tailored for clarity and vision. An in-depth understanding of corneal physiology helps optometrists diagnose and manage conditions like dry eye, edema, infections, dystrophies, and refractive errors more effectively. This knowledge is especially critical when dealing with contact lens use, refractive surgery, or corneal transplants.

Aqueous Humor, Vitreous Humor and Intraocular Pressure – Physiology

Introduction

The eye contains two types of intraocular fluids: the aqueous humor and the vitreous humor. These fluids play critical roles in maintaining the optical clarity of the eye, providing nutrition to avascular structures, maintaining the shape of the globe, and regulating intraocular pressure (IOP). The physiology of aqueous production, circulation, and drainage is directly tied to IOP regulation — a key factor in the pathogenesis of diseases like glaucoma. Meanwhile, the vitreous humor provides structural and metabolic support to the posterior segment of the eye. This article focuses on the detailed physiological mechanisms associated with aqueous and vitreous humor, and intraocular pressure.

I. Aqueous Humor – Physiology

1. Definition and Characteristics

Aqueous humor is a clear, watery fluid that fills the anterior and posterior chambers of the eye. It is produced continuously by the ciliary processes and is crucial in:

Nourishing avascular structures like the lens and cornea
Maintaining intraocular pressure (IOP)
Removing metabolic waste
Supporting optical transparency

Volume: ~250 µL
Production rate: 2.5 – 2.75 µL/min
pH: ~7.2
Osmolality: Slightly hypertonic compared to plasma

2. Site of Production

The aqueous humor is produced by the non-pigmented ciliary epithelium of the ciliary body, located in the pars plicata region.

3. Mechanisms of Aqueous Humor Formation

Mechanism of Aqueous Humour Formation

Aqueous humor formation involves three primary physiological mechanisms:

i. Diffusion

Passive movement of small, lipid-soluble solutes across a concentration gradient from ciliary stromal capillaries to the posterior chamber.

ii. Ultrafiltration

Hydrostatic pressure difference between capillary blood and intraocular space forces plasma-derived fluid across fenestrated capillaries into the stroma.
Limited to large molecules; contributes ~20% of aqueous volume.

iii. Active Secretion (Primary Mechanism)

Accounts for 80–90% of aqueous production.
Involves active transport of ions (mainly Na+, Cl-, HCO₃⁻) by Na⁺/K⁺-ATPase, carbonic anhydrase, and ion co-transporters.
Osmotic gradient created pulls water into the posterior chamber.

4. Composition of Aqueous Humor

Aqueous humor contains electrolytes, amino acids, glucose, ascorbic acid, and small amounts of proteins.

Lower protein content than plasma (important for transparency)
High levels of ascorbate (Vitamin C) – antioxidant role
Low immunoglobulins – maintains immune privilege

5. Aqueous Humor Circulation

Secreted by ciliary processes into the posterior chamber
Flows between the lens and iris through the pupil into the anterior chamber
Drains via conventional or uveoscleral pathways

Factors Facilitating Circulation:

Temperature gradient (warmer iris vs cooler cornea)
Eye movements and blinking
Pupil movement (dilation/constriction)

6. Aqueous Humor Drainage Pathways

Aqueous Humour Drainage Pathways

i. Trabecular (Conventional) Outflow – 85% of drainage

Aqueous exits through:

Trabecular meshwork → Schlemm’s canal → collector channels → episcleral veins
Pressure-dependent system (regulated by IOP)

ii. Uveoscleral (Unconventional) Outflow – 10–15%

Flows through ciliary muscle spaces → suprachoroidal space → scleral veins
Pressure-independent and affected by age, drugs, prostaglandins

7. Regulation of Aqueous Humor Dynamics

Sympathetic stimulation: Increases aqueous production via β2-receptors
Parasympathetic stimulation: Enhances trabecular outflow by contracting ciliary muscle
Drugs: Carbonic anhydrase inhibitors, β-blockers, and prostaglandin analogs affect production and outflow

8. Physiological Role of Aqueous Humor

Maintains intraocular pressure and globe shape
Provides nutrition to cornea, lens, and trabecular meshwork
Removes metabolic waste products
Supports the immune privilege of the anterior chamber

II. Vitreous Humor – Physiology

1. Definition and Characteristics

Vitreous Humour

The vitreous humor is a transparent, gel-like substance filling the posterior segment of the eye, between the lens and the retina. It occupies ~80% of the total eye volume (~4 mL).

99% water + 1% collagen, hyaluronic acid, proteins, and cells (hyalocytes)
Viscoelastic and transparent
Attached to retina (strongest at vitreous base, optic disc, and macula)

2. Composition

Water: 99%
Collagen: Mostly Type II (forms a fibrillar network)
Hyaluronic Acid: Gives gel structure, retains water
Cells: Hyalocytes (immune role), fibroblasts
Other solutes: Glucose, electrolytes, proteins, ascorbate

3. Development and Aging

Formed during embryonic life (primary and secondary vitreous)
Age-related liquefaction (synchysis) begins in middle age → posterior vitreous detachment (PVD)

4. Functions of the Vitreous

Maintains spherical shape of the eyeball
Acts as a shock absorber for the retina
Serves as a metabolic reservoir for the lens and retina
Supports optical clarity and light transmission
Maintains attachment between retina and RPE
Barrier to cell and protein movement between anterior and posterior segments

5. Metabolic Functions

Stores glucose and oxygen for inner retinal metabolism
Contains high ascorbate levels – antioxidant defense
Regulates growth factors, cytokines during inflammation or retinal detachment

6. Vitreous and Disease

PVD: Common with aging; may cause floaters or retinal tears
Hemorrhage: Blood accumulation due to diabetic retinopathy or trauma
Endophthalmitis: Infections can affect vitreous and threaten vision

III. Intraocular Pressure (IOP) – Physiology

Intraocular Pressure

1. Definition and Normal Range

IOP is the fluid pressure inside the eye
Normal Range: 10–21 mmHg (mean ~15 mmHg)
Measured by tonometry

2. Determinants of IOP

Rate of aqueous humor formation
Resistance to aqueous outflow (mainly at trabecular meshwork)
Episcleral venous pressure

3. Aqueous Humor and IOP

As aqueous is produced and drained, a pressure gradient is established. A balance between production and outflow determines IOP.

Increased resistance to outflow → increased IOP (e.g., in glaucoma)
Decreased production or enhanced outflow → decreased IOP

4. Factors Influencing IOP

Age (IOP increases with age)
Diurnal variation (higher in the morning)
Posture (lying down increases IOP)
Blood pressure and systemic medications
Stress and physical activity

5. Clinical Relevance of IOP

i. Glaucoma

Chronic elevation of IOP damages optic nerve → visual field loss
Open-angle glaucoma: resistance at trabecular meshwork
Angle-closure glaucoma: blocked access to drainage angle

ii. Ocular Hypotony

Abnormally low IOP (<6 li="" mmhg="">
May cause retinal detachment, choroidal effusion, and vision loss

6. Measurement of IOP

Goldmann Applanation Tonometry – Gold standard
Non-contact tonometry (air puff)
Indentation tonometry (Schiotz)

7. Drugs That Affect IOP

β-blockers: Decrease aqueous production
Prostaglandin analogs: Increase uveoscleral outflow
Carbonic anhydrase inhibitors: Reduce aqueous secretion
Alpha agonists: Dual effect (reduce production, increase outflow)

8. Autoregulation

The eye uses autoregulation to maintain stable perfusion despite changes in IOP and blood pressure. This ensures consistent oxygen supply to the retina and optic nerve.

Conclusion

The physiology of the aqueous humor, vitreous humor, and intraocular pressure plays a central role in maintaining ocular health and vision. Aqueous humor dynamics maintain corneal and lens nutrition, intraocular pressure, and optical clarity. The vitreous body supports retinal integrity and acts as a metabolic buffer for the posterior segment. Proper regulation of intraocular pressure is crucial to prevent optic nerve damage and glaucoma. A thorough understanding of these physiological processes enables optometrists and ophthalmologists to assess, diagnose, and treat various anterior and posterior segment pathologies effectively.

For more units of Ocular Physiology click below on text 👇

👉 Unit 2

👉 Unit 3

👉 Unit 4

👉 Unit 5