EYE

EXERCISE 14: THE EYE

Overview of the Eye

• The eye is located in a bony socket of the skull. It converts light energy into a visual experience.

• Two eyes provide depth perception, spatial awareness, and color vision.

• Comparison with a camera:   - Camera design is modeled after the eye's structure.   - Differences: human eye has a vascularized retina instead of film and is a living organ.

Objectives

1. Identify the structural components of the eye using models, diagrams, or specimens.

2. Describe the function of light detection cells and understand the neural pathways for visual information to the brain.

3. Compare the human eye with the cow eye.

I. The Accessory Structures

• The accessory structures include parts that protect and support the eye.

• Components:   - Palpebrae (eyelids): Protect about one-sixth of the eye; form medial and lateral canthus.   - Caruncle: Fleshy mound at the medial canthus; secretes oily substance for lubrication.   - Conjunctiva: Mucous membrane lining eyelids and covering eyeball not covered by the cornea.       - Forms secreted mucus; infections (e.g., conjunctivitis).   - Eyelashes: Short hairs providing additional protection.     - Between lashes lies ciliary glands (modified sweat glands) for lubrication.     - Sebaceous glands at eyelash bases secrete sebum, an oily substance.     - Meibomian glands: Located posteriorly to eyelashes; secrete oil preventing evaporation of tears.

• Lacrimal Apparatus:   - Composed of lacrimal gland, canals, sac, and nasolacrimal duct.   - Lacrimal glands, located above and lateral to the eye, secrete dilute salt solution (tears).   - Tears cleanse the eye and contain lysozyme, which has bactericidal properties.   - Aging reduces tear production, increasing susceptibility to infections.

II. Anatomy of the Eye

• Eyeball Tunics:   - Comprised of three tunics:     1. Fibrous Tunic:        - The sclera (white part of the eye) and cornea (transparent anterior part where light enters).     2. Vascular Tunic (Uvea):        - Contains the choroid (dark layer preventing light scattering).        - Anterior part includes ciliary body (attaches to lens) and iris (controls light amount entering eye).   - Iris Functions:     - Contains both radial (dilates pupil) and circular muscles (constricts pupil).     - Color is due to pigmentation; brown indicates pigmentation, while blue indicates lack of pigmentation.

Internal Structure of the Eye

• Sensory Layer (Retina):   - Converts light energy into action potentials sent to the occipital lobe.   - Contains,     - Pigmented epithelial layer (next to choroid).     - Neural layer: Illuminated structure housing rods and cones (photoreceptors).     - Optic Disc: Area where optic nerve exits; referred to as blind spot due to lack of photoreceptors.     - Macula Lutea: Area rich in cones; the center is the fovea centralis, crucial for sharp vision.

• Accommodation Mechanism:   - Focus adjustment achieved by lens shape change:     - Lens becomes thicker for close vision and thinner for distant vision.     - Ciliary Muscle: Attached to lens; its activity alters lens shape.     - Aging leads to presbyopia (loss of elasticity in lens).

Maintenance of Internal Eye Pressure

• The lens divides the eye into two segments: anterior and posterior.

• Anterior Segment:   - Contains aqueous humor: Watery fluid providing pressure and nutrients to the eye.   - Compartmented into anterior and posterior chambers (divided by iris).   - Glaucoma: Condition resulting from poor drainage of aqueous humor leading to high pressure, nerve damage, and potential blindness.

• Posterior Segment:   - Holds vitreous humor: Gel-like substance supporting retina attachment. Floaters (bits of debris) may occur due to aging.

III. Microscopic Analysis and Function of the Retina

• Retinal Structure:   - Two layers: pigmented layer and light-sensitive neural layer.

• Neural Layer Components:   - Photoreceptors (Rods and Cones):     - Rods: Sensitive to low light levels, do not detect color (gray shades only).     - Cones: Function in bright light, differentiate between three wavelengths for color vision.     - Density of cones is highest at fovea, contributing to visual acuity.

• Light Pathway in Retina:   - Incoming light goes through ganglion cell layer, then bipolar, finally reaching rods and cones.   - Stimulation yields action potentials, transmitted through bipolar cells to ganglion cells, forming the optic nerve at the optic disc.

IV. Visual Pathways in the Brain

• Optic Nerve Exit:   - Each optic nerve exits at the posterior of the eyeball, converging at the optic chiasma, where medial fibers cross to opposite sides while lateral fibers remain.   - Optic Tracts:     - Formed post-chiasma; carry visual information to the brain's lateral geniculate nucleus (thalamus) for relay to the visual cortex in the occipital lobe.

• Additional synapses occur with superior colliculus (midbrain) for reflex eye movements.

V. The Extrinsic Muscles of the Eye

• Muscle Types and Functions:   - There are six extrinsic muscles facilitating eye movement:     - Superior Rectus: Elevates eye.     - Superior Oblique: Depresses eye and moves it to the side.     - Inferior Rectus: Depresses eye.     - Inferior Oblique: Elevates and laterally moves eye.     - Lateral Rectus: Moves eye sideways.     - Medial Rectus: Turns eye towards the nose.

• Muscle Naming Definitions:   - Superior: Above, Inferior: Below, Rectus: Straight, Oblique: Angled.

VI. Dissection of the Cow Eye

1. Obtain and prepare the cow eye for dissection.

2. Identify external structures:     - Optic Nerve: Rear of eyeball.     - Cornea: Anterior transparent structure, may appear milky.     - Sclera: White coverage of the eye.

3. Make an incision behind the cornea to extract aqueous humor.

4. Separate the anterior from the posterior segment.

5. Identify internal structures:     - Ciliary Body: Circular structure anterior to lens.     - Lens: Oval and milky in preservation.     - Suspensory Ligaments: Connect the lens to ciliary body.     - Iris: Circular, with a central pupil.

6. Examine the lens and note differences in live versus preserved specimen.

7. Retina: Collapsed mass at the optic disc; can detach during dissection.

8. Choroid Coat: Reflected surface (tapetum lucidum) aids night vision (not present in humans).

9. Clean up after dissection.

Terminology

• Accommodation: Ability to adjust eye focus for distance clarity.

• Aqueous Humor: Maintains intraocular pressure.

• Bipolar Cells: Connect photoreceptor and ganglion cells in retina.

• Caruncle: Fleshy mound in the medial canthus.

• Choroid Layer: Vascularized layer of the uvea.

• Ciliary Body: Contains ciliary muscle; connects to lens.

• Ciliary Gland: Lubricates the eyeball.

• Ciliary Muscle: Changes lens shape for focusing.

• Conjunctiva: Mucous membrane lining the eyelids.

• Cornea: First transparent layer for light entry.

• Extrinsic Eye Muscles: Six muscles allowing directional movement.

• Fovea Centralis: Small area of best visual acuity, contains cones.

• Ganglion Cells: Transmit impulses within the optic nerve.

• Glaucoma: Condition due to excessive intraocular pressure.

• Iris: Controls light entering the eye.

• Lacrimal Apparatus: Produces and drains tears.

• Lateral Geniculate Nucleus: Relay center between optic tract and visual cortex.

• Lens: Focuses light onto retina.

• Macula Lutea: Contains fovea centralis for acute vision.

• Optic Chiasma: Convergence point of optic nerves.

• Optic Disc: Exit point for optic nerve; blind spot.

• Photoreceptors: Sensing light (rods and cones).

• Presbyopia: Age-related focus loss.

• Retina: Sensory layer containing rods/cones.

• Sclera: Outermost layer of the eye.

• Scleral Venous Sinus: Aqueous humor reabsorption site.

• Sebum: Eye-lubricating oily fluid.

• Tapetum Lucidum: Reflective layer in some animals.

• Uvea: Middle layer of the eyeball.

• Visual Cortex: Processing center for visual information.

• Vitreous Humor: Gel-like substance maintaining retinal position.