The Eyeball and Accessory Structures

Eyeball Location and Structure

  • Eyeballs are located in the orbits on either side of the nasal cavity.

  • The eyeball occupies the anterior one-third of the orbit.

  • Adipose tissue in the posterior two-thirds of the orbit provides protective cushioning.

Accessory Structures of the Eye

  • These structures protect and support the eyeball.

Eyelids (Palpebrae)

  • Thin folds covering the anterior region of the orbit.

  • Prevent entry of foreign objects.

  • Help distribute tears during blinking.

Tarsal Plates

  • Thin pieces of dense, regular collagenous connective tissue.

  • Reinforce each eyelid.

Tarsal Glands

  • Modified sebaceous glands within the tarsal plates.

  • Secrete oil to prevent eyelids from sticking together.

Medial and Lateral Commissures (Canthi)

  • Where the upper and lower eyelids meet.

Lacrimal Caruncle

  • Fleshy structure at the medial commissure.

  • Contains sebaceous glands that secrete a whitish lubricating substance.

Orbicularis Oculi

  • Muscle responsible for closing the eyelids.

Levator Palpebrae Superioris

  • Muscle that elevates the upper eyelid.

Eyebrows

  • Hairs on the ridge of the brow.

  • Prevent sweat from entering the eye.

  • Reduce glare from bright light.

  • Important for facial expression.

Eyelashes

  • Stiff hairs on the edge of the eyelids.

  • Sensitive nerve endings cause blinking when touched by objects.

  • Reduce eye injury.

Conjunctiva

  • Thin, continuous epithelial membrane.

  • Lines the posterior surface of the eyelid (palpebral conjunctiva) and the anterior surface of the eyeball (bulbar or ocular conjunctiva), except for the cornea.

  • Translucent with tiny blood vessels.

  • The palpebral conjunctiva covers the inner surface of the eyelid and folds back to form the bulbar conjunctiva, which covers the sclera.

Lacrimal Apparatus

  • Produces and drains tears from the eye.

Lacrimal Gland

  • Releases tears and mucus into lacrimal ducts.

Lacrimal Ducts

  • Enter the conjunctival sac.

  • Stimulated by autonomic neurons.

  • Lubricate and wash away debris.

  • Blinking sweeps tears medially and inferiorly across the eye.

Lacrimal Puncta

  • Two small dots on the medial edge of each eyelid.

  • Drain tears into the lacrimal canaliculi.

Lacrimal Canaliculi

  • Drain into the lacrimal sac.

Lacrimal Sac

  • Drains into the nasolacrimal duct.

Nasolacrimal Duct

  • Drains into the nasal cavity, causing the nose to run when crying.

Extrinsic Eye Muscles

  • Six muscles that control eye movement.

Rectus Muscles

  • Superior, inferior, lateral, and medial rectus muscles.

  • Named for their movements of the eye.

Oblique Muscles

  • Superior and inferior oblique muscles.

  • The superior oblique travels from the posterior eye and hooks through the trochlea before inserting on the top of the eye.

  • Contraction of the superior oblique depresses the eye and moves it laterally.

Cranial Nerve Innervation

  • Cranial nerve IV (trochlear nerve) innervates the superior oblique muscle.

  • Cranial nerve VI (abducens nerve) innervates the lateral rectus muscle.

  • Cranial nerve III (oculomotor nerve) innervates the remaining four muscles.

Strabismus (Lazy Eye)

  • Eyeballs are not properly aligned.

  • Can lead to diplopia (double vision).

  • Corrected with glasses or surgery.

Layers of the Eyeball

  • The eyeball is a hollow sphere with chambers to support the lens and focus light onto the retina.

Fibrous Layer

  • Outermost layer.

Sclera

  • The white part of the eye.

  • Covers the eye entirely except for the cornea.

  • Made of irregularly arranged collagen fibers.

  • Resists deformation from external and internal forces.

  • Maintains the shape of the eye.

Cornea

  • The anterior-most part of the eye.

  • Translucent due to parallel arrangement of collagen fibers.

  • Avascular to maintain clarity for light transmission.

Vascular Layer

  • Middle layer, directly beneath the cornea and sclera.

Choroid

  • Most extensive component.

  • Contains capillaries and pigment.

  • Pigment minimizes scattering of incoming light rays.

Ciliary Body

  • Ring of smooth muscle surrounding the lens.

  • Produces aqueous humor.

Suspensory Ligaments

  • Connect the ciliary body to the lens.

  • Allow contraction and relaxation of the lens to change shape for focusing light.

Iris

  • The colored region of the anterior eye.

  • An extension of the vascular layer.

  • Varying amounts of pigment determine eye color (more pigment = brown eyes, less pigment = blue eyes).

Pupil

  • Opening in the center of the iris.

  • Light enters the eye through the pupil.

  • Diameter controlled by two muscles:

    • Pupillary Sphincter Muscle: Contracts during parasympathetic stimulation to reduce pupil size and reduce the amount of the light entering the eye.

    • Pupillary Dilator Muscle: Controlled by sympathetic activation to increase pupil size and allow more light to enter, so that we can see possible dangers.

Neural Layer (Retina)

  • Innermost layer.

Superficial Layer

  • Thin pigmented epithelium.

  • Reduces light scattering and nourishes photoreceptors.

Deep Layer

  • Consists of photoreceptor cells that form the optic nerve.

Macula Lutea

  • Yellowish region on the back of the eye.

Fovea Centralis

  • Located deep to the macula lutea.

  • Area of concentrated cones for day vision and high visual acuity.

  • Tightly packed cones provide detailed vision.

Macular Degeneration

  • Progressive loss of visual acuity, particularly in the center of the visual field.

  • Causes distortion and changes in color perception.

Optic Disc

  • Area where axons exit the retina to form the optic nerve.

  • Lacks photoreceptors, creating a blind spot.

Retinal Detachment

  • The inner layer of the retina separates from the pigmented epithelium due to trauma or diabetes.

  • Requires lying flat in the dark for weeks to reattach the retina properly.

  • Separation of photoreceptors from their blood supply can cause permanent vision loss.

Testing the Blind Spot

  • The brain fills in the blind spot, so it is not normally perceived.

Lens

  • Slightly flattened sphere behind the pupil and iris.

  • Focuses light on the retina from near objects.

  • Connected to the ciliary body by suspensory ligaments.

  • Contains tightly packed lens fibers lacking nuclei, making it translucent.

Posterior Cavity

  • Filled with vitreous humor, a gelatinous material made mostly of collagen and water.

  • Presses the retina against the choroid to keep it attached.

  • Helps maintain the shape of the eyeball.

Anterior Cavity

  • From the lens to the cornea, divided into anterior and posterior chambers filled with aqueous humor.

Posterior Chamber

  • Between the lens and the iris.

Anterior Chamber

  • From the iris to the cornea.

Aqueous Humor

  • Made by the ciliary body, flows through the posterior chamber, through the pupil to the anterior chamber.

  • Drains out of the anterior chamber through the scleral venous sinus (canal of Schlemm).

Glaucoma

  • Aqueous humor cannot drain, causing fluid build-up in the anterior and posterior chambers.

  • Elevated intraocular pressure compresses and damages the retina and optic nerve.

  • Can lead to blindness due to lack of blood supply to photoreceptors.

  • Caused by eye infections, certain medications, congenital defects in the scleral venous sinus, or unknown reasons.

  • Cannot be restored, but progression can be slowed with medications or surgery.

Cataracts

  • Clouded lens due to trauma, UV radiation exposure, diabetes, or aging.

  • Cannot be reversed but can be treated surgically by removing the clouded lens and replacing it with a new one.

Light and Refraction

  • Visible light is electromagnetic radiation with wavelengths that can be seen.

  • Photons are basic units of light that stimulate photoreceptors.

  • Light rays can be bent or refracted when passing through a translucent object.

  • The refractive index measures the amount of refraction exerted on light rays.

    • Air has a refractive index of one.

    • Water has a higher refractive index.

  • The lens has a similar refractive index to water.

  • The cornea and lens help to focus light onto the retina.

  • Refraction depends on the angle at which light strikes the surface; curved surfaces bend light more at their edges.

Lens Types

  • Convex lens: Bulges in the middle and causes light rays to converge.

  • Concave lens: Thicker on the edges and depressed in the middle, causing light rays to diverge.

  • Focal point: When light rays converge on one point, it is said to be focused.

Focusing Light on the Retina

  • The lens and cornea help focus light on the retina.

  • Two-thirds of the eye's refractive power comes from light passing through the cornea.

  • The lens is used for fine-tuning and refractive adjustment.

Amotropic State

  • When the eye is relaxed, focusing on distant objects.

  • Parallel light rays are minimally refracted by the cornea and focused on the retina.

Accommodation

  • Light rays from closer objects need more refraction.

  • The lens becomes thicker to refract light more than the flattened lens.

Ciliary Muscle and Suspensory Ligaments

  • In a relaxed state, the ciliary muscle pulls on the suspensory ligament, resulting in a flattened lens shape.

  • To see something up close, the ciliary muscle contracts.

  • This reduces the strain on the suspensory ligaments, allowing the lens to fatten up.

  • A fatter lens has a higher refractive power, allowing convergence of light rays onto the retina.

20-20-20 Rule

  • Every 20 minutes, look away for 20 seconds and visualize an object 20 feet away.

  • This allows the ciliary body to relax, reducing eye strain and headaches.

Pupillary Constriction and Convergence

  • Pupillary constriction decreases scattered light that would make vision blurry.

  • Convergence involves moving the eyes more medially to focus light onto the fovea centralis, where visual acuity is highest.

Errors of Refraction

  • Can be due to aging or the shape of the eyeball.

Point of Accommodation

  • The closest point at which the eye can focus on an object.

  • Decreases with age as the lens becomes less flexible.

Presbyopia

  • The near point of accommodation is 10-20 inches or greater.

  • Difficulty reading; correctable with reading glasses or bifocals.

Emmetropia

  • Ideal length of the eyeball in the anterior-posterior direction for light to focus directly on the retina.

Hyperopia (Farsightedness)

  • Eyeball is too short or cornea is too flat.

  • The lens is unable to accommodate enough to focus light on the retina.

  • Light focuses behind the retina, resulting in blurry vision.

  • Convex lenses converge light rays to focus them on the retina.

Myopia (Nearsightedness)

  • The distance between the cornea and the lens is too great, or the cornea is too curved.

  • The lens isn't able to flatten enough, and incoming light focuses in front of the retina.

  • Concave lenses diverge incoming light before it contacts the lens to redirect it to focus on the retina.

Astigmatism

  • Irregular curvature of the lens or cornea.

  • Light rays are not evenly refracted, causing blurred vision at all distances.

  • Corrective lenses or LASIK surgery can flatten out or correct the irregularity of the cornea or lens.

Photoreceptors: Rods and Cones

  • Located next to the pigmented layer of the retina.

Cones

  • Best in bright light for processing high-resolution color vision.

Rods

  • Do not detect colors.

  • Most sensitive in low light and provide peripheral vision.

Neural Connections

  • Photoreceptor cells synapse with bipolar cells, which communicate with ganglion cells.

  • Ganglion cells are in the anterior-most region of the retina and form cranial nerve II (optic nerve).

  • Horizontal and amacrine cells help with image processing.

Rod Structure and Function

  • Contain thousands of flattened disks with the pigment rhodopsin, which absorbs light.

  • All rods contain rhodopsin, so they do not distinguish between different wavelengths of light.

  • Rhodopsin consists of the protein component opsin and the pigment retinal, which is derived from vitamin A.

  • In the dark, retinal is in a bent configuration (cis-retinal).

Cone Structure and Function

  • Cones contain iodopsin, which consists of retinal and photopsin.

  • Photopsin is similar to opsin but has a different structure that allows it to absorb different wavelengths of light (red, blue, or green).

Phototransduction

  • The process of converting light into electrical signals.

  • When a photon encounters a disk in a rod or cone:

    • In the absence of stimulation (in the dark), photoreceptor cells are depolarized and continuously release neurotransmitters.

    • In the presence of light, photoreceptors become hyperpolarized and stop releasing neurotransmitters.

    • This alters the activity of neighboring retinal cells, which send information to the brain.

G Protein-Coupled Receptors

A detailed mechanism of G-protein coupled receptor activation is presented in the transcript.

Light and Dark Adaptation

  • The process of adjusting to changes in the amount of light in the environment.

Dark Adaptation

  • Occurs when light is suddenly reduced.

  • Rhodopsin regeneration is slow, so it takes time for rods to become fully functional.

  • It can take up to 40 minutes for rods to be completely functional.

  • The longer you are in the dark, the easier it is to see over time.

Light Adaptation

  • Occurs when light is suddenly increased.

  • Bleaches the rods and cones, causing a blinding glare.

  • Rods become nonfunctional because rhodopsin is bleached as fast as it can be regenerated.

  • Cones regenerate faster, reaching full function within about 90 seconds.

Image Processing in the Retina

*In the dark:
* The photoreceptor is depolarized and releases glutamate onto bipolar cells.
* Glutamate inhibits the bipolar cell, reducing its release of neurotransmitter.
* Ganglion cells do not produce action potentials, so no signals are sent to the brain.

*In the light:
* Light hyperpolarizes the photoreceptor, and it stops releasing glutamate.
* The bipolar cell is free and depolarizes.
* Bipolar cell sends a neurotransmitter to the ganglion cells.
* Ganglion cells produce action potentials, and information is sent to the brain via the optic nerve.

Color Blindness

  • Occurs when someone lacks a functional gene for one or more of the cone pigments.

  • Cannot see the colors that would be picked up by the affected cone(s).

  • The most common is a defective gene for red or green pigment.

  • Both colors appear grayish brown.

  • 8-10% of males are more commonly affected, compared to fewer than 1% of females.

  • This is because it is sex-linked on the X chromosome.

  • Men have only one copy of the X chromosome, while women have two, which can override the bad gene.

Visual Pathway

  • Light from a specific direction crosses (decussates) at the optic chiasm.

  • Information from each side of the visual field goes to the opposite side of the brain.

Optic Tracts

  • Carry visual stimuli.

  • All visual stimuli from the left visual field go to the right optic tract.

  • All visual stimuli from the right visual field go to the left optic tract.

Pathway

  • Axons in the optic tracts synapse in the lateral geniculate nucleus (LGN) of the thalamus.

  • Neurons from the LGN project to the primary visual cortex in the occipital lobe.

  • Then information goes to the association area for further processing and fine-tuning of eye movements.

Additional Image Processing in the Brain

  • Helps interpret motion, process colors, and form the different objects.

Dorsal Pathway

  • Goes to the parietal lobe and interprets motion.

Ventral Pathway

  • Goes to the temporal lobe and processes colors and forms of objects.

Consensual Pupillary Response

  • Both pupils constrict in response to light entering one eye.

  • Axons terminate in the midbrain and communicate with the oculomotor nerve, which innervates the pupillary muscles in both eyes.

  • Prevents damage to the retina, optic nerve, and brainstem.

Stereoscopic Vision

  • Depth perception, the perception of the distance of an object from the eyes.

  • Since both eyes face the same way, our visual fields overlap.

  • This is called binocular vision.

  • The brain compares the overlapping information to determine the distance of an object from the eyes.

Summary of the visual Pathway

  • Light comes in and is refracted.

  • Photoreceptors release the break off the bipolar calls so they can stimulate other ganglian cell, and then send an action potential out through the optic nerve.

  • Information is decussated at the optic chiasma and combined so you could get a full picture.

  • Goes to the thalamus and goes to the primary visual cortex in the occipital lobe.