The Eye - BIOL 2401

Special Senses & The Eye

Special Senses

  • Vision
  • Taste
  • Smell
  • Hearing
  • Equilibrium
  • All use special sensory receptors.

The Eye & Vision

  • 70% of the body's sensory receptors are in the eye.
  • Half of the cerebral cortex is involved in visual processing.
  • Most of the eye is enclosed and protected by a fat cushion and bony orbit.
  • Consists of accessory structures and the eyeball itself.

Accessory Structures of the Eye

  • Protect the eye and aid eye function.
  • Include:
    • Eyebrows
    • Eyelids
    • Conjunctiva
    • Lacrimal apparatus
    • Extrinsic eye muscles
Eyebrows
  • Overlie the supraorbital margins.
  • Functions:
    • Shade the eye from sunlight.
    • Prevent perspiration from reaching the eye.
Eyelids
  • Also called palpebrae; thin, skin-covered folds that protect the eye anteriorly.
  • Separated at the palpebral fissure (slit).
  • Meet in corners at medial and lateral commissures.
  • Lacrimal caruncle located at the medial commissure contains oil and sweat glands.
  • Tarsal plates: connective tissue for the folds, anchoring orbicularis oculi and levator palpebrae superioris muscles.
  • Eyelids blink reflexively every 3–7 seconds.
    • Protection from foreign objects.
    • Spreads secretions to moisten the eye.
  • Eyelashes have innervated follicles.
    • Nerve endings initiate reflex blinking.
  • Lubricating glands:
    • Tarsal (Meibomian) glands: modified sebaceous glands that produce oily secretion to lubricate the lid and eye.
    • Ciliary glands: modified sweat glands between hair follicles.
Conjunctiva
  • Transparent mucous membrane, produces a lubricating mucous secretion.
  • Palpebral conjunctiva: membrane that lines the underside of the eyelids.
  • Bulbar conjunctiva: membrane that covers the white of the eyes (not the cornea).
    • Small blood vessels are found in this membrane; easily seen in "bloodshot" eyes.
  • Conjunctival sac: space between the palpebral and bulbar conjunctiva.
    • Area where contact lens rests.
Lacrimal Apparatus
  • Consists of the lacrimal gland and ducts that drain into the nasal cavity.
  • Lacrimal gland in orbit above the lateral end of the eye.
  • Secretes lacrimal secretion (tears), a dilute saline solution containing mucus, antibodies, and antibacterial lysozyme.
  • Blinking spreads tears toward the medial commissure, where they enter paired lacrimal canaliculi via lacrimal puncta.
  • Tears then drain into the lacrimal sac and nasolacrimal duct, which empties into the nasal cavity.
Extrinsic Eye Muscles
  • Six straplike muscles.
  • Originate from the bony orbit and insert on the eyeball.
  • Enable the eye to follow moving objects, maintain the shape of the eyeball, and hold it in the orbit.
  • Four rectus muscles originate from a common tendinous ring.
    • Names indicate movements: superior, inferior, lateral, and medial rectus.
  • Two oblique muscles move the eye in the vertical plane and rotate the eyeball.
    • Superior and inferior oblique muscles.

Actions of the Extrinsic Eye Muscles and Innervating Cranial Nerves:
* Lateral rectus: Moves eye laterally; controlled by cranial nerve VI (abducens).
* Medial rectus: Moves eye medially; controlled by cranial nerve III (oculomotor).
* Superior rectus: Elevates eye and turns it medially; controlled by cranial nerve III (oculomotor).
* Inferior rectus: Depresses eye and turns it medially; controlled by cranial nerve III (oculomotor).
* Inferior oblique: Elevates eye and turns it laterally; controlled by cranial nerve III (oculomotor).
* Superior oblique: Depresses eye and turns it laterally; controlled by cranial nerve IV (trochlear).

Conjunctivitis
  • Inflammation of the conjunctiva resulting in reddened, irritated eyes
  • Pinkeye: conjunctival infection caused by bacteria or viruses
    • Highly contagious

Structure of the Eyeball

  • The wall of the eyeball contains three tunics:
    • Fibrous layer
    • Vascular layer
    • Inner layer
  • The internal cavity is filled with fluids called humors.
  • The lens separates the internal cavity into anterior and posterior segments.
Fibrous Layer
  • Outermost layer; dense avascular connective tissue.
  • Two regions: sclera and cornea
  • Sclera (white of the eye):
    • Protects and shapes the eyeball.
    • Anchors extrinsic eye muscles.
  • Cornea:
    • Transparent anterior one-sixth of the fibrous layer.
    • Forms a clear window that lets light enter and bends light as it enters the eye.
Vascular Layer (Uvea)
  • Three regions: choroid, ciliary body, and iris
  • Choroid region:
    • Posterior portion.
    • Supplies blood to all layers of the eyeball.
    • Brown pigment absorbs light to prevent scattering, which would cause visual confusion.
  • Anteriorly, the choroid becomes the ciliary body.
    • A thickened ring of tissue surrounding the lens.
    • Ciliary Muscles: smooth muscle bundles that control the shape of the lens.
    • Ciliary zonule (suspensory ligament): holds the lens in position.
  • Iris:
    • The colored part of the eye between the cornea and the lens.
    • Pupil: the central opening that regulates the amount of light entering the eye.
      • Close vision and bright light cause pupils to constrict.
      • Distant vision and dim light cause pupils to dilate.
      • Pupils also dilate when the subject matter is appealing or requires problem-solving skills.
  • Pupil constriction and dilation:
    • Sphincter pupillae: Contraction constricts the pupil (parasympathetic innervation).
    • Dilator pupillae: Contraction dilates the pupil (sympathetic innervation).
Inner Layer (Retina)
  • Originates as an outpocketing of the brain.
  • Contains millions of photoreceptor cells that transduce light energy.
  • Delicate two-layered membrane:
    • Outer pigmented layer.
    • Inner neural layer.
Neural Layer of the Retina
  • Optic disc:
    • The site where the optic nerve leaves the eye.
    • Lacks photoreceptors; referred to as the blind spot.
  • The retina has a quarter-billion photoreceptors of two types:
    • Rods
    • Cones
Rods
  • Dim light, peripheral vision receptors.
  • More numerous and sensitive to light than cones.
  • No color vision or sharp images.
  • Numbers greatest in the periphery.
Cones
  • Vision receptors for bright light.
  • High-resolution color vision.
  • Macula lutea: area lateral to the blind spot; contains mostly cones.
  • Fovea centralis: a tiny pit in the center of the macula lutea that contains all cones, the region with the best visual acuity.
  • Eye movement allows us to focus on an object so that the fovea can pick it up.
Retinal Detachment
  • A condition where retinal layers separate (detach), allowing jellylike vitreous humor to seep between them.
  • Can lead to permanent blindness.
  • Caused by a traumatic blow to the head or sudden stopping of the head during movement (e.g., bungee jumping).
  • Treatment: reattachment of retina with laser surgery.
Chambers of the Eye

  • The lens and ciliary zonule separate the eye into two segments:

    • Posterior segment:
      • Contains vitreous humor, a fluid that:
        • Transmits light.
        • Supports the posterior surface of the lens.
        • Holds the neural layer of the retina firmly against the pigmented layer.
        • Contributes to intraocular pressure.
      • Vitreous humor forms in the embryo and lasts a lifetime.
    • Anterior segment:
      • Divided into 2 chambers:
        Anterior chamber: between the cornea and the iris.
        Posterior chamber: between the iris and the lens.

  • Contains aqueous humor, a plasma-like fluid continuously formed by capillaries of the ciliary processes.

  • Drains via the scleral venous sinus (canal of Schlemm) at the sclera-cornea junction.

  • Supplies nutrients and oxygen mainly to the lens and cornea, but also to the retina, and removes wastes.

Glaucoma
  • A condition in which the drainage of aqueous humor is blocked, causing fluid to back up and increase pressure within the eye.
  • Pressures may increase to dangerous levels and compress the retina and optic nerve, leading to blindness.
  • Symptoms: few early signs, but late signs include seeing halos around lights and blurred vision.
The Lens
  • Biconvex, transparent, flexible, and avascular.
  • Changes shape to precisely focus light on the retina.
  • Crystallin: a transparent protein that forms the bulk of the lens.
  • Lens fibers are continually added, so the lens becomes more dense, convex, and less elastic with age.
Cataracts
  • Clouding of the lens.
  • Consequence of aging, diabetes mellitus, heavy smoking, frequent exposure to intense sunlight.
  • Crystallin proteins clump.
  • Lens replacement surgery is a treatment.
Focusing Light on the Retina
  • Convex lenses bend light passing through them, so that rays converge at the focal point.
  • The image formed at the focal point is upside-down and reversed from left to right.
Pathway of Light
  • The pathway of light entering the eye: cornea, aqueous humor, lens, vitreous humor, entire neural layer of the retina, and finally photoreceptors.
  • Light is refracted three times along the path:
    • (1) entering the cornea,
    • (2) entering the lens,
    • (3) leaving the lens.
  • The majority of refractive power is in the cornea; it cannot change focus.
Lens Adjustment
  • The lens can adjust its curvature to allow for fine focusing.
  • Can focus for distant vision and for close vision.
Focusing for Distant Vision
  • Eyes are best adapted for distant vision.
  • Far point of vision: no change in lens shape is needed for focusing distant vision.
    • Approximately 20 feet for an emmetropic (normal) eye.
  • Ciliary muscles are completely relaxed in distance vision; the lens is stretched flat.
Focusing for Close Vision
  • The lens must change shape to focus light on the retina.
  • Accommodation: the change in lens shape.
    • Ciliary muscles contract, loosening the ciliary zonule and allowing the lens to bulge.
  • Pupils constrict to prevent divergent light rays from entering.
  • Eyeballs converge: Medial rotation toward the object being viewed.
Vision Problems Associated with Eyeball Shape
  • Myopia (nearsightedness):
    • The eyeball is too long, so the focal point is in front of the retina.
    • Corrected with a concave lens.
  • Hyperopia (farsightedness):
    • The eyeball is too short, so the focal point is behind the retina.
    • Corrected with a convex lens.
  • Astigmatism:
    • Unequal curvatures in different parts of the cornea or lens.
    • Corrected with cylindrically ground lenses or laser procedures.