the eye: part a

Copyright © 2010 Pearson Education, Inc.

THE EYE: PART A


The Eye and Vision

• 70% of all sensory receptors are in the eye

• Nearly half of the cerebral cortex is involved in processing visual information!

• Most of the eye is protected by a cushion of fat and the bony orbit

Copyright © 2010 Pearson Education, Inc. Figure 15.1a

Eyelashes

Sclera(covered byconjunctiva)

Site whereconjunctivamerges withcornea

Lateralcommissure

Iris

Medialcommissure

Lacrimalcaruncle

Eyelid

Eyelid

Eyebrow

Pupil

Palpebralfissure

(a) Surface anatomy of the right eye


Eyebrows

• Eyebrows-Overlie the supraorbital margins

• Shade the eye

• Prevent perspiration from reaching eye


Eyelids (Palpebrae)• Protect the eye anteriorly

• Levator palpebrae superioris muscle—gives upper eyelid mobility

• Tarsal plates—internal supporting CT sheet

• Palpebral fissure—separates eyelids

• Medial and Lateral Canthi – eye angles

• Caruncle—fleshy elevation at medial canthus

• Contains oil and sweat glands

• Eyelashes

• Nerve endings of follicles initiate reflex blinking


Sclera(covered byconjunctiva)

Site whereconjunctivamerges withcornea

Lateralcommissure

Medialcommissure

Lacrimalcaruncle

Eyelid Pupil

Palpebralfissure

(a) Surface anatomy of the right eye


Eyelids

• Tarsal (Meibomian) Glands

• Modified sebaceous glands

• Produce oily secretion; prevents eyelids from sticking, lubricates eyelid

• Embedded in tarsal plates

• Ducts open posterior to eyelashes


Eyelids

• Ciliary Glands

• Sebaceous and sweat glands

• Found between eyelashes


Conjunctiva

• Transparent mucous membrane

• Palpebral conjunctiva: lines eyelids

• Bulbar conjunctiva: covers white of the eyes anteriorly


(b) Lateral view; some structures shown in sagittal section

Levator palpebraesuperioris muscleOrbicularis oculi muscle

Tarsal plate

Palpebral conjunctiva

Tarsal glandsCornea

Palpebral fissure

Bulbar conjunctiva

Orbicularis oculi muscle


Lacrimal Apparatus

• Consists of: Lacrimal gland and ducts that connect to nasal cavity

• Releases: Lacrimal secretion (tears)

• The solution also contains: mucus, antibodies, and lysozyme (bacteria destroying enzyme)

• The solution drains via: paired lacrimal puncta into lacrimal canaliculi then into nasolacrimal duct

Copyright © 2010 Pearson Education, Inc. Figure 15.2

Lacrimal gland

Excretory ducts of lacrimal glands

Lacrimal punctumLacrimal canaliculus

Nasolacrimal duct

Inferior meatusof nasal cavityNostril

Lacrimal sac


Extrinsic Eye Muscles

• The movement of the eye is controlled by six muscles on the external surface of each eye

• Superior rectus: elevates eye

• Inferior rectus: depresses eye

• Lateral rectus: moves eye laterally

• Medial rectus: moves eye medially

• Inferior oblique: Moves eye up and out

• Superior oblique: Moves eye down and out


Inferior rectusmuscle

Inferior obliquemuscle

Superior obliquemuscle

Superior obliquetendon

Superior rectusmuscle

Lateral rectusmuscle

(a) Lateral view of the right eye

Copyright © 2010 Pearson Education, Inc. Figure 15.3b

Superior obliquemuscle

Commontendinous ring

Trochlea

Superior obliquetendon

Superior rectusmuscle

(b) Superior view of the right eye

Axis at centerof eye

Medialrectus muscle

Inferiorrectus muscle

Lateralrectus muscle


• The innervation to each muscle can be remembered by the following equation: (LR6SO4)O3

• Which means:

• Lateral rectus: is innervated by CN # 6 (Abducens)

• Superior oblique: is innervated by CN #4 (Trochlear)

• All others: are innervated by CN #3 (oculomotor)

Extrinsic Eye Muscles


Structure of the Eyeball

•Wall of eyeball contains three layers (tunics)

• Fibrous layer - Outermost layer; dense avascular CT with two regions: sclera and cornea

• Vascular layer - Middle pigmented layer with three regions: choroid, ciliary body, and iris

• Sensory layer (retina) -Delicate two-layered membrane


Lens

Anteriorsegment (containsaqueous humor)

Posterior segment(contains vitreous humor)


Structures or Substances to Know:

• Sclera

• Cornea

• Choroid

• Ciliary body, muscle and processes

• Suspensory ligaments

• Ora Serrata

• Iris

• Pupil

• Anterior segment- divided into anterior and posterior chambers

• Posterior segment

• Vitreous humor

• Aqueous humor

• Lens

• Macula Lutea

• Fovea Centralis

• Optic disc

• Optic nerve

• Central artery and vein

• Canal of Schlemm


Iris (two muscles) • Sphincter pupillae • Dilator pupillae

Pupillary Sphinctermuscle contractiondecreases pupil size.

Dilatormuscle contractionincreases pupil size.

Sympathetic +Parasympathetic +


Optic disc(blind spot)

Optic nerve

ChoroidSclera

Ciliary bodyCiliary zonule(suspensoryligament)

Pupil

Lens

Iris


Sensory Layer: Retina

• Delicate two-layered membrane

• Pigmented layer

• Outer layer

• Absorbs light and prevents its scattering

• Stores vitamin A


Sensory Layer: Retina

• Neural layer

• Photoreceptor cells: transduce light energy

• Cells that transmit and process signals: bipolar cells, ganglion cells, amacrine cells, and horizontal cells


(a) Posterior aspect of the eyeball

Neural layer of retinaPigmentedlayer of retina

Central arteryand vein of retina Optic

nerve

ScleraChoroid

Optic disc

Pathway of light


The Retina

• Ganglion cell axons

• Run along the inner surface of the retina

• Axons of ganglion cells leave eye as the optic nerve

• Optic disc (blind spot)

• Site where the optic nerve leaves the eye

• Lacks photoreceptors


(a) Posterior aspect of the eyeball

Neural layer of retinaPigmentedlayer of retina

Central arteryand vein of retina Optic

nerve

ScleraChoroid

Optic disc

Pathway of light


Photoreceptors

• Rods

• More numerous at peripheral region of retina, away from the macula lutea

• Operate in dim light

• Provide indistinct, fuzzy, non color peripheral vision


Photoreceptors

• Cones

• Found in the macula lutea; concentrated in the fovea centralis

• Operate in bright light

• Provide high-acuity color vision

Copyright © 2010 Pearson Education, Inc. Figure 15.6b

Pigmentedlayer of retinaPathway of light

Pathway of signal output

(b) Cells of the neural layer of the retina

Amacrine cellHorizontal cell

• RodPhotoreceptors

• Cone

BipolarcellsGanglion

cells


Blood Supply to the Retina

• Two sources of blood supply

• Choroid supplies: the outer third (photoreceptors)

• Central artery and vein of the retina supply: the inner two-thirds


Maculalutea

Centralarteryand veinemergingfrom theoptic disc

Optic disc

Retina


Internal Chambers and Fluids

• The lens and ciliary zonule separate the anterior and posterior segments


Optic disc(blind spot)

Optic nervePosterior poleFovea centralisMacula luteaRetinaChoroidSclera

Ciliary bodyCiliary zonule(suspensoryligament)

Pupil

Anteriorsegment (containsaqueous humor)Lens

Posterior segment(contains vitreous humor)



• Posterior segment contains vitreous humor that:

• Transmits light

• Supports posterior surface of the lens

• Holds the neural retina firmly against the pigmented layer

• Contributes to intraocular pressure

• Anterior segment is composed of two chambers

• Anterior chamber—between the cornea and the iris

• Posterior chamber—between the iris and the lens



• Anterior segment contains aqueous humor

• Plasma like fluid continuously filtered from 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: compression of the retina and optic nerve if drainage of aqueous humor is blocked


Scleral venoussinus

Posteriorchamber

Anteriorchamber

Anteriorsegment(containsaqueoushumor)

Corneal-scleral junction

Cornea

Cornea

Aqueous humor

Iris

Lens

Lens

Posteriorsegment(contains vitreoushumor)

Ciliary zonule(suspensoryligament)

Ciliaryprocesses

Ciliarymuscle

Ciliary body1

2

3


Lens

• Biconvex, transparent, flexible, elastic, and avascular

• Allows precise focusing of light on the retina

• Cells of lens epithelium differentiate into lens fibers that form the bulk of the lens

• Lens fibers—cells filled with the transparent protein crystallin

• Lens becomes denser, more convex, and less elastic with age

• Cataracts (clouding of lens) occur as a consequence of aging, diabetes mellitus, heavy smoking, and frequent exposure to intense sunlight


Central arteryand vein ofthe retinaOptic disc(blind spot)

Optic nervePosterior poleFovea centralisMacula luteaRetinaChoroidSclera

Ora serrata

(a) Diagrammatic view. The vitreoushumor is illustrated only in thebottom part of the eyeball.

Ciliary bodyCiliary zonule(suspensoryligament)CorneaIris

Anterior polePupil

Anteriorsegment (containsaqueous humor)LensScleral venoussinusPosterior segment(contains vitreous humor)

the eye: part a

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