section 3, chapter 12: vision

section 3, chapter 12

vision

ivyanatomy.com

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Sense of Sight

• Visual accessory organs• Eyelids• Lacrimal apparatus• Extrinsic eye muscles

Eyelid• Palpebra• Composed of four (4) layers:

• Skin• Muscle • Connective tissue• Conjunctiva

• Orbicularis oculi – closes eyelid• Levator palpebrae superioris – opens eyelid• Tarsal glands – secrete oil onto eyelashes• Conjunctiva – mucous membrane; lines eyelid and covers portion of eyeball

Eyelash

Cornea

Conjunctiva

Eyelid

Tendon of levatorpalpebrae superioris

Superiorrectus

Orbicularisoculi

Inferiorrectus

Tarsal glands

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Lacrimal Apparatus

• Lacrimal gland• Lateral to eye• Secretes tears

• Canaliculi• Collect tears

• Lacrimal sac• Collects from canaliculi

• Nasolacrimal duct• Collects from lacrimal sac• Empties tears into nasal cavity

Lacrimal gland

Lacrimal sac

Superior andinferior canaliculi

Nasolacrimalduct


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Extrinsic Eye Muscles

Inferior rectus Inferior oblique

Medialrectus

Superiorrectus

Superioroblique

Lateralrectus(cut)


• Superior rectus• Rotates eye up and medially

• Inferior rectus• Rotates eye down and medially

• Medial rectus• Rotates eye medially

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Extrinsic Eye Muscles

Inferior rectus Inferior oblique

Medialrectus

Superiorrectus

Superioroblique

Lateralrectus(cut)


• Lateral rectus• Rotates eye laterally

• Superior oblique• Rotates eye down and laterally

• Inferior oblique• Rotates eye up and laterally

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Structure of the Eye

The eye has three distinct layers “tunics”1.Fibrous tunic 2.Vascular tunic3.Nervous tunic

Outer Tunic

• Sclera• Posterior portion• Opaque• Protection

• Cornea• Anterior 1/6th of eye• “window of the eye”• Transparent• Light transmission

Middle Tunic

• Iris “rainbow”• Anterior portion• Pigmented• Controls light intensity

• Ciliary body• Anterior portion• Pigmented• Suspensory ligaments holds lens• Ciliary muscles moves lens for focusing

• Choroid coat• Provides blood supply• Pigments absorb extra light

Anterior Portion of Eye

• Filled with aqueous humor

Lens• The lens is an elastic, biconvex, and transparent structure largely composed of epithelial cells, called lens fibers.

•The lens lies behind the iris and is held in place by suspensory ligaments of ciliary body

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Accommodation

• Changing of lens shape to view objects

(a)

Lens thick

Lens thin

(b)

Ciliary musclefibers contracted

Suspensoryligaments relaxed

Ciliary musclefibers relaxed

Suspensoryligaments taut


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Aqueous Humor

• Fluid in anterior cavity of eye• Secreted by epithelium on inner surface of the ciliary body• Provides nutrients• Maintains shape of anterior portion of eye• Leaves cavity through Canal of Schlemm

Sclera

Iris

Lens

Aqueous humor

Cornea

Vitreous humor

Ciliary process

Ciliary muscles


Posteriorchamber

Ciliarybody

Scleral venous sinus(canal of Schlemm)

Anteriorchamber

Ciliary Body

• Forms internal ring around the front of the eye• Ciliary processes – radiating folds• Ciliary muscles – contract and relax to move lens

Figure 12.28 Lens and ciliary body viewed from behind.Figure 12.28 Lens and ciliary body viewed from behind.

Iris

• Composed of connective tissue and smooth muscle

• Pupil is hole in iris

• Dim light stimulates radial muscles and pupil dilates

• Bright light stimulates circular muscles and pupil constricts

Inner Tunic

• Retina• Contains visual receptors• Continuous with optic nerve

• Fovea centralis – center of macula lutea; produces sharpest vision

•Macula lutea – yellowish spot in retina

• Optic disc – blind spot; contains no visual receptors

• Vitreous humor – thick gel that holds retina flat against choroid coat

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Posterior Cavity

• Contains vitreous humor – thick gel that holds retina flat against choroid coat

Ciliary body

Retina

Choroid coat

Sclera

Fovea centralis

Optic nerve

Lens

Iris

Pupil

Cornea

Lateral rectus

Medial rectus

Optic disc

Posterior cavity

Vitreous humor

Posteriorchamber

Anteriorchamber

Aqueoushumor

Suspensoryligaments


Anteriorcavity

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Layers of the Eye

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Light Refraction

• Refraction • Bending of light• Occurs when light waves pass at an oblique angle into mediums of different densities

Light wave

Perpendicular line

Air

Glass

Refractedlight wave


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Types of Lenses

• Convex lenses cause light waves to converge

• Concave lenses cause light waves to diverge

Air

Glass

(a) (b)

Diverginglight waves

Convexsurface

Lightwave

Converginglight waves

Concavesurface


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Focusing On Retina

• As light enters eye, it is refracted by:• Convex surface of cornea• Convex surface of lens

• Image focused on retina is upside down and reversed from left to right

Light waves

ObjectCornea

Image

Retina


Major Groups of Retinal Neurons

• Receptor cells, bipolar cells, and ganglion cells - provide pathway for impulses triggered by photoreceptors to reach the optic nerve

• Horizontal cells and amacrine cells – modify impulses

Figure 12. Notice that photoreceptors (rods and cones) are the posterior most cells in this circuit. Light waves stimulate the photoreceptors, which send impulses forward to horizontal cells and ganglion cells before leaving the eye

through the optic disc.

• Rods• Long, thin projections• Contain light sensitive pigment called rhodopsin• Hundred times more sensitive to light than cones• Provide vision in dim light• Produce outlines of objects

• Cones• Short, blunt projections• Provide vision in bright light• Produce sharp images• Produce color vision• Pigments include:

• Erythrolabe – responds to red• Chlorolabe – responds to green• Cyanolabe – responds to blue

Visual Receptors

Color perceived depends on which sets of cones are stimulated

Figure 12.38 Rods and cones. (a) Several rods converge onto a single sensory nerve fiber to the brain (b) separate sensory nerve fibers transmits impulses from the cones to the brain. (c) Scanning electron micrograph of

rods and cones.

Figure 12.38 Rods and cones. (a) Several rods converge onto a single sensory nerve fiber to the brain (b) separate sensory nerve fibers transmits impulses from the cones to the brain. (c) Scanning electron micrograph of

rods and cones.

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12.6 Clinical Application

Refraction Disorders• Concave lens corrects nearsightedness

• Convex lens corrects farsightedness

Light waves

Light waves

Light waves

CorneaLens

Retina

(a) Eye too long (myopia)

(b) Normal eye

(c) Eye too short (hyperopia)

Pointof focus

Pointof focus

Pointof focus

Light waves

Concave lens

Convex lens

(a)

(b)

Uncorrectedpoint of focus

Correctedpoint of focus

Uncorrectedpoint of focus

Correctedpoint of focus


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Visual Pigments

• Rhodopsin• Light-sensitive pigment in rods• Decomposes in presence of light• Triggers a complex series of reactions that initiate nerve impulses• Impulses travel along optic nerve

• Pigments on cones• Each set contains different light-sensitive pigment• Each set is sensitive to different wavelengths• Color perceived depends on which sets of cones are stimulated• Erythrolabe – responds to red• Chlorolabe – responds to green• Cyanolabe – responds to blue

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Stereoscopic Vision

• Provides perception of distance and depth• Results from formation of two slightly different retinal images

Lightwaves

Right eyeLeft eye


Visual Nerve Pathways

Axons from ganglion cells in retina leave the eyes to form the optic nerves.

Some of the fibers decussate at the optic chiasm.• Fibers from the nasal (medial) half of retina cross over• Fibers from temporal half (lateral) of retina do not cross over

Most fibers of the optic tract continue to the lateral geniculate nucleus of the thalamus, which relays impulses towards the visual cortex.

Figure 12.41 The visual pathway includes the optic nerve, optic chiasma, optic tract, and optic radiations.

Figure 12.41 The visual pathway includes the optic nerve, optic chiasma, optic tract, and optic radiations.

End of Chapter 12

section 3, chapter 12: vision

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