lecture material is adapted from © 2013 pearson education ... · lacrimal apparatus •lacrimal...

THE SPECIAL SENSES

Lecture Material is adapted from © 2013 Pearson Education, Inc. Human

Anatomy and Physiology

Dr. Henrik Pallos

© 2013 Pearson Education, Inc.

Special Senses

• Special sensory receptors

– Distinct, localized receptor cells in head

• Vision

• Taste

• Smell

• Hearing

• Equilibrium


The Eye and Vision

• 70% of body's sensory receptors in eye

• Visual processing by nearly half cerebral cortex

– Primary visual cortex

– Visual association area

• Most of eye protected by cushion of fat and

bony orbit


Accessory Structures of the Eye

• Protect the eye and aid eye function

1. Eyebrows

2. Eyelids (palpebrae)

3. Conjunctiva

4. Lacrimal apparatus

5. Extrinsic eye muscles


Eyebrow

Eyelid

(palpebrae)

Eyelashes

Site where conjunctiva merges with cornea

Palpebral fissure

Lateral commissure

Iris

Eyelid

Surface anatomy of the right eye

Pupil Sclera (covered by conjunctiva)

Lacrimal caruncle

Medial commissure

The eye and accessory structures.

My wife’s epicanthic fold


Eyebrows

• Overlie supraorbital margins

• Function

1. Shade eye from sunlight

2. Prevent perspiration from reaching eye


Eyelids

• Protect eye anteriorly

• Separated at palpebral fissure

• Meet at medial and lateral commissures

• Lacrimal caruncle

– At medial commissure

– Contains oil and sweat glands

• Tarsal plates—

supporting connective

tissue


Eyelid Muscles

• Levator palpebrae superioris

– Gives upper eyelid mobility

• Raises eyelid to

to open eye

• Blink reflexively

every 3-7 seconds

– Protection

– Spread secretions

to moisten eye

• (Orbicularis oculi)

– Closes eye when

contracts


Eyelids

• Eyelashes

– Nerve endings of follicles initiate reflex blinking

• Lubricating glands associated with eyelids

1. Tarsal (Meibomian) glands

• Modified sebaceous glands

• Oily secretion lubricates lid and eye

2. Ciliary glands between hair follicles

• Modified sweat glands


The eye and accessory structures.

Levator palpebrae superioris muscle

Orbicularis oculi muscle Eyebrow

Tarsal plate

Palpebral conjunctiva Tarsal glands

Cornea

Palpebral fissure

Eyelashes

Bulbar conjunctiva

Conjunctival sac

Orbicularis oculi muscle

some structures shown in sagittal section Lateral view;


Conjunctiva

• Transparent mucous membrane

– Produces a lubricating mucous secretion

1. Palpebral conjunctiva lines eyelids

2. Bulbar conjunctiva covers white of eyes

Conjunctival sac between palpebral and

bulbar conjunctiva

- Where contact lens rests


Lacrimal Apparatus

• Lacrimal gland and ducts that drain into nasal cavity

• Lacrimal gland in orbit above lateral end of eye

• Lacrimal secretion (tears) – 1ml/day

– Dilute saline solution containing mucus, antibodies, and

lysozyme:

• cleanses and protects the eye surface as it lubricates it

– Blinking spreads tears toward medial commissure

• Tears enter paired lacrimal canaliculi via lacrimal puncta

• Then drain into lacrimal sac and nasolacrimal duct

• Taste of eye drops

– Lacrimal secretion increases:

• Eyes are irritated (washes away irritant)

• Emotional (Why we have, we do not know.)


The lacrimal apparatus.

Lacrimal sac

Lacrimal gland

Excretory ducts of lacrimal glands

Lacrimal punctum

Lacrimal canaliculus

Nasolacrimal duct

Inferior meatus of nasal cavity

Nostril

Homeostatic imbalance

• Nasal cavity mucosa is continuous with lacrimal duct

system

• Cold/nasal inflammation: swelling of lacrimal mucosa

• Constriction of

nasolacrimal duct

• No tear draining

• Watery eyes



Extrinsic Eye Muscles

• 6 straplike extrinsic eye muscles

– Originate from bony orbit; insert on eyeball

1. enable eye to follow moving objects

2. maintain shape of eyeball

3. hold in orbit

• 4 rectus muscles originate from common tendinous

ring; names indicate movements

– Superior, inferior, lateral, medial rectus muscles

• 2 oblique muscles move eye in vertical plane and

rotate eyeball

– Superior and inferior oblique muscles

H test

http://emedicine.medscape.com/article/1189759-overview


Extrinsic eye muscles.

Trochlea

Axis of rotation of eye

Inferior rectus muscle (CN 3)

Medial rectus muscle (CN 3)

Lateral rectus muscle ( CN 6)

Common

tendinous ring

Superior oblique muscle

(CN 4)

Superior oblique tendon

Superior rectus muscle

(CN 3)

Superior view of the right eye



Superior oblique muscle

Superior oblique tendon

Superior rectus muscle

Lateral rectus muscle

Inferior rectus muscle

Inferior oblique

muscle (CN 3)

Lateral view of the right eye



Muscle Action Controlling cranial nerve

Summary of muscle actions and innervating cranial nerves

Lateral rectus

Medial rectus

Superior rectus

Inferior rectus

Inferior oblique

Superior oblique

Moves eye laterally

Moves eye medially

Elevates eye and turns it medially

Depresses eye and turns it medially

Elevates eye and turns it laterally

Depresses eye and turns it laterally

VI (abducens)

III (oculomotor)

III (oculomotor)

III (oculomotor)

III (oculomotor)

IV (trochlear)

Homeostatic imbalances

• Movements of external muscles of the two

eyes are not perfectly coordinated

• Image is not properly focused on the same

area of the visual field in each eye

• Two images are seen instead of one


• Diplopia: double vision

– paralysis, weakened muscle, temporary

consequences of alcohol

• Strabismus (cross-eyed)

– Congenital weakness of muscle

– Affected eye rotates medially or laterally

• The eyes may alternate to focus and compensate

• Or disregard deviant eye: functional blindness

• Exercising muscle, surgery

http://www.aapos.org/terms/conditions/100


Structure of the Eyeball

• Wall of eyeball contains 3 layers

• Internal cavity filled with fluids called humors

• Lens separates internal cavity into 1. anterior segment

2. posterior segment (cavities)


Structure of the Eyeball

• Wall of eyeball contains 3 layers

1. Fibrous 1. Sclera

2. Cornea

2. Vascular 1. Choroid

2. Ciliary body

3. Iris

3. Inner (Retina) 1. Pigmental layer

2. Neural layer


Internal structure of the eye (sagittal section).

Ora serrata

Ciliary body

Ciliary zonule

(suspensory

ligament)

Cornea

Pupil

Anterior

pole

Anterior

segment

(contains

aqueous humor)

Lens

Scleral venous sinus

Posterior segment

(contains vitreous humor)

Diagrammatic view. The vitreous humor is illustrated only in the bottom part of the eyeball.

Sclera

Choroid

Retina

Macula lutea

Fovea centralis

Posterior pole

Optic nerve

Central artery and

vein of the retina

Optic disc

(blind spot)

Iris


Internal structure of the eye (sagittal section).

Ciliary body

Ciliary processes

Iris

Margin

of pupil

Anterior

segment

Cornea

Ciliary zonule

(suspensory

ligament)

Photograph of the human eye.

Vitreous humor in posterior segment

Choroid

Fovea centralis

Optic disc

Optic nerve Lens

Sclera

Retina


Fibrous Layer

• Outermost layer

• Dense avascular connective tissue

• Two regions: sclera and cornea

1. Sclera

– Opaque posterior region

– Protects, shapes

eyeball

– Anchors extrinsic

eye muscles

– Continuous with

dura mater of brain

posteriorly


Fibrous Layer

2. Cornea

– Transparent anterior 1/6 of fibrous layer

– Bends light as it enters eye

– Numerous pain receptors contribute to blinking

and tearing reflexes

– Transplantation

• Avascular

• One person to

another with

minimal risk of

rejection


Vascular Layer (Uvea)

• Middle pigmented layer

• 3 regions: choroid, ciliary body, and iris

1. Choroid region

– Posterior portion of uvea

– Supplies blood to all

layers of eyeball

– Brown pigment

absorbs light to

prevent light

scattering and

visual confusion


Vascular Layer

2. Ciliary body

– Ring of tissue surrounding lens

– Smooth muscle bundles (ciliary muscles) control

lens shape

– Capillaries of ciliary processes secrete fluid

– Ciliary zonule

(suspensory

ligament) holds

lens in position


Vascular Layer

3. Iris

• Colored part of eye

• 2 smooth muscle layers + elastic fibers

• Random pattern before birth

• Pupil central opening that regulates amount of light entering eye

• Color of iris: only brown pigment • Lot of pigment: brown/black

• Small amount: blue, green, gray • Unpigmented area scatter shorter wavelengths light

1. Close vision and bright light sphincter pupillae (circular muscles) contract; pupils constrict parasympathetic fibers

2. Distant vision and dim light dilator pupillae (radial muscles) contract; pupils dilate – sympathetic fibers

3. Changes in emotional state pupils dilate when subject matter is appealing such as in your eyes during Anatomy and Physiology lecture ;-)



Inner Layer: Retina

• Originates as outpocketing of brain

• Delicate 2-layered membrane

1. Outer Pigmented layer • Single-cell-thick lining

1. Absorbs light and prevents its scattering

2. Phagocytize photoreceptor cell fragments

3. Stores vitamin A


Microscopic anatomy of the retina.

Neural layer of retina

Pathway of light

Optic disc (blind spot)

Central artery and vein of retina

Pigmented layer of retina Choroid

Sclera

Optic nerve

Posterior aspect of the eyeball


Inner Layer: Retina

2. Inner Neural layer

– Transparent

– Composed of 3 main types of neurons

1. Photoreceptors

2. Bipolar cells

3. Ganglion cells

– Signals spread from photoreceptors to bipolar cells

to ganglion cells

– Ganglion cell axons exit eye as optic nerve



Neural layer of retina

Pathway of light

Optic disc (blind spot)

Central artery and vein of retina

Pigmented layer of retina Choroid

Sclera

Optic nerve

Posterior aspect of the eyeball


The Retina

• Optic disc (blind spot)

– Site where optic nerve leaves eye

– Lacks photoreceptors

• Quarter-billion photoreceptors of two types

1. Rods

2. Cones

Why we do not see blind spot…

• When dot/white gap disappears:

1. Visual filling

• The black bar fills the space (or white space in case of

the dark dot)

• Our brain use the surrounding image to fill in the area

with something similar

• Brain assumes that it is better something similar than

disturbing

2. Eye flickering movements (saccades)

• Same area does not always project to the same area

• What an eye does not see now, it will see a millisec

later when the saccades redirect the visual axis

• Fastest muscular movements




Photoreceptors

• Rod • Cone

Ganglion cells Bipolar

cells Axons of ganglion cells

Amacrine cell Horizontal cell

Pathway of signal output

Pathway of light

Cells of the neural layer of the retina

Pigmented layer of retina


Photoreceptors

• Rods

– Dim light, peripheral vision

receptors

– More numerous, more sensitive

to light than cones

– No color vision or sharp images

– Numbers greatest at periphery


Photoreceptors

• Cones

1. Vision receptors for bright

light

2. High-resolution color vision

– Macula lutea exactly at

posterior pole

• Mostly cones

• Fovea centralis

– Tiny pit in center of macula with all

cones; best vision


Blood Supply to the Retina

• 2 sources of blood supply

1. Choroid supplies outer third (photoreceptors)

2. Central artery and vein of retina supply inner

two-thirds

• Enter/exit eye in center of optic nerve

• Vessels visible in living person

• Hypertension can be detected

Hypertensive retinopathy

• damage to the retina from high blood

pressure.

– It occurs as the existing high blood pressure

causes changes to the microvasculature of

the retina.

– first findings: flame hemorrhages

and cotton wool spots

– progression: hard exudates

can appear around the

macula

– causing impairment of vision



Part of the posterior wall (fundus) of the right eye as seen with an ophthalmoscope.

Central artery and vein emerging from the optic disc

Optic disc

Macula

lutea

Retina


Internal Chambers and Fluids

• The lens and ciliary zonule separate eye into

two segments

– Anterior segment

– Posterior segments



• Posterior segment contains vitreous humor that

– Transmits light

– Supports posterior surface of lens

– Holds neural layer of retina firmly against pigmented layer

– Contributes to intraocular pressure

– Forms in embryo; lasts lifetime

• Anterior segment composed of two chambers

– Anterior chamber— between cornea and iris

– Posterior chamber— between iris and lens



• Anterior segment contains aqueous humor

– Plasma like fluid continuously formed by capillaries of

ciliary processes

– Drains via scleral venous sinus (canal of Schlemm) at

sclera-cornea junction

– Supplies nutrients and oxygen mainly to lens and

cornea but also to retina, and removes wastes

• Glaucoma: blocked drainage of aqueous humor

increases pressure and causes compression of

retina and optic nerve blindness


Circulation of aqueous humor.

Iris

Lens epithelium

Lens Cornea

Corneal epithelium

Corneal endothelium

Aqueous humor

Anterior chamber

Posterior chamber

Anterior

segment (contains aqueous humor)

Scleral venous sinus

Corneoscleral junction

Bulbar conjunctiva

Sclera

Ciliary

body

Ciliary zonule (suspensory ligament)

Posterior

segment

(contains vitreous humor)

Ciliary processes

Ciliary muscle

Aqueous humor forms by filtration from the capillaries in the ciliary processes.

Aqueous humor flows from the posterior chamber through the pupil into the anterior chamber. Some also flows through the vitreous humor (not shown).

Aqueous humor is reabsorbed into the venous blood by the scleral venous sinus.

Lens Cornea

1

2

3

1

2

3


Lens

• Biconvex, transparent, flexible, and avascular

• Changes shape to precisely focus light on retina

• Two regions

– Lens epithelium anteriorly

– Lens fibers form bulk of lens

– Lens fibers filled with transparent protein crystallin

• Lens becomes more dense, convex, less elastic with

age

• Lens can be replaced surgically with artificial lens


Cataracts

• Clouding of lens

– Consequence of aging

– Diabetes mellitus

– Heavy smoking: STOP SMOKING!

– Frequent exposure to intense sunlight! Sunglasses

– Some congenital

– Crystallin proteins clump

– Large doses of Vitamin C increases cataract

formation

Functions of the Eye

1. Light/energy strikes the retina

2. Converts energy into action potentials

3. Relayed to brain for processing



Light And Optics: Wavelength And Color

• Eyes respond to visible light

– Small portion of electromagnetic spectrum

– Wavelengths of 400-700 nm

• Light

– Packets of energy (photons or quanta) that travel

in wavelike fashion at high speeds

– Color of objects:

1. They absorb some wavelength

2. They reflect some wavelength

– Objects reflect determines color eye perceives

• Red apple reflects red, green grass reflects green

• Black: do not reflect anything, but absorbs everything


The electromagnetic spectrum and photoreceptor sensitivities.

10–5 nm

Gamma

rays X rays UV Infrared Micro-

waves Radio waves

Visible light

Blue

cones (420 nm)

Rods (500 nm)

Green

cones (530 nm)

Red

cones (560 nm)

Light absorp

tion (p

erc

ent of m

axim

um

)

100

50

0 400

10–3 nm 1 nm 103 nm 106 nm 1 m 103 m (109 nm =)

Wavelength (nm)

450 500 550 600 650 700


Light And Optics: Refraction And Lenses

• Refraction

– Bending of light rays

• Due to change in speed when light passes from one

transparent medium to another

• Occurs when light meets surface of different medium

at an oblique angle

– Curved lens can refract light


Refraction.

As you sight through the side of the glass at the portion of the straw located above the water's surface,

light travels directly from the straw to your eye (to air to glass, glass to air, but this refraction we can ignore)

Since this light does not change medium, it will not refract.

As you sight at the portion of the straw that was submerged in the water, light travels from water to air

(or from water to glass to air). This light ray changes medium and subsequently undergoes refraction.

As a result, the image of the straw appears to be broken.


Refraction and Lenses

• Light passing through convex lens (as in eye)

is bent so that rays converge at focal point

– Image formed at focal point is upside-down and

reversed right to left

• Concave lenses diverge light

– Prevent light from focusing


Light is focused by a convex lens.

Point sources Focal points

Focusing of two points of light.

The image is inverted—upside down and reversed.

A

B

C

D

C

D

A

B


Focusing Light on The Retina

• Pathway of light entering eye:

cornea, aqueous humor, lens, vitreous humor, entire

neural layer of retina, photoreceptors

• Light refracted 3 (-4) times along pathway

1. Entering cornea

2. Entering lens

3. Leaving lens

• Majority of refractory power in cornea

• Change in lens curvature allows for fine focusing


Focusing For Distant Vision

• Eyes best adapted for distant vision

• Far point of vision

– Distance beyond which no change in lens shape

needed for focusing

• 6m for emmetropic (normal) eye

• Cornea and lens focus light precisely on retina

• Ciliary muscles relaxed

• Lens stretched flat by tension in ciliary zonule


Focusing for distant and close vision.

Sympathetic activation

Nearly parallel rays from distant object

Lens

Ciliary zonule

Ciliary muscle Inverted image

Lens flattens for distant vision. Sympathetic input relaxes the ciliary muscle tightening the ciliary zonule, and flattening the lens.


Focusing For Close Vision

• Light from close objects (<6 m) diverges as

approaches eye

– Requires eye to make active adjustments using 3

simultaneous processes

1. Accommodation of lenses

2. Constriction of pupils

3. Convergence of eyeballs


Focusing For Close Vision

1. Accommodation

– Changing lens shape to increase refraction

– Near point of vision

• Closest point on which the eye can focus

– Presbyopia—loss of accommodation over age 50

2. Constriction

– Accommodation pupillary reflex constricts pupils to prevent

most divergent light rays from entering eye

– (an image is always blurry around edges)

3. Convergence

– Medial rotation of eyeballs toward object being viewed

• If can’t converge: diplopia – double vision

• Image falls different parts of the 2 retinas, brain see 2 images

– Press gently on one eyelid as you look at the slide



Parasympathetic activation

Inverted image

Divergent rays from close object

Lens bulges for close vision. Parasympathetic input contracts the ciliary muscle loosening the ciliary zonule, allowing the lens to bulge.



Ciliary muscle

Lens

Ciliary zonule (suspensory ligament)

View

The ciliary muscle and ciliary zonule are arranged

sphincterlike around the lens. As a result, contraction loosens the ciliary zonule fibers and relaxation tightens them.


Problems Of Refraction

1. Myopia (nearsightedness)

– Focal point in front of retina, e.g., eyeball too long

– Close object: no problem

– Corrected with a concave lens

2. Hyperopia (farsightedness)

– Focal point behind retina, e.g., eyeball too short

– Far object: no problem

– Corrected with a convex lens

3. Astigmatism

– Unequal curvatures in different parts of cornea or lens

– Corrected with cylindrically ground lenses or laser

procedures


Problems of refraction. (1 of 3)

Emmetropic eye (normal)

Focal plane

Focal point is on retina.


Myopic eye (nearsighted)

Uncorrected Focal point is in front of retina.

Concave lens moves focal point further back.

Eyeball too long

Corrected


Myopia

Focal point in

front of retina,

e.g., eyeball too

long

– Close object:

no problem

– Corrected with a

concave lens


Hyperopic eye (farsighted)

Eyeball too short

Uncorrected Focal point is behind retina.

Corrected

Convex lens moves focal point forward.


Hyperopia

(farsightedness)

– Focal point behind

retina, e.g.,

eyeball too short

– Far object:

no problem

– Corrected with a

convex lens


Functional Anatomy Of Photoreceptors

• Rods and cones

– Modified neurons

– Receptive regions called

outer segments

• Contain visual pigments

(photopigments)

Molecules change shape as

absorb light

– Inner segment of each

joins cell body


Photoreceptors of the retina.

Process of bipolar cell

Synaptic terminals

Rod cell body

Inner fibers

Nuclei Cone cell body

Mitochondria

Connecting cilia

Outer fiber

Apical microvillus

Discs containing visual pigments

Discs being phagocytized

Melanin granules

Pigment cell nucleus Basal lamina (border with choroid)

In

ne

r

se

gm

en

t

Pig

me

nte

d la

ye

r

Ou

te

r se

gm

en

t

The outer segments

of rods and cones

are embedded in the

pigmented layer of

the retina.

Rod cell body


Photoreceptor Cells

• Vulnerable to damage

• Degenerate if retina detached

• Destroyed by intense light

• Outer segment renewed every 24 hours

– Tips fragment off and are phagocytized


Rods

• Functional characteristics

– Very sensitive to light

– Best suited for night vision and peripheral vision

– Contain single pigment

• Perceived input in gray tones only

– Pathways converge

• causing fuzzy, indistinct images

For those red/green deficient, they will see a 5.

Cones

• Functional characteristics

– Need bright light for activation (have low sensitivity)

– React more quickly

– Nonconverging pathways result in detailed, high-

resolution vision

• Have 1 of 3 pigments for colored view

• Color blindness–lack of one or more cone pigments


Comparison of Rods and Cones


Chemistry Of Visual Pigments

• Retinal

– Light-absorbing molecule that combines with one of

four proteins (opsins) to form visual pigments

• Synthesized from vitamin A

– Retinal has 2 forms: bent form and straight form

• Bent form change to straight form when pigment absorbs

light

• Conversion of bent to straight initiates reactions

electrical impulses along optic nerve


Phototransduction: Capturing Light

• Deep purple pigment of rods–rhodopsin

– Bent retinal + opsin rhodopsin

– Three steps of rhodopsin

formation and breakdown

1. Pigment synthesis

2. Pigment bleaching

3. Pigment regeneration


Phototransduction: Capturing Light

1. Pigment synthesis

– Rhodopsin forms and accumulates in dark

2. Pigment bleaching

– When rhodopsin absorbs light, retinal changes to straight form

– Retinal and opsin separate (rhodopsin breakdown)

3. Pigment regeneration

– Straight retinal converted to bent

– Rhodopsin regenerated in outer segments


Phototransduction In Cones

• Similar as process in rods

• Cones far less sensitive to light

– Takes higher-intensity light to activate cones


Events of phototransduction. Slide 6

G protein signaling mechanisms are like a molecular relay race.

Retinal absorbs light and changes shape. Visual pigment activates.

Receptor G protein Enzyme 2nd messenger

Visual pigment

1

Light

11-cis-retinal

Transducin (a G protein)

All-trans-retinal

2 3 Visual pigment activates transducin (G protein).

Transducin activates phosphodiesterase (PDE).

4 5 PDE converts cGMP into GMP, causing cGMP levels to fall.

As cGMP levels fall, cGMP-gated cation channels close, resulting in hyperpolarization.

cGMP-gated cation channel open in dark

cGMP-gated cation channel closed in light

Phosphodiesterase (PDE)

Light (1st

messenger)


Light Transduction Reactions

• Light-activated rhodopsin activates G protein transducin

• In dark, cGMP holds channels of outer segment open Na+ and Ca2+ depolarize cell

• In light cGMP breaks down, channels close, cell hyperpolarizes

– Hyperpolarization is signal!


Information Processing In The Retina

• Photoreceptors and bipolar cells only generate graded potentials (EPSPs and IPSPs)

• When light hyperpolarizes photoreceptor cells

1. Stop releasing inhibitory neurotransmitter glutamate

2. Bipolar cells (no longer inhibited) depolarize, release neurotransmitter onto ganglion cells

3. Ganglion cells generate APs transmitted in optic nerve to brain

In the dark


Signal transmission in the retina (1 of 2). Slide 8 In the dark

cGMP-gated channels open, allowing cation influx. Photoreceptor depolarizes.

1

Voltage-gated Ca2+ channels open in synaptic terminals.

Neurotransmitter is released continuously.

Neurotransmitter causes IPSPs in bipolar cell. Hyperpolarization results.

Hyperpolarization closes voltage-gated Ca2+ channels, inhibiting neurotransmitter release.

No EPSPs occur in ganglion cell.

No action potentials occur along the optic nerve.

Photoreceptor

cell (rod)

Bipolar

Cell

Ganglion

cell

Ca2+

−40 mV −40 mV

2

3

4

5

6

7

Ca2+

Na+

In the light


Signal transmission in the retina. (2 of 2). Slide 8

−70 mV

No neurotransmitter

is released.

Depolarization opens voltage-gated Ca2+ channels; neurotransmitter is released.

EPSPs occur in ganglion cell.

Action potentials propagate along the optic nerve.

cGMP-gated channels close, so cation influx stops. Photoreceptor hyperpolarizes.

Lack of IPSPs in bipolar

cell results in depolarization.

Voltage-gated Ca2+ channels close in synaptic terminals.

1

Photoreceptor

cell (rod)

Bipolar

Cell

Ganglion

cell

In the light

Light

Ca2+

−70 mV

2

3

4

5

6

7

Below, we look at a tiny column of retina. The outer segment of the rod, closest to the back of the eye and farthest from the incoming light, is at the top.

Light


Light Adaptation

• Move from darkness into bright light

– Both rods and cones strongly stimulated

• Pupils constrict

– Large amounts of pigments broken down

instantaneously, producing glare

– Visual acuity improves over 5–10 minutes as:

• Rod system turns off

• Retinal sensitivity decreases

• Cones and neurons rapidly adapt


Dark Adaptation

• Move from bright light into darkness

– Cones stop functioning in low-intensity light

– Rod pigments bleached; system turned off

– Rhodopsin accumulates in dark

– Transducin returns to outer segments

– Retinal sensitivity increases within 20–30 minutes

– Pupils dilate


Night Blindness

• Nyctalopia

– Not able to see in low light

• Rod degeneration / rod damage

– Commonly caused by vitamin A deficiency

– If administered early vitamin A supplements

restore function

– Also caused by retinitis pigmentosa

• Degenerative retinal diseases that destroy rods

S A Y O N A

R

A

!

lecture material is adapted from © 2013 pearson education ... · lacrimal apparatus •lacrimal...

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