Ch. 15 Special Senses: Vision Slides mostly © Marieb & Hoehn 9 th ed. Other slides by WCR

Ch. 15Special Senses: Vision

Slides mostly © Marieb & Hoehn 9th ed.Other slides by WCR

© 2013 Pearson Education, Inc.

Light And Optics: Wavelength And Color

• Light– Packets of electromagnetic radiation (energy)– Light waves have different wavelengths

• Visible light– The (small) range of electromagnetic wavelengths

which our eyes can detect: 400-700 nm– Different objects reflect different wavelengths, which

we perceive as different colors


Figure 15.10 The electromagnetic spectrum and photoreceptor sensitivities.

10–5 nm

Gamma

raysX rays UV Infrared Micro-

wavesRadio waves

Visible light

Bluecones(420 nm)

Rods(500 nm)

Greencones(530 nm)

Redcones(560 nm)

Light

ab

sorp

tion

(perc

ent

of

maxim

um

)

100

50

0400

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

•Convexly-curved lens refract light to bring it to a focus•Image formed at focal point is upside-down and left-right reversed


Figure 15.11 Refraction.


Figure 15.12 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.


Focusing Light on The Retina

• Pathway of light entering eye: cornea, aqueous humor, lens, vitreous humor, entire neural layer of retina, photoreceptors

• Light refracted at boundaries along pathway– Air to cornea/aqueous humor– Aqueous humor to lens– Lens to vitreous humor

• Most bending happens at air-cornea boundary• Lens curvature is the “fine adjustment”


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

• 20 feet for emmetropic (normal) eye• Cornea and lens focus light precisely on retina

• Ciliary muscles relaxed• Lens stretched flat by tension in ciliary zonule


Figure 15.13a Focusing for distant and close vision.

Sympathetic activationNearly parallel raysfrom distant object

Lens

Ciliary zonule

Ciliary muscle Invertedimage

Lens flattens for distant vision. Sympathetic inputrelaxes 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 three simultaneous processes• Accommodation of lenses• Constriction of pupils• Convergence of eyeballs


Focusing For Close Vision

• 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

• Constriction– Accommodation pupillary reflex constricts pupils to

prevent most divergent light rays from entering eye

• Convergence– Medial rotation of eyeballs toward object being viewed


Figure 15.13b Focusing for distant and close vision.

Parasympathetic activationInvertedimage

Divergent raysfrom close object

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


Problems Of Refraction

• Myopia (nearsightedness)– Focal point in front of retina, e.g., eyeball too long– Corrected with a concave lens

• Hyperopia (farsightedness)– Focal point behind retina, e.g., eyeball too short– Corrected with a convex lens

• Astigmatism– Unequal curvatures in different parts of cornea or lens– Corrected with cylindrically ground lenses or laser

procedures


Figure 15.14 Problems of refraction. (1 of 3)

Emmetropic eye (normal)

Focalplane

Focal point ison retina.


Myopic eye (nearsighted)

UncorrectedFocal point is infront of retina.

Concave lens moves focalpoint further back.

Eyeballtoo long

Corrected



Hyperopic eye (farsighted)

Eyeballtoo short

UncorrectedFocal point isbehind retina.

CorrectedConvex lens moves focalpoint forward.



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


Figure 15.15a Photoreceptors of the retina.

Process of bipolar cell Li

ght

Synaptic terminals

Rod cell body

Inner fibers

NucleiCone cell bodyMitochondria

Connecting cilia

Outer fiber

Apical microvillus

Discs containingvisual pigmentsDiscs being phagocytized

Melanin granules

Pigment cell nucleusBasal lamina (border with choroid)

Inner

segm

ent

Pig

mente

d layer

Oute

r se

gm

ent

Lig

ht

Lig

ht

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


Cones

• Functional characteristics – Need bright light for activation (have low

sensitivity)– React more quickly– Have one of three pigments for colored view– Nonconverging pathways result in detailed,

high-resolution vision– Color blindness–lack of one or more cone

pigments


Table 15.1 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•Isomers: cis- (bent) and trans- (straight)

– Absorbing a photon causes bent-to-straight (cis –to-trans) shape change

– Change from bent-to-straight initiates reactions electrical impulses along optic nerve

Rhodopsin = cis-retinal (bent retinal) + opsin


Figure 15.15b Photoreceptors of the retina.

Rod discs

Rhodopsin, the visual pigment in rods,is embedded in the membrane that formsdiscs in the outer segment.

Visualpigmentconsists of• Retinal• Opsin


Phototransduction: Capturing Light

• Pigment synthesis– Rhodopsin forms and accumulates in dark

• Pigment bleaching– Light absorption causes retinal to change to

trans isomer– Retinal and opsin separate (rhodopsin

breakdown)

• Pigment regeneration– trans retinal converted to cis– Cis-retinal rejoins opsin to form rhodopsin


Figure 15.16 The formation and breakdown of rhodopsin.

Enzymes slowly convertall-trans-retinal to its 11-cis form in cells of thepigmented layer; requiresATP.

Pigment regeneration:

Light absorption byrhodopsin triggers arapid series of steps inwhich retinal changesshape (11-cis to all-trans) and eventually releases from opsin.

Pigment bleaching:

11-cis-retinal, derivedfrom vitamin A, iscombined with opsin to form rhodopsin.

Pigment synthesis:1

2H+

2H+

All-trans-retinal

All-trans-retinal

Rhodopsin

Dark

3

2

11-cis-retinalVitamin A

Oxidation

Reduction

Opsin and

Light

11-cis-retinal

O


Figure 15.17 Events of phototransduction. Slide 1

Recall from Chapter 3 thatG protein signaling mechanismsare like a molecular relay race.

Retinal absorbs lightand changes shape.Visual pigment activates.

Light (1st

messenger)

Receptor G protein Enzyme 2ndmessenger

Visualpigment

1

Light

11-cis-retinal

Transducin(a G protein)

All-trans-retinal

2 3 Visual pigmentactivatestransducin(G protein).

Transducinactivatesphosphodiesterase (PDE).

4 5 PDE convertscGMP into GMP,causing cGMPlevels to fall.

As cGMP levels fall,cGMP-gated cationchannels close, resultingin hyperpolarization.

cGMP-gatedcation channelopen in dark

cGMP-gatedcation channelclosed in light

Phosphodiesterase (PDE)


Phototransduction In Cones

• Similar as process in rods

• Cones far less sensitive to light– Takes higher-intensity light to activate cones


Light Transduction Reactions

• Light-activated rhodopsin activates G protein transducin

• Transducin activates PDE, which breaks down cyclic GMP (cGMP)

• 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– Stop releasing inhibitory neurotransmitter

glutamate– Bipolar cells (no longer inhibited) depolarize,

release neurotransmitter onto ganglion cells– Ganglion cells generate APs transmitted in

optic nerve to brain


Figure 15.18 Signal transmission in the retina (1 of 2). Slide 1In the dark

cGMP-gated channelsopen, allowing cation influx.Photoreceptor depolarizes.

1

Voltage-gated Ca2+

channels open in synapticterminals.

Neurotransmitter isreleased continuously.

Neurotransmitter causesIPSPs in bipolar cell.Hyperpolarization results.

Hyperpolarization closesvoltage-gated Ca2+ channels,inhibiting neurotransmitterrelease.

No EPSPs occur inganglion cell.

No action potentials occuralong the optic nerve.

Photoreceptorcell (rod)

BipolarCell

Ganglioncell

Ca2+

−40 mV−40 mV

2

3

4

5

6

7

Ca2+

Na+


Figure 15.18 Signal transmission in the retina. (2 of 2). Slide 8

−70 mV No neurotransmitter

is released.

Depolarization opensvoltage-gated Ca2+ channels;neurotransmitter is released.

EPSPs occur in ganglioncell.

Action potentialspropagate along theoptic nerve.

cGMP-gated channelsclose, so cation influxstops. Photoreceptorhyperpolarizes.

Lack of IPSPs in bipolarcell results in depolarization.

Voltage-gated Ca2+

channels close in synapticterminals.

1

Photoreceptorcell (rod)

BipolarCell

Ganglioncell

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 theback of the eye and farthest from theincoming light, is at the top.

Light


Visual Pathway To The Brain

• Axons of retinal ganglion cells form optic nerve • Half of the fibers (medial half) of each optic nerve cross

over at optic chiasm; optic tracts exit• Most optic tract fibers go to lateral geniculate nucleus of

thalamus• Fibers from thalamic (LGN) neurons form optic radiation

and project to primary visual cortex in occipital lobes• Other optic tract fibers go to superior colliculi in midbrain

(initiating visual reflexes) • A few ganglion cells contain melanopsin and project to

other brain areas– Regulate pupil diameter, daily rhythms


Figure 15.19 Visual pathway to the brain and visual fields, inferior view.

Rig

ht

eye

on

ly

Both eyes

Fixation point

Right eye

Supra-chiasmaticnucleus

Pretectalnucleus

Lateralgeniculatenucleus ofthalamus

Superiorcolliculus

The visual fields of the two eyes overlap considerably.Note that fibers from the lateral portion of each retinal fielddo not cross at the optic chiasma.

Occipitallobe(primary visualcortex)

Left eye

Left eye o

nly

Optic nerve

Optic chiasma

Optic tract

LateralgeniculatenucleusSuperiorcolliculus(sectioned)

Uncrossed(ipsilateral) fiberCrossed(contralateral) fiber

Opticradiation

Corpus callosum

Photograph of human brain, with the right sidedissected to reveal internal structures.


Depth Perception

• Eyes see world from slightly different angles• Depth perception (three-dimensional vision)

results from detection of the small differences between R & L eye images

• Requires input from both eyes


Visual Processing

• Retinal cells – Color, brightness, edge detection (by amacrine and

horizontal cells)

• Lateral geniculate nuclei of thalamus– Process for depth perception, cone input emphasized,

contrast sharpened

• Primary visual cortex (striate cortex)– Neurons detect edges, object orientation, movement– Provide form, color, motion inputs to visual

association areas (prestriate cortex)


Cortical Processing

• Occipital lobe centers (prestriate cortex) continues processing form, color, movement

• Complex visual processing extends to other regions– "What" processing identifies objects in visual field– "Where" processing assesses spatial location of

objects– Output from both passes to frontal cortex

Documents

Ch. 15 Special Senses: Vision Slides mostly © Marieb & Hoehn 9 th ed. Other slides by WCR