1 Dr W Kolbinger, Visual System (2009)

Visual System

1

Lecture Outline

• Structures of the Eye

• Refraction and Image Formation

• Visual Acuity

• Autonomic Control of Pupil Diameter

• Clinical Correlations •

Anatomic Considerations

3

The Ocular Fundus

Fovea

Macula

Optic disc

The optic disc region itself only contains axons of retinal ganglion cells, the output elements of the retina, but it lacks photoreceptors. As a consequence, the optic disc is responsible for the blind spot, a region inside the boundaries of the visual field, where we don’t receivevisual information.

Optics of the Eye

Concave Lens Diverges Light RaysConvex Lens Focuses Light Rays

Measurement of the RefractivePower of a Lens—“Diopter”

The refractive power in diopters of aconvex lens is equal to 1 meter divided by its focallength.

5

Optics of the Eye

Cornea refractive power: 42 D

Flat lens refractive power: 13 D

Rounded lens refractive power: 26 D

Plasticity: 13 D

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Accommodation

• Far Vision

Focus on the Retina

Flat lens refractive power: 13 D

Ciliary muscle relaxed

Suspensory ligaments tightened

Accommodation Adjusts the Refractive Power of the Eye

7

Accommodation

• Near Vision

Rounded lens refractive power: 26 D

Focus on the Retina

Ciliary muscle constricted

Suspensory ligaments floppy

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• Near Vision

Blurred picture on the Retina

Flat lens

Presbyopia

The variability of the refractive power of the lens between far vision (13 D)and near vision (26 D) is called refractive plasticity. Unfortunately,the lens looses its elasticityduring aging, therebyreducing the ability to focuson near objects, a conditioncalled presbyopia.

Emmetropia (Normal Vision).

Cataracts“Cataracts” are an especially common eye abnormalitythat occurs mainly in older people. A cataract is a cloudyor opaque area or areas in the lens

10

What is Visual Acuity?

• Two point discrimination of the visual system • Normal: an angle of 5 minutes of a degree

Far vision Near vision

5 ‘

Visual acuity is the ability to distinguish between two nearby points. Visual acuity is high when the two-point discrimination threshold is low (high spatial resolution).

• highly dependent on the densities of retinal photoreceptorsvisual acuity also depends on a proper function of the optical apparatus of the eye, including accommodation. When the optical apparatus fails to produce a focused (sharp) picture on the retina, the objects in the visual field appear “blurry”.

11

20 20

20 800

Distance equivalents

Neurological Examination of Visual Acuity

• Eye charts and near cards

Patient

Normal

13 Dr W Kolbinger, The Retina (2009) 13

RETINA-

• Photoreceptors and Phototransduction • Color Vision • Processing of Visual Information • Clinical Correlations

Receptor andNeural Function of the Retina

Layers of the Retina

Photoreceptor layer

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Photoreceptors

Rod Cone

Outer segment-Visual pigment

Inner segmentSynapses with bipolar

and horizontal cells

Disks

Nucleus

Synaptic ending

Rods are highly sensitive to light and enable us to see under low intensity lightconditions (at night).

Cones(3) are less sensitive to light. enable us to see colors.

glutamate

rhodopsin

•Convergence is high in the rodsystem. It is low in the cone system. As a consequence, spatial resolution (visual acuity) is better in bright light, when the cone system is active.•Rods and cones are not evenly distributed over the whole retina and the fovea only contains cones, but no rods. As a consequence, there is no central vision under dim light conditions

• Night Blindness

The Visual PigmentRhodopsin is the visualpigment of rods. It consistsof two components:· Opsin, a proteinwhich is synthesizedin the photoreceptor(cones have differenttypes of opsins).· Retinal, achromophore, is thelight absorbingcompound or thevisual pigment. It isderived from VitaminA and is the chromophore of the visual pigment in rods and cones. Vitamin A is synthesized from beta-carotene contained in our food.

18 Dr W Kolbinger, The Retina (2009)

,

Photoreception -The Dark Current

Na+

Visual pigment-Rhodopsin

G protein (Transducin)

cGMP phosphodiesterase

cGMP-gated channel

GDP cGMP

19 Dr W Kolbinger, The Retina (2009)

Phototransduction

cGMP phosphodiesterase

GTP cGMP

5’GMP

Ligh

t

Na+

20

Color Vision Is Based on Comparison of Activity ofThree Cone Types

Visible Light Is Part of the Electromagnetic Spectrum

The visible part of the spectrum is characterized bywavelengths ranging from 400 to 700 nm (nanometers).

21 Dr W Kolbinger, The Retina (2009)

Monochromatic Light: Colors of a Rainbow

Three Types of Cones Have Different Spectral SensitivitiesThe human retina containsthree types of cones:· S (short wavelengthsensitive) cones, alsocalled “blue” cones,with a maximumsensitivity at 430 nm· M (mediumwavelengthsensitive) cones, alsocalled “green” cones,with a maximumsensitivity of 530 nm· L (long wavelengthsensitive) cones, alsocalled “red” cones, with a maximum sensitivity of 560 nm.

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Stimulation of Cones by Monochromatic Light

Wavelength (nm)400 500 600 700

450 nm monochromatic lightLo

g re

lativ

e se

nsiti

vity

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Stimulation of Cones by Monochromatic Light

Wavelength (nm)

Log

rela

tive

sens

itivi

ty

400 500 600 700

600 nm monochromatic light

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Red-Green Color Blindness

Normal

Protanopia

Deuteranopia

Individuals affected by red-green color blindness can no longer distinguishcertain red colors from certain green colors.

pseudo-isochromatic colorplates like the one on theright are presented to thepatient. The numbersembedded in the pattern ofcolored dots can bedistinguished by individualswith a fully intact colorvision.

Dichromats, who are weakin red-green discrimination,have difficulties inidentifying the numbers on all plates

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Lecture Outline

• The Visual Field • Passage of Light • Neuronal Pathways • The Primary Visual Cortex • Parallel Pathways for Depth, Motion, Form and

Color • Clinical Correlations

Visual Pathways

The Visual FieldTotal amount of space we can see with this eye, when the eye is fixed straight ahead, pointing towards the center of the visual field (point of fixation). The extension of the visual field is measured in degrees of maximum deviation from this straight line in all directions.

For right eye-- left visual hemifield/nasal- right visual hemifield/temporal

The vertical axis together with the horizontal axis divide the visual field into Four quadrants:- superior left, superior right, inferior left and inferior rightquadrant of the visual field.

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Projections of the Visual Field on the RetinaThe four quadrants of the visual field are projected onto the retina.

The superior half of the visual field is projected to the inferior half of the retina, and vice versa.

The left half of the visual field is projected on the right half of the retina, and vice versa.

The nasal visual hemifield of the right eye temporal hemiretina of the right eye.

temporal visual hemifield of the right eye nasal hemiretina of the right eye

visual field defects e.g “bitemporal hemianopia``

Chief Complaint: Headache and NauseaHistory:A 76 year old retired college dean presents with recurrent headaches over the past fouryears. Her headaches have significantly increased in duration and intensity over thepast few months. She also reports episodes of nausea and vomiting. Her husbandadds that she has recently developed difficulties with comprehending spoken language.The patient experiences uncoordinated movements of the right hand. She stoppeddrinking and smoking cigarettes during her first pregnancy, when she was 27.General Examination:76 year old female in no acute distress. No significant cardiac, respiratory, orabdominal abnormalities. Vital signs unremarkable.

Neurological Examination:The patient can speak fluently, but the content is frequently incomprehensible to thelistener. Her ability to read and write is markedly reduced (dyslexia and dysgraphiarespectively.) The patient can not identify a hairbrush, but was able to demonstrate itsuse. Numbness was detected in the right lower face and right hand. In addition, theexamination of the right hand revealed impaired 2-pt discrimination, joint position, andfine touch.Motor examination and reflex testing of the lower extremities were unremarkable.Marked weakness of right arm flexion and extension was present, as well as elevatedright biceps and brachioradialis reflexes.

Brodmann’s Areas - Left Hemisphere.

circadian clock

afferent limb of the pupillary light reflex

contribute to eye movements

relay station between the retina and the primary visual cortex.

Neuronal Pathways, From the Retina Onwards

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The Primary Visual Cortex V1

Area 17

Occipital pole

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The Primary Visual Cortex V1

Area 17

Parieto-occipital sulcus

Calcarine sulcus

Neuronal Pathways, From the Retina Onwards

• Fibers originating in nasal hemiretina cross over at the optic chiasm, fibers

originating in the temporal hemiretina don’t.• LGN fibers carrying sensory

information of the superior half of the visual field follow the temporal radiation pathway, fibers originating in the inferior half of the visual field follow the parietal radiation pathway.

Example1- the axons originating in the right LGN which are carryingsensory information from the superior left quadrant of the visual field, use thetemporal radiation and synapse in the inferior portion (below the calcarinesulcus) of the primary visual cortex (V1).

Example 2 - ?

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Retinotopic Organization of V1

Visual field left eye

Left hemisphere

Right hemisphere

Visual field right eye

Neurological Examination of the Visual FieldsThe visual pathways are commonly tested in neurological examinations and theyhave high localizing value.

Loss of vision is clinically tested in each quadrant of the visual field in a“confrontation visual field test”. In this test, each eye is tested separately byhaving the patient look straight at the examiners eye, while standing in doublearms length distance. While the examiner occludes his left eye with one hand,the patient occludes his right eye (and vice versa). Then the examiner moves hisother hand, with one (or more) of his fingers stretched out, gradually from theperiphery to the center of the visual field, to determine where it is first seen.Assuming the examiner has normal vision, the patient should see theappearance of the hand at the same time as the examiner. He should also beable to tell the number of fingers stretched.

Sparing of the MaculaWhen the macula is not included, Macular sparing is often associated with vascular lesions involving theposterior cerebral artery or its branches,blood supply of the occipital pole of the cerebbral cortex (the area representing macular vision) may stay intact, due to sufficient blood flowfrom the middle cerebral artery.

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Lecture Outline

• Types of Eye Movements • Extraocular Muscles, their Innervation and

Control • Saccadic Eye Movements • Clinical Correlations

Eye Movements

41

Types of Eye Movements

• Conjugate Eye Movements

• Saccadic eye movements (and gaze)

• Vestibulo-ocular reflex

• Optokinetic reflex (and smooth pursuit)

• Non-Conjugate Eye Movements

• Vergence (convergence and divergence)

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Isolating Extraocular Muscle Function

Neurological Examination of Eye MovementsExtraocular movements are examined using the “H-test”.

44

Cranial Nerve Nuclei and Control Units in the Brainstem

III IV

VI

IIIIV

VI

CN III CN IV

CN VI

Midbrain

Pons

Cerebellum

Medulla

nucleus of MLF-control of vertical eye movements

pontine paramedian reticular formation (PPRF)/horizontal gaze center.

vestibulo-cerebellum (flocculo-nodular lobe)-optokinetic eye movements

45

Cortical Control UnitsFrontal eye field

(Area 8) Parieto-occipital eye field

planning and initiationof eye movements-saccadic eye movements

dorsal (parietal)pathway for motion (and depth) led up exactly into this area.-optokinetic movements and smooth pursuit

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Left Right

Saccadic Eye Movements to the Right

Left MLF

Frontal eye field

PPRF

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Fovea Fovea

Visual field of the left eye

Visual field of the right eye

Diplopia

Internuclear OphthalmoplegiaInternuclearophthalmoplegia is basedon a lesion of the mediallongitudinal fasciculus(MLF), which preventsadduction of the eye on theside of the lesion duringattempted lateral gaze.In the example on the rightshows a patient with alesion of the left MLF(adduction of the left eye isimpaired).Convergence does not involve the MLF and is not affected by the lesion. EXAMPLE- Multiple Sclerosis.

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Internuclear Ophthalmoplegia

Left Right

MLF lesion on the right

X

Prevents adduction of the right eye

Chief Complaint: Nausea and VomitingHistory:An 8 year old boy visited his pediatrician, and his mother reported that he was in good health until about two weeks ago. He describes that his initial symptoms include a mild bifrontal headache, which has become progressively worse. During the last few days, he developed nausea and bouts of occasional projectile vomiting accompanying the headaches. Furthermore, he recently noticed that he has tremendous difficulty when walking down the stairs from his bedroom to the kitchen and reported difficulties in sleeping. No family history of abnormal development or mental retardation exists. He has two perfectly healthy younger brothers.General Examination:This 8 year boy presents with pronounced pubic hair growth, a low pitched voice,enlarged genitalia, and acne on his forehead. Cardiac and respiratory examinationswere unremarkable.Neurological Examination:Patient is alert and oriented x 3. No receptive or expressive aphasias were noted.Pupillary light reflex was intact bilaterally. Both the left and right optic discs appeared more pale than normal. Downward gaze (while the eyes were adducted) was impaired bilaterally.

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Left Right

MLF lesion + PPRF lesion = 1 ½ Syndrome

X

Prevents adduction of the right eye

MLF lesion on the right

PPRF lesion on the right

X X

Prevents conjugate gaze of both eyes to the right

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The Basics of Hair Cell Morphology

Apex of Hair Cell with Cilia

Base of Hair Cell with Synapse on Afferent Fiber

Endolymph: High in Potassium

53

Cells of the Retina

Photoreceptor

Horizontal cell

Bipolar cell

Amacrine cell

Ganglion cell

54

Light Depolarizes ON Center Bipolar Cells

Photoreceptor hyperpolarizes during light ON

On Center Bipolar Cell depolarizes during light ON

Sign-converting synapse: metabotropic glutamate receptor

There are two basic types of retinal bipolar cells. Some bipolar cells are activated(depolarized) when the light is ON. They are therefore called ON bipolar cells.Other bipolar cells are activated (depolarized) during darkness, when the light isOFF. They are therefore called OFF bipolar cells.

Some Bipolar Cells are Activated by Light, Others by Darkness

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Parallel Pathways

MotionDepth

Form Color

LGN

Magno

ParvoVentral (inferior temporal) pathway

Dorsal (parietal) pathway

What?

Where?

Specialized for VisualInformation of Depth, Motion, Form and Color

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Light Hyperpolarizes OFF Center Bipolar Cells

Photoreceptor hyperpolarizes during light ON

OFF Center Bipolar Cell hyperpolarizes during light ON

Sign-conserving synapse: ionotropic glutamate receptor