Physiology of Vision (2)

Dr. Abdelrahman Mustafa Department Basic Medical Sciences

Division of Physiology Faculty of Medicine Almaarefa Colleges

بسم الله الرحمن الرحيم

Learning ObjectivesBy the end of this lecture, You should able to •Contrast the transduction process for rods and the three types of cones,•Describe the visual pathway •Predict the visual field defects resulting from the following•Describe the topographic representation of the visual field.•Describe the processing of information in the visual cortex, and the consequence of a lesion in the higher visual association areas.

THE RODS AND CONES

• GRNETAL FEATURES• These are photoreceptors of the

retina • Telereceptor• Slowly to never adapt • Contain specific photosensitive

pigments which are chemically changed on exposure to light

• Such changes initiate action potential in the optic nerve fibers

Rods Cones

120 millions / retina 6 millions / retina

At the periphery At Center (Fovea centralis)

Rhodopsin pigment Photopsin (3types).

More convergence (200 rods : 1 bipolar)

One to one connection

More sensitive to light (even to 1 photon).

Less sensitive to light.

Less accurate (less visual acuity).

More accurate (Clear vision)

More functioning at night (Scotopic vision)

More functioning in day (Photobic vision)

Vision in shades of grey

Color perception (3 types for Red, Green & Blue)

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Photopigments • Undergo chemical alterations when activated by light• Consists of 2 components:

1- Opsin:Protein that is integral part2- Retinal:Derivative of vitamin ALight-Absorbing part of photopigment

– Rod pigment: Rhodopsin that Absorbs all visible wavelengths

– Cone pigments (color pigments):Respond selectively to various

wavelengths of light.

A single photon of light can activate a rod whereas several hundred

photons are required to activate a cone

Photoreceptors

Visual Pigments: RODS• Retinal + Opsin = Rhodopsin• Functions only in dark, dim light & peripheral vision• Regenerate only in dark or dim light situations

RHO DOPSIN OPSIN RETINAL

(DIM Light)

Impulse

Visual Pigments: Cones• Retinal + Red, Green or Blue Opsin = Red, Green

or Blue visual pigments• Function only in bright light (daylight)• Provide sharp color images

Red Cone Red Opsin RETINAL

(Light)

Impulse

Green Cone Green Opsin RETINAL Impulse

DARK

rod cell Na+ channels open

membrane depolarizes

Na+ inflow stimulates glutamate release

glutamate inhibits bipolar cells

LIGHT

rod cell Na+ channels close

membrane hyperpolarizes

no Na+ inflow prevents glutamate release

bipolar cells initiate action potential

visual pathway started

NO signal in optic nerve

DARK VERSUS LIGHT ACTION POTINAL CONDITIONS(TRASDUCTION)

Dark versus light conditions

Color Blindness Causes:• Most commonly hereditary

(recessive sex - linked) and affecting males (8%) more than females (0.4 %).

• Very rarely acquired e.g. post encephalitis

:1-Trichromate: Patient has 3 cones but

one of them is weak. 2Dichromate: One cone is totally

absent & other two are present.3- Monchromate:• Patient has only one cone system

and matches the different colors

tested By Ishehara Chart

Visual Fields

• Each eye has two areasFor vision NASAL FIELDS For front of vision Pathway through Temporal

nerve CONTRA LATERALY

TEMPORAL FIELDS For surrounding vision Pathway through NASAL

nerves IPSILATERALY

visual pathway• 1)Optic disc• Collection of nerve fibers that transmit AP• 2)Optic Nerve • Ipislateral nasal nerve (same eye)• Ipsilateral temporal nerve (same eye)• 3)Optic chiasma• Nasal nerves crosses to go to the opposite side of the

brain DECASSATION • 4)Optic Tract • Ipislateral temporal nerve (same eye)• Contaletral nasal nerve (other eye)

• 5) Lateral geniculonate Body • The LGN contains a topographic representation of

what the retina “sees”.• This retinotopic map is sent to the visual cortex.

• 6)Optic Radiation • From LGN to visual cortex • Ipislateral temporal nerve (same eye)• Contaletral nasal nerve (other eye)

visual pathway• 7)Primary visual cortexArea 17 (=Primary visual area)

- Concerned with the

appreciation of visual sensations.

Area18(=Secondaryvisualarea/

visual association area)

- Concerned with correlation and

integration of visual sensations.

Area 19 (=Occipital eye field)

- Concerned with movement of

the eye.

point sourceof light

Fovea

RetinaVitreous

IrisLens

Cornea

Optic nerve

Right eye

nasal temporal

Ganglioncells

Conephotoreceptors

Conebipolar cells

Optic axons

Optic nerve

Opticchiasm

Opticradiation

Visualcortex

Optic tract

Lateralgeniculatenucleus

Superiorcolliculus

Hypo-thalamus

Pretectum

Whole brain

ventral surface Optic nerve

Optic tract

Optic chiasm

LateralGeniculateNucleus

Optic

radiation

Lesions of Visual Pathway

Lesions of Visual Pathway

V1 V1

LGN LGN

Optic Chiasmbitemporal hemianopaia

Q1

• Which of the following NOT feature of cons receptor

• A) about6 millions / retina• B)Found at At Center (Fovea centralis)• C) has Photopsin (3types).• D)More sensitive to light.

Q2

• Transduction of dark vision inhibited by• A)glutamate releasing • B)Na influx• C)Bipolar cell block • D) Hyperoplrization

Q3

• Which of the following DOSE NOT considered as visual pathway Station

• A)Optic disc• B)Optic tract • C)Medial geniculate Body of the Thalamus• D)Visual cortex

Q4

• Decassation of nasal nerve occur in:• A)Optic disc• B)Optic chisma• C)Thalamus• D)Visual cortex

References

• Human physiology by Lauralee Sherwood, 7th edition

• Text book physiology by Guyton &Hall,12th edition

• Text book of physiology by Linda .s contanzo,third edition

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