Physiology of Vision by Dr Shahab

7/30/2019 Physiology of Vision by Dr Shahab

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DR SHAHAB SHAIKH


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Visible light is just a part of the electromagnetic spectrum


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The Eye and VisionVisual organ the eye

70% of all sensory receptors are in the eyes

40% of the cerebral cortex is involved in processingvisual information


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Medial View of the Eye

Figure 16.7a


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The Fibrous Tunic Most external layer of the eyeball

Composed of two regions of connective tissue

Sclera posterior five-sixths of the tunic

White, opaque region

Provides shape and an anchor for eye muscles Cornea anterior one-sixth of the fibrous tunic

Limbus junction between sclera and cornea

Scleral venous sinus allows aqueous humor to drain


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The Vascular Tunic The middle coat of the eyeball

Composed of choroid, ciliary body, and iris

Choroid vascular, darkly pigmented membrane Forms posterior five-sixths of the vascular tunic

Brown color from melanocytes

Prevents scattering of light rays within the eye

Choroid corresponds to the arachnoid and pia maters


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Posterior View of the Anterior Half

of the Eye

Figure 16.9a


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Copyright 2005 Pearson

Education, Inc., publishing asBenjamin Cummings

The Vascular Tunic Ciliary body thickened ring of tissue encircles the

lens

Composed of ciliary muscle Ciliary processes posterior surface of the ciliary body

Ciliary zonule (suspensory ligament)

Attached around entire circumference of the lens


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The Vascular Tunic

Figure 16.8


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The IrisVisible colored part of the eye

Attached to the ciliary body

Composed of smooth muscle Pupil the round, central opening

Sphincter pupillae muscle (constrictor or circular)

Dilator pupillae muscle (dilator or radial)

Act to vary the size of the pupil


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Pupillary dilation and constriction


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The Sensory Tunic (Retina) Retina the deepest tunic

Composed of two layers

Pigmented layer single layer of melanocytes Neural layer sheet of nervous tissue

Contains three main types of neurons

Photoreceptor cells

Bipolar cells Ganglion cells


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Education, Inc., publishing asBenjamin Cummings Figure 16.5b

Accessory Structures of the Eye

Lacrimal apparatus keeps the surface of theeye moist

Lacrimal gland

produces lacrimalfluid

Lacrimal sac fluidempties into nasal

cavity


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Microscopic Anatomy of the Retina

Figure 16.10a


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Photoreceptors Two main types

Rod cells more sensitive to light

Allow vision in dim light

Cone cells operate best in bright light

Enable high-acuity, color vision

Considered neurons


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Photoreceptors

Figure 16.11


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Regional Specializations of the

Retina Macula lutea contains mostly cones

Fovea centralis contains only cones

Region of highest visual acuity

Optic disc blind spot


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Medial View of the Eye

Figure 16.7a


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The Pupillary Muscles


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Education, Inc., publishing asBenjamin Cummings Figure 16.10c

Blood Supply of the Retina

Retina receives bloodfrom two sources Outer third of the

retina supplied bycapillaries in thechoroid

Inner two-thirds of theretina supplied bycentral artery and vein

of the retina


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Internal Chambers and Fluids The lens and ciliary zonules divide the eye

Posterior segment (cavity)

Filled with vitreous humor Clear, jelly-like substance

Transmits light

Supports the posterior surface of the lens

Helps maintain intraocular pressure


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Internal Chambers and FluidsAnterior segment

Divided into anterior and posterior chambers

Anterior chamber between the cornea and iris

Posterior chamber between the iris and lens

Filled with aqueous humor

Renewed continuously

Formed as a blood filtrate

Supplies nutrients to the lens and cornea


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Internal Chambers and Fluids

Figure 16.8


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The LensA thick, transparent, biconvex disc

Held in place by its ciliary zonule


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L Z l Fib & Cili


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Lens, Zonule Fibers, & Ciliary

Muscles


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Fig. 10.33


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Lens Epitheliumcapsule

epithelium

fibers


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The Eye as an Optical Device Structures in the eye bend light rays

Light rays converge on the retina at a single focal point

Light bending structures (refractory media) The lens, cornea, and humors

Accommodation curvature of the lens is adjustable

Allows for focusing on nearby objects


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REFRACTION


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FACTORS AFFECTING FOCAL LENGTH


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IMAGE FORMATION BY BICONVEX LENS


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Image Formation


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Figure 17.10

Accommodation


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Visual Abnormalities

RETINA


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The Organization of the Retina

RETINA

RETINA


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RETINA


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SEM OF MACULA & FOVEA


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Retina

Consists of single-cell-thick pigmentedepithelium, layers of otherneurons, andphotoreceptor neurons

(rods and cones). Neural layers are forward

extension of the brain.

Neural layers face outward,toward the incoming light.

Light must pass throughseveral neural layers beforestriking the rods and cones.


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Cones under electron microscope


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Retina (continued)

Rods and cones synapse with other neurons. Each rod and cone consists of inner and outer segments.

Outer segment contains hundreds of f lattened discs with photopigmentmolecules.

New discs are added and retinal pigment epithelium removes old tip

regions. Outer layers of neurons that contribute axons to optic nerve

called ganglion cells. Neurons receive synaptic input from bipolar cells, which receive

input from rods and cones.

Horizontal cells synapse with photoreceptors and bipolar cells. Amacrine cells synapse with several ganglion cells.

APs conducted outward in the retina.


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Effect of Light on Rods

Rods and cones areactivated when lightproduces chemicalchange in rhodopsin. Bleaching reaction:

Rhodopsin dissociates intoretinene (rentinaldehyde)and opsin. 11-cis retinene dissociates

from opsin when convertedto all-trans form.

Initiates changes in ionicpermeability to produceAPs in ganglionic cells.

R d d C


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Rods and Cones

WALDS VISUAL CYCLE


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WALDS VISUAL CYCLE


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Photoreception

Animation: Photoreception (see tutorial)

Ph t ti


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Photoreception


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Bleaching and Regeneration of

Visual Pigments


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Visual Acuity and Sensitivity

Each eye oriented so thatimage falls within foveacentralis. Fovea only contain cones.

Degree of convergence ofcones is 1:1.

Peripheral regions containboth rods and cones. Degree of convergence of

rods is much lower.

Visual acuity greatest andsensitivity lowest whenlight falls on fovea.


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Neural Processing of Visual Information Receptive field:

Part of visual field that affects activity of particularganglion cell.

On-center fields:

Responses produced by light in the center of visualfields.

Off-center fields: Responses inhibited by light in the center, and

stimulated by light in the surround.


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Fig. 10.35


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Fig. 10.36


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Fig. 10.38


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Fig. 10.39


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Fig. 10.41


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Fig. 10.42


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NEURAL FUNCTION OF RETINARODS AND CONESHORIZONTAL CELLSBIPOLAR CELLSAMACRINE CELLS

GANGLION CELLS


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Copyright 2005 PearsonEducation, Inc., publishing asBenjamin Cummings

Visual Pathways Most visual information travels to the cerebral cortex

Responsible for conscious seeing

Other pathways travel to nuclei in the midbrain anddiencephalon


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Visual Pathways to the Cerebral

Cortex Pathway begins at the retina

Light activates photoreceptors

Photoreceptors signal bipolar cells

Bipolar cells signal ganglion cells

Axons of ganglion cells exit eye as the optic nerve

Fibers from the Nasal halves of Retina cross to opp side inthe Optic Chiasma while the fibers from the temporalside of retina do not cross.

Thus together Nasal side fibers of opposite side andTemporal side fibers of same side continue as Optic Tract

Visual Pathways to the Cerebral Cortex


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Visual Pathways to the Cerebral Cortex

Optic tracts terminates in Lateral geniculate nucleus

(LGB) of the thalamus. From the LGB Fibers of the optic radiation reach the

primary visual cortex

This nucleus is composed of six nuclear layers.

Layers II, III, and V receive signals from the Temporalhalf of the ipsilateral retina, whereas layers I, IV, and VIreceive signals from the Nasal half of the retina of theopposite eye


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The dorsal lateral geniculate nucleus is divided in anotherway:

(1) Layers I and II are calledMagnocellular layersbecausethey contain large neurons.

(1) These receive their input almost entirely from the large typeY retinal ganglion cells. provides a rapidly conductingpathway to the visual cortex. However, this systemtransmits only black-and-white information. Also, its point-to-point transmission is poor

(2) Layers III through VI are called Parvocellular layersbecause they contain small to medium-sized neurons.(1) These neurons receive their input almost entirely from the

type X retinal ganglion cells that transmit color and conveyaccurate point-to-point spatial information


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Visual

Pathways


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Visual Pathways to Other Parts of

the Brain Some axons from the optic tracts

Branch to midbrain

Superior colliculi

Pretectal nuclei

Other branches from the optic tracts

Branch to the suprachiasmatic nucleus


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PRIMARY COLORS


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Coding of Visual Information in the Retina

Photoreceptors: Trichromatic Coding


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Photoreceptors: Trichromatic Coding Peak wavelength sensitivities of the three cones:

Blue cone: Short- (420 nm)Green cone: Medium- (530 nm)

Red Cone: Long- (560 nm)


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Light & Dark Adaptation


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Light & Dark Adaptation

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Physiology of Vision by Dr Shahab

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