Phototransduction & Visual Pathway Mmp March 10

Phototransduction & Image perception

Dr. Nisreen Abo-elmaatyPhysiology Department

The Visual PathwayThe Visual Pathway

The Visual PathwayThe Visual Pathway

Perception of a Visible Object

I- Phototransduction:- How light stimulation of photoreceptors is

converted into nerve impulses? II- Visual Pathway from Retina- How nerve impulses are carried from

retina to higher centres?III- Processing of Visual Information for

perception of an image

1st , u have to know what is the retina & its major elements;

• It is the innermost photosensitive layer.

• Blood supply of retina;

- receptors (rods & cones) supplied by the choroid blood vessels (retinal detachment is so damaging to receptor layer),

- inner layers of retina supplied by central retinal artery.

•3 mm medial to post. Pole of eye ball. exit of optic nerve, (+ retinal bl. vessels) no photoreceptors →blind spot.

Special Parts of the Special Parts of the RetinaRetina

Optic Disc (Blind Optic Disc (Blind Spot)Spot)

Fovea Centralis Fovea Centralis (yellow spot)(yellow spot)

An oval depression An oval depression lateral to optic lateral to optic disc, ~ 0.4 mm disc, ~ 0.4 mm diameterdiameter

Present in the Present in the centre of a small centre of a small yellow pit (Macula yellow pit (Macula Lutea; 4.5 mm Lutea; 4.5 mm diameter). diameter).

Fovea contains Fovea contains cones only cones only →→ highest visual highest visual acuityacuity

10 Histological Layers of the Retina

4 retinal layers of physiological significance

Why Fovea is the most sensitive spot in retina?

• All layers are shifted aside leaving outer segments of photosensors to be hit directly by light

• High density of small diameter Cones with long outer segments

• 1:1 convergence (cone-BC-GC)

• Small RF of foveal ganglion cells

• Wide presentation in occipital primary visual area

Why Fovea is the most sensitive spot in retina?

What is the Role of the 4 retinal layers of What is the Role of the 4 retinal layers of physiological significance?physiological significance?

1)- Retinal Pigmented Epithelium

• Absorption of light (due to presence of black pigment melanin) → reduction of glare (Albinos!!)

• Production of extracellular matrix → keeping outer segments of photoreceptors straight

• Storage of vitamin A (precursor of 11-cis retinal) for regeneration of photosensitive pigments

• Phagocytosis of the tips of outer segments after their shedding off by the photoreceptors → continual renewal of outer segments

2- Photoreceptors; Rods & Cones

The 1st order neuron in visual pathway

Convergence vs. No Convergence

Rods Cones

NumberNumber

DistributionDistribution

~ 120 millions in each retina

More in periphery

Non in Fovea

~ 6 millions in each retina

More in centre

Present

PhotosensitivPhotosensitive pigmente pigment

Rhodopsin 3 types (iodopsin)

ConnectionConnection Convergence

(300:1 connection)

No convergence (1:1; direct private line)

FunctionFunction ↑light sensitivity↓ visual acuity- colour visionNight vision

↓ light sensitivity↑ visual acuity+ colour vision Day vision

In darkness, RMP of photoreceptors is ~ - 40mV In darkness, RMP of photoreceptors is ~ - 40mV (Dark Na current)(Dark Na current)

Light Light →→ certain reactions certain reactions →→ blockage of Dark blockage of Dark current current →→ hyperpolarization (MP of -60:-70 mV) hyperpolarization (MP of -60:-70 mV)

3)- Bipolar Cells• The 2nd order neuron in visual pathway• All bipolar cells secrete excitatory transmitter

(glutamate)• 2 types:• On-bipolar cells; Light →↓ release of inhibitory

transmitter from the ends of photoreceptors→ disinhibition of BC [depolarized] → ↑release of excitatory transmitter → facilitation of GC (↑ ↑fR).

• Off-bipolar cells; darkness →↑ release of glutamate from ends of photoreceptors → inhibition of BC [hyperpolarized] → ↓release of excitatory transmitter → disfacilitation of GC (↓ ↓fR)

4)- Ganglion cells• The 3nd order neuron in visual pathway.• ~ 1.6 million in number.• The only retinal neuron that respond to

stimulation by a full regenerative action potential; depolarization

(all others transmit signals by electrotonic currents)

• Ganglion cells are always active; when unstimulated they keep sending signals at a rate of 5-40/sec, the visual signals are superimposed on this background.

• There are 3 Types of Ganglion cells:1- Magnocellular (M) • Less common (10%)• of large receptive field corresponding to 100s of BC• gross analysis of visual image & location of objects in visual

field2- Parvocellular (P) • Numerous (80%)• of small receptive field corresponding to 1 or few BC• Responsible for fine detailed vision (shape & texture)3- Coniocellular• Few (10%)• Medium in size• Controlling pupillary reflexes.

4- Ganglion cells

♥ The Receptive Field (RF) of Ganglion Cells

• Is the part of the retina it sees through its photoreceptor input;

The retinal region from which stimulatory or inhibitory effect originates → modulating the rate of firing of ganglion cell

• In fovea, the RF of ganglion cell is ~ 2μm; corresponding to width of 1 cone (one line connection)

• In peripheral retina, the RF of ganglion cell is ~ 1mm; (connected to hundreds of photoreceptors).

♥ The Receptive Field (RF) of Ganglion Cells

2 types of 2 types of ganglion cellsganglion cells

: According to : According to the effect the effect exerted by the exerted by the receptive field receptive field on them:on them:

On-centre GCOn-centre GC Off-centre GCOff-centre GC

• Thus, wide illumination of the retina has weaker effect than pinpoint illumination

• Because wide illumination does not give the ganglion cells clear orders

• Whereas pinpoint illumination by its antagonistic nature sharpens the orders to ganglion cells

The Receptive Field (RF) of Ganglion CellsResponse of ON-centre Ganglion cell to Light

What is the role of What is the role of Lateral Cells?Lateral Cells?

Horizontal CellsHorizontal Cells Lateral Lateral

connection connection between rods & between rods & cones, between cones, between bipolar cellsbipolar cells

Responsible for Responsible for lateral inhibition lateral inhibition giving a stop to giving a stop to lateral spread of lateral spread of excitation excitation →→↑visual accuracy ↑visual accuracy

Amacrine CellsAmacrine Cells Lateral Lateral

connection connection between BC & between BC & GCGC

~ 30 types~ 30 types Helping to Helping to

analyse visual analyse visual signals before signals before leaving retinaleaving retina

Phototransduction Phototransduction

= Ionic Basis of photoreceptor Potential= Ionic Basis of photoreceptor Potential= Genesis of electric responses in retina= Genesis of electric responses in retina

Phototransduction

• Light falling on photoreceptors

• Causes a change in photoreceptor potential (what is the ionic basis for this change)

• This releases BC from inhibition (stimulated)

• Excite GC whose firing rate is increased & transmitted along optic nerve.

Effect of Light on Retinal Effect of Light on Retinal NeuronsNeurons

1- Decomposition of photosensitive pigment (Visual purple) in rods & cones by light

• Rhodopsin (Visual purple) = protein (scotopsin + pigment (11-cis retinal).

• Rhodopsin Light conversion of angulated 11-cis retinal to straight all-trans retinal

• This change → splitting between scotopsin & all-trans retinal, thus

• Rhodopsin → → metarhodopsin II (Activated Rhodopsin).

Ionic Basis for Phototransduction

2- Generation of receptor potential by activated rhodopsin

• Activated rhodopsin → activates Transducin, Gt (in disc membrane) → cascade activation of the enzyme phosphodiestrase → cascade hydrolysis of cGMP(splinter of Na+ Ch) → cascade closure of Na+ Ch → ↓ Na entry to outer segment → ↑ the intrarod negativity (Hyperpolarization) → ↓transmitter (glutamate) release in synaptic terminal → disinhibition of bipolar cells → excitation of B cells → excitation of GC that respond by generation of depolarizing action potential → optic nerve → visual pathway.

Ionic Basis for Phototransduction

Ionic Basis of Light-evoked Hyperpolarization in Ionic Basis of Light-evoked Hyperpolarization in PhotoreceptorsPhotoreceptors

• Termination of excitation

Occurs when the activated rhodopsin is inactivated by the enzyme rhodopsin kinase → reversal of all reactions → reopening of Na+ Ch. → termination of excitation.

• Regeneration of photopigment

• All-trans retinal is taken up by RPE which contain isomeraze enzyme that converts it again into 11-cis retinal sent back to rods to recombine with opsin

Retinal on pathwayRetinal on pathway

The Visual Pathway

Neural Processing of Visual Information

On, Off retinal pathways Lateral inhibition serving to sharpen

visual image & increases contrast Thalamic LGB; Cortical Visual areas;

- Primary (area 17; V1)

- Secondary (areas 18, 19; association area, V2,3)

Lateral Geniculate Body; LGB• The Subcortical

thalamic relay nucleus of vision, starting the process of co-ordinating vision from the two eyes

• C-shaped 6 defined layers

• Layers 1, 2 receive from large M ganglion cells → Magnocellular division

• Layers 3,4,5 & 6 from small P ganglion cells → Parvocellular division

LGB• Each layer receives

input from one eye only - Layers 1,4,6 from

contralateral eye - Layers 2, 3,5 from

ipsilateral eye• Responses of neurons

are similar to retinal GC (on-centre & off-centre organization)

• LGB also receives input from brain stem, reticular formation & feedback from cerebral cortex

Primary Visual area (Area 17, V1)

• On medial aspect of each occipital lobe

• Its neurons arranged in the form of columns (1x1x2 mm) forming 6 distinct layers

• The input from the two eyes remain separated (2 columns; Ocular dominance columns)

• Fovea has broad presentation

• Its neurons (layers 2,3,4) project into areas 18 & 19 (association or 2ry visual areas)

Visual Projection to Area Visual Projection to Area 1717

Role of Area 17 (V1)Perception of visible objects without

knowing the meaning of these objects:

• Details of image; shape, borders & colours

• Orientation of object in space

• 3D vision (stereoscopic)Fusion of visual images perceived from the

two eyes (conscious perception of a single image of visual field occurs only after signals reaching association parietal areas)

• For fusion of the two images (perception of two images as one), the 2 images must fall on corresponding points on 2 retinas

• This makes images in register (precisely fused) in cortical neurons

• When the 2 images are not in register; cortical neurons → excitation of interference cortical neurons → signals to Oculomotor apparatus → convergence or divergence or rotation to re-establish fusion

Secondary Visual Processing:Association Areas (18 &19; V2,3 &

V4,5)

• In parietal & temporal lobes

• Dealing with complex perception of patterns & forms responsible for object recognition

• 10% of cells in inferotemporal cortex have large complex receptive fields & selective for specific stimuli (familiar faces)

♣ Lesion → visual agnosia

Retinotopic Organization & Retinotopic Organization & Processing of visual informationProcessing of visual information

COLOUR VISION

• Colour vision is the ability to discriminate different wavelengths that constitute the visible spectrum

COLOUR VISION

• Our perception of light is related to wavelengths of light that are

• transmitted, reflected or absorbed by objects of our visual world.

• An object appears red because this object absorbs short wavelengths of light (would perceived as blue) & reflect long wavelengths which excite retinal cones most sensitive to red

• Colour vision is cone-mediated function• explained by (Trichromatic Theory):• There are 3 types of cones; according to their

sensitivity to a certain light wavelength: S-cones most sensitive (maximally absorb &

respond optimally) to short wavelength (peak absorption at 420nm; perception of blue, absent in central fovea)

M-cones most sensitive to medium wavelength (530nm; perception of green, greatest in central fovea)

L-cones most sensitive to long wavelength (560nm, perception of red)

COLOUR VISION

Colour-sensitive ConesColour-sensitive Cones

• Although each type of cones is stimulated maximally by a certain wavelength but still can be stimulated by other but at a less degree

E.g. in response to light of 531nm wavelength, the green cones respond maximally, red cones less & blue cones not at all

• When red & green cones are stimulated equally, one sees yellow

• When all 3 types are equally stimulated →perception of white

COLOUR VISION

Colour BlindnessColour Blindness Sex-linked recessive disorderSex-linked recessive disorder Defect in colour gene Defect in colour gene →→ deficiency of colour deficiency of colour

photosensitive pigment photosensitive pigment →→ inability to see inability to see one or more coloursone or more colours

Common in males (9%) than females Common in males (9%) than females (0.4%)(0.4%)

Protanopia (red blindness)Protanopia (red blindness) Deuteranopia (green blindness)Deuteranopia (green blindness) Tritanopia (blue blindness)Tritanopia (blue blindness)

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