The eye and vision

Lens

• squashes

• Focuses light to the retina

Retina

• 2 hemispheres per eye (Left and right)

• All around outside of eye

• Photoreceptive part of eye

• Fovea and optic nerve

Retina light detection

• detects light over a huge dynamic range

• Vision isn’t static, subject is scanned to bring the image to fovea

Fovea

• Densely packed w photoreceptors

Retina images

• inverted and smaller than reality

• Brain must interpret the retinal image to construct reality

Structure of retina

• pigment epithelium

• Outer segments of photoreceptors

• Outer nuclear layer

• Outer plexiform layer

• Inner nuclear layer

• Inner plexiform layer

• Ganglion cell layer

• Nerve fibre layer

Nuclear layers of retina

• cell bodies

Plexiform layers retina

• synapses

Photoreceptors

• site of light transduction

3 key layers of photoreception

• photoreceptors

• Bipolar cells

• Ganglion cells

Rod cells

• high sensitivity

• Low temporal resolution - slow response , longer integ time

• More sensitive to scattered light

• Low acuity

• Achromatic

Cone cells

• low sensitivity

• High temporal resolution- fast response, short integ time

• Most sensitive to direct axial rays

• High acuity

• Trichromatic

Link to earlier topic

• GPCR - rhodopsin

Phototransduction step 1

• cis retinal to trans retinal in rhodopsin

Phototransduction Step 2

• transduction activated

Phototransduction Step 3

• cGMP phosphodiesterase activated

Phototransduction Step 4

• cGMP levels fall

Phototransduction Step 5

• cGMP gated ion channels close, no Na and Ca ions enter

• Photoreceptor hyperpolarises

Light adaptation

• cGMP channels close and stops Ca entry

• Guanylate cyclase activated, remakes cGMP

• Opens cGMP gated channels

Guanylate cyclase inhibition

• calcium ions

Transducin

• G protein coupled to rhodopsin

• Dissociates from receptor

• Alpha subunit dissociated and binds to cGMP phosphodiesterase, activates it

Initial response to light

• all cGMP channels close

Photoreceptors in the dark

• depolarised

• Sodium channels open

Photoreceptors in the light

• Hyperpolarised

• Sodium channels close

Cyclic nucleotide gated channels

• odorant receptors

• neurons

Visual processing

• light travels towards outer segments of photoreceptors

• Transduced to electric activity which travels towards nerve fibres, and along nerve fibres

Receptive field

• named by Charles Sherrington

• If many sensory receptor cells converge to form synapses with a single neuron, they form the receptive field of that cell (the neuron)

• Describes the distribution of cells

Receptive fields in retina

• central and peripheral regions

• Circular

• Made of photoreceptors

• If light falls in any part of the field, something happens at the bipolar cells

Ganglion cells receptive fields

• on centr field (off surround)

• Off centre field (on surround)

On centre field

• receptive / visual field

• Light to centre = excited

• Light to periphery = inhibited

• Light all over = no response

• Off surround

• Designed to detect contrast in light

Off centre field

• receptive/visual field

• Light in centre = inhibited

• Light to periphery = excited

• On surround

Convergent signalling in retina principle

• One photoreceptor can contribute to multiple bipolar cells and hence ganglion cells

Convergent signalling mechanism

• overlapping receptive fields

• Each stimulate a number of photoreceptors

• Multiple photoreceptors can contribute to one bipolar cell

• Multiple bipolar cells can contribute to one ganglion

Photoreceptor to bipolar cells signal processing

• bipolar cells have receptive fields

• Receives neurotransmitter from photoreceptor in dark conditions

• Either inhibited or excited by the neuro (on and off centre bipolar cells react differently)

• Upon light stimulation, no transmitter is released from rods/cones so there is a change in transmembrane potential in the bipolar cells

On centre bipolar cells signalling

• inhibited by glutamate - released by cone cells in dark

• Upon light stimulation, neurotransmitter not released

• Membrane depolarises due to loss of inhibition

Off centre bipolar cells signalling

• Excited by glutamate in dark

• Upon light stimulation, neuro not released

• Loss of excitation leads to hyperpolarisation

Firing AP in retina processing

• only ganglion cells

• Mostly neuro, synaptic transmission

• Ganglion cells fire AP to get message to brain

Shaping final output of signal

• more neurotransmitter = stronger signal

• Travels from ganglion cells down optic nerve from on and off centre ganglion cells

Retina in image formation

• Acts as contrast detector

• Detects variations in light

On centre ganglion cells in light detection

• signals rapid increase in light intensity

Off centre ganglion cells in light detection

• signal rapid decrease in light intensity

Visual centre pathways

• 2 hemi retinas per eye

• The 2 left hemi retinas go to left brain and vice versa (crosses over)

Projection of visual information

• Highly ordered fashion du to lateral geniculate nucleus

Lateral geniculate nucleus

• parvocellular layers

• Magnocellular layers

• Projects visual info to brain in a highly ordered fashion

Stimulation of one hemisphere

• allow eyes to foveate on one spot

• Then can use 2 different stimuli to stimulate each side of brain

Split brain

• sever corpus callosum

• Sees word in left visual field, but left hemisphere didn’t see it so cant say the word

• Right hemisphere saw word, but doesn’t control speech

• Left hand, controlled by right hemisphere, picks out correct object via touch corresponding to word

Corpus callosum limitation

• limits no. Of axons which can go to either side of brain

Visual fields of lateral geniculate neurons

• concentric

• Signal to M and P pathways, division of info

Concentric

• circular

M pathway

• analyses movement

P pathway

• analysis of fine detail and colour

Layers of visual cortex

• 7

• Analyse visual information from the lateral geniculate neurons

• Composed of spiny and non spiny neurons

Visual cortex cells

• simple cells

• Rectangular receptive fields

• Detect orientation

Simple cells

• have specific positions in retina

• Have discrete excitatory and inhibitory regions

• Specific axis of orientation to detect light at every axis

• All axes of orientation represented for each part of retina - huge parallel convergent field

• Convergence w cells with concentric receptive fields

Simple cell structure And response

• central area = on area , excited by light

• Outer area = off area , inhibited by light

• Light across whole cell = no response

• Light hits at different orientation = no response

Complex cells

• respond optimally to stimulus w particular orientation

• Don’t have discrete excitatory or inhibitory regions

• As long as orientation of light is the same as the cell, it doesnt matter where on the receptive field the light hits , response is the same

• If light isn’t aligned, there will be no response

Organisation of cells in visual cortex

• highly ordered

• Cells in all orientations

• Consistent patterns

• Neurons present which receive input from one eye or both eyes

IpRGC

• intrinsically photoreceptive retinal ganglion

• Responds to light directly rather than via the retina (like rods and cones)

• 5 classes

Photoreceptors in retina

• rods

• Cones

• IpRGC

Melanopsin

• expressed by ipRGC

• Similar to rhodopsin in its operations

• Primitive opsin, dated v far back

Roles of ipRGC

• projects low acuity images to the lateral geniculate nucleus

• Control circadian clocks and pupil dilation

• Has non vision purposes

• Projects to brain via different nucleus

Circadian clock control

• entrained by light detected by ipRGC

SCN acronym

• suprachaismatic nucleus

Age related macular degeneration

• photoreceptors die

• Lead to blind spots

Photoreceptor transplantation

• removed from development from progenitor cells at intermediate stage

• Transplanted into outer nuclear layer

• Form new synapses and become functional

• Need to treat initial cause of death