Vision 2

What is different about the retina and what are the implications of it?

It is part of the CNS as it is an outgrowth of the embryonic diencephalon

• myelination and glial support

  • optic nerve myelinated by oligodendrytes

  • only starts later so keeps retina transparent and prevents light scattering

• blood-brain barrier

  • protects the neural tissue from fluctuations in plasma composition and toxins but makes drug delivery to retina challenging

What are the layers of the retina from back to front?

• black pigmented epithelium

• outer nuclear layer

• outer plexiform layer

• inner nuclear layer

• inner plexiform layer

• ganglion layer

What is in the outer nuclear layer?

nuclei of photoreceptors

What is in the outer plexiform layer?

synapses between photoreceptors and bipolar

What is in the inner nuclear layer?

nuclei of bipolar cells

What is in the inner plexiform layer?

synapses between bipolar and ganglion cells

What is surprising about the direction of light travel?

it passes through all the layers before it reaches the photoreceptors

What are 2 other types of cells found?

• horizontal

• amacrine

Where do horizontal cells act?

receive input from photoreceptors and project laterally to influence surrounding BP and PR cells

Where do amacrine cells act?

between bipolar and ganglion

Roughly how many cones vs rods vs ganglion cells?

7 million vs 100 million vs 1 million

What are the 3 main things that have massive impact on how photoreceptors process light?

• Negative membrane potential of photoreceptors

  • Hyperpolarised as response to incoming light

  • Unique way to react

  • Sensing light and inhibiting

• Response will be graded

  • Membrane potential that goes negative will increase

  • Different steps

  • More intense light is, more negative response will be

• Temporal dynamics of return to resting potential

  • Return is slow for rods

  • Fast for cones

  • Big consequences for temporal resolution

  • Slow decay = summate photons

complex synaptic contacts with bipolar and horizontal cells

What are the 2 major types of bipolar cells?

• ON-response

• OFF-response

How does ON bipolar cells work?

• metabotropic gluatamate receptors at postsynaptic cleft

• they respond to glutamate by hyperpolarising

• less glutamate since hyperpolarised PR therefore means depolarisation

How do OFF bipolar cells work?

• ionotropic glutamate receptors postsynaptically

• these produce classic EPSPs

• therefore hyperpolarisation of cone causes hyperpolarisation of bipolar

What does the high diversity (13 types) of bipolar cells indicate?

bipolar cells perform the first elementary operation on visual signals and provide ganglion cells with highly pre-processed inputs

Are there also on and off ganglion cells?

yes

How can you identify on and off cells?

apply spot of light and see change in AP or not

What are the 3 types of ganglion cell type?

• midget cells

• parasol cells

• small bistratified cells

Midget cells?

• tiny with small cell bodies and dendritic arbours

• only receive input from one bipolar cell

• only on or off, not both

Parasol cells?

• large with massive soma and arbour

• collect input from lot of bipolar cells and PR

• only on or off, not both

Small bistratified cells?

• dendritic arbour split into two

• can have both off and on

As you move from PR to BP to ganglion are there more or less types?

more - processing of light is increasing

What is formed from on and off BP and ganglion cells?

on and off channels where ganglion cels respond to light in the same sense as theur pre-synaptic bipolar cell partners

What is the first neuron in the retina that fires an AP?

ganglion cells

What kinda receptive fields do ganglion cells have?

circular receptive fields with antagonistic center-surround organisation

• means in the centre has on response and around edges has off response or vice versa

• identified by Kuffler 1953

  • applied light to centre and then to other areas and observed

Diagram for centre surround organisation?

What are horizontal cells involved in?

lateral inhibition in outer plexiform layer

Don’t understand next slide on light around and how horizontal cells involved

Why are there larger receptor fields further from the fovea?

lots of photoreceptors (rods) converging on one ganglion cell

Since lots of overlapping receptive fields within the visual field what?

not processing same information, signalling complementary visual information simultaneously and in parallel (eg colour, motion)

What are the 3 main parallel pathways from retina through the LGN to V1?

• koniocellular

  • from bistratified

  • brightness contrast, colour contrast

• parvocellular

  • from midget

  • detail, colour

• magnocellular

  • from parasol

  • motion, luminance

Why are there lots of different damage possible in eye?

damage at different points in the visual pathway cause different defects

What did Hubel and Wiesel do?

investigate LGN by shwing different amounts of light to cat and recorded activity of a single neuron???