Week 12: Retina and Visual Pathway Flashcards
List the retinal layers from anterior to posterior (hint: 10)
- Retinal pigment epithelium
- Photoreceptors (rods and cones)
- Outer limiting membrane
- Outer nuclear layer
- Outer plexiform layer
- Inner nuclear layer
- Inner plexiform layer
- Ganglion cell layer
- Nerve fibre layer
- Inner limiting membrane
Describe the retina
- Retinal photoreceptors act as transducers
- Converts absorbed electromagnetic radiation into a neural signal
List three functions for the retina/visual system
- Spatial discrimination
- Colour discrimination (trichromatic)
- Detection of movement
Describe the rods and cones functions of the following: Peak Responsiveness, Amplification, Direction of light, Visual acuityFovea / Periphery & Colour vision
Rods:
- Peak Responsiveness = low light levels
- Amplification = high
- Direction of light = sensitive to scattered light
- Visual acuity = low
- Fovea / Periphery = not present / concentrated
- Colour vision = no – achromatic (one type of rod pigment)
Cones:
- Peak Responsiveness = high light levels
- Amplification = low
- Direction of light = sensitive to direct axial rays
- Visual acuity = high
- Fovea / Periphery = concentrated/low
- Colour vision = yes – chromatic (three types of cone pigment)
List the dimensions for a retinal fundus
- Macula 5.5 mm in diameter
- Fovea 1.5 mm in diameter
- Foveola 0.35 mm in diameter
- Optic Nerve Head: 1.5 mm Horizontal, 2 mm Vertical
Describe the fovea
- A small central pit
- Contains closely packed cones
- Limited convergences: area of greatest visual acuity
Describe phototransduction
- Rods & cones contain chemicals that change shape when exposed to light
- Photoreceptor excitation is caused by hyperpolarisation
Describe rods
- Outer segment contains rhodopsin
- When retinal is in a ’11-cis’ configuration, it binds with scotopsin to form rhodopsin
- When rhodopsin absorbs light it begins to decompose
- Retinal changes from the cis to trans form
Describe phototransduction in the dark
- An inward current flows into the outer segment, carried by Na+ ions, through cGMP-gated channels
- Current flows out of the cell, through the outflow of K+ ions
- Then through non-gated K+ selective channels in the inner segment of the rod
- Causing a reduction in membrane potential
- Induces the release of glutamate from synaptic end-feet due to Ca2+ influx
Describe phototransduction in the light
- Rhodopsin decomposition causes breakdown of cGMP
- Closes cGMP-gated channels
- Induces a hyperpolarisation of membrane
- Cell stops releasing glutamate from synaptic end-feet
Describe light adaption
- Sensitivity of the retina needs to be adjusted to detect the dark and light spots on images
List ways of light adaption
- Opening & closing of the pupil
- Neural adaptation
- Modulation of amount of light sensitive pigment present
Describe colour vision
- Being able to distinguish different wavelengths of light
- In humans, it is achieved by our cone photoreceptors:
- S cones = short wavelength sensitive cones (blue cone)
- M cones = medium wavelength sensitive cones (green cone)
- L cones = long wavelength sensitive cones (red cone)
List the retinal interneurons
- Horizontal Cells
- Bipolar Cells
- Amacrine Cells
- Ganglion Cells
Describe Horizontal Cells
- Laterally interconnecting neurons
- Provide lateral inhibitory feedback
Describe Bipolar Cells
- Transmit signals vertically from rods, cones & horizontal cells to ganglion or amacrine cells for communication to the brain
- Dendrites from bipolar cells receive from either rods or cones NOT both
Describe Amacrine Cells
- Aid in the beginning of visual signal analysis
- About 30 different types
Describe Ganglion Cells
- Transmit output signal from retina to brain via optic nerve
- Only ganglion cells communicate using action potentials
List the different types of ganglion cells
- Midget Ganglion Cells
- Parasol Ganglion Cells
- Bistratified Ganglion Cells
Describe Midget Ganglion Cells (X-Cell)
- Largest population of ganglion cells (80%)
- Mostly in fovea
- Involved in fine spatial discrimination & colour perception
Describe Parasol Ganglion Cells (W-Cell)
- Make up around 10% of ganglion cell population
- Synapse with all bipolar cells except midget
- Involved in spatial discrimination & detection of movement
Describe Bistratified Ganglion Cells (Y-Cell)
- Make up around 5% of ganglion cell population
- Involved in perception of yellows and blues
Explain Lateral Inhibition
- Horizontal cells connect laterally between rods, cones & bipolar cells
- Essential for transmitting contrast borders in the visual image
Describe On and Off Centre Cells
- Second mechanism for enhanced detection of borders/ edges
- On-centre cells: excited when stimulated by light in centre, inhibited by light in periphery
- Off-centre cells: inhibited when light strikes centre, excited by light in their periphery
List the visual pathway to the brain
- Retina
- Optic nerves
- Optic chiasm
- Optic tracts
- Lateral geniculate bodies (LGN)
- Optic radiations
- Visual cortex
Explain Lateral Geniculate Nucleus (LGN)
- Located in the thalamus
- Primary relay centre for visual information from retina to primary visual cortex
- Consists of two main zones, and one sub-zone:
1. Parvocellular (layers 3-6)
2. Magnocellular (layers 1-2)
3. Koniocellular (sub-layers 1-6)
Describe Layers 1 & 2: magnocellular layers
- Large neurons
- Mainly receive input from parasol ganglion cells
- Rapid conduct. colour blind, black & white information
Describe Layers 3-6: parvocellular layers
- Small to medium sized neurons
- Mainly receive input from midget ganglion cells
- Moderate conduct, convey colour, accurate point to point spatial information
Describe sub-layers 1-6: Koniocellular sub-layers
- Medium sized neurons
- Mainly receive input from bistratified ganglion cells
- Moderate conductance, convey colour (yellow & blue)
Define Anopia/Anopsia
- A defect in the visual field
Define Hemianopia
- An anopia affecting half of the visual field of one eye
Define Quadrantanopia
- An anopia affecting a quarter of the visual field of one eye
Define Homonymous
- Same side of both eyes
Define Baum’s Loop
- Superior retinal fibres (upper occipital radiation)
Define Meyer’s Loop
- Inferior retinal fibres (lower occipital radiation)
