3.2-3.3 Vision Flashcards
(47 cards)
Eyeball Anatomy: Outer Layer (2)
<ul> <li>Eyeball Anatomy: Outer Layer (1/6th) (2) <ul> <li>Cornea: Transparent to allow light in</li> <li>Sclera: Outer shell to protect eyes</li> </ul> </li></ul>
Eyeball Anatomy: Inner Layer (5/6th) (2)
<ul> <li>Eyeball Anatomy: Inner Layer (5/6th) (2) <ul> <li>Neuronal Layer</li> <li>Macula: On the retina, surrounds the fovea and is responsible for central vision (20/20)</li> </ul> </li></ul>
Factors limiting visual acuity (2)
Neural factors: How neurons are connected<br></br>Optical factors: How much can light hit neurons (3)<br></br>
Optical factors: How much can light hit neurons (3)
1.Pupil Size<div>2. Clarity of Optical Media</div><div>3. Refractive errors (Leads to blurring)<br></br><br></br></div>
Optical Factor:Pupil Size (1)
<ul> <li>Pupil Size (1) <ul> <li>Smaller = More Clear</li> </ul> </li></ul>
Optical Factors: Clarity of Optical Media (2)
<ul> <li>Clarity of Optical Media (2) <ul> <li>Cataracts (Len opacity)</li> <li>Corneal Opacity</li> </ul> </li></ul>
Optical Factors: Refractive errors (Leads to blurring) (4)
<ul> <li>Refractive errors (Leads to blurring) (4) <ul> <li>Myopia (short sightedness, eyeball is too long and light doesn’t focus on the retina)</li> <li>Hypermetropia (long sightedness, eyeball is too short and light doesn’t focus on the retina)</li> <li>Astigmatism (odd shaped eyeball)</li> <li>Presbyopia</li> </ul> </li></ul>
Basic Properties of Rods: Vision, Types, Colour, Number, Fovea
<ul> <li>Basic Properties (6) <ul> <li>Night vision (Scotopic)</li> <li>Very sensitive</li> <li>Only 1 type of rod</li> <li>No colour vision</li> <li>100 million (1Cone:20Rod)</li> <li>Absent from the fovea</li> </ul> </li></ul>
Basic Properties of Cones: Vision, Types, Colour, Number, Fovea
<ul> <li><ul><li>Cones<br></br></li></ul></li><li><ul> <li>Basic Properties (5) <ul> <li>Day vision (Photopic)</li> <li>3 types: red, blue and green</li> <li>Allow colour vision</li> <li>5 million (1 Cone: 20Rod)</li> <li>Most dense in the fovea (middle of macula)</li> </ul> </li> </ul> </li></ul>
Why can photoreceptors respond to light (2)
<ul> <li>Photoreceptors contain photopigments that are activated by light <ul> <li>In cones, the photopigments are 1 or 3 different coneopsins</li> <li>In rods, the photopigments are rhodospin</li> </ul> </li> <li>Opsins bind to vitamin A (all-trans-retinal), which picks up light signals</li></ul>
PhototransductionStep 1 (2): Change in Structure
“<ul> <li>When light hits photoreceptors, 11-cis retinal becomes all trans retinal</li> <li>This change in conformation causes a change in rhodopsin, and converts a light signal into an electrical signal<br></br><br></br><img></img><br></br></li></ul>”
Phototransduction Step 2 (2): Depolarisation
“<ul> <li>Photoreceptors are depolarized in the absence of light and are hyperpolarized by light (Respond via. changes in graded potentials, not APs)</li> <li>Glutamate is the NT used in photoreceptors (Dark: Release NT all the time; Light: Reduce NT)</li></ul><div><img></img><br></br></div>”
PhototransductionStep 3 (2): Dark vs Light
“<ul> <li>In the dark, <ul> <li>cGMP gated Na+ channel has continuous Na+ influx</li> <li>Causes depolarization of the photoreceptor</li> </ul> </li> <li>In the light, <ul> <li>cGMP is broken down into GMP</li> <li>cGMP is no longer present to keep open Na+ channels. Na+ influx ceases and causes hyperpolarisation<br></br><br></br><img></img><br></br></li> </ul> </li></ul>”
<br></br>TLDR Summary of Phototrasnduction (4)
“<ul> <li>Light causes a conformational change in retinal, which in turn causes a conformational change in rhodopsin</li> <li>Rhodopsin activates trasducin</li> <li>Transducin activates phosphodiesterase</li> <li>Phosphodiesterase breaks down cGMP, causing closure of Na+ channels and hyperpolarization</li></ul><div><img></img><br></br></div>”
Retina: General neuralstructure and function (2)
“<ul> <li>Retina contains many cell types (Light must pass through from ganglion to photoreceptor)</li> <li>Many vascular and supporting cell types around photoreceptors important for keeping them alive</li></ul><div><img></img><br></br></div>”
Retina: Pathways and Interactions
”"”Through”” Pathway (3)<br></br>- Photoreceptors (Rods and Cones)<div>- Bipolar Cells<br></br>- Ganglion Cells</div><div><br></br></div><div>Lateral Interactions</div><div>- Horizontal Cells</div><div>- Amacrine Cells</div>”
Bipolar Cells: Function, Types
<ul> <li>Synapses with both photoreceptors and ganglion</li> <li>10 types (1 for rods, 9 for cones)</li> <li>Spatial vision and colour vision</li></ul>
Define Receptive Field? (1)
<ul> <li>Area of the retina when stimulated, causes a change in ganglion cell membrane potential</li></ul>
Types? (4) Ganglion cells. What is the shape of the receptive field?
<ul> <li>Many types (20 Types) <ul> <li>ON: Switch on when light shines in receptive field</li> <li>OFF: Switch off when light shines in receptive field</li> <li>M: Motion</li> <li>P: Colour</li> </ul> </li> </ul>
<div>Hence, ganglion cells respond to light differently, depending on where light falls in receptive field (CONCENTRIC)</div>
Function? (1) Ganglion Cells
<ul> <li>Output of retina neurons</li></ul>
Action? (1) Ganglion Cells
In response to light, they release glutamate and fire APs (Only cell in the retina that does it)
“<img></img>What is the red and green”
Red: Horizontal Cells (Input and Output: Both photoreceptors)<div><br></br></div><div>Green: Amacrine Cells (Input: Bipolar & Output: Bipolar and Ganglion)</div>
Horizontal Cells: Function, Action, Response
<ul> <li>Lateral inhibition of the retinal through pathway <ul> <li>Receive input from photoreceptors</li> <li>Output to other photoreceptors</li> </ul> </li> <li>Uses GABA</li> <li>Respond to light by hyperpolarizing</li></ul>
Amacrine Cells: Function, Action, Importance
<ul> <li>Lateral inhibition of the retinal through pathway</li> <li>Axonless</li> <li>Release inhibitory NTs (GABA and glycine)</li> <li>Important for modulating the synapse between bipolar and ganglion cells <ul> <li>Input: Bipolar</li> <li>Output: Bipolar and Ganglion</li> </ul> </li></ul>
- Large
- 10% of ganglion cells
- Large receptive field
- Motion detection, flicker and analysis of gross features
- Small
- 80-90% of ganglion cells
- Visual acuity and colour vision
- P ganglion cells respond best when a specific wavelength of light is shone on their receptive field
- Axons of ganglion cells form the optic nerve, and project to the lateral geniculate nucleus (LGN) in the thalamus
- LGN neurons project through the optic radiations to the visual cortex in the occipital lobe
- Fibres from left and right optic nerve form Optic chiasm
- Nasal fibres of the optic nerve (from the nasal side of the retina) cross at the optic chiasm
- Temporal fibres of the optic nerve (from the temporal side of the retina) don’t cross at the optic chiasm
- Lies at base of the brain (anterior to the pituitary)
- M cells (layers 1 – 2), which receive input from M ganglion cells (1 Layer for each eye)
- P cells (layers 3 – 6), which receive input from P ganglion cells (2 Layers for each eye)
- Hence, left and right LGN receive inputs from both eyes (segregated information)
- White matter tract where LGN neurons (M/P) axons project through
- Temporal and parietal lobes
- Synapse in V1Optic Radiation = White thick structure
- Area 17 located in the occipital lobe, surrounding the calcarine fissure
- Posterior-most (outer) part of the V1 encodes central vision
- Anterior-most part of the V1 encodes peripheral vision (inner)
- 6 layers of the visual cortex, but input from the LGN projects to layer 4C
- M type GC/LGN cells input to layer 4Cα
- P type GC/LGN cells input to layer 4Cβ
- Inputs are segregated into ocular dominance columns, where inputs from each part of the eye are adjacent and interspersed
Orientation columns: Between layers of the cortex, neurons in a particular location will respond the same way to bars of light, and neurons in a different location will respond differently
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- Retina/Optic Nerve
- Lose ability to see in one eye
- Chiasm
- Lose left hemifield in left eyes and right hemifield in right eyes
- Far Back
- Lose vision in same hemifield on both sides
- Receives retinotopic information from V2 and V3
- Receives input from cells in layer 4B of V1 (i.e. M type cells)
- Neurons respond to abstract shapes and colours
- Important for visual memory, perception, and faces
