The Visual System Flashcards
What determines the degree of refraction?
The difference in refractive index of the two substances and the angulation
What is refractive index?
The ratio of the speed of light in air to the speed of light in a substance
I.e. 300,000/speed of light in substance
What does a convex lens do?
Bends light inwards (focuses light)
What does a concave lens do?
Diverges light
What is refractive power measured in?
Dioptres
Dioptre = 1/focal length
What is the refractive power or the lens?
20-34 dioptres
What is accommodation?
The process of increasing the refractive power of the lens to focus on objects as they move closer
What happens in the eye during accommodation?
Ciliary muscles contract, suspensory ligaments lose tension, lens relaxes (becomes more spherical)
How does pupil size affect depth of focus?
Small pupil = increased depth of focus
What are cataracts?
Cloudy/opaque area of the lens due to a coagulation of lens proteins
What is an astigmatism?
Visual image in one plane focuses at a different distance to other planes or focus due to imperfections in the shape of the cornea
What is myopia and how is it corrected?
Axial length of the eye is too long, corrected with concave lenses
What is hyperopia and how is it corrected?
Axial length of the eye is too short, corrected with convex lenses or accommodation
What are the layers of the retina?
Pigmented, outer nuclear, outer plexiform, inner nuclear, inner plexiform, ganglion cell, stratum opticum, inner limiting membrane
What is the retina responsible for?
Contrast, movement and colour
What is the fovea?
Centre of the retina, contains only comes, area where neuronal cells and blood vessels are displaced so that light strikes the comes directly, allowing for the highest visual acuity and discrimination
What cones make up the foveal cone mosaic?
S cones: short wavelength (blue)
M cones: medium wavelength (green)
L cones: long wavelength (red)
Describe the structure of photoreceptors
Outer segment containing membrane shelves lined with rhodopsin, inner segment containing mitochondria
What is the function of rhodopsin?
Capture light and convert it to a neural signal
What is rhodopsin made up of?
Retinal (derivative of vitamin A) in 11-cis formation and scotopsin
What happens when rods are exposed to the dark?
Rods are depolarised
- inward current flows into outer segment carried by Na+ through CGMP channels
- current flows out through efflux of K+
- glutamate released from synaptic end feet due to opening of calcium channels
What happens when rods are exposed to light?
- Light activated rhodopsin stimulates G-protein transducin and decomposed
- Transducin activates phosphodiesterase which breaks down CGMP
- CGMP gated channels close
- Efflux of Na+ causes hyperpolarosation
- Cell stops releasing glutamate
What happens during light adaptation?
Photochemicals in rods and cones convert to retinal and opsin, remaining retinal converts to vitamin a
Results in decreased light sensitivity
What happens during dark adaptation?
Retinal and opsin covert to light sensitive pigments, vitamin a converts to retinal
Results in increased light sensitivity
What name is used to describe colour blindness involving lack of:
A) red cones
B) green cones
C) blue cones
A) protanope
B) deuteranope
C) tritanope
What is the function of horizontal cells?
Connect laterally and provide lateral inhibitory feedback to allow for spatial resolution
What is the layout of bipolar cells?
Connect vertically from photoreceptors and horizontal cells to ganglion cells, receiving info from rods or cones but never both
What is the function of amacrine cells?
Aid the beginning of visual signal analysis
What are the three types of ganglion cells and what are the responsible for?
1) Midget cells: Fine spatial discrimination, colour perception
2) Paracrine cells: Movement detection, spatial discrimination
3) Bistratified cells: Differences between S, M & L cones (perception of yellows and blues)
How do ganglion cells communicate?
Action potentials
How does the retina communicate?
Electronic conduction
What is lateral inhibition?
The output of horizontal cells is always inhibitory, preventing lateral spread of light excitation (essential for transmitting contrast borders)
What visual information remains on the same side and what visual information decussates?
Same side: Info from lateral/temporal side of the eye
Contralateral sure: Info from medial/nasal side of the eye
What is the LGN and where is is located?
Primary relay centre for visual info, located in the thalamus
Describe the two zones of the LGN
1) Magnocellular
- Layers 1-2
- Input from parasol ganglion cells
- Rapidly conducting
- Colour blind info only, general spatial info
2) Parvocellular
- Layers 3-6
- Input from midget ganglion cells
- Moderate conductance
- Conveys colour and accurate point-to-point spatial info
Describe the sublayers of the LGN
Koniocellular sublayers
- Sublayers 1-6
- Receive input from Bistratified ganglion cells (layers 3&4)
- Moderate conductance
- Convey colour (yellows and blues)
Describe the layers of the primary visual cortex
Layers 1-3: Transmit signals short distances laterally
Layer 4: Entry point for sensory info
Layers 5-6: Transmit signals greater distances
