The eye

• EYE

  • light can be described as an electromagnetic waves and a bunch of photons
  • 2 features of light:

  1. WAVELENGTH: colour of the light, can shorten or increase as it shows peak to peak
  2. AMPLITUDE: peak to trough, shows intensity of light --> larger the amplitude = brighter the light

  • wavelength = speed/ frequency
  • we can only see a narrow wavelength range (380 nm- 750 nm = visible light)
• STRUCTURE OF A HUMAN EYE

  • ability of the light to focus light --> due to lens

  1. adjust the shape and strength to accommodate to objects far or close to the eye
  2. held in place by muscles and fibres

  • iris --> act as a apacture
  • retina is found in the eye but technically is part of the brain
  • cornea --> greatest refracting power in the eye
  • image on the retina is inverted 
  • fat lens --> for near aobjects
  • flat lens --> for close objects 

  
• RETINA

  • the retina is made up of specialised neurons
  • photoreceptors respond to light (back of the retina)

  1. rods and cones
  2. provides support to retina and the sclera provides structural support
• RODS AND CONES

  • light goes through several layers of retinal cells before it reaches rods and cones 
  •  rods --> longer more cylindrical in shape 
  • cones --> more coned shape
  • photoreceptors uses glutamate as neurotransmitter
  • signal is then passed to bi-polar cells --> then retinal ganglion cells  (vertical pathways)
  • multiple inputs converge at one layer 
  • allow noise suppression (converging inputs allows combinations of sources)
  • horizontal cells + amacrine cells (help system reduce noise)
• RETINAL PIGMENT EPITHELIUM

  • makes the pupil look black
  • pigmented layer at the back of the retina
  • essential for recycling retinaldehyde  maintaining function of rods and cones
  • helps rods and cones to cope with oxidative stress

  1. when rods and cones are active, creates a lot of oxidative stress
• RODS AND CONES CONT.

  • aren’t evenly distributed in the retina
  • most cones are in the fovea --> max visual acuity + helps see colours (blind at night)
  • rods  --> help with night vision + peripheral vision 

  1. 1000x more sensitive than cones
  2. harder to discriminate colour in the periphery and at night 

  • retinal absorbs light and changes conformational change --> closes sodium channels + hyperpolarization
  • when photon is absorbed --> rotation in 11-cis-retinal to form all-trans-retinal (active state)
  • activates transducin --> activates phosphodiesterase --> reduces cgmp and closes sodium channels 

  
• OPSIN + RETINAL == PHOTOPIGMENT

  • OPSIN STRUCTURE

  1. gpcr with 7 transmembrane domains
  2. different opsins in 3 types of cones, rodes and melanopsin (rgcs) 
  3. 3 types of cones in humans (trichromats)
  4. 3 different colour information
  5. 5 types of opsin in total
  6. 1 type of cone --> monochromates (aquatic animals)
  7. 2 types of cones --> dichromats (dogs, reptiles)
  8. mantis shrimp --> contains 12-16 cone types
  9. bind to rhodopsin
• OPSIN + RETINAL == PHOTOPIGMENT

  • RETINAL STRUCTURE

  1. same for every opsin
  2. vitamin a derived
  3. absorbs light and changes conformation --> bleaching
  4. causes a downstream transduction effect 
  5. until retinal is reset --> it can’t absorb another photon until it is reset 
• PHOTORECEPTORS DETECT DARKNESS

  • in the dark --> they are depolarized with sodium channels oen
  • in the light --> sodium channels close --> membrane hyperolarisation

  1. presence of a stimulant --> hyperpolarization 
  2. cyclic gmp is degraded to gmp
• IMPERFECTIONS

  • myopia --> short-sightedness

  1. focal point comes too soon
  2. lens is too strong
  3. concave lens in glasses or contacts

  • hyperopia --> far-sightedness

  1. focal point of image comes further beyond retina
  2. convex lets in glasses or contacts

  • astigmatism --> irregularities or cornea and lens
• COLOUR-BLINDNESS

  • red and green opsin is found on the x chromosomes 

  1. Males more likely to be colourblind

  • blue opsin found in chromosome 7
• RETINAL CELLS

  • Bipolar cells

  1. main relay cells between photoreceptors
  2. off and on bipolar cells
  3. dependent on how they respond to light
  4. dependent on what glutamate it expresses
  5. when hyperpolarization occurs --> hyperpolarization of bipolar cell == off bipolar cell
  6.  when hyperpolarization occurs --> depolarized bipolar cells == on biopolar cell

  • horizontal cells

  1. light intensity adaption
  2.  spatial processing 
  3. colour processing (opponency)
• RETINAL CELLS cont.

  • amacrine cells

  1. directional motion
  2. modulate light adaption
  3. modulate circadian rhythm
  4. sensitivity of night vision

  • retinal ganglion cells (roc)

  1. further process of colour, motion and hapes
  2. only output cell --> fire action potentials
  3. some can detect light via melanopsin --> only ipRGC
  4.  melanopsin doesn’t contribute to image formation but affects circadian rhythm, pupil size and body temeperature
• VISUAL PATHWAY