Phototransdution DLA

Question 1

What are photoreceptors?

Answer 1

Convert electromagnetic radiation (light) into neural signals
• Located at the back of the retina, number ~125 million
• Divided into 4 regions
• Cell body
• Synaptic terminal
• Inner segment
• Outer segment
• Membraneous disks containing photopigments -> light triggers changes in receptor membrane potential => phototransduction

Question 2

Describe the output of rods

Answer 2

• Long cylindrical outer segment with large number of disks
• Disks are continuously shed and replaced • More sensitive to light (x1000)
• More photosensitive pigment
• Sole contributor to night-time (scotopic) vision
• Achromatic (i.e. not for color vision)
• Poor spatial resolution (convergence high)
• Poor temporal resolution

Question 3

Describe the output of cones

Answer 3

Cones
• Shorter tapering outer segment with fewer disks
• Three cone types with different photopigments -> color vision
• Mainly responsible for day-time (photopic) vision
• Good spatial resolution (convergence low)
• Good temporal resolution

Question 4

Describe the regional difference in retinal structure

Answer 4

• Periphery:
  
  • Predominantly monopigmental rods
    
    • With large receptive fields
    • Night-time vision
• Central:
  
  • Predominantly multi-pigmental cones
    * With small receptive fields

• Fovea
• Highest density of cones
• No rods
• No blood vessels, bipolar or ganglion
cells -> Cones only