0. Visual System in Brief

Provides long and short range sensory information about the layout of surroundings

• Converts photons into electrical signals
• Processes the signals using several pathways
  • analyses visual scenes, identifies objects and faces
  • provides info about threats
  • estimates self-motion and motion of external objects

1. Anatomy of the Visual System

The Eye

• The Optical Apparatus is suited to focus light onto the Photoreceptors in the Retina

  • By adjusting the curvature of the lens, visual objects located at different distances are focused onto the Retina

• The area of Retina near the optic axis, the Fovea, is where vision is the sharpest

  • Corresponds to the centre of gaze when we look at something
  • Density of PRCs, Bipolar Cells and Ganglion Cells is highest at the Fovea
  • Only Cone Cells present, none are Rods Cells
  • In the centre of the Fovea - the Foveola, cellular layers are pushed aside to allow more immediate access to PRCs

• The Retina consists of multilayer of cells

  • Photoreceptor Cells (PRCs)
    • outermost layer
    • absorb light and convert to a neural signal (Phototransduction)
  • Bipolar Cells
    • receive synaptic signals from PRCs
    • mediate the signal to Amacrine Cells or to Retinal Ganglion Cells
  • Retinal Ganglion Cells (RGCs)
    • Innermost layer
    • input from Bipolar Cells
    • output from Retina, with axons forming Optic Nerve
  • Horizontal Cells
    • provide lateral connection, collect signals from neighbouring PRCs

Photoreceptors

• All PRCs have a common structure with 4 functional groups
  • Outer Segment: at distal surface of Retina
    • membrane folding and transduction apparatus to convert light energy into electrical signals
  • Inner Segment: located proximally
    • where nucleus and most organelles locate
  • Cell Body
  • Synaptic Terminal
• The 2 types of the PRCs are distinguished by their morphology

Rod Cell

<aside> 💡 Light Sensitive + Colour Blind

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• Long, cylindrical Outer Segment within which the stacks of discs are separated from the plasma membrane
• Highly sensitive to light, even to a single photon so dominate vision at Low light Levels
• Saturate at High Light Levels
• None in Fovea

Cone Cell

<aside> 💡 $\downarrow$Light Sensitive + Colour Vision

</aside>

• Shorter, tapered Outer Segment and the discs are continuous with the outer membrane
• Much less sensitive to light, no contribution to light vision
• Enable colour vision
• Fast response
• Concentrated in Fovea

2. Phototransduction and Neural Signalling

Rhodopsin Activation

• Rhodopsin (R) is the visual pigment in Rod cells
  • Opsin component embedded in Disc membrane
  • Retinal is the light absorbing moiety
    • 11-cis Isomer covalently linked to a Lysine Residue of Opsin (Residue 296)
    • Absorption of a photon causes it to flip from 11-cis to the All-trans Configuration

Phototransduction

Rod Cells respond to light:

• Leads to the closure of cGMP-gated $Na^+$and $Ca^{2+}$ Channels
• Hyperpolarises the cell membrane