Eye physiology: The retina

The retina contains a sheet of photoreceptors.  The visual image is focused on the retina with minimal distortion, by refraction through the cornea and lens (see optics).

Behind the photoreceptors is the pigment epithelium containing melanin.  This functions to absorb lights, preventing light that is not absorbed by the photoreceptors from reflecting back into the eye (NB some animals have a reflecting layer instead).

However, light must travel through several layers of cells in the retina before reaching the photoreceptors.  This degreades the image slightly.  To reduce degredation, the proximal cell layers are largely made up of unmyelinated fibres.

The fovea is a special area where the proximal cells’ somata are displaced to allow light to be directly absorbed.  This is also the focal point of the image and the region where the majority of cone cells are concentrated.  We constantly move our eyes to view the foveal image.

At the optic disc, fibres leave they eye.  There are no photoreceptors in this region (blind spot).

The retina has 3 different kinds of neuron:

Photoreceptors: Rods and Cones

These are unevenly distributed across the retina:

Cones

• Mediate ‘day’ vision (photopic vision)
• With the exception of detection of dim stimuli, cones perform superiorly compared to rods in all aspects (acuity, resolution of fast changes, colour detection)

Rods

• Responsible for ‘night’ vision (scotopic vision)
• Achromatic function (i.e. tone/shadow- add depth)

Rods and cones have 3 major functional regions:

1. A cilium that connects the outer and inner segments
2. Outer segment
   1. Outer/distal surface of the retina
      1. Comprises a stack of membranous discs (increased surface area)
         1. In cones, the discs are continuous with the plasma membrane
         2. In rods, the folds close off and become separate intracellular organelles
   2. Specialised for photoreception
   3. Filled with light absorbing visual pigments (around 10^8 molecules)
3. Inner segment
   1. Located proximally
   2. Contains the nucleus and biosynthetic organelles
4. Also the synaptic terminal for communication with target cells.

For other cell types see Eye Physiology: Photosignalling