Retinal Layers Simplified

The ten layers of retina – this microscopic anatomy is frequently asked in examinations and also important from the physiological viewpoint. There are plenty of mnemonics around the web, but we will proceed in a different approach to remember the 10 retinal layers easily.

A. Retina is 3 neuron system composed of 3 layers of cells:

From out to in –

  1. PhotoReceptor layer
    • Pigmented epithelium with Zona occludens (outer blood-retina barrier)
    • Photoreceptors
    • Outer limiting membrane
    • Outer nuclear layer
    • Outer plexiform later
  2. Bipolar cell layer (1st order neuron)
    • Inner nuclear layer
  3. Ganglion cell layer (2nd order neuron)
    • Inner plexiform layer
    • Ganglion cell layer
    • Nerve fiber layer (Axons of ganglion cell layer that form optic nerve)
    • Inner limiting membrane

Remember: RBG (Red, Blue, Green) – the color model for color vision

Where are the 3rd order neurons? In the Lateral Geniculate Body (LGB).

B. Orientation:

  1. Inner – refers to layers which are close to the vitreous humor
  2. Outer – refers to layers which are close to the choroid

C. Nuclei of Retinal cells:

These nuclei are arranged in a layer known as Nuclear layer.

  • Nuclei of Photoreceptors – forms Outer Nuclear Layer
  • Nuclei of Bipolar cells (1st order neuron), Horizontal cells (outer to Bipolar cells), Amacrine cells (inner to Bipolar cells) and Muller cell (supporting glial cells) – forms Inner Nuclear Layer

Horizontal cells: help sharpen the receptive field of bipolar cells

Amacrine cells: help sharpen the response of ganglion cells

D. Axons and Dendrites of Retinal cells:

These axons and dendrites (synapses) are arranged in a layer known as Plexiform layer.

Plexiform layer lies just inside the corresponding Nuclear layer:

  • Outer plexiform layer (OPL): Between outer nuclear layer and inner nuclear layer
    • Synapses between Photoreceptors and Bipolar cells forms Middle Limiting Membrane (MLM).
    • Retina external to MLM is avascular (dependent upon choroidal vasculature) and Internal to MLM is vascular (Central retinal artery).
    • Hence, outer plexiform layer is a watershed zone between dual vascular supply – role in localization of edema fluid and hard exudates in this layer.
  • Inner plexiform layer (IPL): Between inner nuclear layer and ganglion cell layer
    • Synapses between Bipolar cells, Ganglion cells and Amacrine cells.

E. Axons of Ganglion Cells form Nerve Fiber Layer:

  • Which go on to Optic nerve (CN II)

F. Inner Limiting Membrane:

  • The only true basement membrane.
  • Synthesized by foot process of Muller cells.

G. Shape of Photoreceptors:

The name comes from the outer segment of these receptors, which contains photopsin:

  • Rods: Outer segment is Rod-like cylindrical
  • Cones: Outer segment is Conical

Rods: Dark vision and Motion

Cones: Light vision, Sharp vision and Color discrimination

  • 3 types: Red, Blue and Green

With this Knowledge, now lets draw the microanatomy of retina:

Schematic Drawing of Retinal Layers

retinal layers

Rod cells in Light and Dark

rods light and dark

  1. Rods – Bipolar cells synapse = Inhibitory synapse
  2. Bipolar cells – Ganglion cells synapse = Excitatory synapse
  3. Hence, to generate action potential in Ganglion cells: Rod cells must be inhibited (Inhibtion of inhibitory fibers)
  4. There are cGMP gated Na+ channels on rods.
    • cGMP is abundant in dark and decreased in light.
  5. Depolarization of rods occurs in dark and repolarization in light.

Hence, action potetional is generated in ganglion cells in light but not in the dark.

 

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