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 –
- PhotoReceptor layer
- Pigmented epithelium with Zona occludens (outer blood-retina barrier)
- Outer limiting membrane
- Outer nuclear layer
- Outer plexiform later
- Bipolar cell layer (1st order neuron)
- Inner nuclear layer
- 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).
- Inner – refers to layers which are close to the vitreous humor
- 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
Rod cells in Light and Dark
- Rods – Bipolar cells synapse = Inhibitory synapse
- Bipolar cells – Ganglion cells synapse = Excitatory synapse
- Hence, to generate action potential in Ganglion cells: Rod cells must be inhibited (Inhibtion of inhibitory fibers)
- There are cGMP gated Na+ channels on rods.
- cGMP is abundant in dark and decreased in light.
- 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.