The retina is the light-sensitive innermost layer of the eye. It contains photoreceptor cells and neuronal cells arranged in layers. Light is detected by rods and cones which transmit signals through bipolar and ganglion cells. The optic nerve carries signals from the retina to the brain. Key features of the retina include the macula and fovea for sharp central vision and the optic disc where blood vessels enter and nerve fibers exit.

2. Retina
• Innermost
• Light-sensitive
• Nervous layer of the eye (forms from Outpouching of diencephalon)
• Consist of 2 parts outer Pigmented layer and inner Neuronal Layer
• Photoreceptors are present up to ora serrata (halfway between limbus and Equator) while
the double epithelium of both the coats lines up to Iris
• Features includes- Optic disc, retinal blood vessels, Macula lutea, fovea centralis
• Optic disc: Circular, 1.5mm, central depression is called as physiological cup (cup is formed
coz of termination of retinal layers or origin of optic nerve)
• Retinal blood vessels: Branch of ophthalmic artery i.e. Central retinal artery pierce the
duramater and enters the optic neve and enters the eyeball through optic disc passing
through lamina cribrosa and Posterior Ciliary artery reaches choroid via sclera
• Macula Lutea: Exact posterior to pupil is a yellow coloured shiny area of 5.5mm.
• Fovea centralis: depression in he middle of the Macula lutea. It is most sensitive part of
retina
ora serrata

3.  Cells of Retina
• Epithelial cell or pigmented cells
• Photoreceptor cells
• Neuronal cells
• Glial cells
• Retinal pigment epithelium: outermost, hexagonal layer of cells that contains a pigment called
melanosome(absorbs light). Receives bloods supply via chorio-vessels in the form of loops and
sinusoids. Have cilia like structure. The layer is firmly attached to Bruch’s membrane and loosely
attached to layer of rods and cones. Separation occurs at this level called as retinal detachment.
• Rods and cones Layer: These are the transducers that convert light energy into nerve impulse. Rods
are more in number as compared to cones (120:6.5 million). Rods have rhodopsin while cones have
Idopsin. Rods function best in low intensity light and are thus responsible for vision in dim light.
Cones works best in high light intensity and are responsible for perception of color. There number
is highest at fovea. Three types red, blue and green.

4. • Bipolar Neurons: Have 1 dendrite and 1 axon. Connected to photoreceptors by dendritic end and on the inner side have
ganglion cells and are connected to it by axons. 1 bipolar neuron is connected to many rod cells and cone cells but at fovea
1 bipolar is connected to 1 cone cell
• Ganglion cells: Generate action potential
• Current is passing from outer part to inner part
• These 3 cells are interconnected by various other conducting cells
• Horizontal cells: They connect the nucleus of photoreceptor cells to each other. This network of nerve is called as outer
plexiform layer
• Amacrine cells: They connect the inner synaptic junction with each other called as inner Plexiform layer
• Interplexiform cells: They connects the inner and outer plexiform layer to each other
• Glial Cell: Retina is a complex structure with variety of neuronal cells synapsing with each other. Supporting cells are
present to keep these cells intact and ensure proper functioning e.g. muller cells, astrocyte and microglia cells
• Muller cells are present between the 2 plexiform layer its one end connects the photoreceptor and holds them tightly
forming a layer like structure called as external limiting layer and other end forms footplate on inner side separating retina
with vitreous body called as Internal limiting layer.

6. Layers of Retina
1. Retinal pigment epithelium
2. Layer of Photoreceptor Component
3. External Limiting Layer
4. Outer Nuclear Layer
5. Outer Plexiform Layer
6. Inner Nuclear Layer
7. Inner Plexiform Layer
8. Ganglion Cell Layer
9. Nerve Fibre Layer
10. Internal Limiting Layer
The outer 5 layers are supplied by chorio vessels by diffusion

7. Retinal Pigment Epithelium
Layer of Photoreceptor Component
External Limiting Layer
Outer Nuclear Layer
Outer Plexiform Layer
Inner Nuclear Layer
Inner Plexiform Layer
Ganglion Cell Layer
Nerve Fibre Layer
Internal Limiting Layer

8. TRANSDUCTION
Conversion of 1 form of energy into another form of energy is transduction
• Light reaches photoreceptor pigment. It breaks rhodopsin into opsin and trans-retinol
• Opsin activates Transducin protein that further activates another protein called as Phosphodiesterase
• There is a enzyme called as Guanylate cyclase - it converts GTP into c-GMP, that opens Sodium and calcium channel
• Phosphodiesterase breaks c-GMP and thereby Na and Ca channel remains closed
Light
Rhodopsin Opsin + Trans-
Retinol
Light
Activate
Transducin Protein
Phosphodiesterase (PDE)
Activate
Breaks GMP (Na & Ca remains closed)

9. • Closed Na & Ca channels results in increase electronegativity or Hyperpolarization
• Coz of hyperpolarization, voltage gated Ca channel remains closed at synaptic junction
• Low graded potential causes less release of neurotransmitter i.e. Glutamate
• Very low glutamate is excitatory in nature i.e. it stimulate (Depolarize) bipolar neuron but the potential generated is again
graded. Bipolar releases good amount of glutamate that helps in generating Action potential that is taken to occipital lobe
What is the function of Horizontal cells and Amacrine cells?
Glutamate are also released at Horizontal cells synaptic junction and stimulate it and as a result Horizontal cells releases GABA
whose function is to inhibit photoreceptor. Its main function is to help in adaption when a person goes into dim to high light or
vice versa. Amacrine cells does the same but at inner plexiform layer
What if there is no light or very Dim Light?
No light means no breaking of rhodopsin into opsin and no activation of Transducin and PDE. This means cGMP is free to open
Na & Ca channel resulting in depolarization. Voltage gated calcium channel opens and thus high amount of glutamate is
released. Glutamate is inhibitory in nature and thus inhibits bipolar neurons and releases less glutamate and so action
potential generated in ganglion cells is low.

10. Optic Pathway
Visual field: The entire area that can be seen when the eye is directed forward, including that which is seen with peripheral
vision
Information from right visual field is taken to left side and vice-versa
Visual field has two ends nasal end and temporal end. Nasal end of right eye is right sided
While its temporal end is left sided
Retina  Optic Nerve  Optic chiasma  Lateral Geniculate body/Nucleus (LGB) 1° Visual Cortex
Nasal retina always forms contralateral fibres while Temporal retina forms ipsilateral fibres
Optic nerve from both eyes combines at optic chiasma and from there optic tract starts and ends
in thalamus in Lateral Geniculate Body/Nucleus.
LGB is a sensory relay nucleus and has 6 layers. Layer 1, 4,6 corresponds to contralateral fibres
and layer 2,3 and 5 receives information from ipsilateral fibres.
Fibres from LGB forms optic radiation one passing through temporal lobe (Inferior retinal fibres)
and other passing through Parietal lobe (Superior Retinal fibres) reaches 1° Visual Cortex in occipital lobe
Optic Nerve
Optic Chiasma
LGB
Temporal Retina
Optic tract