The Eye and Retina

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A drawing of a section through the human eye with a schematic enlargement of the retina. [1]

The retina is a light-sensitive layer of tissue that lines the rear surface of the eye. Light from one's visual field passes through the eye and projects onto the retina to create an image. Subsequently, retinal neurons detect this image, which initiates a cascade of biochemical and electrical processing that is sent through the optic nerve and eventually to the visual cortex of the brain. These biochemical and electrical signals provide the basis for vision.

All the Parts


Transparent surface of the eye that covers the pupil and iris First refractive surface that light goes through on its way to the retina 3 layers (epithelium, stroma, endothelium) 5, possibly 6 layers, (Epithelium, Bowmans layer, Stroma, Dua’s layer (Discovered last year, still awaiting official confirmation), Descemet’s membrane, endothelium) medical: corneal abrasions, ametropia (any refractive area can be cause), etc


Muscular diaphragm that controls the size of the pupil and amount of light that can enter the eye Color of the iris is due to quantity and type of melanin


Aperture that allows the light to enter the eye


Second refractive surface that light goes through on its way to the retina tidbit: no blood supply to the lens medical: cataracts, presbyopia

Ciliary body

Ciliary muscle (with zonule fibers) Changes the shape of the lens and allows for accommodation

Ciliary epithelium

Makes the aqueous fluid that fills the anterior and posterior chambers Three fluid chambers Anterior Between cornea and iris Posterior Between iris and lens Vitreous Between lens and retina Humours (hehehe)


Clear, watery fluid that supplies nutrients to the structures it surrounds. Movement of aqueous humor is from posterior chamber to anterior chamber through the pupil. Drainage out of the anterior chamber via canal of Schlemm and trabecular meshwork into venous system. The equilibrium maintains appropriate intra-ocular pressure. medical: glaucoma (open and close angle)


thick, gel-like fluid that maintains the shape of the eye 80% of the volume of the eye medical: floaters, retinal detachment Three layers of the eye sphere (outside to inside)


Outermost layer of the globe White fibrous layer that becomes transparent at anterior part of the eye and forms the cornea


Middle layer between sclera and retina It is the vascular layer and source of oxygen and nutrients to the outer layers of the retina (photoreceptor layer) Bruch’s membrane


Inner layer of the globe Light-sensitive neurons transmit visual signals Macula and Fovea Site of high acuity of vision Optic nerve (add this here?) Retinal arteries and veins (add this here?)

Interactive site:

Extraocular muscles of the eye

Muscles insert into the sclera and move the eyeball and keep image focused on the fovea 6 main muscles:
superior rectus
inferior rectus
medial rectus
lateral rectus
superior oblique
inferior oblique
medical: lazy eye, etc.

Anatomy of the human retina

A diagram showing the locations of the Optic cube and disc, Macula, Fovea, veins, and arteries. [1]

Funduscopic examination of the retina

Optic cup and disc

retinal ganglion axons converge here
central area are retinal artery and veins
“Blind spot”

Macula and Fovea

high quantity of ganglion cells and cones for visual acuity and color perception
Interactive site:

10 histological layers of the retina Retinal pigment epithelium
Single layer of hexagonal cells
Located between the choroid and the photoreceptor layer
Forms a blood-retina barrier with tight junctions with the choroid
It is not firmly attached to the the neural aspect of the retina (photoreceptor layer)
medical: a potential site of retinal detachment
Photoreceptor layer
Composed of rods and cones
Outer limiting “membrane”
Site of connection between photoreceptors and Müller cells
Outer nuclear layer
Nuclei of photoreceptor cells
Outer plexiform layer
Photoreceptor fibers
Bipolar cell dendrites
“Two important synaptic interactions that occur at the outer plexiform layer are: the splitting of the visual signal into two separate channels of information flow, one for detecting objects lighter than background and one for detecting objects darker that background the instillation of pathways to create simultaneous contrast of visual objects In the first synaptic interactions, the channels of information flow are known as the basis of successive contrast, or ON and OFF pathways, respectively, whereas the second interaction puts light and dark boundaries in simultaneous contrast and forms a receptive field structure, with a center contrasted to an inhibitory surround.”
Inner nuclear layer
Bipolar cell nuclei
Horizontal cells
Amacrine cells
Interplexiform cells
Muller cells
Inner plexiform layer
Presynaptic dendrites of bipolar cells (axons)
Postsynaptic dendrites of ganglion cells
Amacrine cell dendrites
Ganglion cell layer
Nerve fiber layer
Axons of Ganglion cells
Inner limiting “membrane”
Ends of Muller cells

Image credit: Reference credit:

Cells of the retina

Retinal Pigment Epithelium
Contain pigment granules and absorbs scattered light
Regenerates 11-cis-retinal the chromophore used in photoreceptors
Responds to oxidative stress
Clearing up shed discs of rods and cones
Blood-retinal barrier functions
albinos lack pigment in this layer
macular degeneration
retinitis pigmentosa
Segments (outer, inner, fiber)
Rods: role in peripheral vision, night vision
contain rhodopsins
more rods than cones in the retina
no rods in fovea = night blind
more sensitive to dim light
increases in quantity peripherally
Cones: role in visual acuity and color vision
concentrated at center (fovea and macula) and less at periphery
contains different types of opsins
3 types - each absorb one of 3 colors of light
S-cone: short wavelength - blue
M-cone: medium wavelength - green
L cone: long wavelength -red


if issues with one or more cones types - colorblindness
most common - can’t differentiate red and green
usually X-linked recessive - affects men more
Ishihara test
Vitamin A deficiency
Retinitis pigmentosa
Dark adaptation (discuss in the future)
Phototransduction (discuss somewhere else in the future)

Cells of the Retina

Bipolar cells
Several types
rod-specific bipolar cells (1)
cone-specific bipolar cells (10)
Transmit signals from photoreceptor cells to ganglion cells
processes/neurites are called dendrites
ON and OFF layers
need to explain this* “We know that a photoreceptor neurotransmitter (which is glutamate, see Dowling (24) and Massey (25) for reviews) is released in the dark in the vertebrate retina (26). Thus, the photoreceptor, whether it be rod or cone, is in a depolarized state in the dark. On light stimulation, the photoreceptor responds with a hyperpolarization; transmitter release ceases, but the postsynaptic bipolar cells respond with either hyperpolarization or depolarization of their membranes. The hyperpolarizing type of bipolar cell is called an OFF-center cell, whereas the depolarizing bipolar cell is called an ON-center cell (27, 28).”

Ganglion cells
Bipolar cells contact both dendrites and soma of ganglion cells
Axons → forms optic nerve (CN II)
Association neurons (interneurons)
Modify synaptic transmission in retina

Horizontal cells
located between OPL and INL
3 types on human retina (HI, HII, HIII)
dendrites contact synaptic terminals of photoreceptor cells and with the dendrites of bipolar cells, which they inhibit

Amacrine cells
located between INL and IPL
all dendrites emerge from same side of the cell to branch out and terminate in synaptic complexes between bipolar, ganglion, etc cells
thought to lack axons.
some morphologies might have axons but do not leave retina
Interplexiform cells
post-synaptic to amacrine cells and pre-synaptic to horizontal and bipolar cells
feedback loop

Neuroglial cells

cells of Müller - principal glial cells of the retina
extend through the whole thickness of the retina
Provide architectual support
supporting role and other functions (communications?)

Image credit:

Reference credit:


  1. 1.0 1.1 Kolb, Helga, Nelson, Ralph, Fernandez, Eduardo, Jones, Bryan, The Organization of the Retina and Visual System, Simple Anatomy of the Retina. url=