The Eye and Retina

From Eyewire
Revision as of 15:16, 18 June 2014 by DannyS (Talk | contribs)

Jump to: navigation, search
  1. vitreous body
  2. ora serrata
  3. ciliary muscle
  4. ciliary zonules
  5. Schlemm's canal
  6. pupil
  7. anterior chamber
  8. cornea
  9. iris
  10. lens cortex
  11. lens nucleus
  12. ciliary process
  13. conjunctiva
  14. inferior oblique muscle
  15. inferior rectus muscle
  16. medial rectus muscle
  17. retinal arteries and veins
  18. optic disc
  19. dura mater
  20. central retinal artery
  21. central retinal vein
  22. optic nerve
  23. vorticose vein
  24. bulbar sheath
  25. macula
  26. fovea
  27. sclera
  28. choroid
  29. superior rectus muscle
  30. retina

The eyes are organs of vision. It collects photons from the surrounding environment and translates the photons into colors, and multiple photons into visually perceived images. Some of the visual processing is done within the eye, but most of it is done in the visual cortex in the brain.

The Parts of the Eye

The Cornea

The cornea is the transparent surface of the eye that covers the pupil and iris. Other than at the edges, the cornea contains no blood vessels, so it is nourished by tears. Not only does it protect the rest of the eye, but it is also the first refractive surface that light goes through on its way to the retina.

The cornea consists of three major layers, the epithelium, stroma, and endothelium. The epithelium is responsible for protecting the cornea, the stroma makes the cornea transparent, and the endothelium prevents the cornea from swelling.[2]

The Iris and the Pupil

The iris is the part of the eye located between the cornea and the lens. In the center of the iris is the pupil, which is an aperture that allows the light to enter the eye. The iris' muscles constrict the pupil when exposed to bright light, and dilate it when exposed to dim light. Melanin is responsible for the color of the iris. When melanin is relatively absent, the iris will be blue or green, while when there is a lot of melanin, the iris will appear brown or black. [3]

The Lens

The lens is a structure behind the iris that focuses light onto the retina. It contains no blood vessels and is nourished by the aqueous humour. The Ciliary muscles change the shape of the lens to focus it on objects that are at varying distances.[4]

The Vitreous Body

The vitreous body is a thick, gel-like fluid that maintains the shape of the eye. It takes up about 80% of the volume of the eye, and is composed of about 98% of water.[5]


The Sclera is more commonly known as the white of the eye. It is a white fibrous layer that becomes transparent at anterior part of the eye and forms the cornea.

Anatomy of the human retina

A diagram showing the locations of the Optic cube and disc, Macula, Fovea, veins, and arteries. [6]

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.

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

Reference credit:

Cells of the Retina


Photoreceptors consist of two broad classes of cells: rods and cones. Rods are concentrated at the outer edges of the retina and are used in peripheral vision. They are more sensitive to light than cones, and are almost entirely responsible for night vision (also called scotopic vision). Cones are more concentrated in the center of the retina, and are the only photoreceptor type found in the center of the retina (the fovea). Cones are responsible for color vision (also called photopic vision). Mammals usually have either two or three different types of cone cells, because in order to specify the wavelength of a stimulus (i.e., its color), the outputs of at least two cone types must be compared.

Horizontal Cell

Horizontal cells in the retina

Horizontal cells are thought to exist in two types, each with a distinct shape, which together provide feedback to all photoreceptor cells. Despite the number of cells with which they form synapses, horizontal cells represent a relatively small population of the retina’s cells (less than 5% of cells of the inner nuclear layer). The specific reason for the existence of the two classes of horizontal cells is not yet known; it potentially involves detection of color differences in the red-green system.

Amacrine Cell

A Starburst Amacrine cell reconstructed in EyeWire

Amacrine cells appear to allow for ganglion cells to send temporally correlated signals to the brain: input to two separate ganglion cells from the same amacrine cell will tend to make those ganglion cells send signals at the same time. The amacrine cells whose behaviors are well understood have been shown to have very specific functions.

Bipolar Cell

A reconstruction of 114 rod bipolar nerve cells from a piece of mouse retina. The dense bundles (top) are dendrites, and the sparser processes below are axons (credit: MPI for Medical Research).

Bipolar cells connect photoreceptors and ganglion cells. Their function is to transmit signals from photoreceptors to ganglion cells, either directly or indirectly. Bipolar cells get their name from their shape — they have a central cell body from which two different sets of neurites (axons or dendrites) extend. They can make connections with either rods or cones (but not both simultaneously), and they also form connections with horizontal cells. Unlike most neurons, which communicate with one another using action potentials, bipolar cells “talk” with other cells using graded potentials.=

Ganglion Cell

A ganglion cell reconstructed in EyeWire

Ganglion cells are the output cells of the retina. Their axons leave the eye and travel through the optic nerve to the brain, sending the processed visual stimulus to the lateral geniculate nucleus, forming synapses onto neurons that project to the primary visual cortex, where the stimulus can be further interpreted.


  1. Eye-diagram no circles border
  2. "cornea." Encyclopaedia Britannica. Encyclopaedia Britannica Online Academic Edition. Encyclopædia Britannica Inc., 2014. Web. 16 Jun. 2014.
  3. "iris." Encyclopaedia Britannica. Encyclopaedia Britannica Online Academic Edition. Encyclopædia Britannica Inc., 2014. Web. 16 Jun. 2014.
  4. "lens." Encyclopaedia Britannica. Encyclopaedia Britannica Online Academic Edition. Encyclopædia Britannica Inc., 2014. Web. 16 Jun. 2014.
  5. "human eye." Encyclopaedia Britannica. Encyclopaedia Britannica Online Academic Edition. Encyclopædia Britannica Inc., 2014. Web. 16 Jun. 2014.
  6. 6.0 6.1 Kolb, Helga, Nelson, Ralph, Fernandez, Eduardo, Jones, Bryan, The Organization of the Retina and Visual System, Simple Anatomy of the Retina.
  7. Human Physiology and Mechanisms of Disease by Arthur C. Guyton (1992) p.373