The retina - a part of inner layer of the eye - contains the receptor apparatus of the visual system. Retinal thickness is very small -

Structure and functions of retina (pigment epithelium, photoreceptors, blind spot, neurons)

The retina — a part of inner layer of the eye — contains the receptor apparatus of the visual system. Retinal thickness is very small — 0.1-0.2 mm, but it is composed of many layers, which interconnect by appendages, and entwine in openwork mesh (figure 173).

Retinal pigment epithelium (pigmented layer)

The outside layer of the retina — pigmented — is retinal pigment epithelium. It is formed by epithelium which has a fuchsin pigment. The term comes from the name of the German botanist Fuchs, who in the XVI century described plants which contain such colorant. Fuchsin absorbs light and thereby prevents its reflection and dispersion that promotes vision clarity. Interestingly that between the retinal pigment epithelium and photoreceptors some animals has a reflective layer of cells, due to which their eyes seemed to glow in the dark.

Photoreceptors

Photoreceptors (from Greek “the one that refers to the light”, and “the receptor”) are adjacent to the retinal pigment epithelium. There are two types of receptors — rods and cones. Each eye has 7-8 million cones and 110-125 million rods. They are distributed unevenly in the retina. In the center of it, in front of the pupil, there is the so-called yellow spot — a place of best vision, because there placed most of rods and cones. The retina has a fovea centralis, which contains only cones. Moving to the retina periphery quantity of cones reduces while amount of rods — increases. Periphery has almost only rods. Cones have poor sensitivity and therefore they operate in bright light conditions (provide day vision) and perceive colors, but rods perceive only black-and-white light. They are very sensitive and excited even at low light conditions, such as at dusk. These are photoreceptors of twilight or night vision.

Diagram 173. Structure of retina: 1 — optic fiber; 2 — ganglion neurons; 3 — bipolar neurons; 4 — OPC layer; 5 — pigment cell layer; 6 — cones; 7 — rods
Diagram 174. The Mariotte (blind spot) test
Diagram 175. The photosensitive pigment rhodopsin (a) and its transformations (b) under the action of light and in darkness

Stimulating different parts of the retina, scientists have found that colors are distinguished better when the light acts on the yellow spot and fovea centralis, where only cones are placed. Retinal periphery does not perceive colors. Therefore, in low light conditions, when the central — cone — vision greatly reduced, peripheral rod vision dominates. Since rods respond only to black-and-white light in the dusk human poorly distinguishes colors. That's why they say “all cats are gray at night”. Taken from http://worldofschool.org

Blind spot

A blind spot — a place of the optic nerve exit from the eyeball, doesn’t contain photoreceptor and therefore does not perceive light. The existence of the blind spot is easy to see, if you execute a Marriott test (diagram 174). If you close the left eye and lock the right on the cross, depicted in the figure, then at a distance of 10-15 cm from the eye the circle disappears, because it gets in the blind spot.

Retinal neurons

The next layer of the retina — is intercalary, bipolar neurons (diagram 173), which connect to the ganglion layer of nerve cells (from Greek “node”). The axons of these cells form the optic nerve. The bipolar and ganglion cells have a lot of connections between them. Due to this in the optic nerve has 100 times less nerve fibers than photoreceptors.

Categories:
Questions:
  • Determine the interrelation of retina structure and function.

  • What is the difference of the rods and cones?

  • Compare the structure and functions of the rods and cones.

  • Where are the rods and cones located in the retina?

  • How important is the placement of rods and cones in the retina?

  • What is the blind spot?

  • What is the structure and functions of retinal neurons?