Retina Physiology Lecture SlideshowRetina Physiology Lecture Slideshow

Flashes and Floaters


  • Light-sensitive portion of the eye
  • Contains
  • Cones—for color vision
  • Rods– for black and white vision and vision in dark
  • After excitation of photoreceptors signals are transmitted from retina
    to optic nerve ending in cerebral cortex

The Fovea Centralis

  • Location– slightly below and on one side of the optic disc
  • Found in the centre of a shallow depression or pit (the macula lutea)
  • Fovea– minute area in the center of retina i.e. slightly larger than 1
    square millimeter
  • Capable of acute and detailed vision
  • Central fovea– 0.3 mm in diameter
  • Entirely composed of cones

The Fovea Centralis

  • Specialized structure of these cones providing clear detail of the image
  • Foveal cones
  • long and slender bodies, in the foveal region
  • Peripheral retina has cones with fat bodies
  • Fovea also contains blood vessels, ganglion cells and inner nuclear cell
  • Arrangement is made in a manner that light passes unimpeded to reach
    the cones
  • Only cones are present at the fovea which have individual connections with
    the bipolar and ganglion cells

Peripheral Retina

  • No cones in the peripheral retina, but only rods
  • Rods here are also shorter and wider than in the central retina.
  • Receptive fields at the periphery are very large with many rods
    converging onto one ganglion cell.

The Rods and Cones

  • Photo receptors present in the outer
    nuclear layer i.e. Receptor Layer of
  • Human receptor layer consists of
    approximately 120 million rods and 6
    million cones arranged side by side.
  • The distribution of these
    photoreceptors varies across the
    surface of the retina.

sensitive pigments.

  • Roads and cones contain light sensitive pigments
  • Each photoreceptor consists of an
    outer segment which contains
    hundreds of thin plates of
    membrane (lamellae or discs).
  • The outer segment is connected
    by a cilium to an inner segment
    which contains a nucleus.
  • Rods are about 500 times more
    sensitive to light than cones
  • cones are responsible for color

Structure of Rod/Cone

• The light-sensitive photochemical is found in the outer segment.

  • In rods, this is rhodopsin • In cones, it is one of three “color” photochemicals, (color pigments) • Function same as rhodopsin except for differences in spectral sensitivity

Discs/Lamellae of Rods and Cones

  • Large numbers of discs are
    present in the outer segments of
    the rods and cones.
  • Each of the discs is an infolded
    shelf of cell membrane.
  • There are as many as 1000 discs
    in each rod or cone.

Properties of Rods and Cones

Rhodopsin and Color Pigments

  • Conjugated proteins.
  • Present in the membranes
    of the discs in the form of
    transmembrane proteins.
  • These Pigment proteins
    constitute about 40 per cent of
    the entire mass of the outer

Pigment Layer of the Retina

  • Black pigment melanin
  • Prevents light reflection through out the globe of the eye ball
  • Clear vision
  • It stores large amount of vitamin A that is an important precursor of
    photosensitive chemicals of rods and cones

Optic Disc

  • Point at which axons leave the eyeball and join the optic nerve
  • Arteries enter and veins leave the retina at the optic disc.
  • There are no photoreceptors at optic disc
  • Also known as ‘blind spot‘
  • It is a pinky-yellow oval, approximately 2mm in diameter

Pigment Layer of the Retina

  • Melanin pigment layer is absent in albino
  • Light reflected in all directions inside the eye ball by unpigmented
    surfaces of the retina & sclera
  • Light excites many receptors
  • Visual acuity of albinos is badly affected 20/100 to 20/200.

Retinal Detachment

Injury to the eyeball

  • Fluid or blood may be collected between the neural retina and the
    pigment epithelium.
  • Contracture of fine collagenous fibrils in the vitreous humor
  • These fibrils pull the retina toward the interior of the globe

The detached retina can resist degeneration for days because of:
1.Diffusion across the detachment gap
2.Independent blood supply by the Retinal artery
(Early surgical placement may save the permanent loss of vision)

Retinal Detachment

Rhodopsin Visual Cycle

  • Outer segment contains 40% photopigment– rhodopsin/visual purple
  • Combination of retinal and scotopsin
  • Retinal– 11-cis type
  • Readily binds to scotopsin

Rhodopsin Visual Cycle

  • Rhodopsin decompose when light falls on retina
  • Rhodopsin decompose in fraction of a second
  • Photoactivation of electrons in retinal portion of rhodopsin
  • cis form of retinal is converted into all-trans form
  • All trans form has no binding sites for scotopsin
  • Decomposed into retinal and scotopsin

Rhodopsin Visual Cycle

  • First product to be formed is BATHORHOPSIN in psec after light falls
  • Followed by formation of LUMINORHODOPSIN in nsec
  • Leads to formation of METARHODOPSIN-I in μsec
  • Followed by formation of METARHODOPSIN-II in msec
  • Then in seconds is converted into SCOTOPSIN and 11 cis retinal
  • It takes minutes to recompose Rhodopsin
  • For synthesis retinal must be converted to trans form(occurs in DARK)

Food sources of Vitamin A

  • Carrots
  • Sweet potatoes
  • Green leafy vegetables
  • Cantaloupe
  • Fish
  • Dried apricots

Role of Vitamin A

  • all trans retinol, one of the source of Vitamin A
  • all trans retinol is converted to all trans retinal
  • all trans retinol is converted to 11 cis retinol
  • Then it is converted to 11 cis retinal
  • Isomerase enzyme plays a role in conversion
  • Finally binds with scotopsin
  • Vitamin A is present in cytoplasm of rods and in the pigment layer of
  • Readily available for synthesis of retinal
  • Excess retinal is re-converted into vitamin A
  • Decreasing light sensitive pigment
  • Will be elaborated in light and dark adaptation

Night Blindness

  • Occurs with severe vitamin A deficiency
  • Synthesis of rhodopsin is reduced
  • For night blindness to occur vitamin a–deficient diet for months
  • Large quantities of vitamin A is stored in liver
  • Reversed in less than 1 hour by intravenous injection of vitamin A

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