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The Function of the 3 Layers                     

  1. Sclera: Protective
  2. Uvea : Vascular (nutritive)
  3. Retina : Light- sensitive
  4. The retina lines the post 2/3rd of the choroids. The lens ligament (zonule) is attached to the lens and to the ciliary body.
  5. The aqueous humour is formed in the ciliary body. The normal intraocular pressure is 10-20 mmHg.
  6. The refractory power of eye is expressed in diopters



Normal is 59 D (44 by conea & 15 by Lens)

1. Accomodation- In accommodation, only the anterior curvature of the lens changes.

Near response consists of:

  1. Accommodation
  2. Convergence of the eyes
  3. Papillary constriction

The pupils can constrict as a part of the near response; the pupils also constrict as a part of the light reflex


2. Reduced or Schematic eye- Since there are many places in the eye where refraction of the light rays takes place, for simplicity, (to make ray diagrams) we can assume that all refraction takes place at the anterior surface of the cornea. (Most of the refraction occurs at the cornea). The wavelengths of visible light range from approximately 397–723 nm 


3. Retina

There are 10 layers in the retina. The neural cells in the retina are the receptor cells (rods & cones), the bipolar cells, the ganglion cells, the horizontal and the amacrine cells. There are 6 million cones and 120 million rods. The glial cells are the Muller cells. There are gap junctions from one retinal neuron to another.


The horizontal cells (H) join one receptor( R) cell to another; the amacrine (A) cells join one ganglion cell to another.

(Ganglion cell)


The rods have connection with bipolar cell. Which in turn has connection with ganglion cell. The axons of the ganglion cell form the optic nerve. Many receptor ( R) cells converge an one bipolar cell and many bipolar cells converge on one ganglion cell; there is overall convergence from the receptor cell to the ganglion cell.

  1. Amacrine cells: are involved in signal processing via lateral inhibition. They Modulates colour contrast & luminosity under various light conditions. Also Increase visual acuity.
  2. Muller Cells: Are retinal Glial cells. Gives rise to b wave of ERG. K+ & Neurotransmitter uptake

(Note that the direction of the light rays is from that

ganglion cell                          bipolar cell                            receptor cell

At the posterior pole of the eye, the macula lutea (with fovea centralis ) is present. The unique features of the fovea centralis are:

i.  It is rod- free; only cones present

ii. The cones here are densely packed there are very few other cells

iii. There are no blood vessels overlying the receptors

iv. It is the point of highest visual acuity.

Note that action potentials are formed only in the ganglion cells; in the other retinal neural cells only local potentials are formed.

The primary visual area (area 17) lies on the sides of the calcarine fissure in the occipital lobe.

The visual pathway has connections, which subserve:

i.  Vision

ii. Papillary reflex

iii. Superior colliculus

iv. Suprachiasmatic nucleus of the hypothalamus

Fibres from ipsilateral temporal hemiretina end in lamina 2,3,5& Fibres from contralateral nasal hemiretina end in lamina 1,4,6.

The rods are more sensitive (lower threshold) than the cones. The visual spectrum is from 393nm to 727 nm.

Photoreceptor mechanism

All or none action potentials are seen only in the ganglion cells. In all others, there are local, graded potentials.

  • Rods/ cones/ horizontal cells: Hyperpolarising potentials
  • Bipolar cells : Hyper or hypopolarising
  • Amacrine: Depolarizing.
  • Ganglion cells: Action Potentials

The photosensitive pigment in the rods is rhodopsin. It consists of 11-Cis retinal bound to protein opsin. Its peak sensitivity to light at a wavelength of 505 nm.

When rhodopsin is exposed to light 11-Cis retinal is converted to all trans retinal, which activates a G protein



  1. Regeneration of rhodopsin requires isomerization of all trans retinal back to 11-Cis retinal by rhodopsin Kinase.
  2. The sodium channels in the outer segment of the receptor cell are open in the dark; in the dark; there is a steady release of neurotransmitter resulting in dark currents.
  3. When light falls, Transducin closes the sodium channels. Causing hyperpolarisation and a decrease in the neurotransmitter release. One photon of light is enough to stimulate the rod.
  4. The cones are for color vision and respond maximally to light at wavelengths of 440, 535, and 565 nm
  5. The retina has 2 types of ganglion cells:
  • Magnocellular: for movement, Flicker & Stereopsis (Depth of Perception)
  • Parvocellular: for color, texture and shape.
  1. True Stereopsis is due to Binocular vision & False is due to Relative sizes , Angles, Parallax etc
  2. The primary colors are blue, red and green. For any color, there is a complementary color that when properly mixed with it, produces a sensation of white. Colour Blobs (V8) are cells in the visual cortex associated with colour vision.

Color Defects:

Weakness- is called anomally

  • Blindness- is called anopia
  • ‘Prot’ refers to red
  • ‘Deuter’ refers to green
  • ‘Tri’ refers to blue
  • Trichromats have all the 3 types of cones
  • Dichromats have a 2-cone system
  • If ‘red’ absent- Protanopia
  • If ‘green’ absent- Deuteranopia
  • If ‘blue’ absent- Tritanopia
  • Monochromats have only 1-cone system.          

Electroretinography measures the electrical responses of various cell types in the retina, including the photoreceptors (rods and cones), inner retinal cells (bipolar and amacrine cells), and the ganglion cells. Electrodes are usually placed on the cornea and the skin near the eye. During a recording, the patient's eyes are exposed to standardized stimuli and the resulting signal is displayed showing the time course of the signal's amplitude (voltage). a-, b- and c-waves are observed in the ERG.

The a-wave is a corneo-negative waveform both rods and cones contribute to the a-wave.

The second wave which is corneo-positive, is the b-wave. its origin is from the Müller's cells. The c-wave is positive like the b-wave, but otherwise is considerably slower. It is generated by the retinal pigment epithelium (RPE) as a consequence of interaction with the rods.


ERG can be useful in:    

  1. Retinitis pigmentosa and related hereditary degenerations
  2. Leber's congenital amaurosis , Choroideremia

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