Module 1 Lesson 2 - 5

The Retina

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By now, you should be familiar with the various structures of the eye and understand the role in vision of each structure. You should understand which structures are needed to alter and focus light on the retina. The retina is the key structure by which light energy is converted into electrochemical impulses. Then, this sensory impulse is communicated to the occipital lob of the brain by way of the optic nerve.

Photoreception Pathway

light→ cornea → aqueous humour → lens → vitreous humour → retina (cones and rods) → optic nerve → brain (occipital lobe)

Cells in the Retina

1. Pigmented cells
   • closest to the choroid
   • specialized to form the tapetum in some animals


2. Rods and cones
   
   
   • the actual photoreceptors
     
   • located above the pigmented layer



You might expect the photoreceptors to be in the direct path of incoming light, but the rods and cones are covered by layers of transparent neurons. 

3. Bipolar cells 
   
   • activated by rods and cones



4. Ganglion cells 
   

   • closest to the vitreous humour

The image on the left is a drawing, and the image on the right is a stained microscopic image. The image on the right depicts retina cells. Note the direction of the light striking the retina. Light travels through the transparent layer and strikes the rods and cones. These photoreceptors are prompted to stop emitting an inhibiting neurotransmitter substance, which you will learn about in lesson 7. Then, the rods and cones can communicate electrochemical messages first to the bipolar cells and then on to the ganglion cells. Axons of all ganglion cells of the retina converge at the back of the retina to form the optic nerve that carries impulses to the brain.

You need to know about only the rods and cones for this course. The other cells in the retina are given to you for further information only.

Rods and Cones

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Rods are necessary to distinguish shades of black and white as well as to distinguish movement.  They are located in the outer edges of the retina and are used to detect dim light.  

Cones respond to specific wavelengths of the visible spectrum, and this allows us to see colour. Cones are needed for acute vision because they are capable of finer discrimination of detail in bright light.  Cones are located near the center of the retina where the lens focuses images.  This area is known as the fovea centralis.  

Based on your knowledge of rods and cones, can you develop a hypothesis to explain why the eyes of birds that roost at night have mostly cones but the eyes of owls and bats that are active at night have mostly rods? Birds that are active during the day have more than 80% of their photoreceptors as cones, and the retinas of the nocturnal owls are dominated by rods.

You have also read about colour blindness. Cones occur as three types: red sensitive, green sensitive, and blue sensitive. One reason we can see so many variations of colour is that the sensitivity ranges of these receptors overlap. For example, yellow light stimulates both red and green cone cells, but if the red cones are stimulated more than the green cones, we see orange instead of yellow.

Can you explain the varying degrees and types of colour blindness in terms of the types of cones? Colour blindness can result from the loss of specific types of cones or having fewer cones of a given type. Colour blindness is a genetically inherited trait that affects more males than it does females. It is studied in more detail in the genetics unit of this course.

Test your own colour vision by looking at the Ishihara plates in the Did you Know? section.

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To review the concepts of vision, watch the following section from Biologix 06 video.

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Colour blindness is tested using eight placards developed by a Japanese ophthalmologist, Shinobu Ishihara, in 1917.