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Retina: Definition & Function

Retina: Definition & Function
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  • 0:00 What is the Retina?
  • 0:25 How Does the Retina Work?
  • 1:20 Rods
  • 2:15 Cones
  • 3:45 Blind Spot & Impulses
  • 5:00 Lesson Summary
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Lesson Transcript
Instructor: Sarah Phenix
In this lesson, you will learn about the retina of the eye and its specialized structures, as well as how it enables us to see the world in living color.

What Is the Retina?

The retina is the photosensitive (light sensitive) tissue that covers approximately 65% of the interior surface of the eye. The retina is not actually attached to the vascular choroid layer that it lays directly against, but is held in place by the pressure of a jelly-like fluid known as the vitreous humor that fills the chamber of the eye behind the lens.

How Does the Retina Work?

Structures of the eye
Diagram of Eye

The retina works much like the film of a camera. It takes the visual information transmitted by beams of light reflecting off of objects and converts that information into a neural 'image' that it then transmits to the brain through the bundle of nerve fibers called the optic nerve.

When a beam of light enters the eye, it passes through accessory structures - such as the iris, pupil and lens - all of which serve the singular purpose of enabling that beam of light to reach the photosensitive retinal layer. So, what makes this layer so important? Well, it houses all of the photoreceptor cells, called rods and cones, which, when triggered by photons of light, result in a cascade of electrochemical events that generate a nerve impulse. It is this nerve impulse that, when received and translated by the brain, allows us the amazing feat of sight!

Rods

Rods are the specialized photoreceptor cells that allow us to see in dim lighting. You could think of them as the high-speed black and white film of the eye (back before the days of digital cameras) in that they are ultra sensitive to dim light (enabling us to see gradations of blacks and grays in low light settings), as well as very sensitive to high-speed movement.

Your retina contains about 125 million rod cells, giving you both a very acute awareness for anything trying to sneak up on you, as well as the ability of twilight sight (the ability for your eyes to adjust and see in dim lighting). What's interesting is that rods are completely insensitive to red light frequencies, so if someone were to shine a red frequency light in your eyes after your eyes had adjusted to the dark, your night vision would be undisturbed. This is why navigational instruments use red lights for night illumination.

Cones

Cones are the specialized photosensitive cells that allow us to see color. Cones are like the low speed color film of the eye because they are great at sensing bright light and detailed colors but are insensitive to low lights and high speed.

You have three different types of cone cells that are each sensitive to either red, green or blue frequencies of light. And, while there are only about 5-7 million cone cells in the retina (compared to about 125 million rods), when they are excited in various combinations, they allow us to see a range of 7-10 million different shades of color! Cones exist in the highest concentration (about 200,000 per square millimeter) in an area of the retina called the fovea centralis.

The fovea centralis is a small indentation of the retina tissue that contains a very dense collection of cones (no rods) and is responsible for your detailed sight. So when you focus on something by looking directly at it, like you are with these very words, your eyes are centering the light entering your eyes on the fovea centralis.

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