Module 1

Module 1—The Nervous System

Module 1: Lesson 5 Assignment

Retrieve the copy of the Module 1: Lesson 5 Assignment that you saved to your computer earlier in this lesson. Complete Part 1—Lab, which is based on the dissection of the eye. Save your completed assignment in your course folder. You will receive instructions about when to submit your assignment to your teacher later in this lesson.

Self-Check

SC 1. Complete the following crossword puzzle to consolidate your understanding of the different structures of the eye and their functions.

The Retina

Read

You should now be familiar with the different structures of the eye and have a good understanding of each structure’s role in vision. You should understand the role of the cornea and lens in bending and focusing light onto the retina. The retina is the key structure by which light energy is converted into an electrochemical impulse. This sensory impulse is then communicated to the brain via the optic nerve. When it reaches the occipital lobe of the brain, the information that was transmitted as an impulse is processed by interneurons. To further understand how the retina converts light energy into electrochemical impulses, read pages 414 to 416 in the textbook.

Study the diagram below to locate the four layers of cells in the retina:

• pigmented cells: closest to the choroid, specialized to form the tapetum in some animals
  
  
• 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 further covered by layers of transparent neurons.
  
  
• bipolar cells: activated by rods and cones
  
  
• ganglion cells: closest to the vitreous humour

Roll your mouse over the labels on the diagram of the retina to highlight the different structures.

Now that you have located the different layers, note the direction of the light striking the retina. Light travels through the transparent layer and strikes the rods and cones. These photoreceptors are then prompted to stop emitting an inhibiting neurotransmitter substance, which you will learn about in Lesson 8. The rods and cones can then communicate an electrochemical message, first to the bipolar cells and then to the ganglion cells. Axons of all ganglion cells of the retina converge at the back of the retina to form the optic nerve, which carries impulses to the brain.

Rods and Cones

When looking at the diagram of the retina, you may have noticed that rods are the most abundant photoreceptors in the retina. They are necessary for distinguishing shades of black and white as well as for distinguishing movement. Cones respond to specific wavelengths of the visible spectrum, allowing people to see colour. Cones are also needed for acute vision, as they are capable of finer discrimination of detail in bright light than are rods.

Based on your new knowledge of rods and cones, could you develop a hypothesis explaining why the eyes of birds that roost at night have only cones, whereas the eyes of owls and bats that are active at night have only rods?

You have also read about colourblindness. There are three types of cones: red sensitive, green sensitive, and blue sensitive. One reason that people 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 strongly than the green cones, you see orange instead of yellow. Look at “Figure 12.14” on page 414 of your textbook. If you see a number 8, you have the full range of colour vision. If you see a number 3, you have red-green colour blindness. Can you explain the varying degrees and types of colourblindness in terms of the types of cones? Colourblindness is a genetically inherited trait that will be studied in more detail in Unit C.

Try This

TR 4. Choose one of the following activities.

To review the concepts of vision, watch the following segments of the video “The Eye: Vision and Perception: A Whole World to See.”

• “The Retina”
• “Bio Bit: Colour Blindness”

You may be required to enter a username and password to access the video. Contact your teacher for this information.

OR

Do an Internet search using search terms such as “colourblindness,” “colourblindness test,” and “Ishihara test.”