Module 1 The Nervous System
Lesson 1.1.5
1.1.5 page 3
Module 1: Lesson 5 Assignment
Retrieve your copy of Module 1: Lesson 5 Assignment that you saved to your computer earlier in this lesson. Complete Part 1. Save your completed assignment in your course folder. You will receive instructions later in this lesson on when to submit your assignment to your teacher.
Self-Check
Complete the following crossword puzzle to consolidate your understanding of the different structures of the eye and their functions:
Read
The Retina
You should now be familiar with the different structures of the eye and have a good understanding of each structure’s role in vision. As such 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 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 which was transmitted as an impulse is processed by inter-neurons. To further understand how the retina converts light energy into electrochemical impulses, read pages 414 – 416.
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.
In the following diagram, 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 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 which carries impulses to the brain. Mouse over the diagram to see each part of the diagram highlighted
Inquiry into Biology (Whitby, ON: McGraw-Hill Ryerson, 2007), 415, fig. 12.16. Reproduced by permission.
Rods and Cones
From the diagram, you may have noted that rods are the most abundant photoreceptors in the retina. They are necessary in distinguishing shades of black and white as well as distinguishing movement. Cones respond to specific wavelengths of the visible spectrum which allows us 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 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 strongly than the green cones, we see orange instead of yellow. Look at Figure 12.14 on page 414. If you see the *, you have normal colour vision. 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? Colorblindness is a genetically inherited trait that will be studied in more detail in the genetics module of this course (Unit C).
TR 4.
To review the concepts of vision, watch the video.