Module 1 The Nervous System

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The Structures of the Eye

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The eye is an amazing organ! It contains many structures that must all be functioning properly in order for you to see. Several of these structures refract and focus light before it even reaches the different photoreceptors that make up the structure called the retina. Once these photoreceptors are stimulated by light energy, they convert this energy into an electrochemical nerve impulse. Sensory neurons in the optic nerve communicate this nerve impulse to interneurons in the occipital lobe of the brain where visual information is then processed. To understand the parts of the eye and how they contribute to vision, read pages 410 to 412 of the textbook. As you work through the reading, keep track of the vision disorders and their respective symptoms. You will need this information for an upcoming assignment.

 

It is always an excellent idea to support your understanding of the concepts you are learning with a diagram. You may choose to include diagrams in the work you will store in your course folder. Should you choose to include diagrams, be sure to include any brief descriptions of the functions of any structures, as well as brief summaries of the steps of different pathways.

 Crash Course - Sight

Try This

Here are three Try This activities you can use to verify your understanding of the parts of the eye and their functions, as well as vision issues related to the dysfunction of one or more of these structures. Please note that there is a choice of two possible activities indicated under each Try This.

 

TR 1.

Review the structures of the eye and their functions by dragging and dropping the terms provided into their respective places in the following chart.

Inquiry into Biology (Whitby, ON: McGraw-Hill Ryerson, 2007), 411, fig. 12.2. Reproduced by permission.

 

Accommodation video

 

The ciliary muscle and suspensory ligaments are excellent examples of how the sympathetic and parasympathetic nervous systems work together to establish homeostasis in vision. By changing the shape of the lens they allow for light rays to be refracted, bent differently allowing you to focus on the words of this sentence as you read it, or on the words on a poster you see on the far wall when you look up. This process is called accommodation and can be summarized by the following activites.

 

TR 2.

Complete the following by putting the lists provided in the correct order. You may wish to look at Figure 12.10 on page 412 of the textbook to help you figure out the correct order of events.

1. Sympathetic Nervous System: 

   • ciliary muscle relaxes

   • tension on suspensory ligament

   • tension pulls on lens

   • lens flattens

   • focus on far objects

    

   Place the numbered list above in the correct order:
   
   ___, ___, ___, ___, ___

    

2. Parasympathetic Nervous System:
   
   

   • ciliary muscle contracts

   • tension on suspensory ligament relaxes

   • less pull on lens

   • lens bulges

   • focus on near objects
     
     

   Place the numbered list above in the correct order:
   
   ___, ___, ___, ___, ___

OR

 

Watch and Listen

Review the accommodation reflex by watching the video

 

The Lens video

 

As we grow older, the flexibility of our lenses decreases. As a result, many people must wear reading glasses to focus the details of nearby objects. Hyperopia is the term used to describe the ability to see distant objects clearly, and the inability to focus on objects that are close. You may have already heard the more common term for this condition, which is farsightedness. Inversely, myopia is the condition of being able to clearly see objects that are close, but not being able to focus on objects that are farther away. The common term for myopia is nearsightedness. To learn what enables the lens to change its shape in order to successfully focus an image, which will allow you to better understand the conditions mentioned above, read pages 412 and 413 of the textbook. When you have finished the reading, make summary notes on the concepts learned. You may also choose to support this information with a labeled diagram. Store your information in your course folder for review purposes.

 

Another condition that affects the lens is cataracts, while astigmatism is a condition which affects the cornea. These conditions distort your vision. Remember to add them to your table of disorders. At the end of this lesson, you will submit your table to your teacher for assessment.

 

To develop your mastery of the concepts you have been reading about, do questions 1 to 3 and 5 on page 418 of your textbook. You may discuss your responses with your teacher if you wish.

 

The Retina and Supporting Structures

The inner layer of the eye consists of the delicate retina which contains two different types of photoreceptors called rods and cones. Directly in line with the middle of the lens, is an area of the retina called the fovea centralis, or fovea. This small, depressed area is where the cones are most highly concentrated. Cones are the photoreceptors that are sensitive to different colours, whereas rods are sensitive to light intensity. Rods are more spread out on the periphery of the retina. The neural fibres from the retina form the optic nerve, which is the sensory pathway out of the eye. The spot where the fibres come together is the blind spot, or optic disc.

 

Organisms that see well at night, like deer and cats, have a reflective layer similar to spray paint on surface of the choroid layer. This structure is called the tapetum. The tapetum increases the animal’s sensitivity to low levels of light and helps these animals see at night. This layer also causes the eyes of these animals to appear iridescent or reflective in the dark.

 

In front of the lens is the anterior chamber filled with aqueous humour. This transparent, watery fluid is produced by the ciliary body. If the aqueous humour is not successfully drained from the anterior chamber, its buildup will eventually result in the eye disorder called glaucoma. Behind the lens is the posterior chamber filled with vitreous humour—a faintly clear to amber-coloured gel-like fluid. Both the aqueous and vitreous humours will aid the refraction of light to a small degree.