Module 1

1. Module 1

1.33. Page 8

Lesson 5

Module 1—The Nervous System

Lesson Summary

In this lesson you explored the following focusing questions:

What are the major parts of the eye?
How do these parts function?
How do these parts communicate with the nervous system to support the integrated act of seeing?

Through your readings, the dissection you completed, your research, and the videos you watched, you learned about the structures of the eye and their functions. By exploring the structures of the photoreceptors, called the rods and cones, you learned how a nerve impulse is initiated and transmitted to the occipital lobe of the cerebrum. It is there that the sensations are interpreted. Things like the frequency of nerve impulses, the origin of the impulse in the retina, and previous experiences and memory all factor into the interpretation of light stimuli. Not everyone will interpret these sensations in the same way, as is observed with optical illusions. In the Lesson 5 Assignment, you researched many common visual defects and disorders that affect the functioning of the eyes and impede proper vision. You also examined numerous technologies that have been devised to correct these disorders.

Lesson Glossary

Consult the glossary in the textbook for other definitions that you may need to complete your work.

accommodation: the process of changing the shape of the lens from round and fat to thin and flat, and vice versa, so that light can be focused on the retina to accommodate vision of objects near and far away

adaptation: the process by which the iris adjusts the diameter of the pupil based on light conditions, thus controlling the amount of light that enters the eye and strikes the retina

anterior chamber: the space in front of the iris and behind the cornea that is filled with aqueous humour

aqueous humour: a clear, watery fluid in the anterior chamber of the eye that maintains the shape of the cornea and provides oxygen and nutrients for the surrounding cells, including those of the lens and the cornea

astigmatism: an uneven curvature of the cornea or lens, resulting in uneven focusing, which results in poor vision

bipolar cells: specialized sensory nerve cells located in the retina that are stimulated by either rods or cones

Cones mostly have a one-to-one ratio with bipolar cells, whereas several rod cells may communicate with one bipolar cell.

blind spot: the area at the back of the eyeball that is deficient in rods and cones; the area where the sensory fibres come together to form the optic nerve

cataract: a cloudy or grey-white area on the lens caused by deterioration of the protein composing the lens

Cataracts prevent the passing of light to the photoreceptors of the retina.

choroid: the middle layer of the eyeball that lies between the sclera and retina and is highly vascular and heavily pigmented

The choroid absorbs stray light rays not detected by the photoreceptors of the retina.

ciliary muscle: a ring of muscle behind the iris that is attached to the lens by suspensory ligaments and is involved in changing the shape of the lens

colourblindness: an x-linked inherited disorder that results in nonfunctional or deficient cone function; inability to see certain colours such as red, green, or blue

cones: one of two types of photoreceptors in the retina of the eye that is sensitive to different wavelengths of light and are, thus, responsible for distinguishing colour

There are three types of cones: one sensitive to red light, one sensitive to blue light, and one sensitive to green light. The cones are responsible for acute vision, or distinguishing detail.

cornea: a transparent portion of the sclera (located at the front of the eye) that allows light to enter the eye and, in the process, refracts or bends the light rays so that they can be focused on the retina

depth perception: the ability to see in three dimensions

fovea centralis: an area of the retina that is located directly behind the centre of the lens and has a very high concentration of cones, which makes this part of the eye responsible for great visual acuity

ganglion cells: special sensory neurons that communicate with bipolar cells in the retina to transmit a nerve impulse to the brain

These cells have long axons that converge at the back of the eye to form the optic nerve.

glaucoma: a disorder caused by the malfunction of ducts that drain excess aqueous humour from the anterior chamber

The resulting pressure created by excess aqueous humour ruptures delicate blood vessels in the eye and causes deterioration of cells in the eye due to lack of nutrients. This can result in blindness if left untreated.

hyperopia: farsightedness, or the inability to focus objects that are close, caused by an eyeball that is too short, which causes light to be focused behind the retina

iodopsin: the general name of any of the three visual pigments found in cone cells that is stimulated by light to initiate a nerve impulse

myopia: nearsightedness, or the inability to focus objects that are far away, caused by an eyeball that is elongated, which causes light to be focused in front of retina rather than directly on it

opsin: a protein that is the result of the decomposition of rhodopsin

optic nerve: a collection of sensory neurons that carries sensory information from the photoreceptors to the brain

refract: to bend light as it passes through a substance with a different desnsity

retina: the innermost layer of the eye that contains the photoreceptors

retinal: a derivative of Vitamin A (retinol) that is the result of the decomposition of rhodopsin and is instrumental in initiating a nerve impulse

rhodopsin: a visual pigment found in rod cells that is decomposed by light into opsin and retinal

The change from rhodopsin to opsin and retinal initiates a nerve impulse.

rods: one of two types of photoreceptors in the retina of the eye that is sensitive to light intensity and detect movement

Rods do not distinguish colour.

sclera: the white, tough, protective outer layer of the eye that helps gives the eyeball its shape; sometimes called the white of the eye

tapetum: a layer in the choroid that increases the absorption of light to stimulate photoreceptors in dim conditions

vitreous humour: the transparent, amber-coloured, jelly-like fluid in the posterior chamber of the eye that helps to maintain the shape of the eyeball