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Chapter 6

Structure and functions of the eye

QuestionAnswer
What mechanisms protect the eye? The orbit, cushioning fat, eyelids, and tears.
What is the orbit? The bony opening in the skull which houses the eyeball.
How do eyelids protect the eye? Blinking protects the eye from incoming objects as well as moistening and cleaning the front of the eye.
Why don't we respond to the blackouts produced by blinking? The parts of the brain responsible for consciousness are also less active during the blink.
Where are tears produced? They are produced in the lacrimal gland at the outer corner of each eye.
What are tears made of? They are made of water, salt and also protein, glucose and substances that kill bacteria.
What do tears do?T They flush away dust and debris as well as moistening the eye so that the eyelids don't scratch the surface of the eye during blinks.
What is the difference between tears produced by emotional events and tears produced by irritants? Tears produced by emotional events contain about 24 percent more protein.
Describe the eye. It is roughly a sphere with a diameter of about 24 mm.
How large are newborns' eyes? 16-17mm.
What is the sclera? What does it do? It is the "white" of the eye. It provides a tough outer covering that helps the fluid-filled eyeball maintain its shape.
What is the cornea? What does it do? It is the outer layer of the eye; it is a clear and blood vessel-free extension of the sclera but has the greatest density of pain receptors in the body. It begins the process of refracting light rays to form an image in the back of the eye.
What is the anterior chamber? It is the area of the eye located directly under the cornea, and contains aqueous humour.
What is aqueous humor? It is the fluid in the anterior chamber that nourishes the cornea and lens.
What is the pupil? The opening in the front of the eye controlled by the iris.
What is the iris? The circular muscle in the front of the eye that controlls the opening of the iris.
What else is pupil diameter affected by? Emotional state, through the autonomic nervous system. The sympathetic division causes it to dilate, and the parasympathetic division causes it to become more constricted.
What is the color of the iris affected by? Amount of melanin pigment in conjuction with the reflection and absorption of light by other things in the iris e.g. reflection and absorption.
Rank the colours of the iris in terms of melanin pigment amount. Highest to lowest. Brown, green, blue or grey. Also, amber is a combiation of brown eyes with a golden look caused by an additional yellowish pigment.
What is the lens? The structure behind the pupil and iris that focuses light on the retina in the back of the eye.
What allows us to adjust the focus of the lens? Muscles attached to the lens allow us to adjust our focus as we look at objects near or far.
What is accomodation? The ability of the lens to change shape to adjust to the distance of the visual stimulus.
What is the vitreous chamber? The large inner cavity of the eyeball, which is filled with vitreous humour.
What is vitreous humour? The jellylike substance in the vitreous chamber.
What is the difference between aqueous humour and vitreous humour? Vitreous humour remains the same throughout life, as compared to the aqueous humour which is constantly renewed.
What is the retina? The network of phtoreceptors and interneurons at the back of the eye that is responsible for seeing light.
What is the property of the image projected on the retina? The image projected on the retina is upside down and reversed relative to the actual orientation of the object being viewed.
During migration, what is the retina part of? It is a part of the diencephalon.
WHtat are photoreceptors? They are specialized sensory cells in the retina that respond to light.
Why don't we normally see our nose, or blood vessels and neural layers in the eye? The visual system responds to change and tunes out stimuli that remain constat.
What is the optic disk? The area in the retina where blood vessels and the optic nerve exit the eye.
What is the optic disk responsible for? It does not contain any photoreceptors. It is responsible for giving each eye a blind spot.
What is the macula? A yellowish 6mm diameter and round area in the retina that is specialized for central vision and does not contain any blood vessels.
What is central vision? The ability to perceive visual stimuli focused on the macula of the retina, such as when staring at something.
What is peripheral vision? The ability to perceive visual stimuli that are off to the side while looking straight ahead.
What is the fovea? A small 1.8mm diameter pit in the center of the retina that is specialized for detailed vision and only contains cones.
What is the epithelium? The pigmented layer of cells supporting the photoreceptors of the retina and absorb random light.
Why does the interior of the eye look black when seen through the pupil? The epithelium absorbs random light.
What are the layers of the retina, outside to inside? 1. Ganglion cell layer 2. Inner plexiform layer 3. Inner nuclear layer 4. Outer plexiform layer 5. Outer nuclear layer 6. Layer of photoreceptor outer segments.
What is the ganglion cell layer? The layer of retinal interneurons farthest from the photoreceptors which contains ganglion cells and gives rise to the optic nerves.
What are ganglion cells? Retinal cells in the ganglion cell layer which have a single axon. These axons collectively form the optic nerve as it leaves the retina.
What is the inner plexiform layer? The layer in the retina containing axons and dendrites that connect the ganglion, bipolar and amacrine cells.
What are bipolar cells? A cell in the inner nuclear layer of the retina that forms part of the straight pathway between the photoreceptors and the ganglion cell.
What are amacrine cells? A retina interneuron in the inner nuclear layer that integrates signals across adjacent segments of the retina.
What is the inner nuclear layer? The layer of retinal interneurons containing amacrine, horizontal and bipolar cells.
What is a horizontal cell? A retinal interneuron located in the inner nuclear layer that integrates signals from across the surface of the retina.
What is the outer plexiform layer? The retinal layer containing axons and dendrites forming connections betwen bipolar cells, horizontal cells and the photoreceptors.
What is the outer nuclear area? The location in the retina containing the cell bodies of the photoreceptors.
What are the two types of photoreceptors? Rods and cones. They are so named because of the shape of their outer segments.
What are the outer segments? The portion of a photoreceptor that contains photopigments.
What are photopigments? A pigment that absorbs light.
How many rods and cones are there in the eye? 90 million rods and 4 to 5 million cones
What photopigment do rods use? Rhodopsin.
Rods are responsible for scotopic vision. What is scotopic vision? The ability to see in dim light.
What do rods lack? They do not provide any information about colour, and do not produce sharp images.
Cones are responsible for photopic vision. What is photopic vision? The ability to see in bright light.
What are the features of photopic vision? It is sensitive to color and provides images with excellent clarity.
What is the difference between the outer segments of rods and cones? Rods have a long, cylinder-shaped outer segment containing large numbers of disks that store only 1 photopigment, rhodopsin. Cones, however, have a shorter and more pointed outer segment that ends in a folded membrane that stores 1 of 3 photopigments.
What happens as we move from the fovea to the outer margins of the retina? What does this cause in vision? The concentration of rods increases and the number of cones decreases. Thus, center of retina is better in bright light but periphery is superior in dim light. Uneven distribution of rods and cones - see better in dim light when not focusing on object
How do rods (and cones) transduce light energy? 1. 11-cis rhodopsin (made of binded opsin and retinal) absorbs photons 2. Light energy changes the 11-cis form to the all-trans form 3. Change in structure causes the rhodopsin molecule to break apart rapidly.
What is opsin? A protein chain found in photopigments.
What is retinal? A chemical contained in rhodopsin that interacts with absorbed light.