Biology 30 - Unit A The Eye Student Handout.pptx

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Sense Overview

Category and type of Receptor Examples of Receptor Stimulus

Photoreceptors

Vision rods and cones in the eye visible light Chemoreceptors

taste taste buds on the tongue food particles in saliva smell olfactory receptors in the nose odour molecules internal sense receptors in the carotid artery

and aorta blood pH Mechanoreceptors

touch/pressure/pain receptors in the skin mechanical pressure hearing hair cells in the inner ear sound waves balance hair cells in the inner ear fluid movement body position proprioceptors in the muscles

and tendons, and the joints muscle contraction, stretching, and movement Thermoreceptors

Temperature heat and cold receptors in the

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Structure of the Eye

Main parts of the vertebrate eye:

• The _________: white outer layer, including

________

• _The_{_________}_{: pigmented layer}

• The _________: regulates the size of the pupil

• _The_{_________}_{: contains photoreceptors} • The _________: focuses light on the retina

• _The_{_________}_{: a blind spot in the retina where the}

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the eye is divided into two cavities separated

by the lens and

________________

:

• The anterior cavity is filled with watery

__________________

• _{The posterior cavity is filled with}

jellylike

_______________________

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Photoreception

the innermost layer of the eye is the ______

which comprises of four different layers

of cells:

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-Pigmented Epithelium

• _{is positioned between the ___________}

and the light sensitive cells

• _{pigmented granules in this layer}

prevent light that has entered the

eye from scattering.

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Light Sensitive Cells

–

_{there are two types of light sensitive}

cells called the rods and the cones.

»

_{The _______ respond to}

low-intensity light

»

_{The _______ that require high}

intensity light, identify colour.

• _{both rods and cones act as sensory}

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Evolutionarily however, the dog and the human each developed the visual system that worked best for them.

Humans have depended on their diurnal ability and a sense of color throughout time to help them find food.

Dogs on the other hand, were not originally diurnal

animals, until humans domesticated them. Consequently, the ability to see at night was originally more important to the dog than color. After all, their prey is often

camouflaged with the surroundings, so they are unable to rely on color vision cues as heavily as humans do to find food.

The retina of the eye is lined with both rods and cones in humans and dogs. The rods are much more prevalent in both species, but even more so in the dog than the

human. The rods are adapted to work best in low light and are used for motion detection. The central retina of the canine eye contains about 20% cones, while humans have an area of 100% cones called the fovea. The cones work best in mid to high levels of light and have the ability to detect color.

Color and Acuity Differences between Dogs and Humans by Jennifer Davis http://www.uwsp.edu/psych/dog/LA/davis2.htm

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Bipolar Cells and Optic Nerves

–

_{once excited, the nerve message is passed}

from rods and cones to the bipolar cells.

• _{the bipolar cells then relay the message}

to the cells of the optic nerve.

–

_{the optic nerve then carries the nerve}

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A Closer Look at Rods and Cones

rods and cones are unevenly distributed on the retina.

• in the centre of the retina there is a tiny depression

referred to as the _______________________.

• _{the fovea centralis}

• _{is the most sensitive part of the eye.}

• has many cones packed very close together.

• when you look at an object most of light falls here.

• is surrounded by rods

(often you can see things in your peripheral vision without being able to identify its colour.)

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A Closer Look at Rods and Cones

There are no rods and cones in the area which the optic

nerve comes in contact with the retina.

• because there is no photosensitive cells we call this

area the “_____________________”

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A Closer Look at Rods and Cones

there are three types of cones each which absorb

different wavelengths of light.

• the combination of cones that can detect red, blue

and green wavelengths of light allows us to see

range of colours.

• colour blindness is an inherited condition

(occurs more in _________) is a deficiency in

particular cones, usually red or green

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Chemistry of Vision

there are about 160 million rods surrounding

the colour-sensitive cones in the centre of the

eye.

• _{The rods contain a light-sensitive pigment}

called rhodopsin.

• _{The cones contain a similar pigment but}

they are less sensitive to light.

• _{______________ is composed of a form of}

____________and large protein called

_________

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Chemistry of Vision

– _{when a single photon of light strikes a______________}

molecule it divides into two components

• _{_________________, the pigment portion} • _{_________________, the protein portion.}

– _{this division alters the cell membrane of the rods and}

produces an action potential.

• _{neurotransmitters are released from the end plates of}

the rods and the nerve message is conducted across the synapse to the _________________ and to a neuron of the optic nerve.

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Chemistry of Vision

–

_{a terminal vitamin A deficiency can damage the}

rods.

Food, Standard Amount Vitamin A

(μg RAE)

Calories

Organ meats (liver, giblets), various, cooked, 3 oza _1490-9126 _134-235

Carrot juice, ¾ cup 1692 71

Sweetpotato with peel, baked, 1 medium 1096 103

Pumpkin, canned, ½ cup 953 42

Carrots, cooked from fresh, ½ cup 671 27

Spinach, cooked from frozen, ½ cup 573 30

Collards, cooked from frozen, ½ cup 489 31

Kale, cooked from frozen, ½ cup 478 20

Mixed vegetables, canned, ½ cup 474 40

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Chemistry of Vision

–

rhodopsin is extremely sensitive to light.

• in bright light rhodopsin is broken down faster

than it is restored

–

the opsins used for colour vision are much less

sensitive and operate best with greater light

intensity.

• only rods are active during periods of limited

light intensity, this is why images appear in

shades of grey.

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Afterimages

an example of an after image is the blue or green lines

that stay in your vision after a camera flash has gone

off.

• _{there are two types of after images, ___________ and}

_________________ ones.

• _{a positive afterimage occurs after you look into a bright}

light and close your eyes.

• _{a negative afterimage occurs when the eyes are}

exposed to bright coloured light for long periods of time.

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Focusing the Image

As light enters the eye it is bent towards the

pupil by the cornea.

• _{as light enters the more dense}

medium it is refracted (bent).

• _{light is bent to a focal point and an}

inverted image is projected on the

light sensitive retina.

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Focusing the Image

Ciliary muscles control the shape of the lens.

Suspensory ligaments maintain a constant tension.

• _{when close objects are viewed the ciliary}

muscles contract and the lens becomes

thicker.

• the thicker lens provides additional

bending of the light for near vision.

• When far away objects are viewed, the ciliary

muscles relax causing the lens to be thinner.

• The adjustment of the lens is known as the

accommodation reflex, objects 6 meters away from

the viewer need no accommodation.

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Focusing the Image

• The importance of the accommodation reflex becomes more pronounced with age.

• _{as the years add up so does layers of transparent protein}

covering the lens making it harder.

• _{by the age of 40 near point accommodation has}

reduced so much people usually have problems reading.

a secondary adjustment occurs during the accommodation reflex.

• _{when objects are viewed from a distance, the pupil dilates}

letting in as much light as possible.

• _{when objects are viewed close up the pupil constricts in an}

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a constricted pupil makes it so light passes

through a small opening and falls on the most

sensitive part of the retina, the

______________________.

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Visual Defects

Glaucoma

• _{caused by a buildup of aqueous humour in the anterior}

chamber of the eye.

• _{tiny ducts usually drain out any excess liquid that is}

produced every day.

• _{if the ducts get blocked, the fluid builds up and}

pressure inside the eye increases.

• _{the retinal ganglion cells slowly die from the increased}

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Visual Defects

Cataract

the lens becomes __________ and prevents

some of the light from passing through.

the traditional solution is to remove the lens and

fit the patient with strong eye glasses.

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Visual Defects

Astigmatism

for most people the lens and cornea are symmetrical.

• _{incoming light is _____________ along identical angles for both}

the dorsal (back) and ventral (front) surfaces,

• _{this forms a sharp focal point.}

• _{in some people, the lens or cornea are irregularly shaped leading}

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Visual Defects

Nearsightedness

Also known as _____________________

• _{occurs when the eyeball is too long.}

• _{The lens cannot flatten enough to project the image on}

the retina

• _{The distant image is brought into focus in front of the}

retina.

• _{Someone who is nearsighted is able to focus close objects}

but has difficulty seeing objects at a distance.

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Visual Defects

Farsightedness

Also known as ________________________

• _{is caused by an eyeball that is too short.}

• _{distant images are brought into focus behind the retina, instead}

of on it.

• _{A farsighted person can focus on distant objects, but has trouble}

seeing objects that are close up.