Lecture 14

Question 1

Vision allows humans (and animals) to…

Answer 1

Detect prey (in modern world = source food)
- mainly predator: eyes on front for better focus
Detect predators (in modern world = danger)
- mainly prey: eyes on sides to increase peripheral vision
Detect mates
Communicate

Question 2

How much of the human cerebral cortex is involved in analyzing the visual world?

Answer 2

More than 1/3

Question 3

What is light?

Answer 3

Electromagnetic radiation that is visible

Question 4

What does light have?

Answer 4

A wavelength - distance between peaks or troughs
A frequency - number of waves per second
An amplitude - difference between peak and trough

Question 5

How does light rays interact?

Answer 5

Reflection
Absorption
- transfer of light energy to a particle or surface
- occurs in some retinal cells
- some cells absorb light to ensure light doesn’t bounce around and produce mixed signals
Refraction
- bending of light rays as they move from one transparent material to another e.g. air to water

Question 6

How does refraction form images in the retina?

Answer 6

Refraction occurs because the speed of light differs between mediums
e.g. slower through water than air
The greater the difference in speed in the two media, the greater the angle of refraction. Refraction occurs towards a line that is perpendicular to the border

Question 7

What does the pupil do?

Answer 7

Lets light inside the eye

Question 8

What does the iris do?

Answer 8

Contains muscles which control pupil size

Question 9

What does the cornea do?

Answer 9

Glassy, transparent, covering of the pupil and iris

Question 10

What does the sclera do?

Answer 10

Continuous with cornea, forms the tough wall of the eyeball

Question 11

What do the extraocular muscles do?

Answer 11

Move the eyeball

- controlled by cranial nerves III, IV and VI

Question 12

What does the conjunctiva do?

Answer 12

Membrane underneath eyelids that attaches to the sclera

Question 13

What does the optic nerve?

Answer 13

Carries axons from retina to brain

Question 14

What is a type of eye disorder?

Answer 14

Strabismus

• Imbalance in the extraocular muscles of the eyes
• Esotropia = convergence of the eyes, cross-eyed when muscles pulling outwards are weakened
• Exotropia = divergence of the eyes, wall-eyed when eye muscles falling inwards are weakened

Question 15

What is the optic disc?

Answer 15

Origin of blood vessels and optic nerve, cannot sense light

Question 16

What is the macula?

Answer 16

Region of retina for central vision, devoid of large blood vessels to improve vision quality

Question 17

What is the fovea?

Answer 17

Retina is thinnest here where acute vision occurs (cone cells)

Question 18

What is the aqueous humor?

Answer 18

A watery fluid that provides nutrients to the cornea and the lens

Question 19

What is the lens suspended by?

Answer 19

zonal fibers (ligaments) which are attached to the ciliary muscle

Question 20

What is the vitreous humor?

Answer 20

A viscous humor that keeps the eyeball attached

Question 21

How is the aqueous humor produced and absorbed?

Answer 21

Normally produced by the ciliary body and absorbed by the canal of Schlemm (between cornea and sclera)

Question 22

What does the aqueous humor contain?

Answer 22

Nutrients e.g. glucose

Question 23

What kind of disorders can increased aqueous humor and functioning cause?

Answer 23

Cataracts (白内障）
- clouding of the lens
- can occur due to changes in the composition of the aqueous humor
- increased risk with age, smoking or diabetes mellitus
- can be treated by lens replacement
Glaucoma (緑内障）
- Slowed uptake of aqueous humor leads to elevated intraocular pressure
- causes compression of the optic nerve and reduced blood supply to the retina
- leads to a progressive loss of vision from periphery inwards
- - aqueous humor needs to be constantly recycled but it is no longer taken out of Schlemm
- - - pressure pushes eye out
- - - starts to affect peripheral vision but people start noticing when central vision becomes affected

Question 24

Where must the light rays be focused on?

Answer 24

Retina

Question 25

How are the light rays focused on the retina?

Answer 25

The cornea: 80% of total refraction

The lens: 20% of total refraction

Question 26

How does refraction occur by the cornea?

Answer 26

Light arrives at the cornea through air but the cornea is mainly water
Light travels more slowly through water than air = refraction occurs
Light that hits the cornea directly perpendicular just moves straight through onto the retina

Question 27

Why do things look blurry underwater?

Answer 27

Refractory index underwater is similar to the one for the cornea - the refractive power of the cornea is eliminated as light passes through water into a similar substance

Question 28

What is focal distance?

Answer 28

Distance from the refractive substance ( the cornea), to where the parallel - light rays converge

Question 29

If the distance between the cornea to the retina is 2.4 cm, what is the refractive power of the cornea?

Answer 29

42 diopters

Question 30

What happens when light hits the cornea directly perpendicular?

Answer 30

Moves straight through onto the retina

Question 31

What is accommodation by the lens?

Answer 31

Light rays from distant objects are almost parallel by the time they reach the eye
But light rays from nearer objects are not parallel
- needs additional focusing power to focus them on the retina
The lens provide greater refractive power to focus these closer (<9m away) objects

Question 32

What happens at the far point?

Answer 32

Ciliary muscles relax, zonal fibers stretch, lens flattensq

Question 33

What happens at the near point?

Answer 33

Ciliary muscles contract, less tension in zonal fibers, lens becomes more spherical

Question 34

Since the lens is naturally elastic…

Answer 34

If not stretched, it will become more spherical

Question 35

What happens when the lens becomes rounder?

Answer 35

Increases the refractive power

Question 36

What is hyperopia? (Far sightedness)

Answer 36

Eye is too short
Near objects are focused behind the retina
Not enough refractive power
Correction: Convex lens straighten out light

Question 37

What is myopia? (near sightedness)

Answer 37

Eye is too long
Far objects are focused after the retina
Too much refractive power
Correction: concave lens makes light more divergent

Question 38

What does the Pupillary light reflex allow?

Answer 38

Iris regulates light levels

• pupillary constrictor (smooth circular muscle)
• regulated by parasympathetic system
• pupillary dilator (smooth radial muscle)
• regulated by sympathetic system

Question 39

Why is the pupillary light reflex useful?

Answer 39

Enables us to adjust to changes in light intensity

- increase in focus (clearer image)

Question 40

What is the consensual light reflex? (occurs in both eyes even if only one is stimulated)

Answer 40

Shining light in each eye

• to see if both pupils will constrict
• if only one constricts, no proper function in brain
• sensory (afferent) neuron
• • optic nerve (cranial nerve II)
• both Edinger-Westphal nuclei are innervated by sensory input from one eye
• Effector (efferent) neuron
• • oculomotor nerve (cranial nerve III)
• sympathetic neurons via the superior cervical ganglion