lecture 25 - nervous system 5: vision and the eye

Question 1

what is the eye made up of

Answer 1

cornea 
iris 
pupil 
lens 
retina

Question 2

what does the cornea do?

Answer 2

responsible for refracting light to bring about focus on the retina

responsable for most of the refractive power of the eye

Question 3

what does the iris do?

Answer 3

pigment determines eye colour

muscles controlled by ANS
• control pupil diameter according to light and emotional signals

Question 4

what does the pupil do?

Answer 4

opening in the centre of the iris that allows light entry

2-8mm range

Question 5

what does the lens do?

Answer 5

responsible for accommodation - adjusting the refractive properties of the eye

Question 6

what does the retina do?

Answer 6

layer at the back of the eye

photoreceptors, horizontal cells, bipolar cells, amacrine cells, ganglion cells and nerve fibres

part of the brain

Question 7

where does laser treatment act?

Answer 7

on the cornea

Question 8

what do photoreceptors on the retina do?

Answer 8

convert light energy into electrical signals which go to the brain

Question 9

what is the anterior chamber of the eye filled with?

Answer 9

aqueous humour

Question 10

what is the vitreous chamber of the eye and what is it filled with?

Answer 10

maintains shape of the eye

filled with vitreous humour

Question 11

what is the optic disk?

Answer 11

where the nerve fibres and optic nerves leave the eye

Question 12

what is the visual field of each eye?

Answer 12

covers 150 degrees

fields of view overlap

image is laterally inverted in the eye

limited by the nose

Question 13

what is accommodation?

Answer 13

the ability of the eye to adjust its focal length for different object distances

lens brings light rays into focus at the focal length at a particular position on the retina

Question 14

accommodation: what happens if the object is close?

Answer 14

lens isn’t powerful enough to bring object into focus

image will form behind the retina so will be out of focus

you need a stronger lens
• rounder lens gives a stronger refractive power
• brings light into focus on the retina

Question 15

how does the lens accommodate?

Answer 15

it is in a capsule which is attached to the ciliary muscles by the zonulas

Question 16

how do the ciliary muscles allow accommodation?

Answer 16

contraction of the ciliary muscles allows zonulas of zinn to slacken, lens expands and becomes more rounded

when relaxed the zonulas are pulled tight and the lens is pulled into a flat configuration

Question 17

what is the retina made of?

Answer 17

fovea

blind spot or optic disk

blood vessels

Question 18

what is the fovea?

Answer 18

pit in the centre of the macula in the retina

area of maximum acuity (highest image resolution) - highest density of cone receptors

Question 19

what is the blind spot / optic disk?

Answer 19

region where the nerve fibres and blood vessels leave the eye

blind in this region

no photoreceptors here

brain fills in the missing area

Question 20

what do the blood vessels in the retina do?

Answer 20

deliver oxygen and nutrients to the retina

Question 21

organisation of the retina

Answer 21

pigment epithelium

2 types of photoreceptors

other cells connecting vertically and horizontally

Question 22

what is the pigment epithelium?

Answer 22

cells at the very back of the retina

photoreceptors embedded in this layer

cells contain melanin black pigment

absorbs light to prevent scattering stray light that would affect the image

Question 23

what are the 2 types of photoreceptors?

Answer 23

rods
• scotopic vision
• low light levels

cons
• photopic vision
• high light levels

more rods than cones

Question 24

what are the other cells connecting vertically and horizontally?

Answer 24

bipolar cells
horizontal cells
amacrine cells
ganglion cells

cones and rods attached to bipolar cells

Question 25

what does light have to pass through to reach photoreceptors?

Answer 25

blood vessels and nerve fibres

except at the fovea for maximum acuity

Question 26

what happens at the fovea?

Answer 26

fovea cells in upper layers of the retina are pushed aside to allow light direct access to photoreceptors

Question 27

features of rods

Answer 27

• scotopic vision
• most sensitive to light
• sensitive to brightness
• only detect brightness
• function at low light levels
• bleached at high light levels

Question 28

features of cones

Answer 28

• photopic vision
• 3 types sensitive to long, medium and short wavelengths
• enable colour vision
• responsible for high acuity (sharpness)
• work only at high light levels

Question 29

what is dark adaptation?

Answer 29

at high light levels rods bleached so rely on cone vision

when switched from bright to low light levels its initially hard to see - cones not sensitive and rods still bleached from light before

gradually visual sensitivity increases ass rods recover - pigment in the rods gradually builds up

this is called dark adaptation

Question 30

distribution of rods and cones

Answer 30

cones
• low density in retina
• high density in fovea
• not at the blind spot

rods
• high density in retina
• not in the fovea
• not at the blind spot

where there aren’t cones there are rods

Question 31

what are the 3 cone types?

Answer 31

short wavelength
medium wavelength
long wavelength

each cone is sensitive to a range of wavelengths

Question 32

what is the trichromatic theory of colour vision?

Answer 32

colour is represented by the unique ratio of outputs from the 3 different cones

Question 33

what happens when theres an absence of a cone type?

Answer 33

colour confusion or colour blindness

Question 34

light sensitive photopigments in rods and cones

Answer 34

rods - rhodopsin

cones - 3 different pigments (but wavelength sensitive)

Question 35

what is rhodopsin?

Answer 35

the light sensitive photopigment in rods

made of 2 chemicals

1) opsin (protein)
2) 11-CIS-retinal (derived from vitamin A)

in the dark opsin and retinal are bound together

on light exposure, retinal molecule changes shape and splits from opsin

this process is called bleaching

Question 36

where are retinal and opsonin found?

Answer 36

in the outer segment of photoreceptors

rhodopsin are embedded in the membranes of the membraneous disks

Question 37

rods in the dark vs in the light

Answer 37

Rods contain channels for Na+ and K+

In dark:
• cyclic GMP levels high, keeps Na+ channel open
• Na+ current into rod > than K+ current out
• rod depolarised to – 40mV & transmitter released
• constant release of neurotransmitter in dark

Light bleaching of rhodopsin activates transducin (G protein):
• reduces level of cGMP so Na+ channels close
• less inward current (fewer positive ions flow in)
• K+ continues to flow out
• rod hyperpolarises to -70 mV
• less transmitter release

light hyperpolarises the cell and reduces transmitter release (not what you might expect)