Topic 8 (Eyes)

Front 1

Reflex arc for response to light in eye

Back 1

Bright light > photoreceptors in eye (retina) > SN (ganglion neurone/optic nerve) > RN (brain BECAUSE close) > MN > circular contract, radial relax causing pupil to constrict (effectors)

Front 2

The iris

Back 2

• 2 sets of muscles (antagonistic pair)

1) circular muscles

2) radial muscles

Front 3

Bright light (the iris)

Back 3

• parasympathetic nerve impulse

• circular contract (cc)

• radial relax (rr)

• pupil constricts

• less light enters eye

Front 4

Dim light (Reflex response)

Back 4

• sympathetic nerve impulse

• circular relax

• radial contract

• pupil dilates

• more light enters eye

Front 5

What carries impulses from the retina to the brain

Back 5

Optic nerve (grp of sensory neurones)

Front 6

What causes light rays to focus

Back 6

Lens focuses light rays onto the retina (as these contain photoreceptors which detect light)

Front 7

Rod cells

Back 7

• (Photo) Receptor cells in the retina > sensitive to light

• see clarity & detail of image (texture gradient due to shading)

• Rods code LIGHT (e.g. black and white)

Front 8

Rhodopsin

Back 8

• Light sensitive pigment contained by rods

• rhodopsin breaks down into retinal & opsin (light) > generates an action potential in neurones of optics nerve

Front 9

Blind spot

Back 9

Where the optic nerve leaves the eye > has no photoreceptor cells so x sensitive to light

Front 10

Fovea

Back 10

An area of the retina with lots of photoreceptors

Front 11

Light- response in rod cells (process)

Back 11

1) Photo receptor - detects light

2) photopigment rhodopsin - absorbs light

2) light energy causes rhodopsin to break down into retinal + opsin (process called bleaching)

3) outer segment > opsin activates a cascade of reactions causing the sodium ion channel to close

4) inner segment > sodium ions actively pumped out of rod cell (active transport, ATP, against conc grad)

4) hyperpolarisation of cell membrane > ^ -ve inside than outside (-70mv)

5) neurotransmitter glutamate is not released > rod cell able to fire an impulse

6) impulse sent out of rod cell > bipolar neurone > ganglion neurone > optic nerve > brain

Front 12

Dark - response in rod cells

Back 12

1) photoreceptor - rod cell > x detect light

2) photopigment - rhodopsin > x absorb light (as not bleached)

Absence of light means conversion of retinal + opsin > rhodopsin

3) outer segment > cation channel remains open > Na+ diffuses into rod cell

4) inner segment > active transport of Na+ out of the rod cell using sodium pump

5) slight depolarisation of the membrane (-40mv)

6) neurotransmitter glutamate is released > inhibits sending of an impulse by binding to the bipolar neurone

Front 13

Cone cells

Back 13

Allow you to see the Colour of an image

(X work in dim light)

Front 14

Role of opsin

Back 14

• binds to membrane of the rod cell/ activates cascade of reactions to close cation channel of outer segment

• to recombine with retinal to create the photopigment rhodopsin

Front 15

Role of retinal

Back 15

• light absorbing molecule (> derived from vitamin A)

• to recombine with opsin to create photopigment rhodopsin

Front 16

Rhodopsin in the light

Back 16

* sodium channel (outer segment) is closed so Na+ x enter the rod cell

* sodium pump releases (inner segment) Na+

* rhodopsin is converted to retinal + opsin

Front 17

Rhodopsin in the dark

Back 17

* sodium channel (outer segment) is open so Na+ enter the rod cell

* sodium pump (inner segment) releases Na+

* retinal + opsin converted to rhodopsin

Front 18

Why do sodium channels of the outer segment close in the light?

Back 18

Opsin activates a cascade of reactions that result in the closing of the cation channel

Front 19

Where is rhodopsin located

Back 19

Membrane of vesicles in the outer segment of rod cells

Front 20

What is the dark adaptation

Back 20

Reforming of rhodopsin (from retinal and opsin)