visual systems

Question 1

importance of vision (3)

Answer 1

detect prey and predators
detect mates
communication

Question 2

what is light + features

Answer 2

visible electromagnetic radiation
wavelength = distance between peaks and troughs
frequency = number of waves per second (Hz)
short wavelength = high frequency
amplitude = difference between peak and trough
visible light = 400-700nm wavelength

Question 3

eye structures and functions:
extraocular muscles
optic nerve
aqueous humor
lens

Answer 3

extraocular muscles = move eyeball, controlled by oculomotor nerve (CN-III) (somatic nervous system moves eyeball and ANS controls dilation and constriction)
optic nerve = CN-II - carries axons from retina to brain, all sensory info out of the eye
aqueous humor = provides substances to cornea so it doesn’t need blood vessels in it
lens = changes how spherical it is for refraction of light and focusing at distances

Question 4

eye structures and functions:
zonal fibres
ciliary muscles
vitreous humor

Answer 4

zonal fibres = attach lens to ciliary muscles
ciliary muscles = focus light onto retina by adjusting lens
vitreous humor = fluid to maintain spherical pressure in eyeball - lack of this can cause eyeball to shrivel and retina to pull away

Question 5

eye structures and functions:
pupil
iris
cornea
sclera

Answer 5

pupil = lets light into eye
iris = muscles control amount of light entering eye depending on brightness
cornea = glassy transparent covering of pupil and iris that refracts light, many nerve endings, cannot change for refractions
sclera = continuous with cornea, tough protective wall of eyeball to give it its shape

Question 6

monocular vs binocular view

Answer 6

alters what can be seen - field of view, depth perception - predator vs prey

Question 7

retina structures

Answer 7

fovea - most receptors here - focus light on this point
macula - surrounds fovea, many receptors still
optic disk - blind spot - optic nerve and blood vessels here, brain fills in gaps

Question 8

image formation - cornea refraction

Answer 8

light rays focus on retina
80% of refraction is in cornea and remaining 20% in lens
degree of refraction determined by difference in refractive index between 2 media - big difference between air and cornea
light travels more slowly in fluid of cornea - high density causes refraction
focal distance = distance from refractive surface to convergence of parallel light

Question 9

accommodation - the lens

Answer 9

close objects (<7 metres) = need additional refraction through accommodation of lens = light rays are not parallel = lens is rounded (ciliary muscles contract, suspensory ligaments are slack) , increasing refractive power

distant objects = almost parallel light rays = cornea provides sufficient refraction to focus them on retina = lens is flattened (ciliary muscle relax, suspensory ligaments are taut), less refractive power

Question 10

issues with accommodation

Answer 10

emmetropia = normal eye can focus on distant objects when lens is flat

hyperopia = far sightedness
focus on distances, close is blurry (focussed behind the retina) - eyeball is too short so light cannot refract enough
correction = convex lens = less refraction from close objects so they are more similar to distant rays = need less refraction in the eye

myopia = short sightedness
focus on close objects but distances are blurry (focussed in front of retina) - eyeball is too long
light refracts too much before it hits the retina
correction = concave lens = light refracts more before hitting cornea - more similar to rays from closer objects

Question 11

retina - laminar organisation and cell types (5)

Answer 11

7 layers of cells from ganglion (inner) to pigmented epithelium (outer)
- ganglion cells = output from retina
- amacrine cells = modulate info transfer between GC and BC
- bipolar cells = connect photoreceptors to ganglion cells
- horizontal cells = modulate info transfer between photoreceptors and BCs
- photoreceptors = sensory transducers - rods and cones

Question 12

photoreceptors

Answer 12

2 types - rods and cones

1 type of rod = monochrome = high sensitivity
3 types of cones = colour = high resolution
duplicity theory = cannot have high sensitivity and high resolution in a single receptor

Question 13

rods and cones (+ distribution (3 areas))

Answer 13

rods =
greater number of disks
high photopigment concentration
1000x more sensitive to light than cones
enable vision at low light (scotopic)
low visual acuity/resolution

cones =
fewer disks
used in daylight (photopic)
colour vision
high visual acuity/resolution
low sensitivity

distribution:
fovea = 5 million cones and no rods

central retina = low convergence, low sensitivity, high resolution

peripheral retina = high convergence, high sensitivity, low resolution –> not many protons will result in glutamate release and action potential starting in gangion cell –> many photoreceptors to one ganglion cell = threshold reached more easily –> low resolution as it doesn’t matter which photoreceptor is hit, the ganglion cell action potential will give the same signal - no differentiation

Question 14

phototransduction - rod and cone photopigments

Answer 14

rod = rhodopsin = 500nm
rhodopsin is made of retinal (absorbs photons and changes shape) and opsin (GPCR)

cones: S,M,L opsins
S = blue/violet = 420nm
M = green/yellow = 530nm
L = yellow = 560nm
greater numbers of long wave photoreceptors (M&L opsins)

Question 15

phototransduction - retinal ganglion photopigment

Answer 15

melanopsin = 475nm = blue/green
responds to big changes in light (night and day differences) - for circadian rhythms

Question 16

scotopic vs photopic

Answer 16

scotopic = night
photopic = day

Question 17

photoreceptors in light vs dark

Answer 17

photoreceptors are hyperpolarised by light - not depolarised like axons with action potentials
at rest glutamate is constantly being released from photoreceptors (excitatory)

cGMP (cyclic guanosine monophosphate) levels in rods change depending on light level - bind to ligand gated non selective cation channels in outer membrane of photoreceptor => produced by guanylyl cyclase

dark = high cGMP –> cGMP-gated channels open –> influx of Na+ to depolarise photoreceptor
K+ channels always open so K+ can move out to make sure photoreceptor doesn’t become too depolarised

light = lower levels of cGMP –> cGMP-gated channels close –> Na+ stops moving in –> photoreceptor is hyperpolarised

Question 18

phototransduction

Answer 18

5-7 photons evoke sensation of light in humans

rhodopsin is activated by light –> retinal absorbs photons and changes shape –> opsin stimulates G-protein transducin –> transducin GTP –> alpha subunit activates phosphodiesterase (PDE) –> reduces cGMP levels, closing Na+ channels

signal is amplified in enzyme cascade

Question 19

saturation of responses in bright light (rods and cones)

Answer 19

rods cannot process bright light - easily saturated –> rhodopsin is bleached and cGMP levels are low so no additional hyperpolarisation can occur

cones are not saturated easily - used in bright light

photoreceptors gradually depolarise with continued bright light after initial large hyperpolarisation

Question 20

role of calcium in light adaptation

Answer 20

dark:
Ca2+ enters cell and blocks guanylyl cyclase
reduces cGMP production and closes some ion channels

light:
channels are shut so Ca2+ cannot enter cells
guanylyl cyclase is no longer blocked
more cGMP produced –> more channels open

Question 21

on and off bipolar cells

Answer 21

classified based on response to glutamate

in light, reduction in glutamate release:
OFF bipolar cells hyperpolarise = have ionotropic glutamate receptors
ON bipolar cells depolarise = have metabotropic glutamate receptors

bipolar cells have centre-surround organisation - receptive field centre surrounded by receptive field surround which all connect to a horizontal cell and then a bipolar cell

Question 22

bipolar cells receptive fields

Answer 22

retinal ganglion cells only fire action potentials when specific areas of retina are illuminated
can map regions of retina which cause spiking in ganglion cells