25- Vision

Question 1

how does the eye naturally invert images?

Answer 1

inverted horizontally and vertically

the upper part of the external world focuses on the bottom part of the retina

the nasal half of the eye corresponds to the temporal visual field

Question 2

how is the stability of the eye maintained?

Answer 2

the sclera is the white, non-stretchy part of the eye which provides a rigid eye structure

intraocular pressure is generated by aqueous humour production. aqueous humous is a fluid produced by the ciliary body - flows outwards, maintains pressure and eye rigidity

the cornea is made up of collagen fibres and cells which align to maintain its transparency

vitreous humour is behind the lens as a plump, transparent structure, hydrated by the aqueous humour.

Question 3

how does age affect the vitreous humour?

Answer 3

the proteins in it can clump with age causing watery patches, pulling the vitreous away from the eye and causing floaters in vision

Question 4

how do the optics of the eye enable vision?

Answer 4

with the cornea, lens and iris

the CORNEA is a front-curved transparent structure - its curved interface bends light rays forwards

the LENS behind the cornea - ciliary body changes the shape of the lens via the suspensory ligaments. muscle contraction makes the lens fatter = closer vision
muscle relaxation makes the lens flatter = distant vision
provides additional variable fine focus

the IRIS is the ring of muscle which controls the size of the pupil and how much light enters the eye. smaller aperture = better focus - size changes depending on light conditions

these structures focus the light rays at a single point on the retina

Question 5

describe the lens of the eye

Answer 5

behind the cornea, suspended by a ring of suspensory ligaments which form the ciliary body

provides additional variable fine focus for the eye by adjusting its shape

Question 6

what are the two functions of the ciliary body?

Answer 6

produce aqueous humour

changes the shape of the lens to adjust the eye’s focus for close/distant vision

Question 7

describe the two parts of the retina

Answer 7

neural retina
- contains photoreceptor and retinal ganglion cells = RG cells form the optic nerve

retinal pigment epithelium
- provides biochemical support for photoreceptors
- hold the retina in place to stop it peeling

Question 8

describe the primary visual pathway - how the eye captures the pattern of light reflected off the world around

Answer 8

cornea as a curved interface bends the light rays forwards

lens, through suspensory ligament contraction and relaxation, can adjust its shape to adjust the eye’s focus for close/distant vision

iris controls how much light enters the eye through pupil aperture size - a smaller aperture = better focus

light rays are focused at a single point on the retina

photoreceptor cells - rods and cones - located on the retina capture light and convert it to electric signals

neural retina ganglion cell have axons that transmit the signal, project and form the optic nerve - exit to the optic chiasm

at the optic chiasm - nasal optic fibres from the halves of each retina cross over, temporal half fibres remain on the same side

from the optic chiasm - optic nerve fibres project to the lateral genicular nucleus/LGN in the thalamus

LGN cells send axons through the optic radiation (a white matter region)

some axons carry visual information to the occipital cortex as the primary visual area of the brain for visual perception

other axons from the optic radiation branch off to the brainstem and innervate various nuclei involved in the subconscious control of eye movements and pupil size

Question 9

describe the types of photoreceptors in the retina?

Answer 9

two types of specialised photoreceptor cells in the retina - rods and cones

RODS are sensitive to light, function in dim conditions and good for night vision

CONES are responsible for colour vision, function in well-lit conditions

both operate as separate conditions but share similar neural circuitry form the retina to the brain

Question 10

describe cone cell structure

Answer 10

inner segment contains a nucleus with protein-making machinery

“axon” present which functions more like a neurite - doesn’t transmit action potentials, only electric signals

synaptic terminal releases glutamate as an excitatory neurotransmitter

outer segment contains phototransduction apparatus with a bag of tightly packed phospholipid membrane layers with chromophores embedded - arrangement is more efficient for trapping light rays

Question 11

why is the resting membrane potential of a cone cell higher than a typical nerve cell?

Answer 11

cone cell resting membrane potential is from -40 to -45mV

two factors contribute to this:

depolarisation due to K+ ion leak contributes to negative internal potential

Na+ channels being open by default causes a lesser degree of depolarisation - also means some glutamate is released at rest

Question 12

describe the membrane potential changes when light INCREASES and is detected by a cone cell

Answer 12

light strikes the outer segment of the cone cell

some Na+ channels close = cell potential becomes more negative inside causing a hyperpolarisation response

prevents glutamate release

Question 13

describe the membrane potential changes when light DECREASES and is detected by a cone cell

Answer 13

region of darkness detected by retina

more Na+ channels open, held open by cGMP = cell potential becomes more positive inside causing a depolarisation response

increases glutamate release

Question 14

what are the three phases in the process of phototransduction?

Answer 14

initiation of light response
amplifying the biochemical cascade
termination of light response

Question 15

describe the structure of photopigment

Answer 15

PHOTOPIGMENTS are G-protein-coupled transmembrane proteins contained within photoreceptors

made up of an opsin protein and retinal molecule

retinal molecule is the chromophore with a C-ring and C-tail

the C-tail C=C bonds are typically in a stable trans configuration with exception to the 11th position, which is in a less stable cis configuration

Question 16

what holds open the outer membrane Na+ channels of cone cells?

Answer 16

cGMP as a second messenger

Question 17

what is a chromophore?

Answer 17

a light-sensitive molecule which absorbs light and emits colour

for example, retinal in photopigment

Question 18

describe the initiation of the light response in the phototransduction process

Answer 18

light strikes the cone outer segment and photopigment - causes the less stable 11-cis retinal molecule to rupture

retinal molecule reforms in more stable all-trans configuration

transformation activates opsin = photopigment is activated

Question 19

describe the ‘amplification of the biochemical cascade’ response in the phototransduction process

Answer 19

a single activated opsin/ photopigment molecule activates multiple G-proteins, which each activate an enzyme

enzymes work to destroy cGMP which holds the outer membrane Na+ channels open

intracellular cGMP conc. decreases = cGMP diffuses away from Na+ channels so they close

decrease in Na+ influx causes hyperpolarisation - this signal is transmitted along the visual pathway of the brain = visual perception

Question 20

describe the ‘termination of the light response’ in the phototransduction process

Answer 20

rhodopsin as a photopigment in rod cells becomes phosphorylated by rhodopsin kinase, and binds to arrestin protein which prevents rhodopsin from interacting with G-proteins

G-proteins inherently deactivate quickly for a transient response

enzymes activated by G-proteins thus also deactivate

cGMP destruction is terminated = stops closing Na+ ICs and stops hyperpolarisation response

the activated all-trans retinal is capped off and 11-cis retinal reattaches to opsin = prevents further G-proteins activating

photopigment is restored to its usual unstable state, ready to respond to the next photon

Question 21

why is it important to quickly terminate the phototransduction cascade?

Answer 21

allows photoreceptor cells to reset and respond to subsequent light stimuli/ photons

can adapt to changing light conditions efficiently

Question 22

describe the features of peripheral vision

Answer 22

peripheral vision is coarse with blurred images and little fine detail

peripheral circuitry isn’t designed for fine detail vision

Question 23

describe the structure of the peripheral retina and how it contributes to the features of peripheral vision

Answer 23

has photoreceptors of cones and rods with 10x more rods than cones

cones are separated by pools of rods which fill in the gaps in the sampling array of light

many cones and rods converge on one ganglion cells which increases ‘pixel’ size, creating a larger receptive field for ganglion cells

the larger receptive field means less fine detail is visible as the input from the several photoreceptors converges and averages out

Question 24

describe the pathway for peripheral vision from light entering the eye to RFC

Answer 24

light passes through structures like capillary beds and cell organelles to reach the cone outer segments - causes scattering and poor focus

retina photoreceptors (rods and cones) receive input

input to bipolar cells

input to ganglion cells with convergence

to the receptive field centre for peripheral vision

(cones and rods > bipolar cells > ganglion cells > RFC)

Question 25

what is a receptive field?

Answer 25

a region of a visual image that directly changes the action potential firing rate of a cell

Question 26

what is a receptive field centre?

Answer 26

a specific point in a receptive field where a stimulus had the most significant impact on a neuron’s activity

visual RFC - presence of light/ stimulus in a particular spot of the visual field directly affects a neuron’s activity

Question 27

name structures associated with central vision

Answer 27

structures: fovea, fovea centralis, foveal pit

Question 28

how is the fovea structurally adapted for central vision?

Answer 28

has only green and red cone photoreceptors at the foveal pit - these cones are very thin, narrow and tightly packed

no rods, no blue cones

single ganglionic cell to a single cone = no convergence

foveal pit provides a direct path for light to reach the cones - optimal sampling, no image blur

Question 29

what is the retinotopic map?

Answer 29

representation of the visual world

visual information from fovea and related structures –> ganglionic cell axons form this map in the LGN and visual cortex

Question 30

describe how photoreceptors adapt to brightness for a dynamic response

Answer 30

light strikes a photoreceptor and stimulates a strong response - photoreceptor resets and adapts to that level of brightness

allows them to respond to subsequent small changes in brightness without saturating, and adapt to changing light conditions

cones - useful for daytime vision and colour discrimination

retinal photoreceptors respond to a relative change in brightness, not absolute brightness

Question 31

how do off-centre and on-centre retinal ganglionic cells differ in responding to brightness

Answer 31

off-centre cells = inhibited by an increase in brightness over the central region of the RFC

on-centre cells = excited by an increase in brightness over the central region of the RFC

Question 32

describe how off-centre ganglion cells respond to changes in brightness

Answer 32

off-centre = inhibited by light in the central region, means an increase in light in surrounding regions

a decrease in light in the central receptive field depolarises the central photoreceptor

depolarises bipolar cell

activates inhibitory neurons which inhibit ganglionic cells through inhibitory synapses and lateral inhibition - balances excitation of central region with surrounding region’s inhibitory signals

net result of lateral inhibition is the signal sent to off-centre ganglionic cells to the cerebral cortex

Question 33

describe how on-centre ganglion cells respond to changes in brightness

Answer 33

increase in light in central region, means a decrease in light in surrounding regions

hyperpolarises central photoreceptor

decrease in light in surrounding regions stimulates excitatory interneurons - through an inverting synapse they depolarise bipolar cells associated with on-centre ganglionic cell

excites on-centre ganglionic cell - information to cerebral cortex

Question 34

what is lateral inhibition?

Answer 34

the excitation of neurons in the central region is balanced by inhibitory signals from the surrounding region

enhances contrast, relative brightness and the edges of visual perception

Question 35

name the two classes of retinal ganglionic cells

Answer 35

parvocellular
magnocellular

Question 36

compare parvocellular and magnocellular retinal ganglionic cells

Answer 36

parvocellular retinal ganglionic cells:
- large receptive field, less convergence = input from fewer photoreceptor cells
- pick up fine details, high resolution, detailed vision
- more prominent in the central retina with large RFs
- input from ‘red’ or ‘green’ photoreceptors for colour vision

magnocellular retinal ganglionic cells:
- small receptive fields, more convergence
- detect broader patterns, more sensitive to fast-moving stimuli and changed in brightness
- smaller RFs in central field

have complementary roles in vision

Question 37

function of parvocellular retinal ganglion cells?

Answer 37

detailed vision
fine details, high resolution

Question 37

function of magnocellular retinal ganglionic cells?

Answer 37

fast-moving stimuli
changes in brightness
detecting broader patterns

Question 38

name the two wavelength selective retinal ganglionic cells

Answer 38

parvocellular
bistratified

Question 39

features of parvocellular wavelength-sensitive retinal ganglion cells

Answer 39

receives input from ‘red’ and ‘green’ photoreceptors

sensitive to differences between ‘red’ and ‘green’ wavelengths

compare responses and encode wavelengths

Question 40

features of bistratified wavelength-sensitive retinal ganglion cells

Answer 40

receives input from ‘blue’ and ‘red& green’ photoreceptors

sensitive to differences between ‘blue’ and ‘yellow’ wavelengths

compare responses and encode wavelengths

Question 41

how do wavelength-photoreceptors adapt for colour vision?

Answer 41

when exposed to a wavelength of light - e.g. red- for a prolonged period of time, they develop decreased sensitivity to that wavelength

allows them to pick up relative changes in colour and brightness

photoreceptors adjust sensitivity overtime based on their level of stimulation

Question 42

role of the lateral geniculate nucleus in relaying information for visual processing?

Answer 42

relays information to visual cortex areas

mirror the receptive fields of their receptive inputs from the retinal ganglion cell - don’t alter the information

inputs from three types of receptive fields:
- parvocellular and magnocellular cells
- cells with ‘off’ and ‘on’ retinal ganglion cell characteristics
- photoreceptor cells sensitive ‘blue’ and ‘yellow’ wavelengths

Question 43

role of the primary visual cortex in relaying information for visual processing?

Answer 43

cortical cell properties:
- respond to relative brightness
- more elongated receptive fields
- some are orientation sensitive
- some are direction and velocity/ speed of stimulus sensitive

Question 44

role of higher visual cortical areas in visual processing?

Answer 44

input from various sources, associated with colour vision

uses prior experience and context to interpret visual stimuli

Question 45

role of the parietal cortex in visual processing?

Answer 45

input from magnocellular cells, responds to broad shapes, spatial vision, movement processing

encodes info. about location and movement of objects

Question 46

how the brain interprets visual stimuli?

Answer 46

encodes info. from the retina

higher visual cortical areas = recognise object identity

parietal areas = location and movement, magnocellular neurone input

inferior temporal lobe = colour vision