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BS32026 > Visual System > Flashcards

Visual System Flashcards

1
Q

Gross anatomy of the eye

A

Pupil - opening where light enters the eye
Sclera - white of the eyes
Iris - gives eyes colour
Conjunctiva - inner part of eyelid
Cornea - glassy transparent external surface of the eye
Optic nerve - bundle of axons from the retina (retina is a neural structure - sheet of photoreceptors)

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2
Q

Eye structure

A

Cornea covers aqeous humor which is in front of the lens, which is attached by zonule fibres to ciliary muscles
Inside of the eye is vitreous humor
Light passes through the eye and focuses onto the fovea at the back of the eye (this is what the visual system is most attuned to) (where cones are concentrated - remainder of the eye is covered by rods)
Division into nasal and temporal retina occur along the fovea
The optic disc (in nasal retina) is a blind spot which our brain fills in (where the optic nerve leaves)
Lens + cornea focus light onto the retina

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3
Q

Image formation

A

Light focuses on retina > inverted image
Flat lens - far focus point
Fat lens - near focus point
Pupillary light reflex

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4
Q

What influences the ability of an eye to focus on an image?

A

Refractive power of cornea + lens, and shape of eye globe

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5
Q

Refractive index

A

Cornea has refractive index of 42 diopters (parallel light rays will be focused 2.4 cm behind it)
Refractive index (diopters)= 1/focal distance (m)
Focal distance is distance from cornea to fovea

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6
Q

Refraction Errors

A

Emmetropic, presbyopia, hyperopia, myopia

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7
Q

Emmetropic

A

Normal eye

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8
Q

Presbyopia

A

Lens hardens with age and ciliary muscles weaken. Causes a decreased ability in accommodation

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9
Q

Hyperopia

A

Far sightedness - refractive power insufficient for close objects
Corrected with a convex lens <|

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10
Q

Myopia

A

Near sightedness - refractive power too strong for distant objects
Corrected with a concave lens ((

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11
Q

Pupillary light reflex

A

Connections between retina and brain stem neurons that control muscle around the pupil and are continuously adjusting to different ambient light levels
Consensual (both pupils react similarly and simultaneously)

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12
Q

Circular muscles

A

Constrictor
Act to decrease pupil size under parasympathetic control

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13
Q

Radial muscles

A

Dilator
Act to increase the pupil size under sympathetic control

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14
Q

Visual field

A

Amount of space viewed by the retina when the eye is fixed straight ahead (image is inverted). Only the central region of the retina provides high resolution, so we see the world by moving our eyes.
Visual field of the left eye and right eye: _/ _ (150 degrees each)
Visual fields overlap

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15
Q

Visual acuity

A

Ability to distinguish two nearby points
Determined largely by photoreceptor spacing (how close they are to each other - closer is better eye sight) and refractive power
Visual angle: distances across the retina described in degrees

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16
Q

20/20 vision

A

When one can recognise the a letter that occupies 0.083 degrees (5 minutes of arc)

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17
Q

How does vision work?

A

Accommodation - the pattern of the object must fall on the vision receptors (rods and cones in the retina)
Light entering eye must be regulated (too much will bleach out the signals)
Energy from photon waves must be transduced into electrical signals
Brain must receive and interpret signals

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18
Q

Vertical pathway for signal transmission to the retina (ascending)

A

Photoreceptors > bipolar cells > ganglion cells > ganglion cell axons project to forebrain (that’s the direction of the signal, directon of light is opposite)
Horizontal cell - modulates signal from photoreceptors to bipolar cells + other photorceptors
Amacrine cell - modulates signal from bipolar cell to ganglion cell + other bipolar cells, or to other amacrine cells
Only the ganglion cell can generate APs
Photoreceptors can release glutamate, but not generate APs

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19
Q

Photoreceptor structure

A

Synaptic terminals (release glutamate upon bipolar cells) > inner segment (contain cell body) > outer segment (either cone or rod photoreceptor, which are membranous disks containing photopigment)
Convert electromagnetic radiation to neural signals (transduction)

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20
Q

Phototransduction (membrane potential)

A

Vertebrate photoreceptors have a depolarised rmp (Vm) (more positive than other neurons at ~20mV). With light exposure, Vm hyperpolarises (becomes more negative).
Dark current - a nucleotide-gated Na+ channel (opened by cGMP) that is open in the dark and closes in the light. In the dark glutamate is released, but upon light entering this decreases
Sodium enters in outer segment
Potassium leaves in inner segment

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21
Q

Dark Current

A

In the dark: Pna ≈ Pk (Na channels in the outer segment)
Vm is thus between Ena and Ek
In the light: Pna is reduced (outer segment channels close) so Pk > Pna
Vm is thus closer to Ek (more negative), causing hyperpolarisation
Change is local and graded - light intensity varies the membrane potential
Each place of light corresponds to a different photoreceptor

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22
Q

What is Rhodopsin?

A

Pigment molecule for rods = retinal (vit A derivative) + opsin (transmembrane GPCR)
Present in disks (membrane folds) of the outer segment
Light changes the conformation of GPCR which converts 11-cis-retinal to all-trans-retinal (the activated form)

23
Q

Molecular Pathway of Response to Light

A

all-trans-retinal activates transducin (a G-protein) > activates cGMP phosphodiesterase (PDE) > hydrolyses cGMP (reducing conc.) > Na+ channels close > hyperpolarisation
1 opsin - 1000 transducin - 1 PDE - 1000 cGMP

24
Q

Rods

A

See in dim light
Peripheral rods have high convergence (larger spacing at lower density), and occupy larger visual space (less clear vision)
More convergence = more sensitivity but lower acuity
Achromatic
E.g. 5 rods > 1 large ganglion cell

25
Q

Cones

A

See in normal daylight
Foveal cones have low convergence (smaller spacing at higher density), and occupy smaller visual space (better acuity/vision and lower light sensitivity)
3 types: short-wave cones (blue)(triggered by 420-550 nm), medium-wave cones (green)(450-630 nm), long-wave cones (red)(triggered by 500-700 nm)
Chromatic
E.g. 2 cones > 1 small ganglion cell

26
Q

Vision on the Electromanetic Spectrum

A

We see in the visible light region (750-350 nm), not infrared or UV

27
Q

The OFF pathway

A

Light: photoreceptor > less glutamate > ionotropic glutamate receptors on bipolar cells > less Na+ channels open > more negative Vm > ganglion cell > more negative Vm

Dark: photoreceptor > more glutamate > ionotropic glutamate receptors on bipolar cells > more Na+ channels open > more postive Vm (depolarisation) > ganglion cell > more positive Vm > more APs

Hyperpolarisation in photoreceptors causes hyperpolarisations in off-centre bipolar cells (which have ionotropic glutRs) and ganglion cells. Detects decreases in luminance.

28
Q

The ON pathway

A

Light: photoreceptor > less glutamate > metabotropic glutamate receptors on bipolar cells > less K+ channels open (K leaves cell) and less Ca2+ channels close (Ca enters cell) > more positive Vm > ganglion cell > more positive Vm > more APs

Dark: photoreceptor > more glutamate > metabotropic glutamate receptors on bipolar cells > more K+ channels open (K leaves cell) and more Ca2+ channels close (Ca enters cell) > more negative Vm > ganglion cell > more negative Vm

Hyperpolarisation in photoreceptors causes depolarisations in on-centre bipolar cells (which have metabotropic glutRs) and ganglion cells. Detects increases in luminance.

29
Q

Which cells in the visual system produce APs?

A

Ganglion cells and some amacrine cells (all other cells produce graded changes in membrane potential)

30
Q

What is the Receptive Field?

A

The part of the retina that needs to be stimulated to elicit APs from a ganglion cell. They are small and concentric and correspond to the visual field of those cells.
E.g. 1 ganglion cell > 3 bipolar cells > 9 photoreceptors (= receptive field)

31
Q

What are centre-surround receptive fields?

A

Receptive fields that are modified due to lateral inhibition by horizontal cells. GABA is released by horizontal cells and diminishes activation of these cells.

32
Q

Off-centre receptive field

A

Dark centre & light surround
Photoreceptors leading to the field surround are hyperpolarised due to horizontal cell hyperpolarisation, photoreceptor leading to the centre is not hyperpolarised.

33
Q

On-centre receptive field

A

Horizontal cells interconnect a group of ‘surround’ neurons. It samples the total amount of excitation in the surround and responds by releasing GABA. Low excitation in surround = less GABA released. Therefore, the response of the centre on bipolar cells is higher.
Bipolar cells are stimulated due to decreased glutamate

34
Q

What is lateral inhibition?

A

Exaggerates the difference in stimulus intensity detected by adjacent neurons - aids with localisation. The region around the receptive field elicits the opposite response.

35
Q

What is red-green opponency?

A

Red ON centre, green OFF surround
Red in centre - highest ganglion cell output
Red in centre + surround - medium ganglion cell output
Red in centre + green surround - lowest ganglion cell output (green in surround has inhibitory effects)

36
Q

Purpose of centre-surround organisation?

A

Serves to emphasize areas of difference (contrast) in light

37
Q

Types of Ganglion Cells

A

3 Types:
M-type (magnocellular) - larger receptive fields, transient activation (send information about movement). Layer 1-2 of LGN
P-type (parvocellular) - smaller receptive fields, sustained activity, colour sensitive (information about form/colour). Layer 3-6 of LGN
nonM-nonP - sensitive to wavelengths (send information about colours). Koniocellular layers of the LGN

38
Q

Laterate Geniculate Nucleus (LGN)

A

Structure where the thalamus connects with the optic nerve. Sends information to layer 4 of the primary visual cortex.
Has two halves; right and left

Receptive fields of LGN neurons are identical to the ganglion cells that feed them (concentric)

Magnocellular LGN neurons: large, monocular receptive fields with transient response
Parvocellular LGN cells: small, monocular receptive fields with sustained response

39
Q

What is Parallel Processing?

A

Simultaneous input from two eyes
Info compared in cortex (depth + distance)
Info about light and dark, ON- and OFF-centre ganglion cells
Different receptive fields and response properties of retinal ganglion cells
Colour info: red vs green and blue vs yellow opponent ganglion cells

40
Q

Relation between retina and visual cortex

A

Left eye -> right visual cortex
Right eye -> left visual cortex

Right visual field connects with right nasal half and left temporal half
Left visual field connects with left nasal half and right temporal half

Axons of the left nasal half hook up with the right temporal axons
Axons of the right nasal half hook up with the left temporal axons
(Nasal halves cross over)

41
Q

LGN Lesions in Parvocellular and Magnocellular Layers

A

Parvocellular:
Chromatic vision
High fine detail vision
Slow motion vision

Magnocellular:
Achromatic vision
Low fine detail vision
Fast motion vision

42
Q

LGN Function Tests

A

Colour discrimination tests
Fine detail discrimination tests

43
Q

Striate Cortex - Hubel and Weisel Study

A

Recording electrode was inserted into a specific neuron in striate cortex to record APs
Stimulus is presented on screen (green box with red line moving)
The neuron showed differing excitation depending on the direction in which the line moved
Shows cortical receptive fields

44
Q

Anatomy of Striate Cortex

A

Input:
Magnocellular LGN neurons - project to layer IVCalpha
Parvocellular LGN neurons - project to layer IVCbeta
Koniocellular LGN axons - bypass layer IV to make synapses in layers II and III (blobs only visible after cytochrome oxidase staining)

(Layer IVC is monocular, but most of layer III is binocular)

45
Q

Monocular

A

Only has input from one eye (retinal synapses in LGN are monocular)

46
Q

Binocular

A

Has input from both eyes (e.g. cortex)

47
Q

Cortical Receptive Fields

A

Orientation selective neurons
Respond to light/dark bars or edges
Peak frequency depends on angle, preferred orientation

48
Q

Association Pathways for Visual Cortex

A

Dorsal pathway:
To parieto-occipital ass. cortex
Analysis of motion and spatial relations (action)

Ventral pathway:
Project to occipito-temporal ass. cortex
Analysis of form and colour (perception)

49
Q

What are occular dominance columns?

A

Sections in the layers of the LGN that, for instance in the right LGN half, correspond to the right temporal retina (ipsilateral) and the left nasal retina (contralateral)

50
Q

Effects of Monocular Deprivation

A

Sensory deprivation early in life can alter the structure of the cerebral cortex.
Monocular deprivation causes the functioning eye to take over more of the visual cortex - can lead to loss of occular dominance column (can’t be reversed)

Monocular deprivation leads to less branching of LGN axons

Monocular deprivation in infancy is permanent (competition hypothesis between eyes), but in adults doesn’t permanently alter brain function

51
Q

Congenital Cataracts

A

Opaque covering of the lens
Impaired vision from birth
Cataracts are typically removed between 10-20 years of age
People have difficulty perceiving shape and form for the remainder of their lives

52
Q

Amblyopia (cortical blindness)

A

Covers visual disorders where optics and retina are fine, yet one eye has better vision than the other
Can be caused by strabismus (wandering eye) if not corrected in infancy
Until surgery can be performed, the good eye is covered so the brain relies on signals from the bad eye - helps brain develop properly

53
Q

Hebb’s Postulate

A

When an axon of cell A is near enough to excite a cell B and repeatedly takes part in firing it, a growth/metabolic process occurs in one or both cells such that cell A’s efficiency in firing to B is increased.
Correlated activity between presynaptic and postsynaptic cells strengthens synaptic connections between them
Same principle as long term potentiation (LTP) in learning and memory

BS32026 (3 decks)

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