20.3 Vision Flashcards
1
Q
What is the retina?
A
The retina is a layer structure composed of an epithelium and neurons that transduce light into an electrical stimulus and process it, before transmitting it via the optic nerve
2
Q
Perceptive opening for retinal processing
A
-
The eye is a clinician’s window into the brain
- not least of all because the retina is an extension of the CNS but the arrangement of blood vessels and the optic nerve sheath can provide information on the intraoccular and intracranial pressures as indicators of disease.
- Similarly, the pupillary light reflex can be used to assess the function of the brainstem and optic nerves.
- This is because the retina has the sole responsibility of transducing the light into an electrical impulse which is then conducted via the optic nerves to higher brain centres.
- However, retinal function is not limited to transduction.
- The retina is comprised of complex circuits that process information from the 127 million photoreceptors and compress it into just 1 million optic nerve fibers.
3
Q
What are the three main histological parts of the anterior eye?
A
- Cornea
- Scelera
- Conjunctiva
4
Q
Outline the layers of the cornea
A
EBSDE
-
Epithelium
- 5-7 cells thick
- High turnover
- Prevents bacterial entry
-
Bowman’s membrane
- Dense, thin connective sheet
- Forms the junction between the epithelium and the stroma
-
Stroma
- Regularly arranged, parallel type 1 collagen fibres are organised into fibrils
- Fibrils form lamellae which are equally spaced, making the cornea clear
- Mucopolysaccharides to embed the fibrils
-
Descemet’s membrane
- Separates the stroma from the underlying endothelium
-
Endothelium
- Singe layer of cells
- Deepest surface is covered in aqueous humour
5
Q
Outline the function of the scelera
A
- Attachment for extraocular muscles
- Anterior 1/3 = joins choroidal tissue to form the lamina cribrosa
-
Posterior 2/3s = continous with the dura mater
- Scelera - ESLEL
- Endothelium
- Stroma
- Lamina fusca
- Episcelera
- Lamina cribosa
- Scelera - ESLEL
6
Q
Outline the histological layers of the scelera
A
ESLEL
-
Episclera
- Loose fibrous elastic tissue
-
Stroma
- Irregular type 1 collagen fibres
-
Lamina fusca
- High count of pigmented cells
- Dense capillary plexus
- Endothelium
-
Lamina cribrosa
- Fenestrated region of the scelera which the optic nerve exits from
- Collagen lattice that maybe compressed
- Supports the toptic nerve as it exits the eyeball
- Forms a boundary to oligodendrocytes
- Myelination would decrease the transparency of the retina
7
Q
What does the conjunctiva cover? What is its structure?
A
- Covers the anterior eye scelera, inner eyelids but not the cornea
- Structure
- Non-keratinised, stratified squamous
-
Interspaced goblet cells
- Secrete gel forming mucins
- Contains:
- Blood vessels
- Lymphoid tissue
- Fibrous tissue
8
Q
What value of intraoccular pressure would be indicative of raised intraoccular pressure and what are the potential consequences?
A
- Normal IOP = 16.5 mmHg
- Raised IOP e.g. in glaucoma results in compression of the collagen lattice
- At around 21 mmHg, the optic nerve head is compressed
- Results in conduction block and can cause permanent damage and blindness
9
Q
What are the two main layers of the retina?
A
-
Inner sensory retina
- Layers of nerve cells that span from the deep pigmented epithelium to the vitreous humour anteriorly
-
Outer retinal epithelium
- Simple cuboidal epithelial cells with melanin granules
- Dark pigments
- prevent the reflection of light within the posterior chamber of the eye
10
Q
Histology of the inner sensory retina
A
-
Deep (pigmented epithelium)
-
Layer of photoreceptor outer segments
- The outer segment of the photoreceptor contains the light sensitive elements
-
Outer nuclear layer
- The outer nuclear layer contains the cell bodies of the photoreceptors
-
Outer plexiform layer
- The outer plexiform layer contains the synaptic connections between the photoreceptors, bipolar cells and horizontal cells
-
Inner nuclear layer
- The inner nuclear layer contains the cell bodies of bipolar cells, horizontal cells and amacrine cells
-
Inner plexiform layer
- The inner plexiform layer contains the synaptic connections between the bipolar cells, ganglion cells and amacrine cells
-
Ganglion cell layer
- The ganglion cell layer contains the cell bodies of the ganglion cells
-
Layer of photoreceptor outer segments
- Superficial (vitreous humour)
11
Q
- Describe the segments of photoreceptors
A
- Inner segment
- Mitochondria
- Golgi body
- RER & SER
- Outer segment
- Stacks of membranous discs with photo pigment
12
Q
- Describe the rods and cones in terms of:
- Outer segment shape
- Termination shape
- What they synapse onto
- Their photo pigment
A
-
Rods
- Outer segment shape -cylindrical
- Termination structure - rod spherule
- Small knob
- Synapse onto - bipolar and horizontal cells
- Photo pigment - rhodopsin
- Night vision
- Low resolution
-
Cone
- Outer segment shape - conical
- Termination structure - cone pedicle
- Thicker
- Synapse onto - bipolar and horizontal cells
- Photo pigment - photopsin
- High resolution detail
- Discriminates primary colours
13
Q
Outline the types of cones and their wavelengths
A
- S
- 445 nm
- Blue
- M
- 535 nm
- Green
- L
- 575 nm
- Red
14
Q
What are the different types of retinal cells? What are their functions?
A
-
Muller glial cells
-
Present in all layers
- Nuclei in the inner nuclear layer
-
Cytoplasm spans the length of the lamina
- Cytoplasmic processes fill the gaps betwen other retinal cells
-
Regulate neuronal exictability
- Monitoring:
- Extracellular K+
- Uptake of neurotransmittter at synapses
- Monitoring:
-
Present in all layers
-
Horizontal cell
- Branch to the photoreceptors and bipolar cells in the outer plexiform layer
- Present in the outer edge of the inner nuclear layer
-
Bipolar cells
- Rod specific cells
-
Cone specific cells
- Cone bipolar cell have 2 subtypes
-
Midget
- Single cone photoreceptor to a single ganglion cell
-
Parasol
- Connect several cone photoreceptors to a single ganglion cell (convergence)
-
Midget
- Each subtype can have ON/OFF cells
- Cone bipolar cell have 2 subtypes
-
Ganglion cell
- 3 main types that feed into the lateral geniculate pathways:
- Midget = parvocellular
- Parasol = magnocellular
- Bistratified = koniocelular
- Each have ON/OFF that can be found in separate sublamina
- On = sublamina A of the inner plexus layer
- Off = sublamina B of the inner plexus layer
- 3 main types that feed into the lateral geniculate pathways:
15
Q
- Outline the difference between midget cells and parasol cells in terms of:
- Soma size
- Dendritic field size
- Concentration
- Colour
- Summation
- Longetivity
- Projections
- Receptive field size
- Conduction velocity
- Pathway to the lateral geniculate nucleus
A
16
Q
What is the fovea?
A
- Area of highest visual colour acuity and is at the centre of the gaze
- There is a 1:1 relationship between photoreceptors and ganglion cells
- Area containing the highest density of cone photoreceptors
- Present within the macula of the retina
- The other cell layers are pushed aside to reduce the blur from light scattering by other cells
17
Q
What is the optic disc?
A
- Where axons of the ganglion cells converge and emerge from the back of the eye
- AKA blind spots
- No photoreceptors are present and is nasal to the fovea
- Light never falls on the blindspots of both eyes simultaneously
18
Q
Describe the photoreceptor distirbution across the retina
A
-
Fovea
- Contains predominanty red/green cone (LMS)
- 575 nm = red
- 545 nm = green
- 445 nm = blue
- Few
- No rods present
- Contains predominanty red/green cone (LMS)
- Peripheral retina
- Some cones are present - but not in high densities like the fovea
- More rods in these areas
19
Q
What is the role of pigmented eptihelium?
A
- Choroid = vascular layer between the retina and scelera
- Prevent reflection of light posteriorly
-
Transport nutrients
- from the choroidal blood vessels to the sensory retina layers
-
Removal of waste metabolic products
- from the sensory layer
-
Active phagocytosis and recycling
- of photoreceptor disks shed from cones and rods
-
Synthesis of basal lamina of Bruch’s membrane
- Bruch’s membrane = inner most layer of choroid layer that attach the pigmented retina
-
Formation of rhodopsin
- converting the bleached pigment into retinal and returning it to the rods via intersitial retinoid binding protein (IRBP)
20
Q
What is retinal detachment?
A
- Separation of the fovea from the optic disc
- Caused by:
- Trauma
- Vascular disease
- Metabolic disorders
- Risk factors
- Myopia
- If detached for long enough, the sensory retina can become necrotic and this can lead to blindness
- Treatment
-
Retinal layers can be surgically reattached
- Cryotherapy
-
Retinal layers can be surgically reattached
21
Q
Outline the equation for optical refractive power of the eye (P) and the contributions of different eye components
A
-
P = µ/f
-
µ = refractive index of media of the eye
- 1.33 (approximately that of water)
- f = focal length of the eye
- 0.022 m
- P = 60 dioptres
-
µ = refractive index of media of the eye
-
Cornea
- 42 dioptres
- Non adjustable
- 42 dioptres
-
Lens
-
18 doptres
-
Adjustable - accommodation via ciliary muscles
- Contraction of annnular ciliary muscles which reduces tension in radial zonular fibres, allowing lens to relax to a more convex state
- Contract to thicken lens
-
Parasympathetic of M3 muscarinic receptors
- Gq
- Allows focus on close by objects
-
Adjustable - accommodation via ciliary muscles
-
18 doptres
- Air/cornea interface (42 dioptres) causes more refraction than lens/aqueous humour interface (18 dioptres)
22
Q
Define 1 dioptre
A
The power of a lens to focus parallel light at a focal point 1 m away
23
Q
How does the eye accommodate to near vision?
A
- Increase in power
- Ciliary muscles contract
- Pulling border of choroid towards lens
- Reduces tension in radial zonular fibres
- Suspensory ligaments relax
- Lens becomes thicker and rounder
24
Q
How does the eye accommodate to distance vision?
A
- Decrease in power
- Ciliary muscles relax and border of choroid moves away from lens
- Suspensory ligaments pull against lens
- Lens become flatter, focusing on distant objects
25
Q
Describe myopia and its correction
A
- Light is focused in front of the retina
- The eyeball is too long (not due to the curvature of the cornea)
- Corrected by negative power concave lenses
26
Q
Describe hypermetropia and its correction
A
- Light is focused behind the retina
- Corrected by postive power - convex lenses
- The lens is convex so can accommodate for it by contraction of the circular ciliary muscles that increase refraction
- Accommodation means it may not manifest until later in life
27
Q
Describe astigmatism and its correction
A
- Astigmatism = curvature of the cornea in different axis
- Results in different focuses in different lanes of light
- Horizontal point of focus = further back
- Vertical point of focus = in front
- Corrected with spectacles that have a cylindrical component on their surface curvature
28
Q
Describe presbyopia
A
- Failure of lens accommodation with age as the lens becomes stiffer and less elastic
- Near sighted vision becomes more difficult
29
Q
What are cataracts and how can they be treated
A
- Lens become cloudy
- Higher incidence with dehydration
- Perhaps due to change in salt conditions
- Surgically removed and replaced with an artificial lens
30
Q
Describe phototopic conditions and the effect on rods and cones
A
- Vision in the day
- Rods
- Bleached and saturated at -65mV
- Cones
- Active
31
Q
Describe mesopic conditions and their effect on rods and cones
A
- Between night and day (indoors - low)
- Rods
- Active
- Cones
- Active
32
Q
Describe scotopic coniditions and their effect on rods and cones
A
- Vision in the dark
- Rods
- Active
- Cones
- Not bleached as easily
33
Q
Define phototransduction
A
- The process by which light hitting the retina is converted into an electrical, graded potential by photoreceptors
- Results in photoreceptor hyperpolarisation in light
- Due to an intracellular biochemical signalling cascade
- Results in photoreceptor hyperpolarisation in light
34
Q
Outline the basic stages of the phototransduction cascade
A
POT PHS
- Photopigment
- Opsin
- Transducin
- PDE
- Hyperpolarisation
- Synaptic transmission
35
Q
Outline in detail step 1) photopigment in the phototransduction cascade.
A
POT PHS
- Light energy (electromagnetic radiation or photon particle) is absorbed by photopigments embedded in the membrane of the photoreceptor outer segment discs
- In rods, the pigment is rhodopsin, a complex of the GPCR opsin and the covalently-linked (prebound) agonist retinal
- In cones, the pigment is photopsin a complex made of one of three opsins (L, M, S) and retinal
- RGB = LMS
- Each type of cone produces a different variant of the opsin protein that distinguishes their absorption wavelength
- Red light excites L the most
- Green light excites M the most
- Blue light excites S cones the most
36
Q
Outline in detail step 2) opsin bleaching of the phototransduction pathway
A
POT PHS
- Each photpigment contains retinal and opsin
-
Retinal is a small molecule in which the C11-C12 double bond assumes the cis configuration in the dark
- Absorption of one photon by retinal converts the bond to the trans configuration, changing its molecular conformation
- Activates the opsin to metarhodopsin II
- Process is known as bleaching of the photopigment because the pigment changes colour from purple to yellow
37
Q
Outline step 3) transducin of the phototransduction pathway
A
POT PHS
- The activated opsin (metarhodopsin II) stimulates the G-protein transducin which then stimulates photphosdiesterase
- Transducin activated through the exchange of GDP for GTP
- The alpha-GTP subunit diffuses laterally through the disc membrane and activates the membrane bound phosphodiesterase (PDE)
- The G protein activity is increased by GAPs (G-protein activating proteins)
- The more metarhdosopin II, the greater the amplification
38
Q
Describe step 4) PDE of the phototransduction pathway
A
POT PHS
- PDE degrades cytosolic cGMP to 5’-GMP leading to a fall in [cGMP]i
39
Q
Describe step 5) hyperpolarisation of the phototransduction pathway
A
POT PHS
- The reduction in [cGMP]i causes closure of non-selective cation channels in the photoreceptor plasma membrane, reducing the inward Na+ current (the dark current) and hyperpolarising the membrane
- Going from -35 mV to -65 mV
- This slows the release of neurotransmitter from the photo receptor
- In the dark, [cGMP]i is high thus channels are open and transmit a larger inward Na+ current (the dark current)
- The resting Em of the photoreceptor is therefore -35mV and stimulation by light drives the Em in the negative direction to a peak of around -65 mV
40
Q
Describe step 6) synaptic transmission of the phototransduction pathway
A
- Hyperpolarisation of the photoreceptor membrane (from reduces Na+ influx) reduces Ca2+ influx through the non-specific cation channel thereby decreasing the rate of glutamate secretion from the synaptic terminal
- In the dark, glutamate is constituvely released becuase the Em is held at a depolarized value (-35 mV)
41
Q
What are the three main mechanisms in which the photoreceptor signalling cascade is terminated? What is the importance of these mechanisms?
A
- Mechanisms (RIT)
- Reduction in calcium increases affinity of cGMP channel for cGMP
- Inactivation of metarhodopsin 2
- Transducin GTPase activity
- Importance
- Allows the photoreceptor to be sensititive to another photon of light
42
Q
Describe how the inactivation of metarhodopsin II results in the termination of the photoreceptor signalling cascade
A
- Metarhodopsin II is inactivated by rhodopsin kinase which phosphorylates it
- This is followed by the binding of arrestin 2- blocking the interaction its interaction transducin (preventing PDE activation)
43
Q
How does transducin GTPase acitivty terminate the photoreceptor signalling cascade?
A
- Active transducin has intrinsic GTPase activity
- Converts the bound GTP to GDP
- Releases the GDP and recombines with its beta-gamma subunits
- cGMP concentration is then restored by guanylate cyclase that converts GTP to cGMP
- The [cGMP] rises, the cGMP channels open and the Na+ current is resumed, membrane back to depolarised dark potential, glutamate is released again
44
Q
How does calcium concentration result in the termination of the photoreceptor signalling cascade?
A
- Reduction in the [Ca2+] in the light results in:
- Start - Accelerated rhodopsin kinase phosphorylation of metarhodopsin II
- Middle - Accelerated guanylate cyclase activity (producing cGMP from GTP)
- End - Increased affinity of the cGMP channel for cGMP
- Calcium channels enter the cell via the cGMP channel but are quickly pumped back out by membrane transporters
- Thus during the dark, the [Ca2+] in the cell are higher than in thelight when cGMP channels close
45
Q
Describe how the rods and cones adapt to a darkly lit environment
A
- In the light, rod photo pigment is bleached
- -65mV
- Trans-retinal
-
Dark Adaptation
- When moving to the dark, the photopigment is able to regenerate and the rods become more sensitive to light
- Metarhodopsin kinase regenerates metarhodopsin
- Trans-retinal back into cis-retinal
- This means overtime, more rods are available to detect the light and transduce it
- Dark adaptation takes around 1 hour to reach maximal sensitivity
- When moving to the dark, the photopigment is able to regenerate and the rods become more sensitive to light
-
Light Adaptation
- The huge increase in light sensitivity in dark adaptation means there is a temporary saturation of photoreceptors upon re-entry into light
- All the rhodopsin is bleached and ganglion cells discharge action potentials, leading to everything appearing white
- After this, light adaptation occurs and the cones become adapted to the light level
- The huge increase in light sensitivity in dark adaptation means there is a temporary saturation of photoreceptors upon re-entry into light
46
Q
Describe how the eyes adapt to well-lit environments
A
- In the dark, the cones are inactive
- In the light, the cones become activated and adapted to the level of light
- The use of rods and S cones is shifted to R/G cones in photopic light = Purkinje shift
- tendency for the peak luminance sensitivity of the eye to shift toward the blue end of the color spectrum at low illumination levels as part of dark adaptation)
48
Q
Describe age-related macular degeneration, its types and its treatments
A
- The pathology occurs between the deep pigment epithelium of the retina and the underlying Bruch’s membrane (innermost layer of the choroid)
- types:
- Wet - due to angiogenesis
- Dry - debris accumulation (drusen)
-
Treatment
- In the early stages, may be slowed down by:
- Antioxidants
- Minerlal supplements
-
Wet can be slowed down by:
-
VEGF antibodies
- vascular endothelial growth factor) antibodies
- Decoy VEGF receptor
-
VEGF antibodies
- In the early stages, may be slowed down by:
49
Q
Outline the rod vertical pathway and when it is activated
A
- In scotopic and mesotopic conditions
- Several photoreceptors increasing converging inputs onto a rod bipolar cell
- This increases the sensitivity of the rod pathway
50
Q
Outline the cone activated pathway and when it is activated
A
- Photopic and mesotopic conditions
- The cone vertical pathway divides into 2 parallel pathways:
- ON
- OFF
51
Q
Summarise the ON pathway in the cone vertical pathway in both scotopic and phototopic conditions
A
-
Dark
- Photoreceptor is depolarised
- Glutamate release from mGLUR6 (metabotropic glutamate receptor 6)
- Glutamate binding closes the cation channel and hyperpolarises the bipolar cell
- No glutamate released from bipolar cell
- No activation of ON ganglion cells
-
Light
- Photoreceptor is hyperpolarised
- No glutamate release from mGLUR6
- No glutamate binding results in the channel remaining open and depolarisation of the bipolar cell
- Glutamate released from bipolar cell
- Activation of the ganglion cell, action potential generated
52
Q
Summarise the OFF pathway in the cone vertical pathway in both scotopic and phototopic conditions
A
-
Scotopic conditions
- Photoreceptor is depolarised
- Glutamate release from photoreceptor
- AMPA - ionotropic receptor at the bipolar cell
- Glutamate binding opens the ion channel and depolarises the cell
- Glutamate release
- Activation of the OFF ganglion cell - activation generated
-
Phototopic conditions
- Photoreceptor is hyperpolarised
- No glutamate released
- AMPA - inotropic receptor at the bipolar cell
- No glutamate binding keeps the channel closed and the membrane is not depolarised
- No glutamate released
- No activation of OFF ganglion cell
55
Q
How did Slaughter & Miller in 1981 show that the ON and OFF responses to light are independent of each other?
A
- They studied the retina of mudpuppies (species of salamander) using electrophysiological intracellular electrode recordings to measure the graded potentials in ON and OFF bipolar cells
- Under control conditions, light depolarised the ON bipolar cell and hyperpolarized the OFF bipolar cell as would be expected
- When the mGLUR6 agonist 2-amino-4-phosphonobutyric acid was added to the solution in which the retina was bathed, upon exposure to light, there was no depolarisation of the ON cell (it remained hyperpolarised) but the OFF cell response was hyperpolarised
- It provides evidence for:
- Two different recpetors on ON/OFF cells for glutamate
- Changes to one pathway did not affect the other, thus they are independent
59
Q
What is meant by the antagonistic centre-surround receptive fields? What does it result in?
A
- Receptive fields where the centre has the opposite response to light as the surround
- This arrangement means:
- The overall illumination does not change the baseline firing of the ganglion cell but the firing changes when there is contrast (different levels of illumination in the centre and surround)
- This allows the visual scene to be economically encoded by the retina as information about the edges is transmitted while the areas of uniform illumination are not
- The temporal pattern of firing by the ganglion cell and in the optic nerve encodes to higher centres where in the visual field light is
60
Q
Describe ON centre fields in terms of their central receptive field and their surrounding receptive field.
A
- Central receptive field
- Activated in response to light
- Photoreceptors synapse onto ON bipolar cells which synapse onto ON ganglion cells - sublamina A
- Increased firing frequency
- Sign inverting
- Surrounding receptive field
- Inactivated in response to light
- Photoreceptors synapse onto OFF bipolar cells which synapse onto OFF ganglion cells - sublamina B
- Decreased firing frequency
- Sign preserving
61
Q
Describe OFF centre fields in terms of their central receptive field and their surrounding receptive field.
A
- Central receptive field
- Photoreceptors inactivated in response to light
- Photoreceptors synapse onto OFF bipolar cells which synapse onto OFF ganglion cells (hyperpolarisation)
- Surrounding receptive field
- Photoreceptors activated in response to light
- Photoreceptors synapse onto ON bipolar cells which synapse onto ON ganglion cells (depolarisation)
62
Q
Describe the field size and describe the variations in the fovea and peripheral retina
A
- The receptive field size of a ganglion cell is determined by the number of bipolar cells that feed into it which corresponds to the size of the ganglion cell dendritic tree
- Midget ganglion cells have a small dendritic tree and are found at the fovea –> synapse with the parvocellular cells of LGN
- Parasol ganglion cells have a larger dendritic tree and are found in the peripheral retina –> magnocellular pathway at the LGN
- Fovea
- 1:1 mapping
- 1 photoreceptor to 1 bipolar cell to 1 Midget ganglion cell –> Parvocellular cells
- This arrangement results in high acuity with low sensitivity
- Peripheral retina
- Convergent input
- Multiple photoreceptors to 1 parasol type ganglion cell –> magnocellular cells of the LGN
- This arrangement results in low acuity and high sensitivity
64
Q
Describe lateral inhibition
A
- Spatial contrast detected by centre surround antagonism set up by the ON/OFF bipolar cells is enhanced by lateral inhibition by horizontal cells via GABAergic innervation
- The horizontal cells act as an indirect pathway between the photoreceptors and bipolar cells
- Each horizontal cell has multiple dendrites that synapse with the terminal of a photoreceptor and the dendron of a bipolar cell to form a three-way synapse (triad) in the outer plexiform layer
- Lateral inhibition also occurs at the level of the amacrine cells that synapse with the bipolar cells and ganglion cells but is less well characterised than the horizontal cell pathway
65
Q
Describe the input and output of horizontal cells in lateral inhibition
A
- Horizontal cell input
- Horizontal cells receive input from its photoreceptors in the centre in a sign preserving manner
- Glutamate in the dark depolairses the horizontal cells
- No glutamate in the light hyperpolarises the horizontal cells
- Horizontal cell output
- The horizontal cells inverts the signal it receives from the photoreceptors and feeds it back to adjacent photoreceptors connected to bipolar cells in the surround
- This means they have to have the opposite response to the centre and enhances contrast
67
Q
What are the types colour opponency?
A
-
Red-green
- 4 types
- ON centre (R/G)
- OFF centre
- 4 types
-
Blue-yellow
- Yellow refers to both the red and green cones found in the centre/surround
- 4 types
- ON centre (B/RG)
71
Q
Define partial decussation
A
- Only the fibres originating from the nasal half of the retina decussate
- The nasal halves of each retina view the temporal (lateral) aspects of the visual scene for each side
- (The axons of retinal ganglion cells form the optic nerve that leaves the eye at the optic disc and enter the orbit through the optic canal.
- Both tracts meet at the optic chiasm where there is partial decussation of the optic nerve fibres
72
Q
- Define the temporal retina and its function
A
- View the nasal half of the visual field
- Send information to the same side of the brain
- The central aspect of the visual scene is seen by both temporal retinas = binocular vision
73
Q
Define the nasal retina
A
- View the temporal half of the visual field
- Send information to the opposite side of the brain
74
Q
What is a bi-temporal hemianopia?
A
- Loss of the peripheral vision (temporal half of the visual field)
- Usually due to compression of the optic chiasm by an enlarged pituitary gland
75
Q
Define the journey of the optic tracts
A
- From the chiasm axons form the optic tracts which run around the cerebral penducles
- Targets of the optic tract
-
Thalamus
- Most of the fibres of the optic tract project to the LGN at the dorsal thalamus
-
Hypothalamus
- A small number of axons form synaptic connections with the hypothalamus
-
Midbrain
- 10% of fibres form connections at the midbrain with the superior collicuus and pre-tectal areas
- Regulate the pupilliary light reflex
-
Thalamus
77
Q
- What does the right optic tract carry?
A
- The right optic tract carries information from the right temporal retina and from the left nasal retina
- Right temporal retina views left central visual field
- Left nasal retina views left peripheral visual field
- Transection results in the loss of the left visual field
81
Q
Complete the diagram
A
84
Q
Describe the output of the lateral geniculate nucleus as well as the optic radiation
A
- The primary visual cortex
- LGN thalamo-cortical neurones that project through the retrorenticular part of the internal capsule
- Pass around the concave part of the ventricle and into a broad sheet - superior and lateral to the lateral ventricles
- Inferior fibres project rostrally to the superior aspect of ht einferior horn of the lateral ventricle
- Terminates in the primary visual cortex in the occipital lobe
85
Q
Define quadrantopia
A
- Loss of one quadrant of the visual field due to a lesion of one of the divisions of the optic radiation of one side
87
Q
Name the following lesions
A
91
Q
Draw a diagram outlining the circuity of the primary visual cortex
A
-
Pulvinar – a thalamic nucleus involved in visual attention.
- Pulvinar = pay attention
-
Superior colliculus – saccadic eye movements
- Superior colliculus = saccadic
92
Q
Describe the different cells and layers at which they are present in the primary visual cortex.
A
-
Spiny stellate cells
- Spine covered dendrites from the cell body
- 4C alpha and 4c beta extend across towards V5
- Excitatory
-
Pyramidal cells
- Single thick apical dendrite that branches as it extends towards the pia mater
- Multiple basal dendrites that extend horizontally
- All layers except 4C alpha and beta (4A and 4B)
- Extend axons out of V1
-
Inhibitory neurones
- Throughout the cortex
- Local connections
93
Q
Describe the location of the parvocellular, magnocellular and koniocellular cells in the lateral geniculate nucleus and describe their receptive fields and colour opponency
A
- Parvocellular
- Layer 4C beta
- Small receptive fields
- Colour opponent
- Magnocellular
- Layer 4C alpha
- Large receptive fieds
- Not colour opponent
- Koniocellular
- Layers 1 & 2 cytochrome oxidase blogs
- Varied receptive fields
- Colour opponent
94
Q
Compare and contrast the 4C beta and 4C alpha layers in terms of colour opponency and intracortical connections
A
-
4C alpha
- Not colour opponent
- Magnocellular LGN
- Projects to layer 4B
-
4C beta
- Small receptive fields
- Colour opponent
- Parvocelllular LGN
- Projects to 2 & 3 interblob region
95
Q
What are cytochrome oxidase blobs?
A
- Neuronal columns rich in cytochrome oxidase are layers 2, 3, 5 and 6
- Receive the input from:
- Koniocellular pathway of the LGN
- Parvocellular/magnocellular input from layer 4C
- Between the blobs are interblob regions with the blobs centred on the ocular dominance column
97
Q
Describe the function of cells in cytochrome oxidase positive blobs in V1
A
V1 = visual cortex
- Respond selectively to colour contrast
- The blob cells are the “color” processing cells of V1
- This pathway is known as the parvoblob pathway.
- Blob pyramidal cell in layers II/III receive input from the parvocellular pathways in layer IV and directly from the single opponant koniocellular layers of the LGN.
- Properties of the neurons found in the blob areas:
- Neurons with similar properties found in clusters known as blobs
- Each blob is centred on an ocular dominance column
- Wavelength sensitive. Double colour opponent circular receptive fields for detection of colour contrast. They are used in the perception of colour, and in colour discrimination.
- 4 classes of double opponent cells characterised by their preferred stimuli
- Insensitive to achromatic contrast
- Monocular (respond to stimulation of one eye only)
- Insensitive to orientation or direction
106
Q
- Outline simple and complex receptive fields
A
- The receptive fields similar to that of the LGN and retina as they also have ON/OFF regions but the arrangement of these differ
- The orientation selective and binocular neurones of V1 can be:
-
Simple cells with simple receptive fields
- ON and OFF regions are segregated - this allows them to be orientation selective
-
Complex cells with complex receptive fields
- ON and OFF regions appear to be throughout the receptive field
- Respond to elongated stimulus at the correct angle in their receptive field
- Hypercomplex cells
- Even larger receptive fields that act as angle detectors that are sensitive to the length, orientation and motion of the bar of light that moves through their receptive field
- Defined by the property of end stopping - decrease in firing strength with increasingly larger stimuli
-
Simple cells with simple receptive fields
