S3C3 (2.0) Flashcards
(191 cards)
1
Q
What is the pathophysiology of myopia?
A
Axial length of the eye is too long for its refraction
This can be due to an elongated axial diameter of the eye or due to increased refraction (less common)
Focal point anterior to the retina
2
Q
What are the symptoms of myopia?
A
Near vision clear
Far vision unclear
3
Q
What glasses are needed to correct myopia?
A
Concave
4
Q
What is the pathophysiology of hyperopia/presbyopia?
A
Axial length too short for its refraction (hyperopia)
Decreased lens accommodation (presbyopia)
Focal point posterior to retina
5
Q
What are the symptoms of hyperopia/ presbyopia?
A
Near vision unclear
Far vision clear
6
Q
What glasses are needed to correct hyperopia/presbyopia?
A
Convex
7
Q
What is the pathophysiology of an astigmatism?
A
Curvature of cornea hinders even refraction
Eye cannot collect light evenly on the retina
Focal point can be anterior or posterior
8
Q
What is the main symptom of an astigmatism?
A
Unclear vision at all distances
9
Q
What glasses are needed to correct an astigmatism?
A
Cylindric lenses
10
Q
What is a strabismus?
A
Abnormal alignment of the eyes
Visual axes of the eyes of not parallel (cross eyed)
11
Q
What is amblyopia?
A
Functional reduction in visual acuity of an eye caused by disuse during visual development.
12
Q
What is the treatment for amblyopia?
A
Spectacles or contact lenses /Cataract removal/Patching
13
Q
What are the 2 types of stabismus?
A
Esotopia (convergent squint) – the directions of gaze of the two eyes cross, and the person is said to be cross-eyed.
Exotopia (divergent squint) – the directions of the gaze diverge, and the person is said to be wall-eyed.
14
Q
What are the extraocular muscles?
A
Superior oblique Superior rectus Medial rectus Lateral Rectus Inferior rectus Inferior Oblique
15
Q
Where do the recti muscles originate from?
A
The common tendinous ring
16
Q
What is transduction?
A
The process by which the sense organs convert energy from environmental events into neural activity
17
Q
What is anatomical coding?
A
Different nerves represent different sensory modalities
Distinctions between stimuli of the same modality (e.g. Arising from different spatial locations)
18
Q
What is temporal coding?
A
Rate of firing or axons represents stimulus intensity
19
Q
What is signal detection theory?
A
Detecting a stimulus involves discriminating between a signal and noise (background stimulus and random neural activity)
20
Q
What is perception?
A
Our interpretation of what is being represented by sensory input - can be influenced by higher-level cognitive processes such as expectations
21
Q
What is visual perception?
A
We perceive objects and background rather than a complex pattern
This is helped by boundaries and the Gestalt principles
22
Q
What are the Gestalt principles?
A
Adjacency/proximity principle The similarity principle Good continuation The law of closure The principle of common fate
23
Q
What is the Adjacency/proximity principle?
A
Elements of a visual scene that are close are grouped together
24
Q
What is the similarity principle?
A
Similar elements are perceived as belonging together
25
What is good continuation?
Elements that smoothly follow a line tend to belong together
26
What is the law of closure?
Missing information is supplied to close or complete a figure
27
What is the principle of common fate?
Elements on the same movement trajectory belong together
28
What are the models of pattern perception?
Templates - stored visual memories of patterns compared with visual input
Prototypes - flexible, idealised stored patterns compared with visual input
Feature detection models - Distinctive features model
(Geons - the simple 2D/3D shapes that form objects)
29
What is the perceptual set?
Selectivity and bias with respect to what we perceive
30
What is the perceptual set influenced by?
```
Previous experience
Current drive state (arousal and motivation)
Environment
Cultural factors
Threshold for perception
```
31
What is attention?
The allocation of awareness to stimuli
32
How can attention be shfited?
Disengagment
Shifting
Focus
Can be
bottom-up - in response to a stimuli appearing suddenly
Top-down - looking out the corner of your eye (conscious)
33
What is unattended stimuli?
Inattentional blindness - when fixating, we can ignore otherwise obvious events
Change blindness - we can miss otherwise obvious changes to stimuli
Unattended stimuli can affect us
Salient events can grab our attention - can be retained in implicit memory
34
What is the individual model of disability?
This suggests that disability has occurred on a personal level due to:
Personal tragedy
Medical problem
Individual adjustment
35
What is the social model of disability?
Disability is caused by the way society is organised, rather than by a person’s impairment or difference –discrimination.
It looks at ways of removing environmental barriers that restrict life choices for disabled people.
When barriers are removed, disabled people can be independent and equal in society, with choice and control over their own lives.
36
What does the social model of disability emphasise?
That community participation is hindered by inaccessible environments.
That those with disabilities are oppressed by medical and social service professionals.
Underestimation of needs, poverty and deprivation.
Disabled people developed the social model of disability because the traditional medical model did not explain their personal experience of disability or help to develop more inclusive ways of living.
37
What is the medical model of disability?
People are disabled by their impairments or differences.
Under the medical model, these impairments or differences should be 'fixed' or changed by medical and other treatments, even when the impairment or difference does not cause pain or illness
38
What is the psychological model of disability?
That activities performed (or not performed) by someone with a “health condition” are influenced by the same psychological processes that affect the performance of these behaviours by non-disabled people
two people with identical medical conditions, living in identical social and environmental situations, may have very different activity limitations because of their cognitions, emotions or coping strategies
39
How is disability assessed?
Disability is typically assessed by measures of activities of daily living (ADL), which assess the person’s ability to perform everyday self-care or mobility activities.
These measures assess activities that virtually everyone would wish to perform and, therefore, do not include activities that may be important for particular individuals.
There are two main methods of assessment;
Self-report
Observation
40
What are the 3 layers of the eye?
Fibrous tunic
Vascular tunic
Nervous tunic
41
What are the 3 chambers of the eye?
Anterior
Posterior
Vitreous
42
What is the refractory media of the eye?
Cornea
Lens
Vitreous humour
Aqueous humour
43
What is in the fibrous tunic?
Cornea
| Sclera
44
Describe the cornea
Anterior
Transparent
No vasculature - requires aqueous humor for nutrients
Function: refracts the light
45
Describe the sclera
Posterior
Opaque
Fibrous
Function: Protect and stabilise
46
What is in the vascular tunic?
Iris
Ciliary body
Choroid
47
What is the Iris?
Pigmented diaphragm with a central opening that adjusts in size in response to light
48
What fibres does the Iris contain?
Radial fibres are arranged in parallel (iris dilator muscle)
Sympathetic stimulation
Dilation (Mydriasis)
Circular fibres arranged around the pupil (iris sphincter muscle)
Parasympathetic stimulation
Contraction (Miosis)
49
What is the function of the Iris?
Regulate the amount of light that impinges on the retina
50
What is the ciliary body composed of?
Ciliary ring
Ciliary epithelium
Outer layer - pigmented
Inner layer - unpigmented
Ciliary processes - fibres the encircle the border of the lens
Ciliary muscle - smooth muscle innervated by parasympathetic fibres
51
What are the functions are the ciliary body?
Anchors the lens
Accommodation: adjusts the lens for seeing at different distances
Produces aqueous humor
52
How does accommodation happen?
Contraction of the ciliary muscle causes relaxation of the zonular fibers, which releases the tension on the lens. This causes the lens to become more spherical
53
What is the choroid formed of?
Formed by two layers
Outer - pigmented
Inner - highly vascularised
54
What is the function of the choroid?
Nourishes the retina
55
What are the layers of the retina? (in order of light hitting them)
```
Ganglion cell layer
Inner Plexiform
Inner nuclear layer
Outer Pelxiform
Outer nuclear
Layer of rods and cones
Pigment epithelium
```
56
What is the function of the pigment epithelium?
Aids in formation of rhodopsin and storage of Vit A
Provides nutrition to photoreceptors
Absorbs light and prevents reflection
57
Describe rod cells
First order receptors of slender, cylindrical cells
Contain rhodopsin
Achromatic - specialised for sim light and motion
Located around fovea centralis
58
Describe cone cells
First order receptors of flask-shaped cells containing photopsin/ iodopsin
Chromatic - RGB, specialised for colour vision, bright light, object recognition and visual acuity
Located in fovea centralis
59
What neurotransmitter do rod and cone cells use?
Glutamate
60
What does the outer nuclear membrane contain?
Nuclei of rods and cones
61
What does the outer plexiform layer contain?
Synapses between bipolar cells and cone/rods
62
What does the inner nuclear layer contain?
Contains the cell bodies of bipolar neurons
Second order
Transmit information to ganglion cells
Contains muller cells (supporting glial cells)
63
What does the inner plexiform layer contain?
The synapses between ganglionic cells and bipolar neurons
64
What does the ganglion cell layer contain?
Contains nuclei and cell bodies of ganglion cells
Third order
Form optic nerve (CN3)
Project to the hypothalamus, pretectal nucleus, lateral geniculate body and superior colliculus
Contain neuroglia
65
How many rod cells are there?
90 million
66
How many cone cells are there?
4.5 million
67
What is the outer segment of a photorecptor cell composed of?
membranous disks that contain light-sensitive photopigment
| Photopigments absorb light, thereby triggering changes in the photoreceptor membrane
68
What is the inner segment of a photorecptor cell composed of?
the cell nucleus and gives rise to synaptic terminals that contact bipolar or horizontal cells
69
What is the foveola?
All cone, no rod - central 300µm of fovea
70
What is the life span of the membranous disks in the outer segment?
12 days
71
What is the life cycle of the membranous disks?
Disks move progressively from the base of the outer segment to the tip, where the pigment epithelium plays an essential role in removing the expended receptor disks
Shedding involves 'pinching off' a clump of receptor disks by the outer segment membrane of the photoreceptor -
Phagocytosed by the pigment epithelium
72
What is the blood supply to the photoreceptors?
Choroidal blood vessel
73
What features of photoreceptor cells contribute to superior visual acuity?
The layers of cell bodies and processes that overlie the photoreceptors retina are displaced in fovea
The retinal blood vessels are diverted away from foveola
Foveola is dependent on the underlying choroid and pigment epithelium for oxygenation and metabolic sustenance
74
What is the functional specialisation of rod cells?
These have a long, cylindrical outer segment containing many disks - making them extremely sensitive to light
Rods have a low spatial resolution - it is therefore specialised for sensitivity at the expense of resolution
75
What is the functional specialisation of cone cells?
These have sorter tapering outer segment with fewer membranous disks - relatively insensitive to light
High spatial resolution
Allows colour vision
76
What happens re: photoreceptor cells at light intensity increases?
cones become more dominant in determining what is seen and they are the major determinant of perception under relatively bright conditions
77
What is photopic vision?
The contributions of rods to vision drops out nearly entirely in "photopic vision" because their response to light saturates
The response of an individual cone does not saturate at high levels of steady illumination
78
Why do rod cells saturate in response to high light intensity?
The membrane potential of individual rods no longer varies as a function of illumination because all of the membrane channels are closed
79
What is scotopic vision?
Only rods, low light level
80
What is mesopic vision?
Both rods and cones
81
What is convergence?
In most parts of the retina, rod and cone signals converge on the same ganglion cells depending on levels of illumination
makes the rod system a better detector of light as small signals from many rods are pooled to generate a larger response in the bipolar cell
82
What is the function of rod bipolar cells?
```
These do not contact retinal ganglion cells
Synapse with the dendritic processes of a specific class of amacrine cell that makes gap junction and chemical synapses with the terminals of cone bipolar cells; these in turn make synaptic contacts on the dendrites of ganglion cells
Each rod bipolar cell is contacted by a number of rods, and many rod bipolar cells contact a amacrine cell
```
83
What is the function of cone bipolar cells?
Each retinal ganglion cell that dominates central vision (midget ganglion cells) receives input from only one cone bipolar cell, which is contacted by a single cone
84
Where is the role of horizontal cells?
The processes of horizontal cells enable lateral interactions between photoreceptors and bipolar cells that maintain the visual system's sensitivity to luminance
85
Where are the processes of amacrine cells connected to?
Te processes of amacrine are postsynaptic to bipolar cell terminals and presynaptic to the dendrites of ganglion cells
86
Where is the retinal pigment epithelium?
Surrounds the tips of the outer segments of each photoreceptor
87
What is the role of the retinal pigment epithelium?
Phagocytosis of expended membranous disks
| Regenerate photopigment molecules after they have been exposed to light
88
What is the optic disk?
```
Formed by axons of ganglion cells that leave the eye to form the optic nerve
Located medial to the fovea centralis
Lacks receptors
Insensitive to light
Physiologic scotoma - blind spot
Central depression - optic cup
```
89
What is the macula?
Oval shaped yellow spot lateral to optic disk
Lacks blood vessels
Contains fovea centralis
90
What are the boundaries of the anterior chamber?
Anterior: posterior surface of cornea
Posterior: anterior surface of lens, iris and ciliary body
Lateral: trabecular meshwork and canal of Schlemm
91
What is the canal of Schlemm?
□ A vessel in the eye that conveys aqueous humor from the anterior chamber to the episcleral vessels.
92
What are the boundaries of the posterior chamber?
Anterior: posterior surface of iris
Posterior: zonular fibres of the lens and ciliary body
Lateral: ciliary process
93
What are the zonular fibres?
A collection of radially arranged fibres that connect the lens to the ciliary body
94
What are the boundaries of the vitreous chamber?
Between lens and retina
95
What is aqueous humor?
Watery fluid that fills the anterior and posterior chambers of the eye
Synthesised from blood plasma by the ciliary body
96
What is the pathway of aqueous humor?
```
Ciliary body
Posterior chamber
Through pupil
Anterior chamber
Trabecular meshwork with spaces of Fontana (in iridocorneal angle)
Schlemm's canal
Episcleral vessels
```
97
What is the function of aqueous humor?
Maintains intraocular pressure
| Provides nutrients the lens and cornea
98
What is vitreous humor?
Transparent gelatinous substance that fills the eyeball posterior to the lens
99
What is the function of vitreous humor?
Provides support to the lens and hold the retina in place
100
What is the lens?
Transparent and flexible biconvex structure
101
What is the function of the lens?
Function: Accommodation
Increased Convexity - near vision
Decreased convexity - distant vision
102
What are the attachments of the lens?
Medial - lens capsule
| Lateral - ciliary processes
103
What is the primary blood supply to the eye?
The ophthalmic artery, a branch of the ICA that reaches the eye via the optic canal
104
What is the central artery of the retina? What does it supply?
Main branch of the ophthalmic artery
Supplies retina and part of optic nerve
Travels in the optic nerve
Divides into superior, inferior, nasal and temporal branches
105
What can occlusion of the central artery of the retina cause?
Blindness
106
What does the supraorbital artery supply?
Supplies the forehead and the scalp
| Passes through the supraorbital foramen
107
What does the frontal artery supply?
Supplies the forehead and the scalp
108
What does the dorsal nasal artery supply?
Lacrimal sac
109
What is the venous drainage for the eye?
Ophthalmic veins
Superior ophthalmic vein: travels parallel to the ophthalmic artery
Inferior ophthalmic vein: divides into two branches
Pass through inferior or superior orbital fissure
Drains into cavernous sinus
110
Where does the Ophthalamic nerve enter the orbit?
The superior orbital fissure
111
What does the Ophthalamic nerve supply?
Supplies sensation to the forehead, anterior scalp, upper eyelid, and eyeball
Contains sympathetic and parasympathetic nerve fibres that facilitate pupil dilation and innervate the ciliary body, iris, lacrimal gland, conjunctiva and cornea
112
What are the branches of the Ophthalamic nerve?
Lacrimal
Frontal
Nasociliary
113
What does the lacrimal nerve innervate?
Enters orbit through the superior orbital fissure
| Innervates the lacrimal gland, the skin of the upper eyelid, and the conjunctiva
114
What does the frontal nerve innervate?
® Divides into
Supraorbital nerve - innervates the frontal sinus, scalp, forehead and upper eyelid
Supratrochlear nerve - innervates the frontal sinus, scalp, forehead and upper eyelid
115
What are the nasociliary nerve branches?
```
Communicating branch to ciliary ganglion
Long ciliary nerves
Short ciliary nerves
Anterior ethmoidal
Posterior ethmoidal
Infratrochlear
```
116
What does the Long ciliary nerves innervate?
Afferent fibres from iris and cornea
| Postganglionic sympathetic fibres to iris dilator muscle
117
What does the short ciliary nerves innervate?
Afferent fibres from iris and cornea
Postganglionic parasympathetic fibres to the ciliary muscle and iris sphincter muscle
Postganglionic sympathetic fibres to iris dilator muscle
118
What does the maxillary nerve innervate? (eye)
Sensory innervation of the lower eyelid
119
Where is the ciliary ganglion?
Located behind the eyeball, between the lateral rectus muscle and the optic nerve
120
What is the role of the lacrimal system?
Supplies tear fluid for the surface of the eyeball and the eyelid, minimizing friction and cleaning the eye
121
Where is the lacrimal gland?
Located on the upper lateral part of the orbit
| On the surface of the levator palpebrae superioris and the lateral rectus muscle
122
What is the lacrimal duct?
Serous gland with tubuloalveolar secretory elements
Secretes the aqueous layer of the tear fluid
Drains via 12 lacrimal ducts into the superior conjunctival fornix
123
What is the innervation of the lacrimal gland?
Innervated by parasympathetic fibres of the facial nerve via the pterygopalatine ganglion
124
What are the tarsal glands?
Sebaceous glands - produce a lipid layer that protects against dehydration of the cornea
Located within the tarsal plate with orifices at the rim of the marginal zone of the conjunctiva
125
What is the lacrimal sac?
Dilated portion of the nasolacrimal duct
| Gathers tear fluid
126
Where does the nasolacrimal duct drain?
Drains into the inferior meatus of the nose
127
What is the composition of tears?
Isotonic fluid (Na+ K+ Cl- HCO3-)
Proteins (Lysozyme, lactoferrin, defensins)
EGF
IgA
128
What is the pathway of tears?
```
Produced in the lacrimal gland
Lacrimal ducts
Superior conjunctival fornix
Anterior eyeball
Lacus lacrimalis
Lacrimal canaliculi
Lacrimal sac
Nasolacrimal duct
Inferior nasal meatus
```
129
How does light affect photoreceptors?
Light stimulation of the photopigment leads to membrane hyperpolarisation rather than depolarisation
130
What happens to photoreceptors in the dark?
The membrane potential of the rod outer segment -30mV.
This depolarisation is caused by the steady influx of Na+ and K+ ions leaving to balance the electrochemical gradient
This is called dark current.
131
How does Na+ get into photoreceptor cells?
Gated sodium channels are simulated to open by an intracellular second messenger called cyclic guanosine monophosphate (cGMP).
cGMP is produced continually in the photoreceptor by the enzyme guanylyl cyclase, keeping the sodium channels open.
132
What happens to sodium channels in photoreceptors during light stimulation?
Light stimulation of the photopigment activates G-proteins, which in turn activate an effector enzyme that reduces cGMP.
This causes the cGMP-gated-Na+ channels to close, thus hyperpolarising the cell.
133
What is the relationship? between luminance changes and the rate of transmitter release from the photoreceptor terminals?
Higher luminance = reduced transmitter release
Transmitter release from the synaptic terminals of the photoreceptor is dependent on voltage-sensitive Ca2+/Na+ channels in the terminal membrane
134
What is rhodopsin?
biological pigment found in the rods of the retina and is a G-protein-coupled receptor
Extremely sensitive to light
135
What is the chromophore of rhodopsin?
Retinal (11-cis retinal), a substance derived from vitamin A.
136
What is the receptor protein linked to rhodopsin?
Transducin (opsin)
137
What does the absorption of the light do to retinal?
Absorption of light causes a change in the conformation of 11-cis retinal to all-trans retinal.
138
What does the confirmation to all-trans retinal cause?
This activates opsin, causing it to be released
This causes the activation of transducin, resulting in the conversion of GTP to GDP (by GTPase).
GDP, along with phosphodiesterase (PDE) hydrolyses cGMP, thus reducing it’s concentration.
This leads to the closure of the cGMP-gates-Na+ channels.
139
What happens to all-trans retinal following release of opsin?
Gets bleached
becomes all trans retinol
{retinOL not AL}
140
What is transducin?
G-protein involved in phototransduction
141
How does termination of transducin occur?
via GTPase activity
142
What is the retinoid cycle?
The restoration of retinal to a form capable of signalling photons is a complex process:
The all-trans retinol is transported out of the outer segment and into the retinal pigment epithelium.
Enzymes convert it to 11-cis retinal.
After it is transported back into the outer segment, the 11-cis retinal recombines with opsin in the receptor disks to form rhodopsin.
143
How does signal amplification work?
Many G-proteins are activated by each photopigment molecule, and each phosphodiesterase enzyme breaks down more than one cGMP molecule.
This amplification gives our visual system the ability to detect as little as a single photon, the elementary unit of light energy.
As levels of illumination increase, sensitivity to light decreases, preventing the receptors from saturating and thereby greatly extending the range of light intensities over which they operate
144
What wavelength do blue cones work at?
Short
145
What wavelength do green cones work at?
Medium
146
What wavelength do red cones work at?
Long
147
How do we perceive the colour white?
All types of cones are equally active, as in broad-spectrum light
148
How long does dark adaptation take?
20-25 minutes
149
What factors affect dark adaptation?
Pupil dilation – this allows more light to enter the eye.
Regeneration of unbleached rhodopsin.
An adjustment of the functional circuitry of the retina so that information from more rods is available to each ganglion cell.
150
How does calcium play a role in dark adaptation?
In the dark, Ca2+ enters the cones and has an inhibitory effect on the enzyme (guanylyl cyclase) that synthesizes cGMP, thus closing the cGMP-gated channels.
When the cGMP-gated channels close, the flow of Ca2+ into the photoreceptor is reduced; as a result, more cGMP is synthesized (because the synthetic enzyme is less inhibited), thereby allowing the cGMP-gated channels to open again.
Essentially, when the channels close, a process is initiated that gradually reopens them even if the light level does not change.
151
What is the receptive field of bipolar cells?
The area of retina that, when stimulated with light, changes the cell membranes potential
152
What does the receptive field consist of?
A circular centre area of retina providing direct photoreceptor input, called the receptive field centre
A surrounding area of the retina providing indirect photoreceptor input via horizontal cells, called the receptive field surround
153
What is the response of a bipolar cells membrane potential to light in the centre of the receptive field?
Response of a bipolar cells membrane potential to light in the receptive field centre is opposite to that in the surround
If ON-bipolar, illumination of centre will depolarise the cell
Illumination of surround with antagonistically hyperpolarise cell
154
What do ganglion cells respond to?
○ Each ganglion cell responds to stimulation of a small circular patch of the retina, which defines the cells receptive field
Turning on a spot of light in the receptive field of an ON-centre ganglion cells produces a burst of action potentials
ON-centred increase their discharge rate to luminance increments in receptive field
155
What happens to ganglion response in uniform illumination
the centre and surround cancel to yield some low level response
156
What is luminance contrast?
Ganglion cells are sensitive to differences between the level of illumination that falls on the receptive field centre and the level of illumination that falls on the surround
157
What do central bipolar cells release when theres a stimulus?
Glutamate which binds to AMPA/kaini?te/NMDA
158
What happens to OFF-centre bipolar cells with dark stimulus?
Ionotropic receptors (AMPA and kainate) that cause the cells to depolarize in response to glutamate released from photoreceptor terminals.
159
What happens to ON-centre bipolar cells with dark stimulus?
Express a G-protein-coupled metabotropic glutamate receptor (mGluR6).
When bound to glutamate, these receptors activate an intracellular cascade that closes cGMP-gated Na+ channels, reducing inward current and hyperpolarizing the cell
160
What happens to ON-centre bipolar cells with light stimulus?
ON-centre bipolar cells contacted by the photoreceptors are freed from the hyperpolarizing influence of the photoreceptor’s glutamate, and they depolarize.
161
What happens to OFF-centre bipolar cells with light stimulus?
The reduction in glutamate represents the withdrawal of a depolarizing influence, and these cells hyperpolarize.
162
What are horizontal cells?
These are cells that are found in the outer plexiform layer (their terminals) and in the inner nuclear layer (their cell bodies).
Horizontal cells release their neurotransmitters directly onto the photoreceptor terminals in the outer plexiform layer.
163
What is the relationship between photoreceptors and horizontal cells?
§ Glutamate release from photoreceptor terminals has a depolarising effect on horizontal cells.
GABA release from horizontal cells has a hyperpolarising effect on the photoreceptor terminals.
164
Describe the cascade effect for On-centred ganglion cells
```
Light on both aspects of receptive field
All photoreceptors hyperpolarise
Glutamate released from surround
Hyperpolarises horizontal cell
Release GABA to depolarise central photoreceptor
Inhibits on-centre ganglion
```
165
What are the types of ganglion cell?
Large M-Type Ganglion cells – 5% of population. [M = ‘magno’ = large]
Smaller P-Type ganglion cells – 90% of population. [P = ‘parvo = small]
The remaining 5% is made up of a variety of nonM-nonP ganglion cell types. These are less well characterised.
166
Describe M-type ganglion cells
Have larger receptive fields than P-type ganglion cells.
Their axons conduct action potentials more rapidly in the optic nerve.
They are more sensitive to low contrast stimuli.
167
How do M-type ganglion cells response to stimulation?
Stimulation of receptive field centres with a transient burst of action potentials, while P-type ganglion cells respond with a sustained discharge as long as the stimulus is on.
168
Which ganglion type can carry colour information?
P-type and nonM-nonP
P-type ganglion cells are sensitive to differences in the wavelengths of light striking their receptive field centre and surround.
169
What are the two types of opponency?
Red v Green
| Blue v Yellow
170
How is light refracted by the cornea?
Light rays are parallel
Hit the curved surface of the cornea
Converge on the retina
171
What is the focal distance?
The distance from the refractive surface to the point where the parallel light rays converge
172
What is the relationship between the cornea and the focal distance?
The more curved the cornea, the shorter the focal distance
173
What is a dioptre?
Dioptre is the unit of the power of refraction
| Dioptres= 1/focal distance (m)
174
What is the refractive power of the cornea?
42 dioptres
| Light rays striking it will be focused about 2.4cm (~distance from the cornea to the retina)
175
What is the refractive power of the lens?
~12 dioptres
176
How does the lens change to view near objects?
The ciliary muscle contracts and swells in size, thereby making he area inside the muscle smaller and decreasing the tension in the suspensory ligaments
The lens becomes rounder and thicker because of its natural elasticity
This increases the curvature of the lens, and thus its refractive power increases
177
How does the lens change to view far objects?
Relaxation of the ciliary muscle increases the tension in the suspensory ligaments, and the lens is stretched into a flatter shape, and its refractive power is reduced
178
What is the role of the pupillary light reflex?
aids the clarity of images formed on the retina
| Narrowing the pupil reduces both spherical and chromatic aberration, leading to sharper images
179
Why does spherical aberration occur?
Due to the increased refraction of lights rays when they strike a lens
180
What is chromatic aberration?
the effect produced by the refraction of different wavelengths of light through slightly different angles, resulting in a failure to focus
181
What does reduction in pupil size do to depth of field?
increases the depth of field - the distance within which objects are seen without blurring
182
Define the visual field
The total amount of space that can be viewed by the retina when the eye is fixated straight ahead
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What is binocular vision?
The visual field when both eyes are open
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What is visual acuity?
The ability of the eye to distinguish between two nearby points
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What % of fibres cross in the chiasm? Where do they go?
60% and head towards the occipital lobe
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What % of optic nerve fibres continue towards the thalamus and midbrain of the same side?
40% - secondary pathway
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What is the primary visual pathway?
Chiasm
Optic tract
ganglion cell axons reach a number of structures in the diencephalon and midbrain.
Main target LGN
LGN send axons to cerebral cortex via internal capsule
Pass through optic radiation and terminate in primary visual cortex
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What is the secondary visual pathway?
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Chiasm
Projections to:
Superior colliculus
Pretectum
Hypothalamus
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What does the superior colliculus coordinate?
Coordinates head and eye movements to visual (and other) targets
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What does the pretectum coordinate?
Coordinates the pupillary light reflex.
Afferent fibres terminate in the pretectal nucleus and pass to the Edinger-Westphal nucleus, which then project to the ciliary ganglion of the oculomotor nerve, causing contraction of the constrictor pupillae muscles
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What does the hypothalamus (suprachiasmiatic nucleus) coordinate?
The circadian cycle.
The fibres that project here express their own light-sensitive photopigment (melanopsin).
The fibres containing melanopsin (fibres to Pretectum and hypothalamus) are capable of modulating their response to changes in light levels in the absence of signals from rods and cones.
