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the eye and visual defects Flashcards

1
Q

what is the lens?

A

it is a transparent structure that is suspended by ligaments attached to the ciliary muscles

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2
Q

what are the ligaments suspending the lens called?

A

zonule fibres

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3
Q

what do the ciliary muscles do?

A

they control the shape of the lens

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4
Q

what is the vitreous humour?

A

it is a viscous jelly like substance that lies between the lens and the retina and keeps the eye spherical

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5
Q

what happens at the retina?

A

it is where the light is transformed into neural activity and therefore it is part of the CNS

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6
Q

what is the fovea?

A

it is the pit - it is the point of highest visual acuity where the light can reach the photoreceptors directly

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7
Q

how does image formation by the eye occur?

A

through refraction from the cornea, lens and lens accommodation by the ciliary muscles

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8
Q

what is refraction?

A

it is the bending of light rays when light passes from one transparent media to another such as the air to the cornea. It occurs as light is slowed down as it changes from one media to another and will bend towards or away from a line that is perpendicular to the border between the medial boundary

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9
Q

what happens as light passes through the cornea?

A

light rays that strike the curved surface of the cornea bend so that they converge on the back of the eye
light rays that enter the centre of the eye pass straight to the retina

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10
Q

how much of refraction is done by the lens?

A

the majority required is done by the cornea however some is also done by the lens when light passes through it in order to produce a sharp image

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11
Q

what is the refraction of both structures good for?

A

if the image is a far distance away as the light rays will strike the cornea parallel to eachother

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12
Q

what happens if there is a closer image?

A

greater refractive power is required to bring them into focus

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13
Q

what carries out focusing?

A

the lens changing shape - done by the ciliary muscles - accommodation

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14
Q

what is an emmetropic eye?

A

it is a normal eye that should focus parallel light rays onto the retina without the need for accommodation

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15
Q

how does accommodation occur?

A

the contraction of the ciliary muscles changes the shape of the lens - this relieves the tension on the zonule fibres so the lens becomes rounder due to its natural elasticity

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16
Q

what is the relationship between distance and power?

A

the refractive power (diopters) = 1/focal distance

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17
Q

what is the focal distance?

A

the distance from the cornea to the retina

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18
Q

what is hyperopia?

A

far sightedness - when the eyeball is too short from the front to the back and therefore the light rays are focused at some point behind the retina - the retina therefore sees a blurry circle
accommodation of the lens is needed for distant objects and near cannot be brought into focus

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19
Q

how can hyperopia be corrected?

A

a convex lens is placed infront of the eye and will provide the necessary refraction to allow near objects to be brought into focus

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20
Q

what is myopia?

A

near sightedness - when the eyeball is too long from front top back and parallel light rays converge at some point before the retina - as a result the retina sees an unfocused blurry circle

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21
Q

how can myopia be corrected?

A

concave lens is places in front of the eye and will provide the necessary refraction to allow distant objects to be brought into focus on the retinal surface

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22
Q

is there any surgery available for myopia or hyperopia?

A

there is corrective laser surgery called photorefractive keratectomy which uses a laser to reshape the cornea and decrease or increase the amount of refraction possible

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23
Q

on the opthalamic view of the retina what structures are seen?

A

the macula - the central vision
the fovea - the central or thinner region of the retina
the optic disk - the origin of the blood vessels, where the optic nerve axons exit the eye through the blind spot

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24
Q

what are the divisions of the retina on opthalamic view?

A

the temporal - anterior and the nasal - posterior retina

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25
how does light travel through the eye to the photoreceptors at the back?
the light is focused by the cornea and the lens and then passes through the vitreous humour to the retina. The retina lies in front of the pigment epithelium that lines the back of the eye, light passes through all the retinal cells to reach the photoreceptors at the back of the retina
26
what is melanin?
it is a black pigment that is found in the cells of the pigment epithelium that absorbs any light that is not absorbed by the retina
27
what is the structure of the retina?
anterior to posterior there is the ganglion cell layer - a thick layer then the inner plexiform layer (thin) and the inner nuclear layer (medium) then there is the outer plexiform layer (thin) and outer nuclear layer (thick) then a thin layer of photoreceptor outer segments and then the pigment epithelium
28
there are retinal biploar cells that extend into the inner plexiform layer, what are these?
the retinal bipolar cells come in two types - this is based on how they respond to light there are ON bipolar cells - depolarise in response to light onset OFF bipolar cells that hyperpolarise in response to light onset
29
what are the structures of rods and cones?
they have an outer and inner segment and a synaptic terminal the outer is one end and is longer in the rod cells then inner is the middle section which is similar lengths in both cells the synpatic terminals are small at the other end of the cells
30
what are the differences between the rod and the cone system?
the cones are high acuity, concentrated in the fovea and have a dispersed pathway. They are trichromatic. Rods are low acuity and not present in the fovea and they have a highly convergent pathway. They are achromatic
31
what is meant by tri and achromatic?
tri means that there are three types of cone pigment - each with different wavelength preference achromatic means that there is one type of rod pigment
32
what are the characteristics of cone cells?
they have lower amplification, higher temporal resolution with a faster response, more sensitive to direct light rays, less photopigments and lower sensitivity as they are specialised to day vision
33
what are the characteristics of rod cells?
they have higher sensitivity to light and are specialised for night vision, they have more photopigments to capture more light, they have higher amplification, they have lower temporal resolution with a slower response and they are more sensitive to scattered light
34
what is the standard polarisation and where does it drop to during hyperpolarisation in phototransduction?
-30mV to -60mV
35
what is the basis of phototransduction?
in the dark, photoreceptors are depolarised and will continuously release glutamate. The light will cause the depolarising ion channels to close and hyperpolarise the membrane potential therefore causing the glutamate release to reduce. The photopigmemnts include rhodopsin
36
what is the structure of rhodopsin?
there are transmembrane domains of opsin and a C and N terminus. There is retinal in the middle. It is a form of vitamine A - we can make retinal from alpha or beta carotene
37
what is the molecular basis of phototransduction?
1 photon is absorbed by 1 opsin, this makes 800 transducin molecules meaning that 800 PDE enzymes are made. This will make 4800 cGMP be converted into GMP and this will close 200 cGMP sensitive ion channels which causes hyperpolarisation and a decrease in glutamate release
38
what are the pixels in the retina?
the photoreceptors
39
what determines the visual acuity/resolution?
the density of pixels
40
what determines the ON and OFF pathways?
the differential expression of glutamate receptors splits the signal into on and off pathways
41
what happens when glutamate binds to an ON bipolar cell?
ON bipolar cells express mGluR6 and TRPM1 receptors. When glutamate binds to the mGluR6 a G protein is activated and this inhibits TRPM1 meaning that there is hyperpolarisation
42
what happens when a glutamate binds to an OFF bipolar cell?
OFF bipolar cells will express AMPA and Kainate receptors - when the glutamate binds the receptor opens and depolarises the cell
43
what is the distribution of rods and cones across the retina?
the rods will increase slowly to fovea and then drop to nothing at the fovea and then increase and decrease slowly as they get further from the fovea the cones will be at almost nothing until they reach the fovea where they spike very high
44
what is the receptive field of the retina?
it is the area of the retina that causes any change in the response of a neuron
45
what are horizontal and amacrine cells?
they are the cells in the retina that shape the receptive fields of the bipolar cells (horizontal) and ganglion cells (amacrine)
46
what is the receptive field centre?
it is due to the direct connection from the retinal ganglion cells to the glutamatergic neuron which corresponds to the dendritic field centre - dendritic
47
what is the receptive field surround for ganglion cells?
it is due to lateral inhibition from inhibitory neurons and is usually much larger than the dendritic field lateral - surround
48
what are the two fields for?
comparison of light between the centre and surround
49
describe the retinal ganglion cell receptive fields in terms of centre-surround?
the receptive field centre is where the ganglion cell is and this is directly connected to an OFF bipolar cell - glutamatergic neuron receptive field surround is comprise of amacrine cells providing lateral inhibition (inhibitory neurons) that are not directly connected to the ganglion cell
50
what happens when a 'shadow' is in the surround only?
it is found only in the surround receptive field - there is hyperpolarisation and therefore decreased firing
51
what happens as the 'edge' moves over to the centre?
the centre becomes depolarised and therefore there is increased firing from the centre
52
what happens when the shadow covers the surround and the centre?
the firing decreases again and there is hyperpolarisation
53
what can be concluded from the shadow patterns?
that the optimal stimulus is when there is a dark light border across the centre and surround receptive fields
54
what is the young helmholtz theory?
every colour in the rainbow can be obtained by mixing the appropriate ratio of red, green and blue light, at each point in the retina there exists a cluster of three receptor types, each being maximally sensitive to blue, green or red
55
how do we perceive colour?
the brain will assign a colour based on the comparison of the three cone types - ie if they are all equally active then we perceive white
56
how can colour blindness occur?
there are genes for cone pigments identified and in colour blind patients some are missing
57
what are the photoreceptor pigments?
the pigment in all rods is called rhodopsin with the receptor protein opsin - in each cone there is one of three types of opsins that are activated by lights of different wavelengths and therefore has a different spectral sensitivity
58
what are the wavelengths for each light?
``` they are maximally activated by: blue - 420nm green - 530nm red - 560 nm these are where the relative absorbance is highest ```
59
what is colour opponency?
colour is coded with an opponent process - two colours are compared with one colour reducing the ganglion cell activity and one increasing it
60
what are the two opponent pathways?
there is red and green, and blue and yellow | each colour can either be ON or OFF and can be in the centre or retina surround field making four combinations
61
what is the main visual pathway?
the retina to the lateral geniculate nucleus in the thalamus to the primary visual cortex
62
what is the path of information from the eyes to the cortex?
the retina to the optic nerve - optic chiasm - optic tract (where can stop at the hypothalamus, pretectum or superior colliculus) - lateral geniculate nucleus - optic radiation - striate cortex
63
what are luminence encoders?
they are intrinsically photosensitive retinal ganglion cells
64
what do luminence encoders contain?
the fifth photopigment in the eye - melanopsin
65
where is melanopsin expressed?
in ipRGCs - even with all synaptic transmission blocked they are still responsive to light
66
what are ipRCGs for?
a stable representation of ambient light, non-image forming, and are required for normal photo entrainment of the circadian clock and the pupillary light reflex
67
what happens if glutamate transmission in ipRGCs is knocked down?
the circadian rhythm is disrupted and the body is retrained to a different cycle
68
what is the olivary pretectal nucleus?
it is neurons connected to the Erdinger-Westphal nuclei (parasympathetic preganglion of the ciliary ganglia)
69
what are the three layers of the lateral geniculate nucleus?
the parvocellular layers 3-6 the magnocellular layers 1 and 2 koniocellular layers
70
what are the characteristics of the layers of the LGN?
parvo - small cell bodies magno - large cell bodies konio - very small cell bodies
71
what are the three types of retinal ganglion cells?
magnocellular M types parvocellular P types nonM non P types - K type
72
what are the characteristics of magnocellular?
they are a larger cell type that are 5% of the population with a large receptive field and will therefore be important for the detection of stimulus movement
73
what are the characteristics of parvo?
they are a smaller cell types that is 90% of the population and are sensitive to stimulus form and fine detail
74
what are the characteristics of the nonM non P type?
medium cell type that are 5% of the population
75
outline information course from the retina to the LGN for P types?
the retinal output is the eye and the ganglion cell type to the LGN is P type. This can carry contra or ipsilateral information for layers 3-6 in the LGN to the parvocellular LGN cells.
76
what information does M type carry?
it will carry contra and ipsilateral information from the eye to layers 1 and 2 to the magnocellular LGN cells
77
what information does K type carry?
ipsi and contralateral information from the eye to the ventral part of each principal layer to koniocellular cells in the LGN
78
how do neurons of the LGN get to the PVC?
through optic radiation using Meyers loop to the inferior and superior rectinal quadrants near the calcarine fissure
79
what is the main LGN target?
the PVC V1 also known as Broadmann's area 17 or the striate cortex located in the occipital lobe
80
what is the functional organisation of the V1 area?
there are orientation and ocular dominance columns as well as colour processing 'blobs' similar cellular architecture to the rest of the cortex
81
what is V1?
it is a module comprised of 2mm^3 that contains all representations - e.g. colours, orientations and both eyes
82
what are the characteristics of orientation columns?
all neurons in the vertical column display the same orientation specificity neurons in the oblique row display heterogenousorientation specificity traverse for 1mm for 180 degree shift
83
what has autoradiography established?
that inputs from one retina created bright stripes on dark background due to ocular dominance
84
what is the main feature of ocular dominance?
the inputs from the two eyes are still largely separate in V1
85
what is cytochrome oxidase?
it is a mitochondrial enzyme that is used in cell metabolism
86
what are found in the ocular dominance columns?
layer I - VI
87
what is found in the layers II, III, V and VI?
pillars - each pillar is centres on an ocular dominance column - staining with cytochrome oxidase reveals these
88
what does layer IVC beta receive?
P cell LGN input
89
how is K cell input received?
directly from the LGN
90
what does the RF of blocks display?
colour opponency
91
how is input from the M channel received?
through the IVC alpha layer
92
what is the striate cortex / V1 role?
it is the first region of visual processing in the cortex
93
although there are dozens of other areas of the cortex involved in extrastriate visual processing what are the two cortical streams of visual processing?
the dorsal stream - striate cortex to parietal lobe for visual motion the ventral stream - striate cortex to temporal lobe for recognition of objects

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