Physiology of Vision Flashcards
1
Q
why do animals use their senses?
A
to perceive the environment around them
2
Q
what does perception of different touch sensations involve?
A
texture, pressure, stretch, heat, cold and pain
3
Q
what does perception of different chemicals involve?
A
taste and smell
4
Q
what does perception of waves in a medium involve?
A
sound
5
Q
what does perception of light involve?
A
intensity, wavelength, images, movement
6
Q
environmental stimulus modifies a tissue structure, what does modified stimulus affect?
A
the membrane of a sensory cell, signal transduced/amplified and encoded to nerve fibres and interpretation by the brain
7
Q
what is a photon?
A
it is a particle of electromagnetic radiation
8
Q
which photons contain more energy?
A
photons of shorter wavelengths contain more energy than photons with shorter wavelengths
9
Q
how much of the electromagnetic does ‘visible’ light occupy?
A
it occupies only a minute portion of the electromagnetic spectrum
10
Q
in what orientation does light travel in?
A
Travels in straight lines
11
Q
what are the ways of becoming visible?
A
light sources like the sun emit their own light, while other objects become visible by reflecting light
12
Q
what light is included in sunlight?
A
sunlight contains all the wavelengths in the spectrum incl. visible light
13
Q
if an object absorbs certain wavelengths of light while others are reflected - what colour does it become?
A
the object takes the reflected colour in sunlight
14
Q
what is responsible for the process of encoding light information?
A
the brain
15
Q
what is the visible spectrum - based on human vision?
A
400-700nm
16
Q
vision of dog+cat?
A
poor colour vision
17
Q
bird vision?
A
see farther into UV wavelengths
18
Q
what type of eyes can animals have?
A
compound eyes, cup eyes, camera eyes
19
Q
what is the lens system of the vertebrae?
A
focuses light from objects in the environment onto the retina at the rear of the eye, where the sensory cells are located
transmits info to the brain where the image is interpreted
20
Q
visual acuity?
A
the ability to see clearly at varying distances
21
Q
refractive index?
A
ratio between the speed of light in a vacuum and in the material
22
Q
why are light rays refracted when passing from one medium to another?
A
due to different in density
and the cornea refracts light entering the eye
23
Q
what happens when parallel light rays strike a biconvex lens?
A
the rays are refracted to a point behind the lens, called the principal focus - the image is inverted
24
Q
principal focus distance?
A
it is the distance between the lens and principal focus
25
Dioptres?
the unit of refractive power of a lens measured in metres
A dioptre is the reciprocal of the principle focal distance (in metres)
26
1 dioptre =
1/PFL
27
the human eye has a refractive power of?
of 6m at rest
28
what is the dioptreic power of the cornea?
40-45 D
29
what is the dioptreic power of the lens?
15-20 D
30
accommodation for children?
+ 14 D
31
accommodation for the elderly?
0.5 D
32
when is the distance between the lens and the focal point the smallest?
when the light source is so far away that the light rays travelling towards the lens are parallel
33
if the light source is moved so close to the lens that the light rays are not parallel, what happens to the focal point?
it is moved farther away
34
when does the distance between the lens and focal point decrease?
decreases with increasing curvature (convexity) of the lens
35
what happens when the ciliary muscles contract?
it releases the tension on the zonulas and the elastic lens return to a more rounded shape suitable for near vision
36
what does the iris control?
controls the quantity of light entering the eye
37
describe the iris in mammals compared to birds/reptiles:
mammals - smooth muscle
birds/reptiles - striated muscle
38
iris innervation?
autonomic innervation
39
what do radial fibres do to the pupil?
they dilate the pupil
40
what do circular fibres do to the pupil?
they constrict the pupil
41
some carnivores have vertical slit pupils, why?
allows for greater light control (135-fold change in ats vs 15-fold change in humans)
Vertical slit enhances distance judgement in brighter light
42
many herbivores have horizontal rectangular pupils, what does this allow?
shields eyes from high sunlight while maintaining panoramic vision to spot predators
eye rotates as head drops to keep pupil horizontal
43
adaptations in constrictor muscles shape to facilitate?
difference in shape
44
aqueous humour produced by?
by the cells in the ciliary body
45
what does aqueous humour do?
bathes lens and cornea providing nutrients
Drains into venous plexus at the filtration angle
Production and drainage must remain balanced
46
vitreous humour is not...?
continually replaced
47
so what is vitreous humour?
gel-like mass of mostly water with a stroma of transparent fibres
functions to retain globe shape
48
two sensory cell types, what are they?
rods and cones
49
rods?
extremely sensitive making it possible to see in weak light (scotopic vision)
50
cones?
convey colour vision
51
retina consists of how many layers?
3
52
what are the 3 layers of the retina?
outermost, innermost, middle layer
53
outermost layer of retina?
closest to the choroid contains the rods and cones
54
innermost layer of retina?
nearest to the vitreous humour, contains ganglion cells
The axons from the ganglion cells form the optic nerve
55
middle layer of retina?
contains several interneuron cell types
56
what interneuron cell types does the middle layer of the retina contain?
bipolar cells
horizontal cells
amacrine cells
57
bipolar cells?
connect sensory cells and ganglion cells
58
horizontal cells?
connects groups of sensory cells
59
amacrine cells?
forms connections between ganglion cells
60
what does light pass through before reaching the sensory cells?
passes through layers of cells
61
what is the fundus?
it is the visible area at the back of the eye when observed through an ophthalmoscope
62
what does the fundus consist of?
optic disc
macula lutea (area centralis.)
fovea centralis
blood vessels
63
Optic disc?
the point where the optic nerve, the convergence of ganglion cell axons, leaves the eye & blood vessels enter
64
what is the optic disc also known as?
it is a blind spot, it doesn't contain any visual receptors
65
what is the macula lutea?
a yellowish area near the posterior pole. it marks the location of the fovea centralis
66
what is the fovea centralis?
a thin, rod-free, cone-packed area
each cone synapses with a single bipolar cells, which in turn synapses with a single ganglion cell
contains NO blood vessels
maximum visual acuity
67
what do retinal vessels supply?
supply the bipolar and ganglion cells
68
what does the choroid plexus supply?
supplies rods + cones
69
some cells contain melanin - why?
in order to absorb scattered light and improve acuity (many diurnal species ad sides of the eye)
70
some cells contain crystals of guanine or riboflavin - what does this do?
these crystals reflet light enhancing vision in low light at expense of acuity (many nocturnal species and central parts of the eye) - tapetum lucidum
71
each rod and cone is divided into?
into outer segment
inner segment
synaptic zone
72
outer segment of rod/cone?
made of modified cilia that form saccules and discs containing photosensitive compounds - continually cycled
73
inner segment of rod/cone?
contains a nuclear region rich in mitochondria
74
synaptic zone of rods/cones?
which synapses with bipolar and/or horizontal cells
75
appearance of rods/cones?
they have a thin, rod-like appearance in the outer segment vs. cones narrower tip compared to base
rods have long outer segments of even thickness, whereas those of the cones are shorter
76
rods?
more numerous usually, more sensitive to low light (scotopic conditions), can be bleached by bright light
77
cones?
more abundant near fovea better in bright light (photopic conditions), colour vision, different types
78
what type of pigments do rods and cones have?
both have photo pigments
79
what do photoreceptor cells do in response to light?
they hyperpolarise
80
cyclic nucleotide-gated Na+/Ca2+ channels kept open by?
by continual binding of cGMP which dissipates the membrane potential
81
how do photoreceptors communicate?
through other cell types of the retina before the brain
82
what is retinal convergence?
many rods converge on a single ganglion cell
individual cones can synapse with individual ganglion cells
83
receptive fields are smaller where?
in the fovea
84
because of convergence - what will a ganglion cell have?
it will have a receptive field fed from the photoreceptors that synapse with it
85
visual processing - what's the journey?
rods/cones --> bipolar cells --> ganglion cells --> action potentials --> optic nerve --> central processing
86
ganglion cells generate?
actional potentials spontaneously
87
the frequency of the action potentials generated by ganglion cells can increase or decrease depending on?
on the stimulus received
the type of ganglion cell receiving the stimulus
88
receptive fields of ganglion cells when there are multiple photoreceptors and bipolar cells?
they have different responses in the centre vs the surround
89
depending on where an image falls on the retina, what photoreceptors are stimulated?
this depends on the response of the ganglion cells whose receptive fields are affected
90
what does difference in stimulation between centre and surround frequency enhance?
enhances edge perception for example
91
colour - animal vision?
most domestic species have two types of colour-sensitive opsins and therefore cone types
but colour perception is independent of spectrum of wavelengths perceived
92
why do animals need depth and field for vision - PREDATOR?
[redator: for hunting/climbing, animals need depth perception
achieved with front-facing eyes and overlapping fields of vision
93
why do animals need depth and field for vision PREY?
prey animals need a wide field of view to spot predators
laterally located eyes increases the field of view at expense of depth perception
94
both eyes have what fields?
have a right and left field
95
the right side of each retina does what?
it collects light from the left visual field and vice versa
96
the nasal (medial) ganglia cross over where?
at the optic chiasm
97
what about the temporal (lateral) ganglia at the optic chiasm?
does not cross over at the optic chiasm
98
contra-lateral processing?
this means that both sides of the brain process images from both eyes called this...
99
What are ciliary bodies?
these are thickened anterior parts of the choroid, contain circular & longitudinal muscle fibres, produce aqueous humour
100
what is the lens?
adjustable focus mediated by the ciliary bodies
101
what is the iris?
pigmented, opaque, contains constrictors of the pupil (circular) & dilators of the pupil (radial)
102
what is vitreous humour?
a gelatinous substance between the lens & retina
103
what is aqueous humour?
a clear liquid that nourishes the cornea and lens
104
what is the filtration angle?
at the junction between the iris and cornea, drains aqueous humour
105
what is the filtration angle also known as?
Schlemm's canal
106
What is the wall of the globe/eyeball made up of?
sclera
cornea
choroid
retina
107
what is the sclera?
outer layer of tough connective tissue which merges with the cornea
108
what is the cornea?
transparent layer at front, for physical protection, refraction of light and highly sensitive
109
what is the choroid?
vascularised middle layer
110
what is the retina?
innermost layer containing sensory cells and neurons
111
112
the distance between the lens and the focal point decreases with what?
with increasing curvature (convexity) of the lens
113
parallel light rays passing through a divergent (concave) lens are...
not focused, but will instead be spread or diverged
114
an object that reflects light constitutes what?
constitutes a mosaic of many point sources of light
light from each of these point sources can be focused onto a screen behind the lens, thus forming a reduced and inverted image of the object
115
what is accommodation?
it is the process by which the curvature of the lens is increased to focus on a near object
116
when looking at a near object, what happens to the muscles in order to focus images?
the ciliary muscles contract, the distance between the edges of the ciliary bodies decreases and the lens ligaments relax, thus the lens becomes more convex
117
what type of process is accommodation?
it is an active process that requires muscular actions
118
at rest. objects closer than 6m to the eye appear?
they appear blurred
the blurriness is diminished by accommodation
119
what attaches the elastic lens to the circular ciliary muscles?
it is attached by the zonulas which is made of inelastic fibres
120
what happens when the ciliary muscles are relaxed?
the zonulas pulls tight and keeps the lens flattened for distant vision
121
what happens when the ciliary muscles contract?
it releases the tension on the zonulas and the elastic lens returns to a more rounded shape suitable for near vision
122
the outer layer to the retina is the?
it is the retinal pigmented epithelium (tapetum lecidum)
and it is attached to choroid overlying retinal vessels
123
why do some cells contain melanin?
to absorb scattered light and improve acuity (many diurnal species and sides of the eye)
124
what do photoreceptor cells do in response to light?
they hyperpolarise in response to light
125
how are cyclic nucleotide-gated Na+/Ca2+ channels kept open?
by continual binding of cGMP which dissipates the membrane potential
126
how does light cause photoreceptor cells to hyperpolarise in response to light?
by removing cGMP and closing those CNG channels and hyperpolarising the photoreceptor membrane
127
rods and cones contain pigments formed from two components, what are they?
retinal and opsins
128
What is retinal?
it is an aldehyde of vitamin A
11-cis-retinal is converted to all-trans-retinal when exposed to light
129
what is opsins?
(proteins) modify retinal's sensitivity to different light wavelengths e.g. rhodopsins in rods (most mammals also have two kinds of colour sensitive opsins, humans and old-world primates have three
130
causing photoreceptors to hyperpolarise results in less?
less glutamate neurotransmitter
131
conversion to all-trans-retinal results in a conformational change of...
of the opsin which activates a G-protein (transducin) which activates a phospohodiesterase which converts cGMP to GMP by removing the cyclic nucleotide that keeps the CNG channels open
132
what series of actions does activated transducin result in?
activation of phosphodiesterase, hydrolysis of cGMP, closure of Na+ channels, Hyperpolarization and then reduced release of transmitter
133
what causes opsin to be activated?
All-trans-retinal and light is needed
134
what visual fields do eyes have?
they both have a right and left visual field
135
where does the right side of the retina collect light from?
from the left visual field and vice versa
136
what is contra-lateral processing?
this is where both sides of the brain process images from both eyes called contra-lateral processing
137
where does the right side of the visual cortex process images on?
in both right retinas (therefore both left visual field)
138
what percentage of fibres cross over in humans?
50%
139
what percentage of fibres cross over in cats?
30%
140
what percentage of fibres cross over in dogs?
25%
141
what percentage of fibres cross over in horses?
15%
142
what does the number of fibres crossing over measure?
degree of binocular vision
143
what is the lateral geniculate nucleus?
it is defined as a structure located in the diencephalon, consisting of six cellular layers that receive input from the optic tract fibres and optic radiations
144
what is the diencephalon?
it is the region of the embryonic vertebrate neural tube that gives rise to anterior forebrain structures incl. thalamus, hypothalamus, posterior portion of pituitary gland and pineal gland. it encloses a cavity called the third ventricle
145
in the lateral geniculate - what forms the geniculocalacrine tracts?
fibres from one nasal hemiretina synapse with the temporal fibres of the other retina
146
where do the geniculocalacrine tracts conduct sensory information to?
to the visual cortex of the occipital lobe
147
how do we get the image that we see?
the inverted image that hits the retina is corrected
the visual cortex constructs images in the brain from the stimuli received, the 2 eyes apart have slightly different images
The stereoscopic vision leads to the production of a 3D image and gives depth perception
148
