Physiology of Vision Flashcards
why do animals use their senses?
to perceive the environment around them
what does perception of different touch sensations involve?
texture, pressure, stretch, heat, cold and pain
what does perception of different chemicals involve?
taste and smell
what does perception of waves in a medium involve?
sound
what does perception of light involve?
intensity, wavelength, images, movement
environmental stimulus modifies a tissue structure, what does modified stimulus affect?
the membrane of a sensory cell, signal transduced/amplified and encoded to nerve fibres and interpretation by the brain
what is a photon?
it is a particle of electromagnetic radiation
which photons contain more energy?
photons of shorter wavelengths contain more energy than photons with shorter wavelengths
how much of the electromagnetic does ‘visible’ light occupy?
it occupies only a minute portion of the electromagnetic spectrum
in what orientation does light travel in?
Travels in straight lines
what are the ways of becoming visible?
light sources like the sun emit their own light, while other objects become visible by reflecting light
what light is included in sunlight?
sunlight contains all the wavelengths in the spectrum incl. visible light
if an object absorbs certain wavelengths of light while others are reflected - what colour does it become?
the object takes the reflected colour in sunlight
what is responsible for the process of encoding light information?
the brain
what is the visible spectrum - based on human vision?
400-700nm
vision of dog+cat?
poor colour vision
bird vision?
see farther into UV wavelengths
what type of eyes can animals have?
compound eyes, cup eyes, camera eyes
what is the lens system of the vertebrae?
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
visual acuity?
the ability to see clearly at varying distances
refractive index?
ratio between the speed of light in a vacuum and in the material
why are light rays refracted when passing from one medium to another?
due to different in density
and the cornea refracts light entering the eye
what happens when parallel light rays strike a biconvex lens?
the rays are refracted to a point behind the lens, called the principal focus - the image is inverted
principal focus distance?
it is the distance between the lens and principal focus
Dioptres?
the unit of refractive power of a lens measured in metres
A dioptre is the reciprocal of the principle focal distance (in metres)
1 dioptre =
1/PFL
the human eye has a refractive power of?
of 6m at rest
what is the dioptreic power of the cornea?
40-45 D
what is the dioptreic power of the lens?
15-20 D
accommodation for children?
+ 14 D
accommodation for the elderly?
0.5 D
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
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
when does the distance between the lens and focal point decrease?
decreases with increasing curvature (convexity) of the lens
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
what does the iris control?
controls the quantity of light entering the eye
describe the iris in mammals compared to birds/reptiles:
mammals - smooth muscle
birds/reptiles - striated muscle
iris innervation?
autonomic innervation
what do radial fibres do to the pupil?
they dilate the pupil
what do circular fibres do to the pupil?
they constrict the pupil
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
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
adaptations in constrictor muscles shape to facilitate?
difference in shape
aqueous humour produced by?
by the cells in the ciliary body
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
vitreous humour is not…?
continually replaced
so what is vitreous humour?
gel-like mass of mostly water with a stroma of transparent fibres
functions to retain globe shape
two sensory cell types, what are they?
rods and cones
rods?
extremely sensitive making it possible to see in weak light (scotopic vision)
cones?
convey colour vision
retina consists of how many layers?
3
what are the 3 layers of the retina?
outermost, innermost, middle layer
outermost layer of retina?
closest to the choroid contains the rods and cones
innermost layer of retina?
nearest to the vitreous humour, contains ganglion cells
The axons from the ganglion cells form the optic nerve
middle layer of retina?
contains several interneuron cell types
what interneuron cell types does the middle layer of the retina contain?
bipolar cells
horizontal cells
amacrine cells
bipolar cells?
connect sensory cells and ganglion cells
horizontal cells?
connects groups of sensory cells
amacrine cells?
forms connections between ganglion cells
what does light pass through before reaching the sensory cells?
passes through layers of cells
what is the fundus?
it is the visible area at the back of the eye when observed through an ophthalmoscope
what does the fundus consist of?
optic disc
macula lutea (area centralis.)
fovea centralis
blood vessels
Optic disc?
the point where the optic nerve, the convergence of ganglion cell axons, leaves the eye & blood vessels enter
what is the optic disc also known as?
it is a blind spot, it doesn’t contain any visual receptors
what is the macula lutea?
a yellowish area near the posterior pole. it marks the location of the fovea centralis
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
what do retinal vessels supply?
supply the bipolar and ganglion cells
what does the choroid plexus supply?
supplies rods + cones
some cells contain melanin - why?
in order to absorb scattered light and improve acuity (many diurnal species ad sides of the eye)
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
each rod and cone is divided into?
into outer segment
inner segment
synaptic zone
outer segment of rod/cone?
made of modified cilia that form saccules and discs containing photosensitive compounds - continually cycled
inner segment of rod/cone?
contains a nuclear region rich in mitochondria
synaptic zone of rods/cones?
which synapses with bipolar and/or horizontal cells
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
rods?
more numerous usually, more sensitive to low light (scotopic conditions), can be bleached by bright light
cones?
more abundant near fovea better in bright light (photopic conditions), colour vision, different types
what type of pigments do rods and cones have?
both have photo pigments
what do photoreceptor cells do in response to light?
they hyperpolarise
cyclic nucleotide-gated Na+/Ca2+ channels kept open by?
by continual binding of cGMP which dissipates the membrane potential
how do photoreceptors communicate?
through other cell types of the retina before the brain
what is retinal convergence?
many rods converge on a single ganglion cell
individual cones can synapse with individual ganglion cells
receptive fields are smaller where?
in the fovea
because of convergence - what will a ganglion cell have?
it will have a receptive field fed from the photoreceptors that synapse with it
visual processing - what’s the journey?
rods/cones –> bipolar cells –> ganglion cells –> action potentials –> optic nerve –> central processing
ganglion cells generate?
actional potentials spontaneously
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
receptive fields of ganglion cells when there are multiple photoreceptors and bipolar cells?
they have different responses in the centre vs the surround
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
what does difference in stimulation between centre and surround frequency enhance?
enhances edge perception for example
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
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
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
both eyes have what fields?
have a right and left field
the right side of each retina does what?
it collects light from the left visual field and vice versa
the nasal (medial) ganglia cross over where?
at the optic chiasm
what about the temporal (lateral) ganglia at the optic chiasm?
does not cross over at the optic chiasm
contra-lateral processing?
this means that both sides of the brain process images from both eyes called this…
What are ciliary bodies?
these are thickened anterior parts of the choroid, contain circular & longitudinal muscle fibres, produce aqueous humour
what is the lens?
adjustable focus mediated by the ciliary bodies
what is the iris?
pigmented, opaque, contains constrictors of the pupil (circular) & dilators of the pupil (radial)
what is vitreous humour?
a gelatinous substance between the lens & retina
what is aqueous humour?
a clear liquid that nourishes the cornea and lens
what is the filtration angle?
at the junction between the iris and cornea, drains aqueous humour
what is the filtration angle also known as?
Schlemm’s canal
What is the wall of the globe/eyeball made up of?
sclera
cornea
choroid
retina
what is the sclera?
outer layer of tough connective tissue which merges with the cornea
what is the cornea?
transparent layer at front, for physical protection, refraction of light and highly sensitive
what is the choroid?
vascularised middle layer
what is the retina?
innermost layer containing sensory cells and neurons
the distance between the lens and the focal point decreases with what?
with increasing curvature (convexity) of the lens
parallel light rays passing through a divergent (concave) lens are…
not focused, but will instead be spread or diverged
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
what is accommodation?
it is the process by which the curvature of the lens is increased to focus on a near object
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
what type of process is accommodation?
it is an active process that requires muscular actions
at rest. objects closer than 6m to the eye appear?
they appear blurred
the blurriness is diminished by accommodation
what attaches the elastic lens to the circular ciliary muscles?
it is attached by the zonulas which is made of inelastic fibres
what happens when the ciliary muscles are relaxed?
the zonulas pulls tight and keeps the lens flattened for distant vision
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
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
why do some cells contain melanin?
to absorb scattered light and improve acuity (many diurnal species and sides of the eye)
what do photoreceptor cells do in response to light?
they hyperpolarise in response to light
how are cyclic nucleotide-gated Na+/Ca2+ channels kept open?
by continual binding of cGMP which dissipates the membrane potential
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
rods and cones contain pigments formed from two components, what are they?
retinal and opsins
What is retinal?
it is an aldehyde of vitamin A
11-cis-retinal is converted to all-trans-retinal when exposed to light
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
causing photoreceptors to hyperpolarise results in less?
less glutamate neurotransmitter
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
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
what causes opsin to be activated?
All-trans-retinal and light is needed
what visual fields do eyes have?
they both have a right and left visual field
where does the right side of the retina collect light from?
from the left visual field and vice versa
what is contra-lateral processing?
this is where both sides of the brain process images from both eyes called contra-lateral processing
where does the right side of the visual cortex process images on?
in both right retinas (therefore both left visual field)
what percentage of fibres cross over in humans?
50%
what percentage of fibres cross over in cats?
30%
what percentage of fibres cross over in dogs?
25%
what percentage of fibres cross over in horses?
15%
what does the number of fibres crossing over measure?
degree of binocular vision
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
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
in the lateral geniculate - what forms the geniculocalacrine tracts?
fibres from one nasal hemiretina synapse with the temporal fibres of the other retina
where do the geniculocalacrine tracts conduct sensory information to?
to the visual cortex of the occipital lobe
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
