Physiology Of Vision Flashcards
What is cornea
Dome-shaped window which provides the majority of the focusing power of the eye. It is clear and avascular and continuous with the sclera.
What is iris
Thin, elastic, circular structure responsible for controlling the diameter and size of the pupil and thus the amount of light reaching the retina. The iris determines eye colour.
What is lens
Crystalline structure which aids focusing of light onto the retina. Its curvature is altered and controlled by the cilliary muscles and the zonule fibres. Clouding of the lens causes cataracts.
Describe aqeous humour
Transparent gel like fluid which fills the anterior part of the eye between the lens and the cornea. It is formed by the ciliary body and drains via the trabecular meshwork. It maintains intraocular pressure (IOP), nourishes and removes debris from the avascular anterior segments of the eye.
How is glaucoma caused?
Glaucoma (open angle glaucoma) is caused by an increase in IOP commonly caused by degeneration of the trabecular meshwork reducing drainage of the aqueous humor.
Ciliary body Zonule fibres
What is sclera
Fibrous white opaque connective tissue layer covering 5/6 of the eye ball. Continuous with the transparent cornea. Both the sclera and cornea are made of Type 1 collagen.
What is choroid
Vascular connective tissue layer. Contains melanin pigment and nourishes outer 1/3 of the retina. Continuous with the ciliary body.
What is retina
Light sensing layer composed of the retinal pigment epithelium and the neural retina.
What is vitreous humor
Transparent gel which provides structure to the eyeball.
How is light passed into eye
Light passes through all the neural retinal cells to reach the photoreceptors at the back of the retina.
How is light focused in eye
Light is focused by the cornea and the lens and then passes through the vitreous humor to the retina.
What are the two types of photoreceptors in retina
Cones – colour system – daylight.
Rods – no colour – low light levels
Where does process of absorption occur?
Process of absorption occurs in the outer segments of the photoreceptors
What does outer segment contain of photoreceptors?
The outer segment contains a stack of membranous disks which contain the light sensitive photopigments.
What is the difference in appearance of rods and cones?
Rod photoreceptors have a long, cylindrical outer segment containing many disks, whereas cones have a short, tapering outer segment with few membranous disks.
What does photoreceptors do?
Photoreceptors transduce light energy into changes in membrane potential
What are the retinal cells?
Horizontal, bipolar, amacrine and ganglion cells
What do the Horizontal, bipolar, amacrine and ganglion cells do?
pre-processing’ before sending the visual information through the optic nerve.
Each photoreceptor is in synaptic contact with which two types of retinal neurones?
bipolar cells and horizontal cells.
What do bipolar cells do?
Bipolar cells create the direct pathway from photoreceptors to ganglion cells.
What do horizontal cells do?
Horizontal cells feed information laterally in the outer plexiform layer to influence neighbouring cells.
What do amacrine cells do?
Amacrine cells control and modulate the majority of the inputs to the retinal ganglion cells and the ganglion cell responses.
What are the major glial cells of retina?
Müller cells
Where are Müller cells located?
They are located in the neuronal retina and span its entire depth.
Role of muller cells
Müller cells ensheath all retinal neurons and contribute significantly in mediating their functions and stability.
What is the pigment in rods called?
Rhodopsin
What are the 3 types of opsin in cones?
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The pigment in all rods is called rhodopsin (rod – opsin).
In each cone there is one of three types of opsins; red, green and blue. Each cone opsin has a different spectral sensitivity:
How is colour perception determined in cones?
The pigment in all rods is called rhodopsin (rod – opsin).
In each cone there is one of three types of opsins; red, green and blue. Each cone opsin has a different spectral sensitivity:
Colour perception is determined by the relative contribution of the blue, green and red cones to the retinal signal.
What is colour blindness?
Colour blindness or colour deficiency is the inability or decreased ability to see colour or perceive colour differences
What is achromatopsia
Total colour blindness
This is rare (3/100,000) and is caused by genetic mutations which result in only rod photoreceptors being functional. Affected patients see no colour but also have poor vision in bright light due to the sensitivity of the rod photoreceptors (one photon activates a response). They also have poor visual acuity due to lack of cones.
What is dichromacy?
One of the three basic colour mechanisms is not functioning.
Which dichromacy is x linked?
Red-green colour blindness”
Deuteranopia (mutation of green opsin) Protanopia (mutation of red opsin)
What is blue yellow colour blindness called?
Tritanopia
What are males mostly affected by in colour blindness.
affected by red-green deficiency due to X-linked inheritance. The red and green opsin genes are on the X-chromosome.
What percentage of men have mutant green opsin ?
5%
What percentage of men have mutated red opsin?
1%
What is phototransduction?
Phototransduction is the process by which light is converted into electrical signals in the rod and cone cells.
What are the photo pigment protein in photoreceptor outer segments?
Phototransduction pathway
- protein coupled receptors which are chemically attached to a vitamin A derived chromophore called 11-cis retinal.
What happens to the G- protein coupled receptors which are chemically attached to a vitamin A derived chromophore called 11-cis retinal. ? Phototransduction pathway
. Light photons strike this chromophore and cause a conformational change to all-trans retinal. This isomerization triggers the visual process by activating a signal transduction cascade.
Describe visual cycle
A photon of light activates the 11-cis-retinal to change it to all-trans–retinal.
All-trans-retinal is not photosensitive so the opsin must release this chromophore and replace it with more 11-cis-retinal. This process is called the visual cycle.
What would happen if the proteins in visual cycle was mutated?
Inherited blindness
What is a vitamin a derivative?
11-cis-retinal is a vitamin A derivative!
What is 11-cis-retinal depletion?
See light brighter than normally is
Describe signal transduction in photoreceptors
- Guanosine diphosphate (GDP)-bound inactive transducin will exchange GDP for guanosine triphosphate (GTP) following interaction with activated rhodopsin.
- The activated GTP-bound α-subunit of transducin dissociates and activates cyclic guanosine monophosphate (cGMP) phosphodiesterase (PDE).
- PDE hydrolyzes cGMP to 5’GMP. The result is decreased levels of cGMP in the cytoplasm. This causes the closing of cGMP-gated ion channels leading to membrane hyperpolarization.
- In photoreceptors, the neurotransmitters are released in depolarized membranes (dark light).
Why can activation of single rhodopsin by single photon be sufficient to cause a significant change in membrane conductance?
This is possible due to amplification steps present in the transduction cascade.
A single photoactivated rhodopsin catalyses the activation of 500 transducin molecules.
What are the structures that transmit the signals received by the retina to the visual cortex for processing and decoding
retina, optic nerves, optic chiasm, optic tracts, lateral geniculate nuclei, optic radiations and the visual cortex in the occipital lobe.
What is the first region of visual processing in cortex?
Striate cortex or V1
What are the 2 cortical streams of visual processing?
Striate cortex towards
– parietal lobe: visual motion.
Striate cortex towards temporal lobe: recognition of objects.
What is lateral inhibition?
The capacity of excited neurons to reduce the activity of their neighbours
How is direct input given by receptive field?
DIRECT input of photoreceptor to bipolar cell from receptive field centre
How is indirect input from receptive field given?
INDIRECT input from photoreceptor to bipolar cells via horizontal cells from receptive field surround
