2. Special senses/ the eye and seeing Flashcards
Trachoma is an infectious disease caused by…
Chlamydia
Top 4 causes of visual impairment?
- URE (uncorrected refractive error)
- Cataract
- Glaucome
- Trachoma, AMD
Top 4 causes of blindness?
- Cataract
- Glaucoma
- AMD
- CO (cornea opacity)
What are the 5 layers of the cornea?
Is 6 when including the tear film
(Outer) + tear film Epithelium Bowman's membrane Stroma Descemet's membrane Endothelium (keep at keeping cornea clear)
What is the major light focussing element of the eye?
The cornea at 40D of “fixed power”
Layers of the tear film?
- LIPID LAYER
Oily lipid outer layer to reduce evaporation - WATER LAYER
Aqueous middle layer containing lysozymes and antibodies - MUCIN LAYER
Mucin layer containing glycoprotein, wets the cornea and allows good coverage
Function of tear film?
- To wash the eye of debris
- To provide the dissolved o2 to the cornea external epithelium
- Lubrication of the cornea
what are the different refractive elements of the eye?
• Cornea – largest element (40D) – Interfaces with air with low RI • Lens – lesser element (20D) – Interfaces with aqueous – similar RI – But can vary in power (accommodation) • Whole Eye Ball – About 60D
How to calculate power?
1/focal length(m)
or
(n2-n1)/ r
Note: radius of curvature = “r”
and “n” is the difference of refractive ability of the object. N2 = cornea, N1 is of the medium it is in.
Why is underwater vision poor?
Due to similar refractive ability of cornea and water. Need to be a difference in “power = (n2-n1)/ r” to produce a greater power.
Hence the light cannot be focused on the retina as corneal refraction has been dampened
When is accommodation needed?
To look at something UP CLOSE
What actions occur in accommodation?
To look at something upclose the eye must:
Constrict and turn in. The lens must narrow so ciliary muscles relaxes to allow the zonule fibres to slack off
What are the sources of the dual innervation of the iris?
PS fibres from CN III –> Cholinergic stimulation of pupillary CONSTRICTOR
Sympathetic fibres from the superior cervical ganglion –>Adrenergic stimulation of pupillary DILATOR
What is the near triad?
Miosis
Convergence
Accommodation
i.e. actions of the eye when focusing on something near
URE?
Uncorrected refractive error
Main type of URE?
Presbyopia (failure to accommodate when you are older) is the main type of URE
Myopia?
Short-sidedness
Hypermetropia? Associated with?
Long sightedness
Hypermetropia (long sightedness) is associated with squint and lazy eye (amblyopia) and acute closed angle glaucoma
Cataract treatment?
SIC (Small incision cataract) surgery
Phacoemulsification surgery
Structure of the retina?
(outer) Layers: Outer nuclear layer Ganglion cell layer Plexiform layers (amacrine, lateral and bipolar cells) Outer nuclear layer Rod/cones Pigmented epithelium (inner)
Note: In fovea the neural components are moved to one side and the degree of convergence ono ganglion cells is minimal
Information from the rods and cones is converged to ganglion cells, lateral cells and amacrine to provide initial processing of the visual signal
Cells present in the plexiform layer of the epithelium?
Amacrine cell
Bipolar cell
Lateral cells
Difference between rods and cones?
Number, convergence, types, sensitivity, distribution
More rods> cones in the retina
Rods have a higher convergence to ganglion cells
Rods have one type (vision in greyscale, cones have three types (blue + green + red)
Rods are 30x more sensitive
Rods are widespread across retrina, cones are concentrated in macula. Specifically the fovea
Why is central and peripheral vision different?
Central vision has colour and is sharp
Peripheral is b/w and blurry
Due to the ganglion to rod/cone ratio i.e. ganglion convergence
what are opsins?
Opsins are a group of light-sensitive proteins found in photoreceptor cells of the retina. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical signal, the first step in the visual transduction cascade.
Wavelength of visible light?
400-700nm
Each opsin gives sensitivity to…
a different range of wavelengths of light
At their most sensitive wavelength our eyes can detect…
as little as 5 photons of light
Photoreceptor are depolarised when? Hyperpolarised when?
Photoreceptor are depolarised when it’s dark
Hyperpolarised when it’s light
Cause of light exposure to retinal?
Causes the protein RETINAL to change from 11-cis isomer to all-trans isomer
This is the protein found in all cones and rods.
When unstimulated it is kinked and bound to opsin. (purple appearance)
When stimulated it’s straight and detached. (appears clear when bleached)
Different types of opsin?
Each specific to different types of cone.
3 colours with rod retinal binding an opsin called a rhodopsin
What is the benefit of the adaptability of the eye?
Vision can be created in bright sunlight and in star lit sky
Means that the eye can function in environments which are 9 fold order of magnitude different in brightness
What is the result of “cone fatigue” and why does it happen?
- When you stare at a specific colour for too long, the cells that detect that frequency of light will get faAgued.
- The afer image is a result your photoreceptors not being ‘in balance’.
- As the photoreceptors become less Ared, which takes between 10 and 30 seconds, the balance is recovered and the afer image disappears.
Why do staff in a submarine work in red light?
To have a strategic effect via cone fatigue
It only stimualtes the cones (allowing the staff to see the charts) leaving the rods dark adapted (allowing them to see lights far away on the horizon)
What is the consequence of emmert’s law? Why?
When looking at an image that stimulates cone fatigue, after if you look at a blank sheet:
- Far away the object feels bigger
- At near the object feels smaller
Why?
Things the same size on the retina appear a different size depending on the distance you are viewing them and how far away you ‘think’ you are viewing them
When causes cone fatigue in ONE EYE, what is the effect?
When looking at a blank sheet after there is no imagine produced in the covered eye.
This proves that it is the retina not the brain i.e. a “pre-chiasmal” phenomenon
What is meant by “dark current”?
The release of neurotransmitter from the retina due to depolarisation in darkness
Difference between effect of dark and light on retina and signal processing?
- Light hyperpolarises photoreceptors (inhibits action potentials)
- Dark causes depolarisation and release of neuro-transmitter –> Dark current
What are the photoreceptors?
Rods and cones
Bipolar cells:
Where found?
Types?
Role?
Found in the retina in the inner nuclear layer.
Bipolar cells receive inputs from a set of photoreceptor cells that define the bipolar cell’s receptive field. The neurotransmitter released from all photoreceptor cells is GLUTAMETE. Because glutamate release is DECREASE upon exposure to LIGHT (light = low glutamate)
Types: ON-centre or OFF-centre
ON= Some bipolar cells respond to glutamate by hyperpolarization, or inhibition. In the dark, the photoreceptors maximally release glutamate and these bipolar cells are maximally hyperpolarized. In the light, the photoreceptors are hyperpolarized and they release less glutamate. These bipolar cells are called on-center bipolar cells because they are active when the light is on.
OFF= Glutamate release is decreased upon exposure to light, a bipolar cell that responds to glutamate by excitation will be excited when the light is off. These are called off-center bipolar cells because they are active when the light is off in the center of their receptive field
Connect bipolar cells and allow for summaAon of informaAon to allow detecAon of edges and contrast, which cells?
Role of horizontal and amacrine cells
Where does processing of the signals detected by the eye take place?
At the level of the retina
then…
Ganglion cell transfers the retinal information to the brain; optic nerve –> chiasm –> optic tract
First synapse is at the lateral geniculate nucleus (LGN) which is part of the thalamus.
Role of LGN?
lateral geniculate nucleus
Major relay station for sensory information
Optic nerve is a collection of?
All the ganglion cells
Exit of optic nerve?
Through back of the eye through a hole in the sclera.
Optic nerve head can be seen at the back of the eye (aka optic disc)
Signs of glaucoma that can be seen when looking at the retina?.
Glaucoma
- Loss of nerve retinal ring
- loss of axons due to damage to optic nerve
- optic cup enlarges
