S3: The Eye and Visual Pathways Flashcards
How does the anatomy of the eye give it a stable shape?
- The outer coating (sclera) of the eye is flexible so it will bend but won’t stretch. The sclera continues as the transparent cornea at the front.
- The shape of the eye is maintained by internal pressure by the aqueous fluid. The aqueous humour fills the front of the eye and the vitreous humour is the transparent jelly at the back. The aqueous humour diffuses back to vitreous to hydrate it.
- As long as the aqueous humour is being produced and removed at the right rate, the pressure in the eye will remain stable and the shape of the eye is kept.
How does the anatomy of the eye give it a the ability to focus a image?
It has an optical apparatus:
- Lens sits in central axis of the eye and held in place by a ring of suspensory ligaments attached to ciliary bodies.
- The cornea us a powerful lens that refracts the right rays.
- The iris (coloured) has a pupil at its centre.
How does the optic apparatus work?
- Light reflects off the point we are looking at and some light rays will hit and pass straight through the cornea and some will hit the lens which refracts light rays and bends them inwards into a single point on the retina. The cornea is also a powerful lens and the lens is less powerful in comparison.
- However, the lens can change shape allowing variable fine focus allowing us to see different distances by changing focal power (becoming thicker and thinner).
- Our pupil becomes larger and smaller to adapt to different levels of light so our iris controls how much light enters our eye by blocking it. Our retina also has the capacity to adapt to light and it is stronger than our pupil.
- Our pupil has one important characteristic - it cuts out light rays that would otherwise go to edge of lens. Our lens is just a bag of proteins and more lens used, the less focused the light rays will be so to have good focus we need to use to centre of the lens which is what the iris and pupil do. So during darkness, the pupil opens up to get more light in eye so image is bright enough to see but it does it at the expense of the quality and focus of the image (it gets blurrier).
Describe the retinal layers in the anatomy of the eye and how they are needed to capture a pattern of light
- The neural retina develops as part of the neural tube so is a part of the brain, the optic nerve hence is a central nerve (of CNS). The axons in the optic nerve are myelinated by oligodendrocytes rather than Schwann cells and are therefore vulnerable to MS.
- Photoreceptors in the retina are the light sensitive cells and are linked via afferents to the retina ganglion cells (these have axons which run along the surface of retina which bundle and form optic nerve which then carry signal back to the brain).
- The neural retina ends at the ora serrata (serrated edge). This is a bit of the retina but is non-neural. The non-neural retina extends back and goes behind the neural retina. So the neural retina is resting on this, here is called the retinal pigment epithelium.
Describe layers of the cross section through the peripheral retina
- At the bottom is the vitreous layer that is inside the eye.
- The neural retina includes everything above this up until the pigment epithelium , behind which is the blood rich choroid.
- The photoreceptors are on the outside of the globe and the light passes from the inside to the outside through the retina.
- The layers of the retina are are (inside eye to outside): the ganglion cells, interneurone layer, photoreceptor layer and the pigment epithelium.
What is the primary role of the retina?
To detect light and this is done by photoreceptors.
What are the two photoreceptors and their role?
- Cone photoreceptors are used for daylight vision (works best in bright light and have a wider range of illumination). These can constantly reset their sensitivity so can work in different levels of light.
- Rod photoreceptors are used for night vision, also called scotopic vision (work best in dim light and in bight light their mechanism saturates so become useless). These are very sensitive so they work at night.
What is the secondary role of the neural retina?
Transmit information they detect to the brain. This is done by afferents and in the visual system, these are the ganglion cells that sit in an layer in the retina. The axons of the ganglion cells run over the surface of the retina and head to the optic nerve.
What does the bipolar interneurone do?
It connects the photoreceptors to the ganglion cells. Interneurones are found in the inner nuclear layer which is found between the photoreceptor layer and the ganglion cells.
What forms the ganglionic cell receptive field?
The rays of the visual image that enter our eye that are detected by these cones ( a few of them), form the retinal receptive field for this ganglionic one ganglion cell. Changes in brightness in this field will ultimately be sent to this ganglion cell directly to relay back to the brain.
What is the inherent weakness of the eye in the peripheral retina?
- When the light enters the eye it has to pass through the vitreous and multiple layers of the retina before it reaches the cones. It is also the outer portion of the cones which actually detect the change in illumination.
- The multiple layers however not 100% transparent rather they are translucent and hence light gets scattered.
- This is a weakness as the light that hits the retina being detecting is blurred because it passes through the inner segments of the retina first.
- Where the light from the visual image is blurred there is no point in sampling that particular visual image and sending it off because it is such poor quality.
- Instead, the image will be sampled by cones that are separated by a large number of rods.
- These pool of cones will converge input onto a single ganglion cell, so the receptive fields in the peripheral retina (where this is most present) are very large and because of this the resolution of the image gets poorer as you go further out.
Describe function of the peripheral retina
- Peripheral retina is only capable of coarsed vision hence the image we see is optically blurred.
- This is due to the fact that cone photoreceptors in the periphery are large and spaced widely apart (separated by rods) to try and gather as much of the scattered/blurred light as possible.
- The bipolar cells pick up input from multiple cones. These cones gather this blurred light and converge it onto a single ganglion cell to transmit the information.
- This is why when reading we can only read well with the centre of our eyes, in the periphery we will be unable to pick out words as the image is blurred due to the large receptive fields.
- The visual receptive fields get lower so resolution falls but also the wiring gets more disorganised and less specific. This is why we don’t have good colour vision in the periphery.
- So convergence gets larger, pixel size bigger and focus worse as we go further out into the periphery of the retina.
What is the central retina?
- Vision from our central retina produces clear vision.
- Within the central retina is an area of yellow pigment called macula lucida. This catches some short wave length light which is damaging so it protects the fovea.
- Within the macula lucida is the fovea centralis. This is specialised and a small area of the eye. The fovea has no overlying blood vessels or capillary bed.
- Surrounding the macula lucida is the global central retina.
Describe how the fovea is specialised for its function
The fovea is specialised because it contains a small region, only 0.35 mm across where the ganglion cells and interneurones have been pushed back and halted to leave a foveal pit.
- This directly exposes the photoreceptors and means that light that passes into the eye and vitrous humour does not have to pass through multiple layers e.g. nerve cells or dense capillary network first. The light is therefore not scattered.
- To further enhance the resolution the cones in the foveal pit are very slender thus a huge number are packed together. Each photoreceptor then synapses onto a single ganglion cell, so there is no convergence. This means the fine image (because no scattering!) detected by each single photoreceptor is signalled to a single ganglion cell so there is no “noise” added to the fine image and instead is signalled back to the brain completely as the fine image.
- There are no rods and no blood vessels.
- The fovea also has the best colour vision because tiny receptor fields can keep input from the difference types of photoreceptors separate (red, green and blue cone).
- The foveal pit is the only part of the eye where vision is very well focused/ resolution and it visualises about the size of your thumbnail at arms-length so the fovea represents a tiny amount of our visual field.
What would damage to fovea lead to?
If there is damage to the fovea or improper function this would have very serious problems and one would be registered blind.
Describe the primary visual pathway
The primary visual pathway is the pathway from the eye to the thalamus to the primary visual cortex.
- This starts at the retinal ganglion cells which project back to the lateral geniculate nucleus (LGN) in the thalamus.
- From here, axons go back to the primary visual cortex which is located in the occipital cortex, or more precisely. embedded in the calcarine sulcus. The axons that go back from the LGN are part of the optic radiation (optic nerve, tract and chiasm).
What other pathway do we have alongside the primary visual pathway?
- Pathway that is associated with reflexes and orientating responses.
- The nuclei involved in this reflexive movement and control of the eye are in the brainstem. The most prominent one being the superior colliculi.
- The superior colliculi are involved in reaching movements, turning the head and eyes towards a stimulus requiring attention and navigating in locomotion.
- Hence the ganglion cells that project back to the lateral geniculate nucleus have branches that go to the brainstem to drive these reflexive processes but these brainstem structures are not directly involved in visual perception.
What is the retinatopic cortex?
Neighbouring points within the retina project to neighbouring points within the visual cortex. The fovea has a far larger area of cortex, as does the macula.
Describe how the image is inverted by the optics
- The nasal axons (from nasal half of the retina) have to cross at the chiasm and run along side the ones at equivalent location of other retina. Axons from the right temporal hemiretina join axons from the left nasal hemiretina in the right optic tract synapsing at the right LGN which projects to the right primary visual cortex.
- The right temporal retina and left nasal retina will be looking at the left visual field, thus the right optic tract, LGN and visual cortex are involved in dealing with the left visual field.
- RHS visual image is mapped to L visual cortex in brain. So each half of the brain is looking at the contralateral side of the world.
- Image is flipped when it passes through the visual pathway, The LHS visual world equated to RHS of two retina to RHS of the brain.
What gives up binocular vision?
The two fields (R and L eye) are not separate, rather most of what we can see we see simultaneously through both eyes as the visual fields cross over. This gives us binocular vision with a small amount on either side that we can see only through one eye.
- Remember we have a nasal retina and temporal retina.
Key rules for visual field defects
- Behind the optic chiasm the defect is symmetrical, before the optic chiasm the defect is independent in both eyes.
- Each half of the brain maps the contralateral visual field.
- The retinal image is inverted (left is right, up is down and vice versa).
