NEURO: Vision Flashcards
What is the visual field?
There is a temporal visual field and a nasal visual field.
The temporal visual field of either eye is focusing on the nasal part of the retina and vice versa. This is because the optics of the eye invert the image. They also invert it top to bottom so the upper part of the world is focusing on the bottom part of the retina.
The nasal visual field in one eye is the temporal visual field in the other.
The eye captures an image of the world. For that, it needs to be a stable shape.
How does it achieve that?
- There is a layer of non-stretchy sclera on the outside of the eye, which becomes the cornea in the front. The sclera is the white part of the eye and runs all the way to the back, creating an anchoring point for the extraocular eye muscles that move the eye around. The sclera provides protection and structural integrity.
- At the front of the eye, the collagen fibres and cells that make up the outer layer align themselves in way that makes the structure transparent. This is the cornea.
- The sclera is flexible and held rigid by a certain amount intraocular pressure. The pressure is generated by production of aqueous humor (fluid inside the cornea) from the ciliary body and flows outwards - eventually being reabsorbed by the ‘angle of eye’. The balance between production and drainage of the aqueous humor will produce enough intraocular pressure to keep the eye rigid.
Behind the lens is a jelly like structure called the vitreous humor. This is hydrated by the aqueous humor, keeping it plump and transparent.
The function of these features is to:
- keep the eye rigid
- keep the back surface of the eye smooth and stable
- keep the distances between the optics and the retina correct
- with old age the vitreous humor starts to clump (the proteins clump) leaving watery patches and starts to pull away from the back of the eye creating fuzzy floaters in vision.
The eye also needs the ability to focus an image.
How does it achieve that?
The cornea is primarily responsible for focusing light rays. The iris controls how much light enters the eye via the pupil. The smaller the pupil aperture, the more accurate the focus and the greater the depth of field. The pupil only opens further when it has to in order to let in more light (in dimmer conditions), otherwise being narrow to provide greater focus.
The lens provide additional, variable ‘fine’ focus. The suspensory ligaments (a ring) suspend the lens from the ciliary body. The ciliary body contains a muscle that can contract or relax, thus making the lens fatter or flatter.
The retina adjusts your eye for different brightness levels and the pupil maintains the smallest aperture it can for the illumination conditions.
Light photons strike cornea and pass through –> some will be stopped by the iris –> those that pass through the pupil will be brought back to focus at a single point by the cornea and lens.
The cornea is the most powerful refractive surface in the eye.
How does the eye transmit the visual information it gather to the visual cortex and describe the primary visual cortex.
At the back of the eye, is the neural retina and the retinal pigment epithelium. The epithelium is a supporting structure that keeps the retina alive (provides a lot of biochemical support and holds the retina in place). They are both CNS structures. The optic nerve is myelinated by oligodendrocytes.
Primary visual pathway:
- The neural retina (outpost of the brain generated from the neural tube) contains a neural circuit which links the photoreceptors (which detect the light) to retinal ganglion cells.
- The retinal ganglion cells project signals via their axons from the optic nerve to the brain.
- These axons from the 2 nerves meet at the optic chiasm where the nerves from the temporal retina and nasal retina swap sides.
- The nerves then project through the optic tract, and eventually to the lateral geniculate nucleus (LGN) - a specific nucleus in the thalamus.
- Cells in the LGN send their axons through a region of white matter known as the optic radiation to the occipital cortex where the primary visual area is.
- Axons form a ‘retinotopic map’ in LGN and cortex, with the maps for the two ends in register.
(image is inverted in the optics so left side of image –> right side of brain)
Some of the axons make branches that run down to the brainstem and innervate a number of different nuclei which are involved in subconscious actions like control of eye movements or pupils.
Multiple sclerosis - problems with optic nerve as multiple sclerosis attacks oligodendrocytes.
What are the 2 types of photoreceptors.
The 2 types of photoreceptors are rods (night vision) and cones (day vision).
They are quite separate systems. We use the cones almost all the time.
Describe the structure of the cone photoreceptors.
In the inner segment, we have the nucleus, the ‘axon’ (it doesn’t fire action potentials), and the synaptic terminal.
The synaptic terminal releases glutamate, so it’s a fast, excitatory synapse.
The outer segment consists of a sac that is filled with tightly packed layers of phospholipid membrane. The layers hold the chromophore (light sensitive part) at a right angle to the light path. Their job is to hold the membrane-bound protein neatly in an array so that they can capture light as it goes through.
Cones don’t fire action potentials as they use electronic potentials to transmit info from one end to another - doesn’t need action potentials as its a small structure.
What is the resting potential of a cone photoreceptor, and how does it come about?
The resting potential of a cone photoreceptor is -45 mV.
They’re depolarised as such because the inner segment has potassium channels that leak K+ out, and the outer segment have sodium channels that are continuously open, so they leak Na+ in.
Describe the response to a cone photoreceptor in increased and decreased light.
In increased light, the cone photoreceptor reacts by closing the sodium channels, causing hyperpolarisation. This reduces the release of glutamate.
In decreased light, it opens more sodium channels, so depolarising the cell and increasing the release of glutamate.
Describe the transduction initiation of a light response in a cone photoreceptor.
On a membrane disc in the outer segment, there are photopigments. On the plasma membrane contains the Na+ channels, these are held open by intracellular messenegers of cGMP.
Two components make up the photopigment:
- the opsin (the protein component)
- retinal (a molecule - 11 cis retinalaldehyde).
The photopigment is the light-sensitive component of the visual system.
The cis bond on retinal makes it unstable. When light strikes the photopigment, it reforms the bond to a trans bond, making it more stable (all-trans retinal). This acts as an agonist for GPCRs.
Describe the amplification of a biochemical cascade in a cone photoreceptor.
The all-trans retinal activates the photopigment, which goes on to activate the corresponding G-protein. This activates the enzyme, which causes a decrease in cGMP.
The concentration of cGMP falls, so some of it will diffuse away from the sodium channels, causing them to close (the cGMP held the sodium channels open).
Describe the transduction termination of the response in a cone photoreceptor.
The opsin gets capped off by enzymatic actions, which end up in the all-trans retinal being taken away to the retinal pigment epithelium. There, it will be reformed with its cis bond, rendering it inactivate again. It is then put back into the opsin.
When activated, the opsin moves around the membrane activating G-proteins. Now that it is inactivated, it will stop. The G-proteins will use up their GTP, and a second enzyme will rebuild cGMP, which then open up the Na+ channels again.
Describe the peripheral retina.
There are lots of rods (which are too sensitive to light) which have cones amongst them. The are big gaps between the cones (in sampling array) separated by the rods. The ganglion cells are receiving input from the bipolar cells which are picking up input from a whole pool of photoreceptors. Therefore the ‘pixel size’ increases (due to convergence). This gets bigger the further you go out in the retina. The concentrated cone cell area of the retina, directly linked to the ganglion is called the ganglion cells’ receptive field centre. The bigger the receptive field centre = the less fine detail seen because input from each cone cell is being converged in summation on the ganglion cell
The image blurs as light passes through the retinal tissue.
Diseases for the loss of peripheral vision:
E.g. Glaucoma, retinitis pigmentosa
Where is the central retina located and describe it?
Central retina is within the optic nerve head, centred around the fovea centralis
In the centre is foveal pit - a region where the photoreceptors are uncovered, no retina between receptors and light path ⇒ no image blur
Image blurs as passes through retinal tissue; scattered
Disorders that can cause the loss in central vision:
Age-related macular degeneration - destroys retina; lose ability to see detail
Why is the sampling array better in the foveal pit compared to the peripheral retina?
Excellent sampling array, as no rod cells present and very THIN cone cells packed closely together to maximise space
Only red and green cone cells are present in the foveal pit (only red and green cells associated with fine detail, blue cone cells aren’t)
Ganglion cells don’t receive converged input from these cone cells, only receive input from a single cell. Thus, the foveal pit allows us to see in greater detail than the peripheral retina.
Interneuron circuitry extracts detail from the photoreceptor signals and transmits it to the ganglion cells.
Summarise the role of peripheral vision
Majority of the retina serves only coarse vision:-
- The visual image is optically blurred.
- The cone photoreceptors are large and widely spaced
(separated by larger number of rods).
- The signals from many cones converge onto single
ganglion cells.
Diseases for the loss of peripheral vision:
E.g. Glaucoma, retinitis pigmentosa
