NEURO: Vision Flashcards
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.
On the inside, there is the liquid called the aqueous humor, which is inside the cornea. It also seeps round to the vitreous humor, keeping it hydrated.
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
The eye also needs the ability to focus an image.
How does it achieve that?
The cornea is primarily responsible for focussing light rays. The iris controls how much light enters the eye via the pupil.
The lens provide additional, variable ‘fine’ focus. The suspensory ligaments 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.
How does the eye transmit the visual information it gather to the visual cortex?
At the back of the eye, we have the neural retina and the retinal pigment epithelium. The epithelium is a supporting structure that keeps the retina alive.
The neural retina is an outpost of the brain (it’s generated from the neural tube). It contains a whole neural circuit which links the photoreceptors (which detect the light) to retinal ganglion cells. These are the cells which have axons that run out via the optic nerve to take signals to the brain.
These axons run project back through the optic nerve, and the two nerves meet at the optic chiasm (where some of the axons swap over). They then run through the optic tract, which dives up into the brain.
Many axons have branches that go down the brainstem to nuclei involved in eye movements; the main branch goes to the lateral geniculate nucleus (LGN) and the thalamus. This is the specific nucleus of the primary vision pathway.
There, they activate relay cells that carry the signal up to the primary visual cortex which is in the occipital cortex. They run in part of the white matter known as the optic radiation.
What are the two types of photoreceptors?
We have rods and cones in our eyes.
The rods are super-sensitive, so they saturate and become non-functional in high-light levels. They are used mostly in night vision.
The cones are less sensitive, but work better in high-light levels. They are used for day vision.
Describe the structure of cone receptors.
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 layer upon layer of membrane. The layers sit 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.
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 polarised 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 initiation of a light response in a cone photoreceptor.
On a membrane disc in the outer segment, there are photopigments. Two components make up the photopigment: the opsin (the protein component) and the 11-cis retinal (or retinal). 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 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 moved 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 action potential of a cone photoreceptor.
At the beginning, with a light change, we see a very rapid response. You have a photo-transduction cascade which amplifies the response to each photon to produce this rapid change.
If the light remains constant, the photoreceptor adapts; it changes the amplification of the light response to bring the membrane potential back down to rest. This is so that it can produce a strong response again.
Describe the difference in nerve signals for peripheral vision and central vision.
PERIPHERAL VISION:
- the visual image is optically blurred
- the cone photoreceptors are large and widely spaced (separated by a large number of rods)
- the signals from many cones converge into single ganglion cells
CENTRAL VISION:
- good focus because the overlaying layers are absent
- only cone photoreceptors are present, primarily red and green
- the photoreceptors are narrow and closely packed
- the signals from the photoreceptors are kept separate throughout the primary visual pathway
What do photoreceptors and retinal ganglion cells react to?
Photoreceptors report changes in illumination from one moment to another.
Retinal ganglion cells report changes in illumination from one location to another.
Retinal ganglion centres may be excited by either decreases or increases in brightness.
Explain.
The retinal ganglion centres can either be ‘off’ centre or ‘on’ centre.
If they’re ‘off’ centre, when the central photoreceptor depolarises by decreased illumination, the bipolar and ganglion cells will be depolarised by the excitatory synapses.
If they’re ‘on centre’, when the central photoreceptor is hyperpolarised by increased illumination, the bipolar cell is depolarised by inverting the synapse, which excited the ganglion cell.
Retinal ganglion cells can be divided into different classes.
Describe their differences based on size.
PARVOCELLULAR:
- small cell with strong surround
- sustained responses
- fine detail vision, but only when image is stable
- half ‘on’ centre and half ‘off’ centre
MAGNOCELLULAR:
- large cell with weak surround
- transient responses
- coarse detail vision, but responds well to fast movement
- half ‘on’ centre and half ‘off’ centre
