Neuro: Vision

Question 1

Describe the eye as a camera.

Answer 1

• Optic nerves lead back to the optic chiasm.
• Temporal half and a nasal half of the eye from above.
• The temporal visual field focuses on the nasal part of the retina and vice versa. This is because the optics of the eye invert the image.
• They also invert the image from top to bottom - the upper part of the world is focusing on the bottom part of the retina and vice versa.

Question 2

How does the eye keep a stable shape?

Answer 2

• At the front, the collagen fibres and cells that make up the cornea.
• Our eye has a non-stretchy outside layer known as the sclera. This is the white part of the eye and it runs all the way around the back. It creates the anchoring point e.g. for the extra ocular eye muscles (eye muscles that move the eye around).

The sclera is flexible - there is intraocular pressure generated by the production of aqueous humor - fluid that fills the eye. Produced at the ciliary body and flows outwards to be reabsorbed through the angle of the eye.

Behind the lens there is a jelly-like structure known as the vitreous. This is hydrated by the aqueous which keeps it plump and transparent.

Question 3

How does the eye focus an image?

Answer 3

• Cornea - as the light passes through the light rays are bent inwards.
• Lens - Can change shape which changes the focus of your eye. It is suspended by a ring of suspensory ligaments from the ciliary body which contains a ring of muscle. When the ring of muscle contracts the diameter becomes smaller, and the lens become fatter and vice versa.
• Iris - Controls how much light enters the eye via the pupil. Smaller aperture = better focus.

Question 4

How does the eye transmit the visual information to the visual cortex?

Answer 4

• 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 part of the brain (created 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 (afferents).
• These axons run project back through the optic nerve. The two nerves meet at the optic chiasm (the nerves on the nasal half of the retina will swap sides. Whereas the ones that came from the temporal half of the retina stay on the same side). They then run through the optic tract, which dives up into the brain.
• The main branch goes to the lateral geniculate nucleus (LGN) in the thalamus. This is the specific nucleus of the primary vision pathway. Relay cells carry the signal up to the primary visual cortex (area 17) which is in the occipital cortex. They run in part of the white matter known as the optic radiation.
• Instead, some branches head down towards the brainstem where they innervate a number of different nuclei e.g. involved in subconscious control of eye moments or pupils.

Question 5

What are the two types of photoreceptors?

Answer 5

• Rods and cones
• 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.

Question 6

Describe the structure of cone receptors.

Answer 6

Inner segment - nucleus, protein making machinery etc stored here.

‘Axon’ - this doesn’t act as an axon (doesn’t fire action potentials)

Synaptic terminal - releases glutamate (fast excitatory neurotransmitter) depending on the level of depolarisation.

Outer segment - bag containing tightly packed layers of phospholipid membrane. Contains the transduction apparatus which produces a graded potential depolarisation

Phospholipid membrane - holds the chromophore. Held in neat layers perpendicular to the light path which ensures efficient trapping of the light rays.

Question 7

What is the resting potential of a cone photoreceptor, and how does it come about?

Answer 7

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.

Depolarised even at rest. This depolarisation occurs because in the outer segment there are Na+ channels which are open by default.

If the light striking the outer segment gets brighter, then some of the channels close. This allows the cells to become more negative inside (hyper-polarises). This prevents some of the release of glutamate. This is the signal that is sent along through the visual pathway.

Whereas if the outer segment region of the retina gets darker, then more of the sodium channels open. This depolarises the cell (less negative) and causes it to release more glutamate.

Question 8

Describe the initiation of a light response in a cone photoreceptor.

Answer 8

• Sodium channels are on the outer membrane. They are held open by an intracellular messenger called cGMP. As long as there is enough cGMP around, the channel through the membrane remains open, leaking sodium and depolarising the cell.
• There is a photopigment on the membrane disc. It is made up of opsin (protein) and a molecule of retinal (11-cis retinaldehyde. The cis configuration is less stable.
• When light strikes the unstable molecule, it causes the unstable bond to rupture. When it reforms, it reforms into the more stable trans configuration. The 11-cis retinaldehyde is photoconverted to all-trans retinaldehyde. The photopigment becomes activated.
• Activates a G proteins which activate enzymes that destroy cGMP. This results in a fall in intracellular cGMP, causing some of the cGMP to diffuse away from its channels allowing the sodium channels to close

Question 9

How is the initiation of a light response terminated (transduction)?

Answer 9

• A cascade of biochemical events cap the retinal off and stop it from acting
• This stops activating new G proteins which stop activating the enzymes
• This allows the second enzyme to rebuild the cGMP levels which reopens the channels.
• Another molecule of 11-cis retinaldehyde is attached to the opsin and is ready to respond to the next incoming photon.

Question 10

Describe the difference in nerve signals for peripheral vision and central vision.

Answer 10

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 (foveal pit):

• 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 (no convergence)

Question 11

What do photoreceptors and retinal ganglion cells react to?

Answer 11

Photoreceptors report changes in illumination from one moment to another.

Retinal ganglion cells report changes in illumination from one location to another.

Question 12

Half of all the retinal ganglion cells respond to increases in illumination and half respond to decreases.

How does this occur given that all of the photoreceptors are depolarised by
decreases in illumination?

Answer 12

• This is because for the ‘on’ centre ones there is an inverting synapse in the pathway.
• 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.

Question 13

Retinal ganglion cells can be divided into different classes.

Describe their differences based on size.

Answer 13

PARVOCELLULAR:

• small field with strong surround
• sustained responses
• fine detail vision, but only when image is stable
• half ‘on’ centre and half ‘off’ centre

MAGNOCELLULAR:

• large field with weak surround
• transient responses
• coarse detail vision, but responds well to fast movement
• half ‘on’ centre and half ‘off’ centre

Question 14

Retinal ganglion cells can be divided into different classes.

Describe them based on the wavelength they receive.

Answer 14

PARVOCELLULAR:

• selective inputs from ‘red’ or ‘green’ photoreceptors
• by comparing these responses they can encode wavelength
• RED vs GREEN

BISTRATIFIED:

• selective inputs from ‘blue’ or ‘red+green’ photoreceptors
• by comparing these responses they can encode wavelength
• BLUE vs YELLOW

Question 15

What is the difference in lateral geniculate cells and primary visual cortical cells?

Answer 15

Lateral geniculate nucleus has the exact same types of receptive fields as you will find in the retina

Visual cortex cells still respond to the relative brightness in adjacent locations but now they have more elongated fields and are orientation sensitive.
Difficult to get visual cortical cells to respond to vision because they are fussy about what they respond to.

Question 16

List the different areas in the brain involved in vision.

Answer 16

• The parietal visual areas encode information about location and movement.
• There is also a cortical area that processes colour.
• The inferotemporal visual areas encode information about object identity.