Vision (Katrin)

Question 1

What is the visual system

Answer 1

The visual system is the physiological basis of visual perception

The system detects, transduces and interprets information concerning light within the visible range to construct an image and build a mental model of the surrounding environment

Question 2

What is a retinal ganglion cell?

Answer 2

Retinal ganglion cells (RGCs) are a type of neuron found in the retina of the eye

Question 3

What is the optic nerve?

Answer 3

It is a bundle of retinal ganglion cell axons

It transmits visual information from the retina to the brain

Question 4

What is the Nasal Retina?

Answer 4

The nasal retina is the portion of the retina located on the inner side of each eye, closer to the nose

It primarily processes visual information from the temporal (outer) visual field of the same eye

Its axons cross at the optic chiasm to project to the opposite hemisphere of the brain

Question 5

What is the Temporal Retina?

Answer 5

The temporal retina is the portion of the retina situated on the outer side of each eye, closer to the temples

It mainly processes visual information from the nasal (inner) visual field of the same eye

Its axons do not cross at the optic chiasm, projecting to the same hemisphere of the brain

Question 6

What is Partial Decussation?

Answer 6

Partial crossover of optic nerve fibers at the optic chiasm

Allows for processing of visual information from both left and right visual fields in both cerebral hemispheres

Question 7

What is the Optic Chiasm?

Answer 7

A structure at the base of the brain where some optic nerve fibers cross over to the opposite side

It ensures integration of information from both eyes and both sides of the visual field

Question 8

What is the Lateral Geniculate Nucleus?

Answer 8

It is a relay station in the thalamus that receives many optic nerve fibers

It relays visual information to the visual cortex, which covers approximately 40% of the cortical area and is responsible for visual perception

Question 9

What is the Pretectal Nucleus?

Answer 9

A brain region where some optic nerve fibers terminate

It is involved in mediating reflexive eye movements, such as pupillary reflexes

Question 10

What is the Suprachiasmatic Nucleus?

Answer 10

A nucleus located above the optic chiasm

It regulates the sleep-wake cycle and circadian rhythms, receiving input from the optic nerve to synchronize biological rhythms with light and dark cycles

Question 11

What are the layers of the eyeball?

Answer 11

1) Sclera (outermost)

2) Choroid (middle layer)

3) Retina (innermost)

Question 12

What is the Fovea Centralis?

Answer 12

The fovea centralis is a small, central depression in the retina of the eye, located near the back of the eyeball

It is the region of the retina responsible for the sharpest and most detailed central vision

The fovea is essential for tasks that require precise visual acuity, such as reading and recognising fine details in objects.

Question 13

What is the Vitreous Humour?

Answer 13

The interior of the eyeball is filled with a clear, jelly-like substance called the vitreous humour

It helps maintain the eyeball’s shape and optical properties

Question 14

Features of Cornea and Lens

Answer 14

Both the cornea (the clear front surface of the eye) and the lens (located behind the iris) are highly transparent structures that allow light to enter the eye and focus on the retina

Question 15

What is the Cornea’s Role in Refraction?

Answer 15

The process of bending or refracting light is essential for focusing it onto the retina

The cornea and lens work together to achieve this, ensuring that incoming light rays converge properly

The cornea is primarily responsible for the initial refraction of light as it enters the eye. It accounts for a significant portion of the eye’s overall refraction power

Question 16

What is Lens Accommodation?

Answer 16

The lens is an adjustable structure that can change its shape to fine-tune the focus of incoming light

This process is called accommodation and is crucial for clear vision at various distances

Question 17

What are some refractive errors?

Answer 17

Emmetropia:
- Ideal eye condition where light focuses on the retina for clear vision at all distances

Myopia (Nearsightedness):
- Light focuses in front of the retina, causing distant objects to appear blurry
- Corrected with concave lenses

Hyperopia (Farsightedness):
- Light focuses behind the retina, making nearby objects blurry
- Corrected with convex lenses

Question 18

The retina basic structure and origin?

Answer 18

Part of the CNS

Forms from diencephalon (optic vesicle)

5 neuronal cell types:
- Photoreceptors
- Bipolar cells
- Ganglion cells
- Amacrine cells
- Horizontal cells

Question 19

What are the Layers of the Retina?

Answer 19

Pigmented Epithelium:
- A layer of darkly pigmented cells located outside the neural retina, adjacent to the choroid. It helps nourish and support the photoreceptor cells, absorbs excess light, and maintains the health of the retina

Photoreceptor Layer:
- The outermost layer of the retina, containing rods (for low-light vision) and cones (for color vision)

Outer Nuclear Layer:
- Contains the cell bodies of photoreceptor cells

Outer Plexiform Layer:
- Where synapses occur between photoreceptors and bipolar cells

Inner Nuclear Layer:
- Contains cell bodies of bipolar cells, horizontal cells, and amacrine cells

Inner Plexiform Layer:
- Where synapses occur between bipolar cells, ganglion cells, and other interneurons

Ganglion Cell Layer:
- Contains ganglion cell bodies, which send axons to form the optic nerve

Nerve Fiber Layer:
- Comprised of ganglion cell axons exiting the eye as the optic nerve

Internal Limiting Membrane:
- The innermost boundary between the retina and the vitreous humour

Question 20

Where are photoreceptors located in the eye?

Answer 20

Photoreceptors are located adjacent to the Retinal Pigmented Epithelium (RPE)

Question 21

What is found in the outer segment of photoreceptors, and what is its function?

Answer 21

The outer segment of photoreceptors contains discs with pigments for light detection

These pigments are responsible for the initial detection of light

Question 22

Where does the regeneration of photopigments occur, and what happens to the discs in photoreceptors?

Answer 22

The regeneration of photopigments occurs in the Retinal Pigmented Epithelium (RPE)

The discs in photoreceptors are continually turned over or “shed”

Question 23

What physiological response do photoreceptors exhibit when exposed to light?

Answer 23

When exposed to light, photoreceptors undergo hyperpolarisation

Question 24

Why is hyperpolarisation important in photoreceptors?

Answer 24

Hyperpolarisation in photoreceptors allows for graded responses to varying levels of light, enabling dynamic adjustments to different lighting conditions

Question 25

What happens to ion flow in rod photoreceptor cells in the dark?

Answer 25

In the dark, rod cells have a baseline ion flow

Sodium (Na+) ions flow into the rod cell through cGMP-gated sodium channels, while potassium (K+) ions flow out of the cell through potassium channels

This continuous inward sodium influx and outward potassium efflux maintain the rod cell’s resting membrane potential

Question 26

How does ion flow change in rod photoreceptor cells when exposed to light?

Answer 26

When light is detected, the ion flow in rod cells is altered

Light leads to the closure of cGMP-gated sodium channels, reducing the influx of sodium ions

Simultaneously, potassium channels remain open, allowing potassium ions to continue flowing out of the cell

As a result, there is a net decrease in intracellular sodium levels and a sustained potassium efflux

This change in ion flow signifies the detection of light and initiates the visual signaling cascade

Question 27

What are Opsins?

Answer 27

Opsins are proteins found within the membrane of photoreceptor cells’ discs in the retina

They play a critical role in the visual system by mediating the detection of light and initiating the visual signaling cascade

Opsins bind with the light-sensitive chromophore molecule called retinal, enabling them to respond to light stimuli

Opsins can adjust their sensitivity to specific wavelengths of light, allowing them to detect different colors within the visible spectrum

Question 28

Opsin Activation and Signal Transduction

Answer 28

Opsins, when exposed to light, undergo a conformational change in their structure

This conformational change leads to the activation of transducin, a protein

Transducin activation subsequently triggers the hydrolysis of cyclic guanosine monophosphate (cGMP) within the photoreceptor cell

The activation of this signaling cascade results in an amplification of the initial light signal

This amplification allows the detection of even small amounts of light and initiates the visual response in the retina, ultimately leading to visual perception

Question 29

Opsin Cascade Termination Steps?

Answer 29

Light detection causes calcium channels to close, decreasing calcium levels

Reduced calcium leads to increased guanylate cyclase activity, restoring cGMP levels

Decreased calcium activates rhodopsin kinase, which phosphorylates and deactivates rhodopsin

Phosphorylated rhodopsin binds arrestin, deactivating rhodopsin and displacing transducin

Arrestin binding halts the visual signaling cascade

All-trans retinol dissociates from opsin, completing cascade termination

Visual system returns to a resting state, ready for subsequent light detection

Question 30

What are the differences between rods and cones?

Answer 30

Rods:
- Low spatial resolution
- High sensitivity (1 photon)

Cones:
- High spatial resolution
- Low sensitivity (>100 photons)
- Less saturation
- Rapid recovery
- Colour vision

Loss of rod function = night blindness
Loss of cone function = legally blind

Question 31

How do cones detect colour?

Answer 31

Our eyes are trichromatic (detects 3 colours)

There are 3 types of cones:
- Blue (short)
- Green (medium)
- Red (long)

The different cones are excited at different wavelengths

Each cone is monochromatic

Medium and Long wavelength cones are located on the X chromosome

Question 32

What is the distribution of cones and rods in the retina?

Answer 32

Around the fovea there is a more even distribution of cones and rods

As you move away from fovea, cones density decreases rapidly until on the outer edges there are not many cones

In the fovea there are only cones, and no rods

Hence loss of cone function leads to legal blindness

Question 33

Rods and Cones overview table

Answer 33

Question 34

What is the most simple retinal circuit?

Answer 34

Photoreceptor –> bipolar cell –> retinal ganglion cell

Ganglion cell detects changes in luminance

Question 35

How do retinal ganglion cells detect changes in luminance?

Answer 35

They contain receptive fields:
- Regions in which a stimulus elicits and action potential

Two types of ganglion cells:
- On-centre
- Off-centre

Equal numbers of both

If one is activated, the other is inactivated

e.g. if centre is brighter than the surround, the on-centre will be activated and the off-centre will be inactivated and vica verca

Question 36

What are on/off-centre bipolar cells?

Answer 36

The on-centre and off-centre ganglion cells connect to on and off centre bipolar cells

These cells have different glutamate receptors

On-centre bipolar cells are sign-inverting
- If the cone cell is depolarised, the on-centre bipolar cell will be hyperpolarised and the ganglion cell will be hyperpolarised

Off-centre bipolar cells are sign-conserving
- If the cone cell is depolarised, the off-centre bipolar cell with be depolarised and the ganglion cell will be depolarised

Question 37

What are horizontal cells?

Answer 37

Horizontal cells are the laterally interconnecting neurones in the retina

They have cell bodies in the inner nuclear layer of the retina of vertebrate eyes

They help integrate and regulate the input from multiple photoreceptor cells

They are connected to photoreceptors by gap junctions

They form a network over a large area of the retina

They allow detection of background illumination

Question 38

How do horizontal cells work?

Answer 38

They provide feedback to the photoreceptor

If there is transmitter release at the photoreceptor, the horizontal cell will depolarise (sign-conserving)

Instead of releasing glutamate, they release GABA (inhibitory neurotransmitter) which then inhibits the photoreceptors (sign-inverting)

Question 39

How do we know where visual neurones project to?

Answer 39

• Retina is injected with radioactively labelled amino acids
• Amino acids will be taken up into proteins and used by vision cells
• Visual neurone cell will be radioactively labelled including the axon
• Area where axon meets its target will be radioactively illuminated
• At axon, neurotransmitter release and reuptake into post-synapse will occur, subsequent neurone therefore is also radioactively labelled/illuminated
• Able to visualise neurone route via radioactive labelling

In modern day, dyes are used instead of radioactive labelling

Question 40

How does the retina project to the dLGN?

Answer 40

It is a retinotopic point-to-point projection

There is no mixing of information

Question 41

What happens when information leaves the dLGN?

Answer 41

It is project to the visual cortex (occipital lobe), which is where information is combined from both eyes

Question 42

Visual field arrangement in the occipital lobe?

Answer 42

Spacial arrangement of the retina is maintained throughout the visual pathway

However

There is a disproportionate representation in the lobe

e.g.
- Fovea has a much larger area in the lobe than it takes up in the visual field

Question 43

What is the primary function of direction-selective neurons in the visual cortex?

Answer 43

• Respond to movement in specific directions
• Key for processing motion in the visual environment

Question 44

How are neurons organized in the visual cortex regarding directionality?

Answer 44

• In columnar formations
• Each column responds to stimuli moving in a particular direction

Question 45

What role do orientation-selective neurons play in the visual cortex?

Answer 45

• Respond to specific orientations of edges and lines
• Contribute to understanding shapes and contours

Question 46

How does the visual cortex integrate motion information?

Answer 46

• By combining data from multiple columns
• Integrates direction, speed, colour, depth for coherent visual perception

Question 47

What are ocular dominance columns, and where are they located?

Answer 47

Ocular dominance columns are stripes of neurons in the primary visual cortex (V1) that preferentially respond to visual input from one eye over the other

These columns are located in the primary visual cortex

Question 48

What is the function of ocular dominance columns

Answer 48

Ocular dominance columns help preserve spatial and binocular information necessary for depth perception and the processing of three-dimensional space

Question 49

How are ocular dominance columns organized

Answer 49

Inputs from the left and right eyes are segregated into these columns in the visual cortex, preserving the spatial and binocular information from each eye