Vision (Katrin) Flashcards
What is the visual system
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
What is a retinal ganglion cell?
Retinal ganglion cells (RGCs) are a type of neuron found in the retina of the eye
What is the optic nerve?
It is a bundle of retinal ganglion cell axons
It transmits visual information from the retina to the brain
What is the Nasal Retina?
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
What is the Temporal Retina?
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
What is Partial Decussation?
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
What is the Optic Chiasm?
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
What is the Lateral Geniculate Nucleus?
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
What is the Pretectal Nucleus?
A brain region where some optic nerve fibers terminate
It is involved in mediating reflexive eye movements, such as pupillary reflexes
What is the Suprachiasmatic Nucleus?
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
What are the layers of the eyeball?
1) Sclera (outermost)
2) Choroid (middle layer)
3) Retina (innermost)
What is the Fovea Centralis?
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.
What is the Vitreous Humour?
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
Features of Cornea and Lens
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
What is the Cornea’s Role in Refraction?
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
What is Lens Accommodation?
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
What are some refractive errors?
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
The retina basic structure and origin?
Part of the CNS
Forms from diencephalon (optic vesicle)
5 neuronal cell types:
- Photoreceptors
- Bipolar cells
- Ganglion cells
- Amacrine cells
- Horizontal cells
What are the Layers of the Retina?
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
Where are photoreceptors located in the eye?
Photoreceptors are located adjacent to the Retinal Pigmented Epithelium (RPE)
What is found in the outer segment of photoreceptors, and what is its function?
The outer segment of photoreceptors contains discs with pigments for light detection
These pigments are responsible for the initial detection of light
Where does the regeneration of photopigments occur, and what happens to the discs in photoreceptors?
The regeneration of photopigments occurs in the Retinal Pigmented Epithelium (RPE)
The discs in photoreceptors are continually turned over or “shed”
What physiological response do photoreceptors exhibit when exposed to light?
When exposed to light, photoreceptors undergo hyperpolarisation
Why is hyperpolarisation important in photoreceptors?
Hyperpolarisation in photoreceptors allows for graded responses to varying levels of light, enabling dynamic adjustments to different lighting conditions
What happens to ion flow in rod photoreceptor cells in the dark?
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
How does ion flow change in rod photoreceptor cells when exposed to light?
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
What are Opsins?
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
Opsin Activation and Signal Transduction
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
Opsin Cascade Termination Steps?
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
What are the differences between rods and cones?
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
How do cones detect colour?
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
What is the distribution of cones and rods in the retina?
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
Rods and Cones overview table
What is the most simple retinal circuit?
Photoreceptor –> bipolar cell –> retinal ganglion cell
Ganglion cell detects changes in luminance
How do retinal ganglion cells detect changes in luminance?
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
What are on/off-centre bipolar cells?
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
What are horizontal cells?
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
How do horizontal cells work?
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)
How do we know where visual neurones project to?
- 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
How does the retina project to the dLGN?
It is a retinotopic point-to-point projection
There is no mixing of information
What happens when information leaves the dLGN?
It is project to the visual cortex (occipital lobe), which is where information is combined from both eyes
Visual field arrangement in the occipital lobe?
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
What is the primary function of direction-selective neurons in the visual cortex?
- Respond to movement in specific directions
- Key for processing motion in the visual environment
How are neurons organized in the visual cortex regarding directionality?
- In columnar formations
- Each column responds to stimuli moving in a particular direction
What role do orientation-selective neurons play in the visual cortex?
- Respond to specific orientations of edges and lines
- Contribute to understanding shapes and contours
How does the visual cortex integrate motion information?
- By combining data from multiple columns
- Integrates direction, speed, colour, depth for coherent visual perception
What are ocular dominance columns, and where are they located?
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
What is the function of ocular dominance columns
Ocular dominance columns help preserve spatial and binocular information necessary for depth perception and the processing of three-dimensional space
How are ocular dominance columns organized
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
