Eyes And Vision Flashcards
What are opsins and the different types?
Opsins- light sensitive receptors
Rhodopsin- high sensitivity rods, = apoprotein (opsin) + chromophore (retinal)
Long wave sensitive opsin- red
Medium wave sensitive opsin- green
Short wave sensitive opsin- blue
How does retinal respond to light?
What is the process of G protein signalling?
Mydriasis vs miosis
Ciliary muscles focussing on distant vs close objects
Cardiac vs smooth muscle signalling
What is glaucoma?
- Visual impairment- Progressive optic neuropathy, optic nerve cupping
- Classification
• Primary vs secondary
• Acute vs chronic
• Open-angle vs closed-angle (open is the ‘normal’ configuration)
• Primary open-angle glaucoma (POAG) is most common (and chronic) - Intra-ocular pressure (IOP) often raised – a significant risk factor
- IOP regulated by production and drainage of aqueous humour
• Impaired drainage (↓outflow) is a common cause of raised IOP
• Increased production (↑inflow) is a rare cause of raised IOP
How is aqueous humour produced?
- Ciliary body synthesises aqueous humour
- Aqueous humour flow:
• INFLOW: Ciliary body → posterior chamber → pupil → anterior chamber →
• OUTFLOW:
• ~90%: trabecular meshwork → Schlemm’s canal → scleral and episcleral veins (pressure SENSITIVE)
• ~10%: uveoscleral route (pressure INSENSITIVE)
What are the parts of a ciliary body?
- Ciliary Muscles: three types:
- Longitudinal (LCM): most external, connects scleral spur and trabecular network anteriorly to choroid sclera posteriorly. Contraction opens trabecular network and Schlemm’s canal.
- Circular (CCM): anterior, inner muscles. Contraction → accommodation
- Radial (RCM): intermediate, connects LCM and CCM. -
Epithelia: double layer on inner surface of ciliary processes
- Inner layer: non-pigmented adjacent to aqueous
humour in posterior chamber
- Outer layer: pigmented adjacent to stroma/vessels - Stroma, incl. mesenchymal cells and connective tissue in ciliary processes
- Vessels, incl. major arterial circle and ciliary process capillaries
- Nerves, incl. parasympathetic and sympathetic to vessels, muscles and stroma/epithelia
What is Age-related macular degeneration?
- Progressive degeneration of central retinal cells → vision loss
- Dry (non-neovascular): degeneration without formation of blood vessels
- Wet (neovascular): new vessels form and damage retina
• Active – may benefit from treatment
• Inactive – changes probably irreversible – unlikely to benefit from treatment
Sensation vs perception
Sensation
- The process of detecting and receiving stimuli from the environment
- In visual perception, sensation describes the reception of light by rods and cones in the retina, which is converted to neural signals via transduction and transmitted to the brain via the optic nerve as action potentials
Perception
- The processing (organisation and interpretation) of sensory information that occurs after information is transmitted to the brain
- Enables us to recognise objects and events (percept)
Visual sensation vs perception
Visual sensation
- Light from environment projected onto retina
- Photoreceptors transform light into electrical impulses
- Transmitted via optic nerve to visual cortex
- Image of visual pathway
Visual perception
- Information processing that combines neural signals with prior
knowledge, expectations and beliefs to help us make sense of the
world - Facilitates the recognition and understanding of objects and scenes
- Active process that allows us to recognise, locate & detect even with frequently changing stimuli
- Results in differential experiences
What is the process of perception?
- Selection- external environment is too detailed so we have to be selective to avoid overload, it can be bottom up or top down
- Organisation- process of organising and grouping items together to perceive them as one, organise in patterns, make assumptions
- Interpretation- information is interpreted into meaningful experience including colour, depth and distance
Bottom up vs top down processing
- Bottom-up processing (data driven)- Based on analyses of details in stimuli that are present (e.g. colour, orientation, size)
- Top-down processing (constructivist)- Based on information provided by context in which stimulus is encountered, past experiences, existing knowledge
What are the gestalt principles?
- The law of prägnanz- what we see is the simplest and most stable interpretation of the elements
- Figure-ground relationship- we distinguish between figures and the ground
- Perceptual grouping- closure (we see things as complete wholes rather than segmented parts) and good continuity (we are more likely to perceive smooth continuities lines rather than abrupt changes)
- Proximity- elements placed together are perceived to be part of the same object rather than separate ones
- Similarity- objects that look the same are perceived as being together
Binocular vs monocular cues
- Binocular cues- only helpful for short distances, use both eyes
- Monocular cues- can operate with just one eye, used to judge gradient, relative size, position on the horizon etc.
What is perceptual bias?
Perceptual bias- a predisposition to interpret a stimulus a certain way, can lead to perceptual error
What are the types of perceptual disorders?
Perceptual disorders- brain damage in visual association areas
- Visual agnosias - ‘unable to know’
- Apperceptive agnosia- cannot recognise by shape; cannot copy drawings
- Associative agnosia- can copy shapes, cannot associate meaning with shapes
- Prosopagnosia- inability to recognise faces only
- Capgras syndrome- Inability to recognise known people; belief that they have been replaced by an imposter
What does each part of the eye do?
What stops us from seeing in the eye?
What is the function of the cornea?
What are examples of refractive errors?
What is astigmatism?
What can go wrong with the cornea?
What is the function of the lens?
What can go wrong with the lens?
What are the 10 layers of the retina?
What is the fovea?
What are photoreceptors and where are they found?
What is the process of phototransduction?
What can go wrong in the retina?
What is colour blindness?
What is central artery/vein occlusion?
What is diabetic retinopathy?
What is dry vs wet age-related macular degeneration?
What is retinal detachment?
What is the optic nerve and what can go wrong with it?
What is optic neuritis?
What is glaucoma?
What is anterior ischaemic optic neuropathy?
What causes optic nerve compression?
What is Papilloedema?
What is the visual pathway?
What are the visual field defects?
What are the pupil response pathways?
What is the accommodation reflex?
What are the conditions which cause abnormal pupils?
How do we turn electrical signals into what we see?
What causes pain in the eye?
• Foreign body
• Trauma
• Reduced tear film
• Corneal epithelial disturbance
• Inflammation
• Raised IOP
What are the points of inflammation in the eye?
What is Blepharitis and meibomianitis?
What are the lumps and bums of the eye?
What is entropion, ectropion and trichiasis?
What are the lid diseases?
What is inflammation of the conjunctiva problems?
What are inflammation of the cornea problems?
What is acute angle closure glaucoma (AACG)?
What are the inflammatory conditions of the iris?
What are the red eye conditions?
What is the visual pathway?
- Light enters the eye through the cornea, which helps to focus the light onto the lens
- The lens changes shape to adjust the focus of the light onto the retina. The retina contains two types of photoreceptor cells: rods and cones.
- When light strikes a photoreceptor cell, it activates a pigment called rhodopsin in rods or one of three cone opsins in cones. Phototransduction, results in a change in the membrane potential of the cell.
Short-wavelength-sensitive (S) cone opsin: also known as the blue cone opsin, sensitive to short-wavelength light (around 420-440 nanometers).
Medium-wavelength-sensitive (M) cone opsin: also known as the green cone opsin, sensitive to medium-wavelength light (around 530-540 nanometers).
Long-wavelength-sensitive (L) cone opsin: also known as the red cone opsin, sensitive to long-wavelength light (around 560-580 nanometers).
4-7. After the change in membrane potential in photoreceptor cells, results in a cascade of chemical reactions
When rhodopsin or cone opsins are activated by light, they stimulate the activation of transducin, which in turn activates PDE. PDE breaks down cGMP, which is normally present at high levels in the photoreceptor cells, into its inactive form, GMP. This results in a decrease in the concentration of cGMP in the cell.
The decrease in cGMP concentration causes a closure of the cation channels, leading to hyperpolarization of the cell membrane. Thisleads to a reduction in the release of neurotransmitters from the photoreceptor cells.
The amount of neurotransmitter released by the photoreceptor cell depends on the amount of light that is absorbed by the rhodopsin or cone opsin. If there is a lot of light, more cGMP is broken down, and the membrane potential of the photoreceptor cell is more hyperpolarized, leading to a decrease in neurotransmitter release. Conversely, if there is less light, less cGMP is broken down, and the membrane potential of the photoreceptor cell is less hyperpolarized, leading to an increase in neurotransmitter release. The graded potential generated by the photoreceptor cell is then transmitted to other cells in the retina, including bipolar cells and ganglion cells.
- The bipolar cells act as intermediaries, transmitting information from the photoreceptor cells to the ganglion cells. The ganglion cells are the output cells of the retina, and their axons form the optic nerve, which carries visual information to the brain.
- The information from multiple photoreceptor cells is integrated by other cells in the retina, including horizontal cells and amacrine cells. These cells help to enhance contrast and adjust sensitivity to different levels of light.
- The axons of the ganglion cells form the optic nerve, which travels to the brain. At the optic chiasm, some of the fibers cross over to the opposite side of the brain, while others remain on the same side.
- The visual information is then processed by a complex network of neurons in the visual cortex, which is located in the occipital lobe of the brain. Different areas of the visual cortex are specialized for processing different types of visual information, such as color, motion, and form.
- As the visual information is processed by the brain, it is interpreted and combined with information from other senses to create a perception of the visual world.
What is the embryological development of the brain?
Brain development
The forebrain (prosencephalon) evaginates to produce two secondary brain vesicles: the telencephalon (cortex, BG) and the diencephalon (thalamus, hypothalamus, and retina). So, the retina is embryologically part of the CNS.
Eye development
Eye development then is a combination of retinal (CNS) and lens (ectodermal placode) development. The presumptive lens and the developing optic cup signal to one another to drive growth, cell division, and morphogenesis. This involves reciprocal FGF signalling.
The apical ectodermal ridge (AER) signals to the underlying lateral plate mesoderm (LPM) with fibroblast growth factor 8 (FGF8). The LPM signals back with a related gene, FGF10.
Pax6 is a homeobox transcription factor that controls all aspects of eye development (and some brain/spinal cord development too).
What are placodes?
The lens develops from a specialized thickening of ectoderm: a placode. These structures give rise to many sense organs of the head, in whole or in part, and are often neurogenic (ie they make cranial PNS
neurons)
What are the 5 types of cells in the retina?
What are the retinal circuits?
Photoreceptors (PRs) comprise rods and cones, and they convert light into electrical information (not action potentials). Photoreceptors synapse with and signal to bipolar cells. Bipolar cells compute the information synapse and synapse with and signal to retinal ganglion cells. Horizontal cells and amacrine cells both provide lateral inhibition to PRs and bipolar cells respectively.
Horizontal and amacrine cells modulate the signalling through the circuit that is received by retinal ganglion cells (RGCs). Only RGCs send action potentials – all of the other circuitry consists of subtle
balances of polarisation and depolarisation. RGCs project their axons across the internal surface of the retina and send signals down the optic nerve to the brain.
What happens in phototransduction?
How are photoreceptors arranged on the retina?
What is the process of the ON and OFF circuit in the light?
What is the process of the ON and OFF circuit in the dark?
What is centre-surround functional organisation?
What side of the brain computes which side of the world?
What are the central pathways?
What is the function of the pretectum?
What is the function of the superior colliculus?
- co-ordinates saccadic eye movements (eyes drawn to moving/bright/differently-coloured objects)
What is the function of the lateral geniculate nucleus (LGN)?
nucleus of the thalamus and responsible for transferring visual information to the cortex for the conscious perception
What are the Thalamocortical projections?
Thalamocortical fibres project through the parietal or the temporal lobe. Meyer’s loop can be an important consideration in surgery for temporal lobe epilepsy.
What are the dorsal vs ventral visual pathways?
What is the process of conscious vision?
