Retina and Vision Flashcards
4 factors that are necessary for an object to be seen?
- Pattern of the object must fall on the vision receptors (rods and cones in the retina) - allows accommodation
- Amount of light entering the eye must be regulated (too much light will “bleach out” the signals)
- Energy from the waves of photons must be transduced into electrical signals
- Brain must receive and interpret the signals
Structure of the retina?
Inside-out laminar structure
Direct (vertical) pathway for signal transmission via the retina?
Photoreceptors to bipolar cells to ganglion cells
IMAGE
Importance of the lateral connection to the direct pathway?
Horizontal cells - receive input from photoreceptors and project to other photoreceptors and bipolar cells
Amacrine cells - receive input from bipolar cells and project to ganglion cells, bipolar cells and other amacrine cells
Function of the photoreceptors?
Converts electromagnetic radiation to neural signals (transduction)
4 main regions of the photoreceptors?
- Outer segment
- Inner segment
- Cell body
- Synaptic terminal
2 types of photoreceptors?
Rods
Cones
Resting membrane potential of a photoreceptor?
Have a DEPOLARISED rmp of -20 mV; compared to other neurons, the resting Vm is more positive
What happens to the membrane potential when there is light exposure?
Vm hyperpolarises
Why is the Vm +ve in a photoreceptor?
Due to the ‘dark current’ - a cGMP gated Na+ channel that is:
• OPEN in the DARK
• CLOSED in the LIGHT
Why is the ‘dark current’ important for vision?
Change in Na+ with light is the signal that enables the brain to perceive objects in the visual field
Action of the dark current in the dark?
Permeability of Na+ and K+ are equal and thus the Vm is between ENa and EK
Action of the dark current in response to light?
Na+ channels close so permeability decreases to below that of K+
Thus, hyperpolarisation occurs (driving the Vm towards the equlibrium potential of K+); this is local and graded (the more light there is, the more hyperpolarisation occurs?)
What are the visual pigment molecules?
Rhodopsin (for rods):
• 11-cis-Retinal (vitamin A derivate) + Opsin (GPCR)
These are present in the membrane folds
Action of light on rhodopsin?
Converts 11-cis-retinal to all-trans-retinal (activated form)
Mechanism of action of all-trans-retinal?
Activates transducin, causing a molecular cascade
cGMP decreases leading to closure of cGMP-gated Na+ channels
Lowered Na+ entry causes hyperpolarisation
Explain what is meant by high gain mechanism in phototransduction?
Amplification means that even minute changes in light can change the membrane potential of photoreceptors, i.e: 1 opsin leads to 1000 transducin OR 1 PDE leads to 1000 cGMP
What is the basis of phototransduction?
Dark current channel which is a nucleotide-gated channel (opened by cGMP) and is permeable to Na+
It opens in the dark and closes in response to light
It keeps the photoreceptor Vm more positive than other neurons and there is a steady release of neurotransmitter (more glutamate in the dark and less in the light)
Define visual acuity? How is it determined?
Ability to distinguish two nearby points; determined largely by photoreceptor spacing and refractive power
What do rods and cones allows?
Rods - seeing in dim light
Cones - seeing in normal daylight
Which photoreceptors have less acuity?
Rods - they have more convergence, allowing increased sensitivity but acuity is decreased; this is what allows sight in dim light
What is meant by high convergence in peripheral rods?
They have larger spacing, i.e: there is a lower density of rods, allowing more convergence on a large ganglion cell
What is meant by low convergence in foveal cones?
They have smaller spacing, i.e: there is a high density of cones; there is less convergence on the small ganglion cell but acuity is increased
Basis of colour vision?
Light is comprised of discrete wavelengths and photoreceptor are only activated by a small portion (not infrared or UV)
What is basis of different colours being seen?
Different opsins perceive different wavelength, e.g: there are 4 types of photoreceptors in the human retina: • Short-wave cone (blue) • Middle-wave cone (green) • Long-wave cone (red) • Rod
Different shades of colour be seen
5 differences between rods and cones?
Rods: • Achromatic • Peripheral retina • High convergence • High light sensitivity • Low visual acuity
Cones: • Chromatic • Central retina (fovea) • Low convergence • Low light sensitivity • High visual acuity
In essence, what does out visual system actually perceive?
Local differences in light intensity, not the absolute amounts of light
What does each eye see?
A part of the visual space, i.e: monocular visual field (+/- 45 degrees)
These overlap extensively to create a binocular visual field (+/- 45 degrees)
How is the retina divided?
In half, relative to the fovea, into:
• Nasal hemiretina
• Temporal hemiretina
Pathway of the nerve fibres and optic tract?
Nerve fibers from the nasal half of each retina cross over at the optic chiasm
Resulting 2 optic tracts allow right and left visual fields to reach the left and right hemispheres separately:
• 60% (nasal retina) cross
• 40% (temporal) do not
What is meant by retinotopy?
Image is picked up at the photoreceptor and is NOT MIXED; it goes all the way to the striate cortex
Where is the visual field mapped?
In the retina, lateral geniculate nucleus (LGN) and cortex
Where are eye specific inputs segregated in the brain?
In the primary visual cortex, more specifically in layer 4
Both eyes project to each visual cortex but, at the primary visual area (17), they remain largely segregated into ocular dominance columns, i.e: each column is dominated by input from 1 of the 2 eyes
Cells outside of layer 4 receive input from both eyes
How is visual perception shaped?
Based on the brain’s interpretation of distributed patterns of activity
Visual perception is shaped by early activity
Consequences of congenital cataracts and treatment?
Opaque covering of lens results in impaired vision from birth; they are typically removed between 10-20 years of age
They have difficulty perceiving shape and form
What is amblyopia?
AKA cortical blindness - variety of visual disorders where there is not issue with the eye but 1 eye has better vision than the other; causes include:
• Strabismus (wandering eye) if not corrected in infancy
Treatment of wandering eye?
Often surgically corrected but delayed until the child is old enough
Until then, cover the infant’s better eye for a few hours each day, making the brain rely on signals from the affected eye; allows the brain to develop properly to process signals from both eyes
What is Hebb’s postulate?
Axon of cell A is near enough to excite cell B, and repeatedly/persistently takes part in firing it, some growth process or metabolic change occurs in one/both cells
Results in cell A’s efficiency, as once of the cells firing cell B, increasing
Correlated activity between pre-synaptic and post-synaptic cells strengthens synaptic connections between them;
cells that fire together, wire together
Describe long-term potentiation (LTP)
Hebb’s postulate is also important for learning and memory
How are the terminal arborisation (branches) of LGN axons affected in monocular deprivation?
Lack of visual activity leads to less branching, i.e: there are less branches in the deprived eye
