46 & 47 Physiology of visual sensation Flashcards
What is the unit of power and what is its equation?
power = 1/focal length (diopter)
What is focal length?
The length between the focal point (fovea) and the lens
The higher the refractive power, the shorter/longer the focal length.
shorter
How is accommodation achieved when a person switches to view a near object?
- lateral geniculate nucleus > blurred image perceived in visual cortex > recognised as out of focus > signals from cortex to brainstem at Edinger-Westphal nucleus > signals to ciliary ganglion > ciliary muscles contract > zonular fibers slacken > lens rounded
What is the range of accommodation?
Near point - far point
In presbyopia, the range of accommodation increases/decreases because?
decreases;
the near point is too far
What happens to the near point and far point in myopia?
both too near ;
far point too near is the culprit
What lens shall be worn in myopia?
Concave lens
When should convex lens be worn?
Hyperopia / Presbyopia
What happen to the near point and far point in hyperopia?
near point too far
far point too far (but can be compensated by accommodation
Which are the 5 major layers in the retina?
- Outer nuclear layer
- Outer plexiform layer
- Inner nuclear layer
- Inner plexiform layer
- Ganglion cell layer
In the ______________ layer, retinal ganglion cells make synaptic contacts with bipolar cells and amacrine cells.
inner plexiform
In the inner plexiform layer: retinal ganglion cells make synaptic contacts with __________ and __________.
Bipolar cells and amacrine cells
Light passes through the layers before/after reaching the photoreceptors.
before
Information is modulated by ______________ cells in the ___________layer and are relayed to bipolar cells, then the retinal ganglion cells.
horizontal;
outer plexiform
Connection of cone to ganglion cell results in small/big receptive field, and low/high visual acuity.
small; high
Multiple rods converging to a single ganglion cell result in a small/big receptive field, and low/high visual acuity.
big; low
Photoreceptors have an outer segment which contain the _________________ located on a stack of membranous discs.
photosensitive pigment
Rods contains ____________ which contributes to its high sensitivity to light.
rhodopsin
Cones contains ___________ thus they have lower sensitivity to light.
Iodopsin
Cones are chromatic because?
They have multiple types of iodopsins
Which type of photoreceptors have higher convergence of retinal ganglion cells?
This results in lower/higher spatial resolution.
Rods;
Cones: one receptive field connect to one retinal cell only ;
lower
Which type of photoreceptors is shorter, thicker and tapered?
Cones
rods: long, thin, cylindrical
Single pigment system cannot differentiate colours because both _________ and _____________affects the response.
wavelength and intensity
Humans have 3 types of cones, which type is much fewer in number than the other 2?
Blue cones
Give the 4 possibilities of color weakness.
- Absence of cones
- Just one cone available (monochromatism)
- 3rd cone missing (dichromatism)
- Abnormality in photopigment (anomalous trichromatism)
Red-green color weakness is more/less common than blue color weakness, and is more frequent in males/females.
more; males
Subtypes of common colour weakness can be tested with the ______________.
Ishihara chart
In the absence of light, the membrane is _______________ due to _______________. __________ pump maintains the concentration gradient
depolarised; high level of Na+ current flowing into the outer segment.;
Na/K+ pump
In bright light, the membrane is more _____________, because _________________.
hyperpolarized,;
the Na+ channels are closed
Give the flow of events happening when light is shone on the retina.
Light activates transducin (G-protein) > activates cGMP phosphodiesterase > break down cGMP > Na+ channels close > membrane hyperpolarization > less glutamate release
The Na+ channels in the membrane is kept opened by?
cGMP
Glutamate is inhibitory to ___________ cells.
On center bipolar cells
Glutamate is excitatory to _____________ cells.
Off center bipolar cells
Surround photoreceptors are associated with _________ cell. In dark light, it will cause ___________ to photoreceptor, and more/less glutamate release from the photoreceptor to the __________ cell. The __________ cell in turn release more/less inhibitory NT , causing the on center// off center bipolar cell to __________ respectively.
horizontal; depolarization; more; horizontal; horizontal; more;
less glutamate release
on center: depolarised;
off center: hyperpolarized
Retinal ganglion cells with small receptive fields (P cells) receiving main inputs from _________ cells are concentrated in the _______.
cones;
fovea
Retinal ganglion cells with ________ receptive fields (__ cells)are found more in the periphery.
large;
M
What type of retinal ganglion cell is intrinsically sensitive to light ?
It plays a role in ____________ by their direct projection to the ___________ nucleus of the hypothalamus, also mediating light reflex.
Photosensitive melanopsin;
circadian rhythm;
suprachiasmatic
State the route of the visual pathway from nerve fibers leaving the eye to the cortex.
> Optic chiasm > cross-over medial bundles > lateral geniculate nuclei > optic radiation > primary visual cortex (V1)
_____________ cells project to the parvocellular layers of the lateral geniculate nucleus consisting of small cells.
Retinal P cells
M cells project to ____________ layers of the lateral geniculate nucleus consisting of larger cells.
Magnocellular
Majority of neurons in primary visual cortex are specialized in detecting ________________ and their _________, and properties like color.
line contrast of specific orientation;
movements
Neurons in primary visual cortex are named ________ cells.
Simple and complex
Contour detection by the primary visual cortex is achieved by convergence of inputs from the ______________ into _____________.
Lateral genicular nucleus nuerons into one simple cell in V1
What are complex cells?
While simple cells respond maximally to a bar at a preferred orientation in a specific location, one type of complex cells have a larger receptive field which are orientation-sensitive but location-insensitive
__________________ represents the initial site of analysis of visual images.
Primary visual cortex (V1, area 17)
The primary visual cortex projects to the ____________________ for further visual processing, from there to visual association cortex and higher visual areas.
Extrastriae visual cortex
What does the dorsal stream detect and via what to reach its final destination?
Where?;
via middle temporal visual area (V5) to the posterior parietal cortex.
What is V5? Which pathway is it involved?
Lesion?
Middle temporal (MT) visual area; dorsal pathway (where); V5: for analysis of motion, seeing world in series of snapshots
What will happen with damage to the parietal lobe?
Hemineglect syndrome, tendency to ignore the contralateral side.
What does the ventral stream detect and via what to reach its final destination?
What?
via V4 to the inferior temporal cortex
Function of fusiform gyrus?
Neurons there have small/large receptor field, therefore?
recognition of face;
large, can code an code different sizes of the faces reflecting different distance from the subject
Spatial orientation, depth perception, location of objects is detected in which pathway?
Where pathway
also movement of objects
What pathway detects what features of vision?
Color and shape, important in language ability.
Describe the flow of events in pupillary reflex.
When light shines onto retina > signals project to pretectal area > Edinger-Westphal nucleus > send out to contract the sphincter muscles > decrease size of pupil
Why pupillary reflex is consensual?
Due to the bilateral projection of the pretectal nucleus of one side to Edinger-Westphal nucleus on both sides.
Briefly describe what is glaucoma.
Excess aqeuous fluid in anterior chamber of the eye, retinal ganglion cells and axons degenerates, causing tunnel vision then blindness
How can cataract be treated?
Lens become cloudy, replacement by artificial lens.
What is macular degeneration? Will it cause whole-blindness?
Atrophy of neural tissue or severe haemorrhage diseases at the macula (includes fovea).
No, peripheral vision is not affected.
_____________ is progressive degeneration of the retina resulting from diabetes mellitus. Blood vessels burst and neural cells die.
Diebetic retinopathy
In retinal detachment, _______ becomes separated from the _________.
Retina; choroid
