B&B Vision Flashcards
How many muscles control the position of the eyeball in the socket and what cranial nerves are they innervated by?
6
lateral (VI) & medial (III) rectus
superior (III) & inferior (III) rectus
superior (IV) & inferior (III) oblique
In bright light, what contracts to restrict the pupil?
The circular sphincter muscle of
the iris
In dim light, what contracts to widen the pupil?
The radial dilator muscle
Describe the muscles that contract/relax when focusing near
Ciliary muscles contract
Zonular fibres relax
Lens become more spherical
Eyes converge and pupils restrict
Describe the muscles that contract/relax when focusing far
Ciliary muscles relax
Zonular fibres contract
Lens become less spherical
Eyes diverge and pupils dilate
What is the cause and symptom of Myopia?
Myopia causes nearsightedness so close objects are clear but far objects are blurred. This happens because the cornea/lens is too powerful or the eyeball is too long. The image is focused in front of the retina
What is the cause and symptom of Hyperopia or Presbyopia?
Hyperopia causes farsightedness so far objects are clear but close objects are blurred. This happens because the cornea/lens is too weak or the eyeball is too short. The image is focused behind the retina.
How do you correct Myopia?
Concave corrective lens
negative
How do you correct Hyperopia?
Convex corrective lens
positive
What is an astigmatism?
Caused by the irregularities in the curvature of the cornea
How do you correct an astigmatism?
Need cylindrical (not spherical) corrective lens
What is retinal isomerization?
There are cis/trans isomers of the retinal
When a photon of light is absorbed in the visible range, 11-cis retinal can isomerize to all-trans retinal
It goes from being kinked to straight
Purple to bleached
Can also be reversed but enzymes are needed
What is phototransduction?
Process by which photons are converted to electrical signals. Occurs in photoreceptors
Describe the process of phototransduction
eg phosphoryltion and completion and stabilisinf g protein
1) Phosphorylation of metarhodopsin II (R)
2) Completion of phosphorylation and stabilising
(“capping”) by arrestin (Arr): inactivation of R
3) Inactivation of G-protein (transducin) and
phosphodiesterase (PDE) by a GTPase activating
protein (GAP)
4) Regeneration of cG by guanylyl cyclase (GC):
depends on low [Ca2+] and requires GTP
5) cG-gated channels re-open, Na+
influx
Describe the process of brightness (or light) adaptation?
1) During illumination, cGMP gated channels close and
intracellular Ca2+ concentration falls (because of activity of Na/Ca pump)
2) Ca2+ no longer inhibits guanylyl cyclase, which recycles cGMP from GTP
3) cGMP is replaced quickly, and cGMP gated channels reopen, keeping the light response relatively small and brief
What is the distribution of rods and cones?
Receptor density decreases towards periphery
Cone density peaks in the fovea and falls off rapidly
Rod density peaks at ~18° eccentricity
Acuity declines with receptor density
Compare rods and cones
Cones mediate photopic vision and rods mediate scotopic vision
Cones are for day time light levels and rods are for low light levels
Cones are for a range of colours (trichromatic) and rods are for shades of grey
Cones are densest in the fovea and there are no rods in the fovea
The cone density falls off sharply in periphery but rod density rises near periphery
Cones have a low density to light and rods have a high sensitivity to light
Cones have a quick recovery in the dark and rods have a slow recovery in the dark
What is the receptive field (RF)?
The area on the receptor surface where an appropriate
stimulus causes a change in neuronal activity.
In the visual system, it is the region on the retina
where illumination influences a cell’s response.
What are bipolar cells?
They mediate the path of visual information between photoreceptors and ganglion cells
They receive direct excitory input from the photoreceptors in the centre and indirect inhibitory input from those in the surround
On/off bipolar cells
If you shine a light onto the center of a on-center cells do you get action potentials?
Yes
If you shine a light onto the center of an off-center cells do you get action potentials?
No
Take light away and then you get APs
If you shine a light onto the outside of a on-center cells do you get action potentials?
No
Take light away and then you get APs
If you shine a light onto the outside of an off-center cells do you get action potentials?
Yes
In your eye, where do the optic nerves meet?
In the chiasm
In the chiasm, hemidecussation takes place. What is this?
Half of the optic nerve fibers, from the temporal half of the retina eg, stay on the same side of the brain while the other half from the nasal part eg, cross over to the other side of the brain so the left half of the visual field is mapped on the right side of the brain vice versa
Describe the Lateral geniculate nucleus LGN
Consists of 6 layers
Receives input from both eyes
Each layer receives input from only 1 eye
Layers 1,4,6 are contralateral
Layers 2,3,5 are ipsilateral
At this point info from 2 eyes is still separated
Layers 1,2 are magnocellular
Layers 3-6 are parvocellular
The receptive fields are similar to retinal ganglion cells
Describe the primary visual cortex V1
Also known as the striate cortex or area 17
Consists of 6 layers
Contains retinotopic map
Receives inputs from both eyes
Input primarily arrives in layer 4
The left and right eye inputs in layer 4 segregate into ocular dominance columns
Cells outside layer 4 are mostly binocular
This means they receive inputs from both eyes
Are the V1 neurons sensitive to the orientation of a bar of light?
Yes
Only preferred orientation gives APs (diagonal) - Hubel and Wiesel 1962
What is a simple cell receptive field?
Adjacent stripes of on and off responses
What are orientation columns?
In the visual cortex
Neurons with similar orientation preferences are clustered