Problem 2 Flashcards
Cornea
transparent
–> most light photons are transmitted through it
(deutsch - Hornhaut )
–> accounts for 80% of the eyes focusing power
Aqueous Humour
fluid derived from blood
–> fills the space immediately behind the cornea
Iris
- Gives the eye its distictive color
- controls the size of the pupil
–> the amount of light that reaches the retina
Pupil
A hole in the iris
–> light enters here
Lens
Shape of it is controlled by the ciliary muscles
–> enables the changing of focus
–> accounts for 20% of the eyes focusing power
Vitreous Humour
Transparent fluid that fills the vitreous chamber in the posterior part of the eye
Retina
Is the network of neurons that covers the back of the eye
–> contains rods and cones
Optic Disc / Blind Spot
The point where
- the arteries + veins that feed the retina, enter the eye
- the axons of ganglion cells leave the eye
- there are no photoreceptors
Anatomy of rods and cones
- outer segment –> adjacent to pigment epithelium
- inner segment –> cell body
- synaptic terminal –> contain connections to horizontal + bipolar cells
Peripheral retina
Includes all of the retina outside of the fovea
- -> contains both rods and cones (S cones)
- -> used when detecting + localizing stimuli that we aren’t looking at directly
ex.: seeing a moving truck out of the corner of the eye
Opsin
Its structure determines which wavelengths of light the pigment molecule absorbs
Rhodopsin
Pigment that is found in rods
Ciliary muscles
Increase the focusing power of the lens by increasing the lens curvature
Accomodation
Change in the lens shape that occurs when the ciliary muscles tighten
–> this increases the curvature of the lens so that it gets thicker
Why is accommodation necessary ?
The increased curvature of the lens increases the bending of the light rays, pulling the focus point back to Point A
–> image is not blurred anymore
Near point
Distance at which the lens can no longer accommodate to bring close objects into focus
Presbyopia
Increasing distance of the near point as one gets older
- loss of this ability occurs because
a) lens hardens with age
b) ciliary muscles become weaker
Myopia / Nearsightedness
Inability to see distant objects clearly
–> myoptic optical system brings parallel light into focus at a point in front of the retina ( result : blurred image)
What are possible causes of Myopia ?
- Refractive myopia
–> cornea or lens bend the light too much
- Axial myopia
–> eyeball is too long
What are solutions for Myopia ?
- Moving objects closer
- Corrective glasses
- Surgical procedure
Far point
Distance at which light becomes focused on the retina
Hyperopia / Farsightedness
Inability to see near objects clearly
–> focus point for parallel rays of light is located behind the retina
What is the cause of Hyperopia ?
Too short eyeball
Solution for Hyperopia ?
Corrective glasses
Functions of Visual pigments
- Trigger electrical signals
2. Determine our ability to see dim light + light in different parts of the visual system
What are the components of a visual pigment ?
Opsin –> long protein
Retinal –> small, light sensitive component
Isomerization
Describes the retinals change of shape when the visual pigment molecule absorbs one photon of light
Dark adaptation
The process of increasing sensitivity in the dark
–> rod and cone receptors adapt to the dark at different rates
Fovea
small area on the retina, contains only cones
–> image falls on fovea when we look directly at an object
–> specialized for seeing fine details
(M + L cones)
Macular degeneration
condition that destroys the fovea and a small area surrounding it
–> results in a blind region in central vision
Why is the blind spot hard to detect ?
a) Mechanisms in the brain fill in the space where the image disappears
b) it is located off to the side of the visual field
Process of dark adaptation in steps
- Sensitivity of cones + rods begins to increase
–> we still see with our cones cause they are much more sensitive
- After 3-5 min the cones have reached their maximum sensitivity, while rods are still adapting
- ROD CONE BREAK: after 7 min rods become more sensitive than the cones
- After 20-30 min rods reach their maximum sensitivity
Visual pigment bleaching
Process whereby the molecule becomes lighter in color, after the retinal separates from the opsin after isomerization
–> when pigments are bleached, they are no longer useful for vision
Visual pigment regeneration
Process of reforming the visual pigment so hat it can change light energy into electrical energy
–> retinal returns to its bent shape + becomes reattached to the opsin
What are two important connections between perception and physiology ?
- Our sensitivity depends on the concentration of the visual pigment
- The speed at which our sensitivity increases in the dark depends in the regeneration of the visual pigment
Pigment Epithelium
Layer that contains enzymes that are necessary for pigment regeneration
Detached Retina
Condition where the retina becomes detached from the pigment epithelium
–> bleached pigments can no longer be recombined, which results in blindness
How are the electrical signals transmitted ?
- Receptors transmit signals to the retina
- Signals are then transmitted through the optic nerve to the LGN
- Then to the visual receiving area of the cortex
Convergence
A number of neurons synapse onto a single neuron
–> Signals form rods converge more than those from cones
=> High convergence leads to high sensitivity + poor acuity and vice versa
e.g.: 120 rods –> 1 GC ; 6 cones –> 1 GC
Rods
Photoreceptor of the eye
–> result in better sensitivity in the dark
—> in general there are more rods than cones in the eye
Cones
Photoreceptor of the eye
–> result in better detail vision
–> there are 3 types of cone receptors, each having a different sensitivity to the wavelengths of light
–> operate at brighter levels than rods
Why are rods more sensitive than cones ?
- It takes less light to generate a response from an individual rod receptor
- Rods have grater convergence
–> rods ganglion cells fire at a lower intensity than cones due to their greater convergence
Visual Acuity
Refers to the ability to see details
–> cones have better acuity because they have less convergence
Receptive field
The retinal region over which a cell in the visual system can be influenced (excited or inhibited) by light
Center surround organization
The area in the center of the receptive field responds differently to light than the area in the surround of the RF
–> excitatory center - inhibitory surround RF vs.
Inhibitory center - excitatory surround RF
Center surround agonism
When the spot of light becomes large enough to cover the inhibitory area, this stimulation counteracts the centers excitatory response
–> decrease in the neurons firing rate
–> neurons will respond best to spots of light that have the size of the excitatory center
Process of turning light energy into electrical energy
- PHOTOACTIVATION:
Photon enters the outer segment of the photoreceptor + is absorbed by a molecule of the visual pigment
–> photon transfers its energy to the retinal
- ISOMERIZATION
- REGENERATION
- ENZYME CASCADE
- -> Sequence of reactions that results in transduction - HYPERPOLARIZATION:
Ca+ channels close which causes a decrease in glutamate concentration
–> this signals the bipolar cell that the rod has captured a photon
Horizontal cells
Retinal cell that contacts photoreceptors + bipolar cells
Amacrine cells
Retinal cell that contacts
- bipolar cells
- ganglion cells
- other amacrine cells
On Bipolar cell
Responds to an increase in light captured by the cone
Off bipolar cell
Responds to a decrease in light captured by the cone
Role of on and off bipolar cells ?
- provide information about whether retinal illumination increased or decreased
–> each foveal cone contacts both
Bipolar cell
Retinal cell that synapses with
- -> photoreceptors
- -> horizontal cells
=> it then passes the signals on to ganglion cells
Ganglion cell
Retinal cell that receives signals from photoreceptors via bipolar + amacrine cells
–> Transit information to the brain + midbrain
P Ganglion Cell
Receives excitatory input from single midget bipolar cells
–> feed the parvocellular (small/petit) layer of the LGN
M Ganglion cell
Receives excitatory input form diffuse bipolar cells
–> feed the magnocellular (large) layer of the LGN
Characteristics of photoreceptors
Info is passed unto bipolar cells via graded potentials
–> electrical potential that varies continuously in amplitude
–> do not respond in an all-or-nothing fashion
Why do cones have better visual acuity than rods ?
- Because they have less convergence
2. It takes more units of light to generate a response from individual cone receptors
Lateral inhibition
Refers to an effect in which illumination of receptors ‘inhibits’ the firing of neighboring receptors
–> if you illuminate a single receptor (A) you will get a large response; however, when you add illumination to A’s neighbors, the response in A decreases
Mach Band
Refers to an optical illusion that exaggerates the contrast between edges of the slightly differing shades of gray, as soon as they contact one another, by triggering edge-detection in the human visual system
Hermann grid
Refers to an optical illusion where you see dark patches appear in the “street crossings”, except the ones which you are directly looking at
