Physiology of the visual system Flashcards
the fact or phenomenon of light being deflected in passing between one medium and another is the definition of?
Refraction
Where is the first site of refraction in the eye?
Cornea
2/3 of light bending
What adds a variable amount of light bending?
The lens
A rounder lens = ______ refraction
more
A flatter lens = _______ refraction
less
The ciliary muscle, suspensory ligaments allow for what?
The lens to curve
What muscle has to contract, to increase the curvature of the lens, leading to more refraction?
The ciliary mucle contracts, which allows the lens to loosen
What happens when the lens flattens(decreasing curvature) and therefore, decreasing refraction?
The ciliary muscle relaxes –> causing the suspensory ligaments to tighten and the lens is pulled tight
What is the term for when the lens becomes stiff with age and loses its elasticity?
Presbyopia
When focusing on a close object (the near response), what happens? (3)
- contraction of ciliary muscles (lens = rounder)
- convergence of eyes to point of focus
- constriction of pupil –> focus
Describe the path of light from where it enters the cornea to when it activates a photoreceptor:
There are 5 types of neurons in the retina, these are divided into either vertically oriented cells or horozontally oriented cells.
What 2 types are classified as horizontally oriented cells?
Horizontal cells
Amacrine cells
What 3 types of cells found in the retina are classified as vertically oriented cells?
- Receptor cells (rods + cones)
- Bipolar cells
- Ganglion cells (subtype = MG cells)
Rods utilize what property to allow them to operate in dim light?
Convergence
Many rods + bipolars –> one ganglion cell (sensitivity)
*cones do not do this*
Where are cones + rods the most dense in the retina?
Cones = fovea
Rods = outside ~ 20 degrees
What are rods + cones constantly releasing?
Glutamate!
When is glutamate released the most?
In the dark
How much glutamate is released when the cells are hyperpolarized?
a little bit of glutamate is released when there is light
light = little
Cones allow in light (photons) which do what to the photoreceptors?
Causes them to hyperpolarize, which decreases glutamate release
What are the 2 types of bipolar cells?
ON-center
OFF-center
Which part of an ON-center cell is depolarized upon its activation?
The center
What does an OFF-center cell mean?
Its depolarized in the periphery.
And hyperpolarized in the center
How is a bipolar cell depolarized?
The glutamate receptor in depolarizing bipolar cells is a GPCR and when it sees light it closes its channel, so less glutamate gets through.
In which type of bipolar cell does a sign change occur?
Depolarizing bipolars
What type of glutamate receptor is found in hyperpolarizing bipolar cells?
Common, excitatory glutamate receptor (non-nmda: ampa + kainate)
Dark + ON-center =
Hyperpolarized
Light + ON-center =
Depolarized
Light + OFF-center =
Hyperpolarized
Dark + OFF-center =
Depolarized
In order to see in “low-light” situations, many rods converge onto a ____ cell.
ON-center bipolar cell
What is the function of the ON + OFF center cells?
To help us increase our ability to detect edges + sharpen our vision
What do ON-center cells tell us?
Where something is
(excited by bright spot)
What do OFF-center cells tell us?
Where something ends
(excited by dark spot)
What are the direct targets of the retina?
Lateral geniculate body
Superior Colliculus
Pretectum (pupillary light reflex)
Hypothalamus
Accessory optic nuclei
What is the function of the lateral geniculate body?
- Control the motions of the eyes to converge on a pointo of interest
- Control the focus based on distance
- Create a map of objects in space
- Detect movement relative to an object
Area 17 =
Primary visual cortex
Area 18 =
Parastriate cortex
Area 19 =
Peristriate cortex
V1 =
Primary visual cortex (area 17)
V2 + V3 =
Brodmann 18
V4 =
area 19
V5 =
middle tempoaral + 19
Which layer recieves input from LGB?
Layer 4
Which layers are the main output layers?
5 + 6
Where are the ocular dominance (one column = one eye L or R) columns found?
The primary visual cortex
What columns are oriented perpendicular to the cortical surface + excited by visual line stimuli?
Orientation columns
(ex. vertically responsive orientation columns see things running up + down)
What layer are columns found in?
They span all 6 layers of the cortex!
Organized regions that are sensitive to color are called?
Blobs (in the primary visual cortex)
Red cones =
564 nm
Green cones =
~ 500 nm
Blue cone =
437 nm
How are these 3 things mapped?
- ocular dominance columns
- Orientation columns
- Cytochrome oxidase
- stripes
- swirls
- blobs
What is the major function of V1 of the primary visual cortex?
To identify edges + contours of objects
What is the function of V2?
Depth perception (via looking at the disparities between the two eyes)
What is the function of V3a?
Identification of motion
What is the funcion of V4?
Color input
The dorsal “where” pathway runs from _____ to ______.
Function?
Primary visual cortex –> parietal/frontal cortex
It passes through V3 to complete motor acts based on visual input.
The ventral “what” pathway runs from ______ to ________.
Function?
From primary visual cortex to inferior temporal cortex
Interpretes images + patterns (copying/naming objects + facial recognition)
What is the importance of the non-image-forming light-responsive melanopsin ganaglion cells?
Help synchronize circadian rhythms via melanopsin stimulating MG cells w/ Ca+
(might be another photosensitive cell (like rods + cones–> project onto hypothalamus))
