Physiology of the Visual System Flashcards
Rounder lens =
Flatter lens =
More refraction
Less refraction
To increase curvature of the lens:
What’s it used for?
Ciliary m. contracts, allowing suspensory l. to loosen. Lens becomes rounder.
Used for near vision.
To decrease curvature of the lens:
What’s it used for?
Ciliary m. relaxes, which causes suspensory l. to tighten. Lens flattens.
Used for far vision.
Presbyopia
When the lens becomes stiffer in aging, w/ loss of elasticity.
Near response (3 steps)
- Contraction of ciliary ms.
- Convergence of eyes to point of focus.
- Constriction of pupil.
Vertically oriented cells in the retina (3)
Receptor cells (rods and cones)
Bipolar cells
Ganglion cells
Horizontally oriented cells (2)
Horizontal cells
Amacrine cells
Rod system
Convergence: Many rods and bipolars converge onto 1 ganglion cells.
This allows rods to operate in dim light.
Cone system
Less convergence: Cone goes to 1 bipolar cell which goes to 1 ganglion cell.
Where do amount of cones peak in the retina?
Where do rods peak?
Cones: at fovea.
Rods: eccentricity of 20 degrees.
When is glutamate release greatest?
When is it lowest?
Greatest when it is dark.
Lowest when there is light.
Glutamate is constantly being released, though.
ON-center bipolar cells
Activation in the center causes depolarization.
Activation of the periphery causes hyperpolarization.
OFF-center bipolar cells
Activation in the center causes hyperpolarization.
Activation in the periphery causes depolarization.
Activation of ON-center bipolar cell by cone photoreceptor in the dark (3)
Center only
- Glutamate activates Gi on the ON-center cell.
- Decrease in Na+ influx into cell.
- Hyperpolarizes cell.
Activation of ON-center bipolar cell by a cone photoreceptor when light is present (4)
Center only
- Light decreases glutamate levels.
- Less activation of Gi.
- Increase in Na+ influx.
- Depolarization.
Activation of OFF-center cell by a cone photoreceptor in the dark (3)
Center only
- Glutamate activates AMPA/kainate.
- Increase in Na+.
- Depolarizes cell.
Activation of OFF-center cell by a cone when light is present (4)
Center only
- Less light –> less glutamate.
- Less activation of AMPA/kainate.
- Less influx of Na+.
- Hyperpolarization.
Activation of bipolar cell by rod photoreceptor (4)
- Many rods converge on one ON-center cell.
- Connects to a “rod-bipolar cell” and a “rod amacrine cell” which act as interneurons.
- Connects to a “cone-bipolar cell”.
- Connects to a ganglion cell.
ON-center cells tell us where _______
OFF-center cells tell us where _______
Where something is.
Where something ends.
What’s the function of the amacrine and horizontal cells?
They provide inhibitory modulation of nearby cells in the retina.
Direct targets of the retina (5)
LGN Superior colliculus Pretectum Hypothalamus Accessory optic nuclei (AON)
Function of the LGN/LGB
- Control motion of eyes to converge on a point.
- Control focus of eyes based on distance.
- Determine relative position of objects to map them in space.
- Detect movement relative to an object.
Brodmann’s areas of:
Primary visual cortex
Parastriate cortex
Peristriate cortex
Primary visual cortex - 17
Parastriate cortex - 18
Peristriate cortex - 19
Visual areas V1 V2 V3 V4 V5
V1 - primary visual cortex (17) V2 - greater part of Brodmann 18. V3 - narrow strip of Brodmann 18. V4 - area 19 V5 - middle temporal area (part of 19)
Which layers of the retina receive input from LGB?
Which layers are main output from LGB?
Layer IV
Layers V and VI.
Ocular dominance column
A slab of cells that preferentially respond to input from one eye or the other.
Orientation columns
How are they oriented?
Region of neurons that are excited by visual line stimuli of varying angles.
Perpendicular to cortex.
Blobs in primary visual cortex
Neurons that are sensitive to color. Requires all 3-colo-coding cones for accurate detection.
Which colors are absorbed at lowest wavelength to highest wavelength?
Lowest to highest: blue, green, red.
Major job of V1
Identify edges and contours of objects
Major job of V2
Detects depth perception by analyzing disparities between two eyes.
Major job of V3a
Identifies motion
Major job of V4
Complete color processing of color inputs.
Dorsal pathway
From primary visual cortex and goes to parietal/frontal cortex.
Primary path associating vision w/ movement.
Completes motor acts based on visual input.
Passes thru V3.
Ventral pathway
From primary visual cortex to inferior temporal cortex.
Primarily involved in interpreting images and complex patterns.
Explain melanopsin and circadian rhythms
MG cells detect light via melanopsin.
Lights, via melanopsin, causes changes in Ca++ levels in MG cells.
This is considered a non-image-forming light-responsive system.
Projects to thalamus.
