Visual System Flashcards
What are the components of the anterior visual system?
Globe
Retina
EOM
What is vision?
A perceptual phenomenon initiated by the capture of light incident upon the retina in the posterior eye. Most of what we see is actually light that is being reflected off of objects. However, we can visualize light that is being emitted (flame, lightbulb, sun, etc.)
Light is […] in the cornea
Refracted
What does the lens do?
Focus the incoming light on the retina in the back of the eye
What does the pupil do?
THE PUPIL IS NOT A PHYSICAL THING. IT IS THE HOLE IN THE MIDDLE OF THE IRIS. Essentially, the iris constricts and dilates resulting in a change in the size of the hole that it allows light to pass through before hitting the lens. The hole is the pupil.
What is the macula of the eye?
It contains the highest density of photo receptors, region of highest visual acuity
What is the choroid?
Layer containing blood vessels that lines the back of the eye and is located between the retina (the inner light-sensitive layer) and the sclera (the outer white eye wall).
What is the ciliary body?
FOCUSES LENS
Structure containing muscle and is located behind the iris
What is the iris?
The colored part of the eye which helps regulate the amount of light entering the eye. Control the aperture of the eye (how dilated or constricted the light is coming into the lens)
What is the optic disc?
The optic disc is the site where blood supply and nerves enter and exit eye. There are no photoreceptors at the optic disc –> we’re blind to the part of the world that corresponds to that part of our visual field. Our brain interprets the other information it receive to fill in the gaps so we don’t have blind spots in vision.
What is this structure?
Macula
What cells capture light?
Rods (white light)
Cone (Red green blue)
Describe the pathway of phototransduction.
Light hits a rod (or cone) in the most posterior layer of the retina. The light hits a GPCR called Rhodopsin, causing a conformational change. This change in shape stimulates transducin, which stimulates phosphodiesterase, which converts cGMP into GMP and closes CNG which is a Na+ channel. This hyperpolarizes the cell. The opposite is true in the dark (cell is depolarized).
Photoreceptors can only signal the presence of light by […] or the absence of light by […]
Hyperpolarizing
Depolarizing
Describe the microcircuitry that is occurring between photoreceptors, bipolar cells, horizontal cells and retinal ganglion cells.
Light hits a group of photoreceptors in retina. Leads to hyperpolarization of cells. Group synapses with 1 bipolar “ON” cell, which then synapses with 1 ganglion “ON” cell. This pathway allows us to turn on specific neural circuitry when light is present.
When absence of light is detected, the photoreceptors that surround this “ON center” are activated and depolarized and synapse with a different bipolar “OFF” cell, which synapses with a different ganglion “OFF” cell which sends that information to the brain.
Horizontal cells collect input from surrounding photorecetors and antagonize the bipolar cell with which they are associated.
What is the purpose of organizing photoreceptors in the manner where there are a group of receptors that are turned “ON” by the presence (or absence) of light and those receptors are surrounded by other photoreceptors that, through the action of horizontal cells, antagonize that stimulus? Why not just have an excitatory center?
The information from the horizontal cells allows us to detect edges. If we just had an excited group of photoreceptors only then we’d see a blurry image (for ON receptors). The feedback from the surroundings allows us to discern more detail about the image.
Explain what is happening in this image.
For an On-Center Ganglion cell circuit:
- Light stimulates on center –> leads to excitatory impulses
- Light stimulates off center photoreceptors –> leads to inhibitory impulses
- Light stimulates both groups of photoreceptors –> tonic levels of firing
For an Off-center ganglion cell
- Light stimulates on center –> inhibition
- Light stimulates off center –> excitation
- Light stimulates both –> tonic levels
What structure is shown in this image?
The LGN
For lesions at each of the following locations, how would the person’s visual field be imparied?
Describe what the different layers of the LGN do. For simplicity, there are only 2 layers that we can chunk everything into.
Magnocellular layer - large receptive field, information about gross movement sent here, large diameter neurons
Parvocellular layer - small receptive field, information about fine details, smaller diameter
The blue and red lines drawn on the brain below represent pathways that convey information from the occipital lobe to the nearby parietal and temporal lobes. Each of these pathways is responsible for conveying different information about the visual information that was received at the visual cortex. What does each pathway convey?
Dorsal pathway = Where is the thing (analysis of movement and spatial relations)
Ventral pathway = what is the thing (analysis of form and color)
