Retina and Central Visual Pathways Flashcards
What are the characteristics of the fovea?
- Everything past the outer plexiform layer is displaced to the macula area.
- Only cones are found here
- No blood vessels
- Highest visual acuity
What is the ora serrata?
Junction of the retina and ciliary body
What is the shape of rods? What’s in them?
Outer cylindrical segment which contains membranous discs, having the protein opsin in them. Each opsin has an 11-cis retinal, making up rhodopsin
What is the mechanism for rhodopsin work?
Light striking 11-cis retinal converts it to all-transretinal. Opsin is GPCR, which uses the protein transducin to decrease glutamate release by that rod cell. Rhodopsin is recycled by retinal pigment epithelium
What is the convergence of rods like?
100 rod cells per 1 ganglion cell, favors sensitivity but loss of spatial resolution. This lets rods have a high sensitivity to dim light (scotopic vision), also they are greater length to pick up more light.
What type of light conditions are rods used in? How is the convergence?
Photopic conditions, to see RGB sensitivity via 3 different opsins. Low convergence: 1:1 for rod to ganglion cell in fovea, 5:1 in rest of eye. High resolution, poor sensitivity
What is the role of the retinal pigment epithelium?
- Contain melanin pigment to prevent reflection off retina
- Phagocytose portions of outer segment to help replenish the photopigment
- Contribute to blood-retinal barrier.
What do bipolar cells synapse with?
Receive input from cones and rods, synapse with ganglion + amacrine cells (3rd order)
What does a specialized population of retinal ganglion cells do?
Called intrinsically photosensitive ganglion cells, contain melanopsin, they control circadian rhythms and the pupillary light reflex
What are the two types of interneurons in the retina and where do they synapse? What neurotransmitter do they use and what is their function?
- Horizontal cells - synapse between photoreceptors in outer plexiform layer
- Amacrine cells - synapse between bipolar cells
Use GABA or glycine, lateral inhibition
What neurotransmitter do photoreceptors, bipolar, and ganglion cells use?
Glutamate
What describes a group of inherited disorders in hwhich photorecptors / RPE abnormalities lead to retinal degeneration / visual loss? What is the progression of symptoms?
Retinitis pigmentosa. Night blindness -> tunnel vision
What are the two types of AMD? What type of vision do they affect?
They affect central vision
Dry: 80-90%, due to drusen buildup
Wet: Due to abnormal blood vessel growth
What is protanopia?
X-linked, red photopigment colorblindess. Very near green photopigment
What is tritanopia?
Autosomal linked, blue photopigment colorblindess
What is deuteranopia?
X-linked, green photopigment colorblindess. Very near red photopigment
Where does retinal detachment occur and what does this cause?
Between RPE and photoreceptors. Deprives outer segments of oxygen and nutrients leading to blindness.
How does the optic tract project to reach the thalamus?
On ventral lateral surface of hypothalamus, courses around cerebral peduncle to reach lateral geniculate nucleus.
What two nuclei do the intrinsically photoreceptive retinal ganglion cells project to and what is their function?
- Suprachiasmatic nucleus - in hypothalamus. Regulate Circadian rhythm
- Pretectal nucleus - adjacent to posterior commissure. Regulate pupillary diameter
What is the function of the superior colliculus in the visual pathway?
Rapid eye movements, shifting direction of gaze
What nucleus is used for object recognition?
Lateral geniculate nucleus of the thalamus, medial to the inferior horn of the lateral ventricle.
What is considered the center of the visual field, and where does the optic disc lie relative to this?
The fovea / macula complex is the center, with the optic disc being more medial (nasal).
What fibers cross the midline in the optic chiasm (decussation)?
All ganglion cell axons which are nasal (medial) to the fovea in each eye. Thus, the right side of the visual field in the right eye, and the left side of the visual field in the left eye (since they are reversed via the lens).
What is the ultimate result of the visual field contained in each optic tract?
Right optic tract contains left visual field, left optic tract contains right visual field (contralateral).
What is the structure of the lateral geniculate nucleus?
Has 6 layers, with each layer getting input from one eye, alternating between each eye.
What axons are sent from the lateral geniculate nucleus?
Geniculocalcarine tract, or “optic radiations”, following the retrolenticular portion of the internal capsule. This passes around the lateral ventricle
What are the two places where the optic radiations end up? What is their path?
Both cortical lobes in posterior midline (area 17)
- Cuneus - from direct, dorsal projection thru parietal lobe from LGN to the cortical lobe
- Lingula - from Meyer’s loop, ventral projection into temporal lobe prior to reaching the cortical lobe
What are the two separations of area 17 carrying? Where is it generally located?
Cuneus - receives fibers more directly. Inferior visual field (above calcarine sulcus).
Lingula - Superior visual field (below calcarine sulcus).
Generally located posterior to parieto-occipital sulcus in occipital lobe, near occipital pole.
What is the organization of the fibers in area 17 (striate cortex)?
Macula / fovea are overrepresented, and present at occipital pole. Horizontal axis runs away from the pole.
What are the visual association areas?
Areas 18 and 19. 19 is particular important in color, form, and motion analysis.
What are the two major pathways of visual analysis originating in the occipital lobe?
Ventral to ventral temporal lobe: Recognition of faces and objections
Dorsal to parietal lobe: Spatial orientation - relating position of body to objects around it. Near areas 5,7 for processing of conscious proprioceptive info
What is a scotoma and what causes it?
Deficit in visual field. Caused by damage in retina or visual pathway up to striate cortex (not processing / analysis of info).
What is homonymous vs heteronymous damage?
Same vs different field affected in each eye. I.e. damage to one optic tract = homonymous. Damage to optic chiasm = heteronymous.
What is heteronymous hemianopsia which is bitemporal caused by?
Damage to optic chiasm
What is quadranopsia?
only a single quadrant of the visual field is affected
What causes “pie in the sky” visual loss? What is the name of this defect?
Damage to Meyer’s loop fibers in the temporal lobe. Superior homonymous quadranopsia.
What does damage to upper or lower cortex alone calcarine sulcus cause?
Homonymous quadranopsia in lower or upper contralateral visual field, respectively
What is the blood supply to the area 17 cortex, aside from macula?
Posterior cerebral artery
What artery supplies the macula?
Middle cerebral artery
