Retina and Central Visual Pathways

Question 1

What are the characteristics of the fovea?

Answer 1

1. Everything past the outer plexiform layer is displaced to the macula area.
2. Only cones are found here
3. No blood vessels
4. Highest visual acuity

Question 2

What is the ora serrata?

Answer 2

Junction of the retina and ciliary body

Question 3

What is the shape of rods? What’s in them?

Answer 3

Outer cylindrical segment which contains membranous discs, having the protein opsin in them. Each opsin has an 11-cis retinal, making up rhodopsin

Question 4

What is the mechanism for rhodopsin work?

Answer 4

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

Question 5

What is the convergence of rods like?

Answer 5

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.

Question 6

What type of light conditions are rods used in? How is the convergence?

Answer 6

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

Question 7

What is the role of the retinal pigment epithelium?

Answer 7

1. Contain melanin pigment to prevent reflection off retina
2. Phagocytose portions of outer segment to help replenish the photopigment
3. Contribute to blood-retinal barrier.

Question 8

What do bipolar cells synapse with?

Answer 8

Receive input from cones and rods, synapse with ganglion + amacrine cells (3rd order)

Question 9

What does a specialized population of retinal ganglion cells do?

Answer 9

Called intrinsically photosensitive ganglion cells, contain melanopsin, they control circadian rhythms and the pupillary light reflex

Question 10

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?

Answer 10

1. Horizontal cells - synapse between photoreceptors in outer plexiform layer
2. Amacrine cells - synapse between bipolar cells

Use GABA or glycine, lateral inhibition

Question 11

What neurotransmitter do photoreceptors, bipolar, and ganglion cells use?

Answer 11

Glutamate

Question 12

What describes a group of inherited disorders in hwhich photorecptors / RPE abnormalities lead to retinal degeneration / visual loss? What is the progression of symptoms?

Answer 12

Retinitis pigmentosa. Night blindness -> tunnel vision

Question 13

What are the two types of AMD? What type of vision do they affect?

Answer 13

They affect central vision

Dry: 80-90%, due to drusen buildup
Wet: Due to abnormal blood vessel growth

Question 14

What is protanopia?

Answer 14

X-linked, red photopigment colorblindess. Very near green photopigment

Question 15

What is tritanopia?

Answer 15

Autosomal linked, blue photopigment colorblindess

Question 16

What is deuteranopia?

Answer 16

X-linked, green photopigment colorblindess. Very near red photopigment

Question 17

Where does retinal detachment occur and what does this cause?

Answer 17

Between RPE and photoreceptors. Deprives outer segments of oxygen and nutrients leading to blindness.

Question 18

How does the optic tract project to reach the thalamus?

Answer 18

On ventral lateral surface of hypothalamus, courses around cerebral peduncle to reach lateral geniculate nucleus.

Question 19

What two nuclei do the intrinsically photoreceptive retinal ganglion cells project to and what is their function?

Answer 19

1. Suprachiasmatic nucleus - in hypothalamus. Regulate Circadian rhythm
2. Pretectal nucleus - adjacent to posterior commissure. Regulate pupillary diameter

Question 20

What is the function of the superior colliculus in the visual pathway?

Answer 20

Rapid eye movements, shifting direction of gaze

Question 21

What nucleus is used for object recognition?

Answer 21

Lateral geniculate nucleus of the thalamus, medial to the inferior horn of the lateral ventricle.

Question 22

What is considered the center of the visual field, and where does the optic disc lie relative to this?

Answer 22

The fovea / macula complex is the center, with the optic disc being more medial (nasal).

Question 23

What fibers cross the midline in the optic chiasm (decussation)?

Answer 23

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).

Question 24

What is the ultimate result of the visual field contained in each optic tract?

Answer 24

Right optic tract contains left visual field, left optic tract contains right visual field (contralateral).

Question 25

What is the structure of the lateral geniculate nucleus?

Answer 25

Has 6 layers, with each layer getting input from one eye, alternating between each eye.

Question 26

What axons are sent from the lateral geniculate nucleus?

Answer 26

Geniculocalcarine tract, or “optic radiations”, following the retrolenticular portion of the internal capsule. This passes around the lateral ventricle

Question 27

What are the two places where the optic radiations end up? What is their path?

Answer 27

Both cortical lobes in posterior midline (area 17)

1. Cuneus - from direct, dorsal projection thru parietal lobe from LGN to the cortical lobe
2. Lingula - from Meyer’s loop, ventral projection into temporal lobe prior to reaching the cortical lobe

Question 28

What are the two separations of area 17 carrying? Where is it generally located?

Answer 28

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.

Question 29

What is the organization of the fibers in area 17 (striate cortex)?

Answer 29

Macula / fovea are overrepresented, and present at occipital pole. Horizontal axis runs away from the pole.

Question 30

What are the visual association areas?

Answer 30

Areas 18 and 19. 19 is particular important in color, form, and motion analysis.

Question 31

What are the two major pathways of visual analysis originating in the occipital lobe?

Answer 31

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

Question 32

What is a scotoma and what causes it?

Answer 32

Deficit in visual field. Caused by damage in retina or visual pathway up to striate cortex (not processing / analysis of info).

Question 33

What is homonymous vs heteronymous damage?

Answer 33

Same vs different field affected in each eye. I.e. damage to one optic tract = homonymous. Damage to optic chiasm = heteronymous.

Question 34

What is heteronymous hemianopsia which is bitemporal caused by?

Answer 34

Damage to optic chiasm

Question 35

What is quadranopsia?

Answer 35

only a single quadrant of the visual field is affected

Question 36

What causes “pie in the sky” visual loss? What is the name of this defect?

Answer 36

Damage to Meyer’s loop fibers in the temporal lobe. Superior homonymous quadranopsia.

Question 37

What does damage to upper or lower cortex alone calcarine sulcus cause?

Answer 37

Homonymous quadranopsia in lower or upper contralateral visual field, respectively

Question 38

What is the blood supply to the area 17 cortex, aside from macula?

Answer 38

Posterior cerebral artery

Question 39

What artery supplies the macula?

Answer 39

Middle cerebral artery