Visual System

Question 1

What are the major functional groups of neurons in the retina?

Answer 1

• Outermost = pigmented epithelial cells
• Outer layer = rods and cone receptor cells
• Intermediate layer = network of interneurons and bipolar cells
• innermost layer = ganglion cells

Question 2

How is the visual signal transferred to the optic N?

Answer 2

photoreceptors –> bipolar cells –> ganglion cells –> optic N.

Question 3

Rods:

Answer 3

Rods:

• monocolor
• best for transient light
• DO saturate in daylight
• High sensitivity = night vision
• more photopigmentation than cones
• high amplification
• low temporal resolution
• low acuity
• achromatic, one type

Question 4

Cones:

Answer 4

Cones:

• color
• do NOT saturate in daylight
• most numerous at fovea
• low sensitivity = day vision
• high temporal resolution
• direct axial stimulation
• high acuity
• three types of cones

Question 5

Where could you find the highest visual acuity?

Answer 5

At the fovea.

Here, bipolar cells, interneurons and ganglion cells are pushed laterally to allow the light an unobstructed path to photoreceptor layer = unobstructive pathway. Also, there are no large blood vessesl here.

Mostly cones; almost 1:1 ratio cone:ganglion cell

Foveola = avascular zone

Question 6

What are the important components of the optic disc?

Answer 6

Where the retinal blood vessels come into the eye and the axons of the ganglion cells come together to form the optic nerve

Question 7

What axons cross at the optic chiasm?

Answer 7

axons from nasal retina

Question 8

How is information sent to the visual cortex?

Answer 8

From the retina, information travels through the optic nerve, nasal retinal fibers cross at the optic chiasm, continue through the optic tract to synapse at the lateral geniculate (in the thalamus. From here, optic radiations (geniculocalcrine tract) travel to the visual cortex.

Question 9

Where does the visual information terminate?

Answer 9

• Area 17, primary visual cortex in the occipital lobe
• Calcrine sulcus, specifically, the lower visual fields, terminate at the upper portion, where as the lower visual fields, terminate at the lower portion.

Question 10

What is the lateral geniculate?

Answer 10

• Relay nucleus part of the thalamus that receives the information from the optic tract and sends it to the primary visual cortex.
• Has 6 distinct layers, with M and P type cells

Question 11

From where is the majority of the information sent to the visual cortex coming from?

Answer 11

70% comes from the fovea

Although fovea is only 5% of the whole retina, it has the greatest concentration of cones; cones here have an almost 1:1 or 1:2 ratio with bipolar cells. Therefore, periphery information is diluted.

Question 12

What are the two major classes of ganglion cells found in the lateral geniculate?

Answer 12

M-type and P-type:

Question 13

What does the magnocellular layer consist of?

Answer 13

Layers 1 and 2

M type cells = large cell bodies, dense arborization, large receptive fields, huge dendritic fields that integrate info from a lot of bipolar cells (50:1 relation)

Respond best to movement and large objects; start/stop signals.

Rods

Question 14

What does the parvocellular layer consists of?

Answer 14

P-type cells = small cell body, 1:1 relation with cones, small receptive fields, more numerous near the fovea, wavelength selective. More layers and thicker = higher visual acuity, finer detail. Highly represented in lateral geniculate (layers 3-6)

Respond best to color and fine detail

Cones

Question 15

Where does perception of vision begin?

Answer 15

primary visual cortex, area 17 (striate cortex, V1)

Question 16

Not all information from the optic nerve travels to the lateral geniculate, where does a portion of the information travel to?

Answer 16

• Pretectal area for pupillary reflexes and the superior colliculus (head and neck movements).
• Superior colliculus, more from peripheral parts of the retina, helps to orient the head and eyes to that particular

Question 17

How does the primary visual cortex accommodate to the information it receives?

Answer 17

Visual cortex has an expanded layer IV.

Question 18

What are hypercolumns?

Answer 18

specialized columns within the cortical layers that process specific information:

1. Orientation (spatial representation/edges)
2. Blobs (color specificity)
3. Occular dominance (left or right eye)

Question 19

How is the information processed?

Answer 19

Each layer of the cortex has projections to other sites in the CNS via large pyramidal cells.

As the information ascends, it gets more complex.

Two pathways:
Dorsal Stream –> posterior parietal lobe
Ventral Stream –> inferior tempora cortex

Question 20

Explain the parietal pathway:

Answer 20

A dorsal stream of non-color discriminative info from the occipital cortex is sent to the POSTERIOR PARIETAL LOBE and contains info on WHERE an object is in the visual field.

Question 21

Explain the temporal pathway:

Answer 21

A ventral stream of info from both the color and non-color discriminitative cells in the occipital cortex flows towards the INF. TEMPORAL CORTEX, organized to respond to WHAT an object is- form, shape of object in the visual field.

Lesions to the inferior temporal gyrus, can cause visual agnosia (inability to identify an object)