2 - Visual System Pathways

Question 1

How does the lens alter images?

Rods?

Cones?

Fovea?

Answer 1

Reverses, Inverts

Rods - Low light, periphery

Cones - Color, center (fovea)

Fovea - Center of cone field; no blood vessels/interneurons, etc.–nothing in the way

Question 2

Temporal vs Nasal Visual Field

Temporal vs Nasual Retinal Field

Answer 2

Question 3

Retinal blood supply

Answer 3

Opthalmic Retinal Artery (CRA) - Penetrates meninges surrounding optic nerve and supplies inner retina

Ciliary Artery - Penetrates sclera near optic nerve exit and supplies the choriocapillaris

Question 4

Overall Route of Phototransduction

Answer 4

1. RPE - Retinal pigment epithelium - absorb/scatter light
2. ONL - Outer Neuronal Layer - rods/cods
3. OPL - Outer Plexiform Layer - horizontal/bipolar cells
4. INL - Inner Nuclear Layer - bipolar
5. IPL - Inner Plexiform Layer - bipolar/amacrine cells
6. GCL - Ganglion Cell Layer - sends signals

**Don’t need to memorize layer names or oder; just rough concept of signal detection, modulation, and transmission**

Question 5

What fibers is the orientation of data transfer to the optic nerve with respect to the visual field?

Answer 5

Only Nasal fibers cross optic chiasm (sees temporal part of visual field)

Temporal Fibers (sees nasal part of visual field) do NOT cross optic chiasm

Question 6

Optic N. vs Optic Chiasm vs Optic Tract

Answer 6

Optic Nerve - Pre-Chiasm

Optic Chiasm - Cross over

Optic Tract - Post-Chiasm

Question 7

Four Promary Targets for RGC fibers in brain?

Answer 7

1. Lateral Geniculate Nucleus (LGN) (thalamus)
2. Superior Colliculus (midbrain)
3. Pretectum (midbrain)
4. Hypothalamus

Question 8

Geniculocalcarine vs Extrageniculate Tracts

Answer 8

Geniculocalcarine - Primary projection to LGN

Extrageniculate - Primary projection to Superior Colliculus

Question 9

Outermost portion of the retina?

What lies beneath?

What synapses occur here?

Answer 9

Retinal Pigment Epithelium (RPE) - Melanin cointaining cuboidal cells

Outer Nuclear Layer (ONL) - Location of the photoreceptors

Synapses with Bipolar Cells, Horizontal Cells occur in the Outer Plexiform Layer (OPL)

Question 10

Müller Glia (M) Cells

Answer 10

Processes spread the entire retina

Perform many functions, involved in synapse formation in the retina

Question 11

Blood supply for Lateral Geniculate Nucleus (LGN)?

Answer 11

Anterior Choroidal Artery

Posterior Cerebral Artery (PCA)

Question 12

Organization of LGN?

Role in binocular vision?

Answer 12

Layered structure with each layer receiving input from one eye

For binocular zone, all points in space of the retina of each eye are represented in their entirety via a series of vertical columns in the LGN

Question 13

Parvocellular Layer of LGN

Answer 13

Color and Form Vision from Central Retina

Question 14

Magnocellular Layers of LGN

Answer 14

Movement information of peripheral retina

Question 15

How is information sent from the LGN to the Visual Cortex?

Answer 15

Geniculocalcarine Tract

Question 16

Geniculocalcarine Tract

Function

Fibers + Travel Path

Blood Supply

Answer 16

Function: Connects LGN to Primary Visual Cortex

Fibers + Travel: Optic Radiations, travel through retrolenticular portion of posterior limb of the internal capsule

Blood Supply: Middle and Posterior Cerebral Arterys (MCA/PCA)

Question 17

Geniculocalcarine Tract

Major Divisions

Answer 17

2x

Upper Division: Projects from MEDIAL LGN, travels OVER lateral ventricle towards upper portion of Visual Cortex–Cuneus Gyrus

Lower Division: Projects from LATERAL LGN, trabels UNDER inferior horn of lateral ventricle (Meyer’s Loop) towards lower portion of Visual Cortex–Lingual Gyrus

Question 18

Geniculocalcarine Tract

Contralateral Hemifield of Vision

Answer 18

Upper and Lower Optic Radiations in each hemisphers; represents half visual field

***Upper Division = LOWER half of visual field

Lower Division = UPPER half of visual field***

Question 19

Extrastriate Visual Pathways

Major Divisions

Common Target

Answer 19

Dorsal Extrastriate Projections: Connect V1/BA17 to Pareito-occipital association cortex (V5) - encode Motion and Spatial Relationships

Ventral Extrastriate Projections: Connect V1/BA17 to Occipitotemporal Association Cortex - encode Form (V3) and Color (V4)

- - -

In BOTH stream, connections to V2 (Prestriate Cortex/Secondary Visual Cortex) - provide further redinement of properties encodes by V1

Question 20

Superior Colliculus

Function

Type of Information

Connection to LGN

Answer 20

Tectal Structure

Function: Secondary Pathway that supplements Geniculocalcarine Tract; may compensate for lesions

Information: Moving stimuli, regulates Saccades (high velocity eye movements); important for “refreshing” eye gaze

Connection: Brachium connects to LGN

Question 21

Superior Colliculus:

Targets

Answer 21

1. Cranial Nerves III and IV: Regulate horizontal eye movement

2. Upper Spinal Cord - via tectospinal tract; reflexive movement toward visual input
3. Pons/Cerebellum - Coordinate eye/head movement; quick adaptation, balance, posture
4. Pulvinar - Medially located thalamic nucleus

Question 22

Pulvinar pathway / function

Answer 22

Function: Projects to extra striate visual areas, assimilate different modalities to create construct of objects in visual space

Pathway: Retina - Superior Colliculus - Pulvinar - Extrastriate Cortex

**Works in parallel with geniculocalcarine tract**

Question 23

Pretectum and Hypothalamus

Answer 23

RGCs project here (in addition to LGN/Superior Colliculus) to regulate eye response to changes in ambient light

Question 24

Pretectum:

Function

Target

Regulates

Clinical Use

Answer 24

Function: ACUTE changes to ambiet light

Target: Edinger-Westphal Nucleus; parasympathetic fibers project to the eye to regulate pupil constriction, lens accommodation, and eye convergence

Regulates: Pupillary Light Reflex through Edinger-Westphal Nucleus

Clinical: Test CN II and III

Question 25

Hypothalamus (and vision)

Function

Clinical Relevance

Answer 25

Function: Suprachiasmatic Nucleus (SCN) located here; receives input from primitive light-sensitive RGCs that respond to ambient light

Clinical Relevance: Links Circadian rhythem and hormonal cycles with levels of ambient light

Question 26

Progressive Encephalization

Answer 26

Mammals have increased cortical processing of visual stimuli

Question 27

Clinical: Cortical Blindness

How can “blindsight” occur?

Answer 27

Lesion of BA 17 / V1

Loss of conscious visual perception

Can still respond to stimuli w/out conscious perception

“Blindsight” is thought to occur via the Extrageniculate Pathway (retina-superior colliculus-pulvinar-extrastriate cortex)

Question 28

Clinical: Anton’s Syndrome

Answer 28

Visual Anosognosia

Denial of losing vision

Question 29

Clinical: Visual Hallucinations cause?

Answer 29

Seizures in the inferior occipitotemporal cortex

Question 30

Clinical: Prosopagnosia

Answer 30

Face blindness

right or bilateral fusiform gyrus lesion

“man who thought his wife was a hat”

Question 31

Clinical: Achromatopsia

Answer 31

Disorder of color perception

Color perception occurs, but fails at identification stage; caused by fusiform gyris lesion

Question 32

Clinical: Metamorphopsia

Answer 32

Distorion of size and shape (“Alice in Wonderland” syndrome)

Lesions in Inferior or Lateral Visual Association Cortex

Question 33

Clinical: Balint’s Syndrome

Answer 33

Caused by bilateral lesions of Dosolateral parieto-occipital cortex (“where” stream)

Symptoms:

1. Simultanagnosia (random shifting of visual field perception)
2. Optic Ataxia (lack of hand eye coordination)
3. Ocular Apraxia (impaired gaze direction/saccades)