2 - Visual System Pathways Flashcards
How does the lens alter images?
Rods?
Cones?
Fovea?
Reverses, Inverts
Rods - Low light, periphery
Cones - Color, center (fovea)
Fovea - Center of cone field; no blood vessels/interneurons, etc.–nothing in the way
Temporal vs Nasal Visual Field
Temporal vs Nasual Retinal Field
Retinal blood supply
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
Overall Route of Phototransduction
- RPE - Retinal pigment epithelium - absorb/scatter light
- ONL - Outer Neuronal Layer - rods/cods
- OPL - Outer Plexiform Layer - horizontal/bipolar cells
- INL - Inner Nuclear Layer - bipolar
- IPL - Inner Plexiform Layer - bipolar/amacrine cells
- GCL - Ganglion Cell Layer - sends signals
**Don’t need to memorize layer names or oder; just rough concept of signal detection, modulation, and transmission**
What fibers is the orientation of data transfer to the optic nerve with respect to the visual field?
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
Optic N. vs Optic Chiasm vs Optic Tract
Optic Nerve - Pre-Chiasm
Optic Chiasm - Cross over
Optic Tract - Post-Chiasm
Four Promary Targets for RGC fibers in brain?
- Lateral Geniculate Nucleus (LGN) (thalamus)
- Superior Colliculus (midbrain)
- Pretectum (midbrain)
- Hypothalamus
Geniculocalcarine vs Extrageniculate Tracts
Geniculocalcarine - Primary projection to LGN
Extrageniculate - Primary projection to Superior Colliculus
Outermost portion of the retina?
What lies beneath?
What synapses occur here?
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)
Müller Glia (M) Cells
Processes spread the entire retina
Perform many functions, involved in synapse formation in the retina
Blood supply for Lateral Geniculate Nucleus (LGN)?
Anterior Choroidal Artery
Posterior Cerebral Artery (PCA)
Organization of LGN?
Role in binocular vision?
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
Parvocellular Layer of LGN
Color and Form Vision from Central Retina
Magnocellular Layers of LGN
Movement information of peripheral retina
How is information sent from the LGN to the Visual Cortex?
Geniculocalcarine Tract
Geniculocalcarine Tract
Function
Fibers + Travel Path
Blood Supply
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)
Geniculocalcarine Tract
Major Divisions
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
Geniculocalcarine Tract
Contralateral Hemifield of Vision
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***
Extrastriate Visual Pathways
Major Divisions
Common Target
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
Superior Colliculus
Function
Type of Information
Connection to LGN
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
Superior Colliculus:
Targets
1. Cranial Nerves III and IV: Regulate horizontal eye movement
- Upper Spinal Cord - via tectospinal tract; reflexive movement toward visual input
- Pons/Cerebellum - Coordinate eye/head movement; quick adaptation, balance, posture
- Pulvinar - Medially located thalamic nucleus
Pulvinar pathway / function
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**
Pretectum and Hypothalamus
RGCs project here (in addition to LGN/Superior Colliculus) to regulate eye response to changes in ambient light
Pretectum:
Function
Target
Regulates
Clinical Use
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
Hypothalamus (and vision)
Function
Clinical Relevance
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
Progressive Encephalization
Mammals have increased cortical processing of visual stimuli
Clinical: Cortical Blindness
How can “blindsight” occur?
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)
Clinical: Anton’s Syndrome
Visual Anosognosia
Denial of losing vision
Clinical: Visual Hallucinations cause?
Seizures in the inferior occipitotemporal cortex
Clinical: Prosopagnosia
Face blindness
right or bilateral fusiform gyrus lesion
“man who thought his wife was a hat”
Clinical: Achromatopsia
Disorder of color perception
Color perception occurs, but fails at identification stage; caused by fusiform gyris lesion
Clinical: Metamorphopsia
Distorion of size and shape (“Alice in Wonderland” syndrome)
Lesions in Inferior or Lateral Visual Association Cortex
Clinical: Balint’s Syndrome
Caused by bilateral lesions of Dosolateral parieto-occipital cortex (“where” stream)
Symptoms:
- Simultanagnosia (random shifting of visual field perception)
- Optic Ataxia (lack of hand eye coordination)
- Ocular Apraxia (impaired gaze direction/saccades)
