Visual Pathways Flashcards
These convert light energy into a neuronal signal via the release of NT in a processes known as phototransduction
Photoreceptors
Specialized for vision in dim light (motion detection) and dominate the midperipheral and peripheral retina
Rods
Responsible fo day time vision and diffrentailly sensitive to particular wavelengths of light
Cones
Where are cones most populated
Macula
Where are there only cones
Very center of macula
Where are there no rods
Very center of macula
The signal from the photoreceptors are passes on to the ____________ and modified
Retinal internuerons (bipolar cells, Amacrine cells, and horizontal cells)
Located between the receptor cell layer and the bipolar cell layer. Their processes lie parallel to the plane of the retina. Lateral processing of information between cells
Horizontal cells
Receive inputs from photoreceptor cell and project to ganglion cells
Bipolar cells
Located between the bipolar cell layer and the ganglion cell layer. These cells modulate activity between the two layers
Amacrine cells
Internuerons relay the signal to the _______, the output cells of the retina
Retinal ganglion cells
Encoding aspects of visual information such as color, image velocity, movement direction, and contrast
Subsets of retinal ganglion cells will selectively encode aspects of visual information such as color, image velocity, movement direction, acuity, and contrast. The ganglion cells are encoding these aspects of vision simultaneously, but through separate channels.
The retinal ganglion cells send their output via axons that collectively will form the ________ to communicate with the brain
Optic nerve
Axons of the nerve fiber layer at the optic disc
The axons of the nerve fiber layer make a 90 degree turn in the retina at the optic disc and will exit the eye as the optic nerve.
When do the axons of the nerve fiber layer get myelinated
After they exit the eye.
Where do 90% of the axonal fibers synapse from the optic nerve
In the LGN
Where do 10% of the axonal fibers from the optic nerve go
Project to areas involved with pupil responses and circadian rhythm
What kind of VF loss results from a complete transaction of the optic nerve
Complete loss of vision in that eye
The optic nerves continue posteriorly giving rise to a rectangular structure, the __________
Optic chiasm
What happens on the optic chiasm
Visual information from the right and the left eye will cross to the contralateral side of the visual system
How does the visual pathway continue after the optic chiasm
Optic tract
What fibers cross to the contralateral side at the optic chiasm
The nasal retinal fibers. The temporal fibers remain in ipsilateral tract and do not cross over
What lies above the optic chiasm
3rd ventricle
What lies below the optic chiasm
Pituitary gland
What is the significance of a pituitary adenoma to the visual system
The gland will enlarge and compress the optic chiasm that usually will cause visual deficits seen on VF tests as a bitemporal hemianopsia
What is the visual field defect that typically results from a pituitary adenoma
Bitemporal hemianopsia
What kind of VF deficit will result from an internal carotid aneurysm
Binasal hemianopsia
Why does the an aneurysm on one ICA cause compression on both sides of the optic chiasm
It may compress the temporal fibers of the optic chiasm. In doing so, the optic chiasm is often displaced and compressed against the contralateral ICA, which in turn compresses the temporal fibers on that same side
This is a flat band that extends from the posterolateral corners of the optic chiasm to the LGN
Optic tract
90% of all fibers from the optic tract synapse where
LGN
10% of the fibers from the optic tract synapse where?
In other structures. The afferent pupillomotor reflex fibers will leave the optic tract before reaching the LGN and synapse in the pretectal nucleus of the midbrain
What kind of fibers are in each optic tract
Fibers from the temporal retina of the ipsilateral eye and fibers from the nasal retina of the contralateral eye
What kind of information do the temporal retinal fibers carry
Visual information regarding the nasal VF
What kind of information do the nasal retinal fibers carry
Visual information associated with the temporal VF
What is the VF deficit that would result from right optic tract transaction?
Left hemianopsia
Where is visual information from the optic tracts related
LGN
What is the largest and most important primary visual nucleus in humans
LGN
Where is the LGN located
In the dorsolateral aspect of the thalamus
This is a cone shaped structure that is multilaminar
LGN
LGN and layers
It is 6-layered, each layer containing the same type of cells. The retinal ganglion cells that project to a specific layer each have their own distinct characteristics
The axons leaving the LGN are known as
Optic radiations
How do the optic radiations course
Sweep laterally and inferiorly around the anterior tip of the temporal horns of the lateral ventricle.
What optic radiations loop into the temporal lobe on their way to the occipital lobe
Meyers loop
A lesion where would give you superior quandrantonopia (pie in the sky)?
Meyers loop (temporal lobe)
What are the optic radiations that course through the parietal lobe called
Baum’s loop
What forms the superior radiations
Fibers leaving the medial LGN
What kind of information is carried in the Meyers loop?
Inferior retinal fibers (superior VF)
What kind of information carried in Baum’s loop
Superior retinal fibers (inferior VF)
What is another name for the primary visual cortex
V1 or Broadmann area 17
Where is the primary visual cortex located
Predominantly on the medial surface of the occipital lobe with just a small bit extending around the posterior pole onto the lateral surface
What divides the parietal lobes from the occipital lobes
The parieto-occipital sulcus
What is the upper portion of the visual cortex
Cuneus
What goes to the cuneus?
Superior retinal fibers (inferior VF)
What is the lower part of the visual cortex
Lingual
What goes to the lingual gyrus
Inferior retinal fibers (superior VF)
What separates the cuneus and the lingual gyri
Calcarine sulcus
Horizontal layers of the visual cortex
Most superficial is layer I and that which is most inward is layer VI. Each layer has a specific function
What horizontal layer of the visual cortex contains the white myelinated fiber layer known as the strip of gennari?
Layer IV
This is where the term striate cortex comes from
What is the striate cortex involved in
Binocular vision and contour analysis, which depend on both horizontal and vertical organization of the striate cortex
Where do the superior optic radiations terminate?
In the striate cortex superior to the calcarine fissure (cuneus gyrus)
Where do the inferior optic radiations terminate
In the striate cortex below the calcarine fissure (lingual gyrus)
VF defect with lesion at cuneate gyrus
Contralteral lower quadrantonopia
VF defect with lesion at lingual gyrus
Contralateral upper quadrantonopia
What are the areas of higher visual association that analyze, recognize, and interpret environment?
Area 18 (parastriate) Area 19 (peristriate) And others
How are areas in one hemisphere connected to the other hemisphere
Posterior commissure
Visual information is ultimately analyzed in the
Dominant parietal lobe
What has to happen when visual information is analyzed in the dominant parietal lobe
Visual information arriving in the right occipital cortex must be passed through the corpus callosum to the left parietal lobe to the recognized and interpreted.
Lesions of the left angular gyrus
Result in faulty integration of visual information despite intact primary visual pathway
Gives rise to LMN supplying EOMs (levator palpebrae superioris, superior rectus, inferior rectus, medial rectus and inferior oblique)
Oculomotor nucleus
Gives rise to preganglionic parasympathetic neurons to ultimately supply sphincter pupillae and ciliary muscles
EW nucleus
Fives rise to LMN supplying EOMS (superior oblique)
Trochlear nucleus
Gives rise to LMN supplying EOMS (lateral rectus )
Abducens nucleus
Accommodative pathway
- area 19 (visual association cortex) receives feedback from area 17 (primary visual cortex)
- area 19 projects to pretectal nucleus
- pretectal nucleus projects bilaterally to EW nucleus
- preganglionic fibers synapse in the ciliary ganglion
- postganglionic fibers supply the sphincter pupillae muscle to constrict the pupil and the ciliary muscle to thicken the lens
- CNIII also carries out convergence of the eyes
Accommodative triad
Accommodation
Pupillary constriction
Convergence
Pupillary dilation pathway (sympathetic innervation)
- post hypothalamus projects to intermediolateral cell column of the spinal cord at the upper thoracic levels
- preganglionic fibers synapse onto postganglionic neurons in the superior cervical ganglion
- postganglionic fibers form the ICA plexus and follow the ICA and its branches
- postganglionic fibers supply the dilator pupillae muscles via the long ciliary nerve
General schematics of primary visual pathway
Retina-optic nerve-chiasm-optic tract-LGN-optic radiations-visual cortex-visual association areas
Retinal fibers and the horizontal raphe
They don’t cross over it
Are peripheral or central retinal axons more superficial
Peripheral
What si the organization of the retinal nerve fibers at the optic disc
- inferonasal and inferotemporal take up 1/3
- macular fibers take up 1/3
- superonasal retinal fibers and superotemporal retinal fibers take up 1/3
What is the anterior knee of Willbrand
At the chiasm, when the nasal fibers cross over, the inferior nasal fibers will shot straight over and kind of bend upward before going down into the contralteral tract
What is the posterior knee of willbrand
The superior nasal fiber crossing the optic chiasm will go way down, almost into the ipsilateral optic tract and then come back up a little and shoot across the optic chiasm to contralateral tract
What layers of the LGN do the magnocellular fibers synapse
Layers 1 and 2
What layers of the LGN do the parvocellular fibers synapse in
Layers 3,4,6
What layers of the LGN synapse ipsilateral fibers
2,3,5 same side
What layers of the LGN synapse contralteral fibers
1,4,6
Lesion of the left inferior optic radiations (Meyers loop)
Right homonomyous superior quadrantonopia (pie in the sky)
Lesion of the superior optic radiation in the left parietal lobe (baums)
Right homonomyous inferior quadrantonopia
PITS
Parietal inferior temporal superior (referring to VF not retinal fibers)
Dorsal or parietal visual stream
Where
Ventral or temporal stream
What
Normal patient with the pupillary light reflex
Direct and consensual response to light shined in OD
Afferent pupillary defect OD
- No direct or consensual response to light shined in OD
- direct and consensual response to light shined OS
Blood supply to central visual structures
Vertebrobasilar
Difference between wet and dry AMD
- dry is essentially drusen in Bruch’s layer
- wet is where the RPE is sick, metabolic waste builds here, new vessels try to grow here from choroid and they are leaky
- VEGF is what causes the new vessels to form
Optic nerve transecting VF loss
Blindness in ipsilateral eye
Optic tract transection VF loss
Contralteral hemianopsia
Optic chiasm midsaggital transection
Bitemporal hemianopsia
Horners syndrome
- pupil constriction
- ptosis
- redness of skin
- facial anhydrosis
