Visual Systems Flashcards
List the pathway of light as it enters the eye
Through the cornea, lens, vitreous and formaing an image on the retina (inverted and reversed). In the retina photorecptors transduce the light stimuli into eletctrical signals and relay the eletrical signals through a series of neurons to form the visal pathway
What is the pigmented layer?
Represented as a single layer of epithelial cells adjacent to the choroid of the eyeball
Where do the pigmented cell processes extend to?
Extend slightly into the neural layer
What do pigmented cells contain?
melanin pigment
What is the function of the pigmented layer?
Support to rods and cones
Absorb light and reduce light that can be reflected from the choroid
What is the neural layer?
Includes the photoreceptors of the retina and neurons that relay visual data to the brain
What is the ratio of rods to cones?
20:1
What photosensitive chemical is found in rods?
Rhodopsin
What does light do to rhodopsin?
Light changes the configuration of rhodopsin and results in a change in the membrane potential of the rod cell
What is the result of membrane potential change of the rod cells?
It leads to the rods transducing light information into visual data
Are the rod distribution even across the retina?
No the distribution of rods is uneven across the retina
Where are rods more prevalent in the retina?
More prevalent in the peripheral parts of the retina and absent from the most central retina
What is the function of rods?
Peripheral vision
Vision in low light conditions (low resolution vision)
What factors allow the rods to function?
Properties of rhodopsin, distribution of rods in the retina and details of the visual pathway
are there more rods or cones?
Rods
How many types of cone receptors are there?
3
What is different about each type of cone receptor?
Each includes one of three photosensitive pigments that are similar to rhodopsin and absorb either red, green or blue light
Where are cones found?
In the central-most retina
What happens to the ratio of cones to rods as it moves from the central retina to the periphery?
The ratio of cones to rods decreases
What is the function of cones?
Color vision
Vision of high resolution
What is the macula lutea?
“yellow spot”
Central part of retina or visual center
WHat is the relationship between the macula lutea to the visual axis?
Macula lutea is in line with the visual axis
WHat is the ratio of cones to rods at the macula lutea?
Ratio is high (cones > rods)
WHat type of vision is the macula lutea responsible for
Vision of high resolution or acuity
What is the fovea centralis?
Depression in center of macula
How does the size of the fovea centralis compare to that of the macula lutea?
The Macula lutea is about 3 times bigger
What is the center of the fovea known as?
foveola
What does the foveola contain?
Contains only cones
Where is the highest vision resolution found?
At the foveola
What is the ora serrata?
Peripheral border of the functional retina
What is the optic papilla also known as?
Optic Disc
What is the optic papilla?
Area of the retina where neuron fibers converge to form the optic nerve
What is the size of the optic papilla in comparison to the macula lutea?
About the same size
Where is the optic papilla located relative to the macula?
Medial or nasally and a little superior
HOw is the optic papilla easily identified?
It is the area of the retina where there is divergence/convergence of retinal vessels
What is the blind spot?
Where the optic disc forms and therefore no rods or cones. Or where retinal vessels converge (retinal artery, vein and optic nerve
What is a condition of increased intracranial pressure present as?
Papilladema
What are bipolar cells?
True neurons that innervate either rods or cones
How many rods are typically in contact with a bipolar that engages rods near the macula?
10 rods
How many rods are typically in contact with a bipolar that engages rods near the ora serrata?
100 rods
What is the significance of the summation of information from many rods onto a single bipolar cell?
The rod system is sensitive to low light vision and provide vision of low resolution
What is the ratio of cones to bipolar cells in the fovea?
1:1
What is the significance of the low ratio of cones to bipolar cells?
The cone system provides for vision of high resolution
What are ganglion cells?
Neurons that innervate bipolar cells
What is the ratio of ganglion cells to bipolar cells in the fovea?
1:1
Where are axons of the ganglion cells found?
They course along the inner retina adjacent to the vitreous body
Where do ganglion cell axons converge?
On the optic disc where they penetrate perforation in the sclera
What is the area where the ganglion cell axons pentetrate perforations in the sclera called?
lamina cribrosa
What do the ganglion cell axons penetration the lamina cribrosa form?
The optic nerve
How do the ganglion cell axons travel in regards to the macula lutea?
Axons diverage around the macula lutea
What is the visual field?
Area seen by both eyes
Does the visual field of one eye overlap the other?
Yes the visual field of one eye partially overlaps the visual field of the opposite eye
What are the two parts each visual field be identified as?
Peripheral or macular portions
How can the visual field be divided in right and left portions?
Divided into left and right halves by a vertical line through the center of the macular portion
How can the visual field be divided into upper and lower quadrants?
A horizontal line through the macula can divide each half into upper and lower quadrants
What portions make up the retinal field?
Retinal field has a macular portion for central vision and a peripheral portion
How can each retinal field be divided into right and left halves?
Retinal field can be divided by a vertical line through the center of the fovea
What are the two halves named?
Nasal and temporal half
Which half is lateral and which half is medial?
Lateral: temporal
Medial: nasal
How is each half of the retinal field divided into quadrants?
By a horizontal line through the center of the fovea
How are the images from the visual field relayed to the retinal fields?
Images are inverted and reversed onto the retinal folds
Where do visual images in the upper left quadrant of the visual field projects onto?
Lower nasal quadrant of the left retina
Lower temporal quadrant of the right retina
What is the organizational pattern for visual data from the retinal fields known as?
Retinotopic organization
Where is the visual data from both right halves of the retina organized and where does it project?
Visual data from the right halves of the retina or visual images from the left half are organized in the central pathway and project to the right side of the brain
What are optic nerves made of?
Axons of ganglion cells of the retina
What composes the right optic nerve?
Axons from the right retina
What is the optic chiasma?
It allows reorganization of optic nerve fibers
Where do fibers from the nasal halves of both retinae travel in the optic chiasma?
Fibers from the nasal halves decussate in the optic chiasma
Where do fibers from the tempora halves of both retinae travel in the optic chiasma?
Fibers from the temporal halves of the retinae do not decussate in the optic chiasma
What do optic tracts do?
Fibers of the optic tract carry visual data that represents images from the contralateral half of the visual field
The fibers in the right optic tract are from ganglion cells located where?
Nasal half of the left retina and temporal half of the right retina
Where do fibers of the optic tracts project to?
Fibers of the optic tract project to the lateral geniculate
What do lateral geniculate bodies do?
Relay nuclei of the thalamus of visual data
Where does the right lateral geniculate body receive visual data?
From the left half of the visual field
Where do the neurons of the lateral geniculate bodies project the axons?
Into the cerebral hemisphere through the posterior limb of the internal capsule
What fibers make up the optic radiations?
Geniculocalcarine tract
Where does the geniculocalcarine tract travel in regards to the ventricular system?
Tract passes around the lateral ventricle, collateral trigone and inferior horn of lateral ventricle
What is the Meyer’s loop?
A portion of the optic radiations that loop around the inferior horn of the lateral ventricle in the temporal lobe
What does the Meyer’s loop do?
Carry visual info from the contralateral upper quadrant of visual field
Where do optic radiation project the visual information?
To medial aspect of the occipital lobe
Where is the primary visual cortex found?
Located along the upper and lower borders of the calcarine sulcus
Where does most of the functional area of the primary visual cortex lie?
Lies deep in the sulcus and is represented only minimally on the medial surface of the hemisphere
What Brodmann’s area is the visual primary cortex found in?
17
Where is macular field of vision mapped in the visual cortex?
Posterior 1/3 of primary visual cortex
Where is peripheral vision mapped in the primary visual cortex?
Peripheral vision mapped in anterior 2/3 of primary visual cortex
What area of the visual field is found along the upper banks of the calcarine?
Contralateral lower quadrant of visual field
What vision area is found along the lower banks of the calcarine?
contralateral upper quadrant
What are the functions of the primary visual cortex?
permits conscious perception of vision or seeing
What would a unilateral destructive lesion of the calcarine cortex cause?
a loss of vision/blindness in the contralateral visual field
Cortical blindness
What areas of the brain does the visual association cortex correspond with?
Corresponds mostly to the medial and lateral parts of the occipital lobe (and the inferolateral surface of the temporal lobe)
What brodmann’s map area is the visual association cortex found in?
Area 18 & 19
Where does the visual association cortex receive information from?
Receives input from primary visual cortex and other areas of the brain
What is the function of the visual association cortex?
Permits integration of visual data with previous experiences
Permits recognition and appreciation of: color, objects faces, shapes, motion, spatial relations and compensations for movement
What would the effect of a lesion in the significant areas of the visual association cortex (assuming the primary visual cortex is intact)?
Not blind
Will have visual agnosia
What is visual agnosia?
Inability to appreciate the significance of visual field and visual data
What is an alternate pathway of projections of the visual pathway?
Some fibers of the optic tract bypass the lateral geniculate body and traverse through the superior brachium and terminate on neurons in the tectum of the midbrain
What do the projection that terminate on the tectum via the superior brachium influence?
Response of the pupil changes in light (pupillary light reflex - still present if cortically blind)
Reflex of involuntary movements of the eyes and head in response to an object in the visual field moving at high velocity
What does the reflex of ducking and avoiding require to be active?
Reflex requires that retinal fibers reach neurons of the superior colliculus which in turn influence motor areas of the brainstem and spinal cord
What are the fibers that connect the superior colliculus to the brainstem called?
Tectobulbar fibers
What are the fibers that connect the superior colliculus to the spinal cord called?
tectospinal fibers
How is the occipital lobe able to control reflexes that are visually dependent?
Some neurons of the primary and associative visual cortex project corticalfugal fibers through the geniculocalcarine tract past the lateral geniculate body, transverse throught the superior brachium and terminate in the superior colliculus.
Which cranial nerves do the tectobulbar tract fibers associate with?
CN III, IV, VI
What do the corticotectal projections thought to do?
Thought to help in coordinating involuntary movements of the eyes as in following a moving object across the visual field. Allows for smooth pursuit movements
What are smooth pursuit movements?
Automatic tracking of movements such as visual tracking maneuver such as maintenance of visual fixation on a moving object
