visual tracts Flashcards
visual system
formation of a light image (photons) by the phoreceptive retina
-info transduced by rods and cones —-> ganglion cells –> higher order cells –> thalamus –> visual pathway
retinotopic
arrangement of fibers that is maintained at each relay location
-small regions of the retina are represented in specific regions centrally
visual fields
area that a person is able to see when both eyes are fixed in one position
retinal field
light passes from objects in the visual field through the pupil to subtend an image upon the retina
fovea centralis and macula lutea
location where the object of attention is focused and centered in the retinal field
optic disc
medial to the macula
- region where retinal axons leave the eye as the optic nerve
- no photoreceptors –> blind spot
visual field zones
- binocular zone
- monocular zone (R/L)
binocular zone
broad central region seen by both eyes
monocular zone
seen only by the corresponding eye
retinal hemifields
nasal and temporal halves
quadrants
each hemifield is divided into upper and lower parts
visual system lesions are described in terms of their ____
visual deficits
What way is the image formed on the retina inverted?
both lateral and vertical dimensions
decussation of CN II
two optic nerves partially decussate in the optic chiasm
- nasal half of each retins –> contralateral optic tract
- temporal half of each retina –> ipsilateral optic tract
optic tract
fibers from temporal retina (ipsilateral eye) + fibers from nasal retina (contralateral eye)
depth perception
higher order function that needs to bring together info from comparable areas of both retinas
Where does the optic tract terminate?
lateral geniculate nucleus of the thalamus
lateral geniculate nucleus (LGN)
thalamus
- six layers with myelinated fibers sandwiched between them
- fibers terminate in a precise retinotopic pattern
optic tract (retinogeniculate) fibers
ventral base of lateral geniculate nucleus
optic radiations
dorsal and lateral borders of lateral geniculate nucleus
layers of the LGN
1-6
- magnocellular
- parvocellular
magnocellular (M) layers
1-2 ventral
- large cells
- ganglion cell inputs relaying from rods –> larger receptive fields and thick, rapidly conducting axons, sensitive to moving stimuli
parvocellular (P) layers
3-6 dorsal
- small cells
- ganglion cell inputs relaying from cones –> small receptive fields, slower conducting axons, tonically responsive to stationary stimuli, high-acuity color vision
temporal retina
remain uncrossed and terminate in layers 2, 3, 5 of ipsilateral LGN
nasal retina
cross and terminate in layers 1, 4, 6 of contralateral LGN
how many times is the same point represented in the LGN?
6 times
same medial-lateral position in layers 1-6 of LGN
layers 2, 3, 5 from ipsilateral temporal retina
layers 1, 4, 6 from contralateral nasal retina
optic radiation
secondary neurons from LGN extend a large bundle of myelinated fibers
-retinotopic organization maintained, but individual fibers carry info from one eye
-relay to primary visual cortex (striate cortex)
AKA geniculostriate or geniculocalcarine pathway
primary visual cortex
input from LGN optic radiation
-upper and lower banks of the calcarine sulcus
Where do fibers from the lower quadrant of the contralateral hemifields
- originate
- arch
- target
- originate from dorsomedial LGN
- arch caudally to pass retrolenticular limb of the internal capsule
- target superior bank of calcarine sulcus on the cuneus
Where do fibers from the upper quadrant of the contralateral hemifields
- originate
- arch
- target
- originate from the ventrolateral portion of the LGN
- do not pass directly caudal to the visual cortex
- arch rostrally passing into white matter of the temporal lobe to form a broad U-turn: Meyer loop
- target inferior bank of the calcarine sulcus on the lingual gyrus
Where to fibers conveying info from the macula and fovea
- originate
- target
- originate from central region of the LGN
- target caudal portions of the visual cortex
What can temporal lobe damage deficit?
superior visual field - Meyer’s loop
macula is represented by disproportionately ____ volumes relative to size of the LGN and visual cortex
large
Where is the macula represented in the occipital lobe?
occipital pole
What Brodmann’s area is the primary visual cortex?
17 - above and below the calcarine sulcus
- inferior fields above sulcus
- superior fields below sulcus
- macula posteriorly
visual association cortex
Brodmann’s areas 18-19 (rest of occipital lobe)
AKA extrastriate cortex
-parieto-occipito-temporal area
-heavily interconnected with 17
-LGN also sends targets - helps interpret location, motion, form, color
superior colliculus - visual cortical area
spatially directs head movements and visual reflexes
- retinal input: select fibers that bypass LGN, pass over MGN in the brachium of the superior colliculus, terminate retinotopically in the superior colliculus
- cortical input: area 17 via brachium, pattern coincides with retinotopic map
- spinotectal (somatosensory) and auditory inputs
DIRECTING EYE MOVEMENT
pretectal/pretectum area
bilateral group of interconnected nuclei near midbrain/forebrain junction
- inputs: afferent bilateral fibers from optic tract, LGN, and suprachiasmatic nucleus
- pretectal nuclei respond to varying intensities of illuminance
- mediate non-conscious behavioral responses to acute changes in light
PUPILLARY LIGHT REFLEX
pupillary light reflex
sympathetic postganglionic fibers (superior cervical ganglion) --> pupillary dilator m parasympathetic postganglionics (Edinger-Westphal nucleus) --> pupillary constrictor m
pupillary constriction
visceral motor efferents of CN III arise in Edinger-Westphal preganglionic nucleus
- preganglionic fibers terminate in ciliary ganglion and axons of postganglionic cells innervate: constrictor pupillae m of the iris and ciliary m
- cholinergic, preganglionic motor neurons travel with the ipsilateral oculomotor N and synapse in the ciliary ganglion
- cholinergic, postganglionic motor neurons send axons via the short ciliary nerve to supply ciliary m and pupillary constrictor
4 neuron arc of pupillary reflex
- retinal axons terminate in olivary pretectal nucleus (pretectum)
- bilateral projections to Edinger-Westphal preganglionic nucleus (contralateral travel with posterior commissure)
- parasympathetics exit w/ CN III synapse in ipsilateral ciliary ganglion
- postganglionic fibers (short ciliary nerves) excite pupillary constrictor m
What permits consensual response of pupillary constriction in opposite eye?
decussation of retinal fibers
What does the pupillary light reflex test?
CN III and brainstem function
lesion: loss of direct or consensual pupillary response or in uneven pupil size
anisocoria
uneven pupil size
What is a grave sign in an unconscious person?
dilated, unresponsive (fixed) pupils
What happens if CN III is lesioned?
loss of preganglionic fibers result in ipsilateral mydriasis (dilation of pupil) and paralyisis of accomodation
What happens if short ciliary nerves are damaged?
tonic dilated pupil
optic N partially damaged
shining light into damaged eye will produced diminished direct and consensual responses, but both will be present when the undamaged eye is illuminated
total lesion of optic N
blind eye, will not induce direct nor consensual response when eye is illuminated
lesion of optic tract or pretectum
neither direct or consensual response is lost, although reflexes may be weaker
large lesion in posterior (dorsal) midbrain
weaken pupillary responses bilaterally
lesion in oculomotor nucleus or nerve
both direct and consensual responses lost ipsilaterally, but will be present contralaterally
hemianopia
blindness (-anopia or -anopsia) in 1/2 of the visual field
quadrantanopia
blindness of a quadrant of a visual field
homonymous visual fields
conditions in visual field losses are similar in both eyes
heteronymous visual fields
conditions in which the two eyes have non-overlapping field losses
macular sparing
visual field loss that preserves vision in the center of the visual field
conguous
visual field loss of one eye can be superimposed on that of the other eye (symmetrical)
-more likely closer to visual cortex
inconguous
not symmetric
-more anterior lesion of optic tract or radiations
damage anterior to the chiasm affects
only the ipsilateral eye
damage at the chiasm causes
heteronymous deficits
damage behind chiasm causes
homonymous deficits
associative visual agnosia
- infarction of the left occipital lobe and posterior corpus callosum
- typically due to secondary occlusion of the posterior cerebral artery
- disconnects the language area from the visual association cortex
- pt cannot name or describe an object in the visual field, but he can recognize and demonstrate its use
- visual perception intact
- may be alexic (unable to read) and writing abiliaty may be affected agraphia
