Visual pathways Flashcards
1
Q
Overview of the visual pathway
A
- Begins with neural retina forming the optic nerve
- Partial decussation of fibers at optic chiasm leading to two optic tracts
- Optic tracts synapse onto the lateral gesticulate nucleus (LGN) in the thalamus
- Axons of the LGN form the optic radiations and terminate in the primary visual cortex (v1) of the occipital lobe
2
Q
3 functional layers of the retina
A
- Photoreceptors
- Bipolar cells and integrators (amacrine, horizontal)
- Retinal ganglion cells (form optic disc and axons make optic nerve)
3
Q
Retinal quadrants and visual fields (VFs)
A
- Two visual fields (one from R and L eye) and each VF is broken up into 2 halves: nasal (medial) and temporal (lateral)
- Each half is broken up into quadrants upper and lower
- Thus each VF has an upper nasal, an upper temporal, a lower nasal, and a lower temporal
- Two eyes means that the two VFs overlap to produce a single binocular field in the center and two peripheral rims of vision on both temporal extremes (of the VF)
4
Q
Inversion and reversal of the visual fields
A
- The right half of both visual fields is picked by the temporal side of the left eye and the nasal side of the right eye (left sides of both retinas)
- The left half of both VFs is picked up by the temporal side of the right eye and nasal side of the left eye (right sides of both retinas)
- This is reversal (things to the right are picked up by the left sides of each eye)
- The upper quadrants of each VF are picked up by the lower 1/2 of each retina
- The lower quadrants of each VF are picked up by the upper 1/2 of each retina
- This is inversion (things in the upper part of VF are picked up by the lower part of each retina)
5
Q
Optic nerves, chiasm, and tracts 1
A
- Each optic nerve carries both halves of the VF for that eye
- Therefore damage to an optic nerve results in loss of all vision in the ipsilateral eye (anopsia)
- Nasal fibers of each optic nerve (carrying info about the temporal 1/2 of each VF) cross at the optic chiasm (53%), temporal fibers remain uncrossed
- Therefore damage to the medial part of optic chiasm only affecting crossing fibers leads to loss of temporal half of both VFs (bitemporal hemianopsia)
6
Q
Optic nerves, chiasm, and tracts 2
A
- Each optic tract contains uncrossed fibers from the temporal half of the ipsilateral retinal (carrying the nasal half of the VF) and the crossed fibers from the nasal half of the contralateral retina (carrying the temporal half of the VF)
- At this point all info in one optic tract covers the same half of both visual fields (i.e. the right optic tract contains the L VFs for both the L and R eyes)
- Damage to an optic tract results in homonymous hemianopsia, where both eyes lose the L VF (if the R optic tract is damaged)
7
Q
Lateral genticulate nucleus
A
- LGN receives the axons from optic tracts, layers 2, 3, 5 (prime numbers) are the ones that receive info from ipsilateral eye (about contralateral VF)
- The axons that do not end on the LGN will mediate visual reflexes by synapsing on the brainstem
- In the LGN the fibers from the lower retinal quadrants (containing info about the upper quadrants of the VFs) are laterally to those form the upper retinal quadrants (which contain info about the lower quadrants of the VFs)
- Macular fibers are in the center of the peripheral fields
- Axons project to cortex as geniculocalcarine tract or optic radiations
8
Q
Optic radiations 1
A
- After leaving the LGN the optic radiations pass thru the posterior limb of the internal capsule for a short distance (posterior to the sensory and motor fibers)
- After the IC the fibers spread out in the white matter of the temporal and parietal lobes (breaks each VF half into quadrants- upper and lower)
- The fibers that run along the parietal lobe (superior fibers) project to v1 and are derived from the upper quadrant of the ipsilateral side of each retina (thus contain info about the contralateral lower quadrant of each VF)
- Therefore damage to the superior fibers result in loss of vision in the lower contralateral VF of each eye (contralateral inferior quadrantanopsia)
9
Q
Optic radiations 2
A
- The inferior fibers that run through the temporal lobe (around the inferior horn of the LV forming the loop of Meyer) project to v1 and are derived from the lower quadrant of the ipsilateral side of each retina (and thus contain info about the contralateral upper quadrant of each VF)
- Therefore damage to the inferior fibers result in loss of vision in the upper contralateral VF of each eye (contralateral superior quadrantanopsia)
- Lesions to both superior and inferior fibers (or v1) leads to contralateral hemianopsia, where the entire contralateral half of the VFs for each eye is lost (contralateral to side of lesion)
10
Q
Primary visual cortex (v1) 1
A
- Optic radiations from one side innervate the same side of v1 (do not decussate), meaning that each v1 receives info about the VFs of the contralateral side
- The cortex occupies the gyro within and adjacent to the calcarine sulcus on the medial surface of the occipital lobe
- Axons terminating in the upper calcarine sulcus are derived from the superior fibers from that side (i.e. have info about the lower contralateral quadrant of the VFs)
11
Q
Primary visual cortex (v1) 2
A
- Axons terminating in the lower calcarine are derived from the inferior fibers from that side (i.e. have info about the upper contralateral quadrant of the VFs)
- Macula is represented most posteriorly and the peripheral fields are more anterior (large cortical devotion to macula/fovea since it is the highest resolution)
- The cortex functions in visual perception and initial processing of visual info (sends out fibers to association cortices)
12
Q
Association cortices for vision
A
- Visual association cortex found in the medial and lateral surface of occipital lobe, receiving input from v1
- Functions of major attributes of visual stimuli (form, color, depth, location, motion
- Additional visual association areas receive input from visual association cortex, there are 2 additional visual association areas
- The first is the ventral pathway (what pathway- P cells) where the perception of shape and color are used for object ID
- The second is the dorsal pathway (where pathway- M cells) which analyzes motion and location of objects in the visual field
- P and M cells divide from each other at LGN
13
Q
Terms for visual defects
A
- Anopsia: loss of vision in entire field
- Hemi: loss of vision in 1/2 of a VF
- Bi: loss of vision in 2 parts of the VFs (usually bitemporal)
- Quadrant: loss of vision in 1/4 of VFs
- Homonymous: related to the same halves/quadrants of the two VFs (i.e. contralateral homonymous hemianopsia means both eyes lost info from 1/2 of their VF, with this loss being contralateral to the side of the lesion)
14
Q
General principles of lesions to eye pathways
A
- Lesions anterior to chiasm only affects corresponding eye
- Lesions posterior to chiasm affect both eyes, and of the homonymous side of the VFs
- Lesions at the chiasm affect both eyes but opposite sides of the VFs
15
Q
Damage to retina
A
- Focal damage results in scotomas (small blind spots) which may go unnoticed due to binocular compensation. Causes include degenerative diseases, infarcts, hemorrhage, diabetic neuropathy, infection
- Papilledema: edema of the optic disc due to increased intracranial pressure. Seen as blurring of the optic disc (mild) or obliteration of the vessels at the disc margin and hemorrhage (severe)
- Retinal detachment (often diabetic neuropathy) causes blindness
