Vision and Ocular Pathways- Barber

Question 1

what are the 3 groups of neurons in the retina?

Answer 1

1. photoreceptors (rods and cones)
2. bipolar neurons
3. ganglion (multipolar)

axons of ganglia all fuse at the optic disk (blind spot, myelinated) and exit the orbit via the ocular nerve (CN II, unmyelinated)

Question 2

what are the 2 major fates for axons of the optic nerve?

Answer 2

1. most (80%) go to the lateral geniculate nucleus in the thalamus for the visual pathway
2. some (20%) go to the midbrain, mostly to the pretectal nucleus for the pupillary light reflex

Question 3

describe pendular nystagmus

Answer 3

no fast of slow phase, NOT vestibular disease, just congenital disruption of the very organized visual system

Question 4

what are the 4 important structures in the brain for ocular and visual pathways?

Answer 4

1. lateral geniculate nucleus in the thalamus; houses CN II
2. pretectal nucleus in the midbrain: houses CN III
3. occulomotor nucleus in the midbrain: houses CN III
4. occipital/visual cortex: most caudal part of the cerebrum

Question 5

describe the pathway for visual response

Answer 5

1. light enters at retina
2. light is sent via the optic nerve to crossover at the optic chiasm to the optic tract on the contralateral side
3. information reaches the lateral geniculate nucleus in the thalamus
4. optic radiations from the lateral geniculate nucleus reach the occipital cortex in the cerebrum where the animal now consciously recognizes what they saw

Question 6

describe the result of a lesion in the retina, in the optic nerve, and in the prosencephalon would do to the visual pathway

Answer 6

at retina: no light/info can enter the eye = no vision

at optic nerve: no light/info can enter eye = no vision

at prosencephalon: animal will technically be able to see out of the contralateral eye (if lesion in L side, can still technically see out of right side), but will not be able to consciously recognize that or do anything with the information, so also = no vision

Question 7

describe the pathway of the menace response (7)

Answer 7

1. menacing gesture is seen/info goes into the retina and is sent via the optic nerve
2. information crosses at the optic chiasm to the contralateral lateral geniculate nucleus in the thalamus
3. optic radiations send the information to the occipital cortex in the cerebrum (which is why this is a conscious response, not a reflex)
4. (extra: info goes rostral to the primary motor cortex and then comes back caudal to the pontine nucleus)
5. information then goes to the cerebellar nucleus IPSILATERAL to the side that received the stimulus
6. information is sent bilaterally to the facial nerve nuclei (CN 7)
7. information courses cranially through the facial nerve and BOTH EYES BLINK!!!!!

Question 8

describe the consequence of a lesion in the retina and optic nerve, in the prosencephalon, in the cerebellar nucleus, and along the path of cranial nerve 7 on the menace response

Answer 8

retina and optic nerve: no vision = no menace (ipsilateral)

prosencephalon lesion: no perception of vision = no menace (contralateral)

lesion in the cerebellar nucleus = MAY cause an ipsilateral menace deficit, but animal will still be visual (more commonly cerebellar lesions do not cause a menace deficit)

lesion along CN 7: CAN’T blink, even though can see! (ipsilateral)

Question 9

describe the pupillary light reflex response pathway

Answer 9

1. light enters retina and travels through optic nerve to cross at optic chiasm
2. information travels to the contralateral pretectal nucleus
3. most of the information goes to the bilateral parasympathetic nuclei on the ipsilateral side that received the stimulation
4. some of the information, however, goes right to the bilateral parasympathetic nuclei on the side originally contralateral to the side that received the stimulus (if stimulus in L eye, this info stays across chiasm on R side)
5. information travels up each side (depending on if its respective parasympathetic nucleus received input) via CN III to constrict the pupil
   (this reflex is bilateral but skewed toward ipsilateral pupil, meaning that if light shine in L eye, L pupil (ipsilateral) directly constricts to a greater degree than the right pupil, but the right/contralateral pupil will still constrict

Question 10

describe the effects of a lesion in the retina/optic nerve, in the prosencephalon, and in CN III on the pupillary light reflex

Answer 10

retina/optic nerve: no vision = no PLR either side

lesion in prosencephalon: PLR intact because this reflex does not involve the prosencephalon

lesion in left CN 3: no L pupil constriction, but still some right pupil constriction!! same for other side; if lesion in in left CN 3, L pupil will be huge because cannot constrict at all

Question 11

describe the effects of a lesion rostral to the optic chiasm, caudal to the optic chiasm, prosencephalon, in the retina/optic nerve and in CN 3 on menace and PLR

Answer 11

rostral to optic chiasm: menace AND PLR affected together

caudal to optic chiasm: affect either PLR or menace separately

damage to retina/optic nerve (rostral to optic chiasm): no vision, no menace, no PLR

lesion is prosencephalon: no menace, but PLR intact

lesion in CN 3: no PLR but menace intact