Motor Innervation of the Eye

Question 1

anatomy of the pupillary light reflex

Answer 1

*afferent limb: axons of retinal ganglion cells follow the usual route (optic nerve, optic chiasm, optic tract) after exiting the retina
*these axons enter the brachium of the superior colliculus en route to their sire of termination, the PRETECTAL NUCLEUS
*from pretectal nucleus, the efferent limb begins
*fibers exit the pretectal nucleus and project BILAERALLY to the Edinger-Westphal nucleus; preganglionic parasympathetic fibers exit as part of CN III and travel to the ciliary ganglion and innervate the pupillary sphincter

Question 2

anatomy of the pupillary reflex (short version)

Answer 2

1) retinal ganglion cell axons exit the retina
2) optic nerve -> optic chiasm -> optic tract
3) brachium of superior collilcus
4) pretectal nucleus
5) fibers project BILATERALLY to Edinger-Westphal nucleus
6) preganglionic parasympathetics join CN III
7) ciliary ganglion
8) postganglionic fibers enter eye as short ciliary nerve and innervate pupillary sphincter (constrictor)

Question 3

pretectal nucleus - location

Answer 3

*level of posterior commissure, just anterior to the superior colliculus and just posterior to the thalamus
*midbrain-diencephalon junction

Question 4

why do we get a consensual response during pupillary constriction

Answer 4

because of the bilaterality of the projection from the pretectal nucleus to the EW nucleus

Question 5

normal pupillary light reflex

Answer 5

*when light is shone on one eye, that pupil constricts (direct response) AND other pupil constricts the same amount (consensual response)
*afferent and efferent pathways are both intact

Question 6

afferent pupillary defect (LAPD or RAPD)

Answer 6

*when light is shone in the “unaffected” eye: normal direct and consensual response
*when light is shone in the “affected” eye: no response in either eye (absent direct response AND absent consensual response)
*often results from a lesion in the optic tract (light cannot get IN); often the affected eye is termed a “blind eye)
NOTE: afferent defects need not be “all or none”

Question 7

efferent pupillary defect

Answer 7

*lesion in the pathway anywhere from the pretectum to the short ciliary nerve
*when light is shone on the eye on the side of the lesion: NO direct response, but a consensual response is observed
*when light is shone on the unaffected eye: a direct response is observed, but NO consensual response (in the affected eye)

Question 8

overview of afferent lesion (pupillary light reflex)

Answer 8

light on pupil IPSILATERAL to lesion: NO direct response; NO consensual response

light on pupil CONTRALATERAL to lesion: YES direct response; YES consensual response

Question 9

overview of efferent lesion (pupillary light reflex)

Answer 9

light on pupil IPSILATERAL to lesion: NO direct response; YES consensual response

light on pupil CONTRALATERAL to lesion: YES direct response; NO consensual response

Question 10

pupillary DILATION

Answer 10

*sympathetic response
*regulated by the hypothalamus
*lots of opportunities for disruption because it has a long pathway

Question 11

chronically small pupil

Answer 11

*paralysis of the pupillary dilator
*can result from lesions anywhere in the pathway of the pupillary dilator (sympathetic pathway)
*effect is on the same side as the lesion

Question 12

near reflex

Answer 12

*involves the coordinated effort of multiple systems and enables one to view near objects
*3 main components:
1. ocular convergence
2. accommodation
3. pupillary constriction

Question 13

convergence

Answer 13

*contraction of the medial rectus muscles to make the eye converge
*necessary to put the image of a near object on the fovea
*part of the near reflex

Question 14

accommodation

Answer 14

*contraction of the ciliary muscles cause the lens to become more spherical (increasing refractive power)
*the nearer an object, the greater refractive power needed to focus the image on the retina
*part of the near reflex

Question 15

Argyll-Robertson pupil

Answer 15

*pupils can constrict during the near reflex, but do not constrict when light is shone on the eye

Question 16

function of the medial rectus muscle

Answer 16

*pulls eyes IN (adduction)
*innervated by CN III

Question 17

function of the lateral rectus muscle

Answer 17

*pulls eye OUT (abduction)
*innervated by CN VI

Question 18

function of the superior rectus muscle

Answer 18

*pulls eye up
*innervated by CN III

Question 19

function of the inferior rectus muscle

Answer 19

*pulls eye down
*innervated by CN III

Question 20

function of the superior oblique muscle

Answer 20

*rotates eye DOWN AND IN (depresses) + CCW rotation (intorsion)
*innervated by CN IV
*muscle travels through a bony protrusion called the trochlea

Question 21

function of the inferior oblique muscle

Answer 21

*rotates eye UP AND OUT (elevates) + CW rotation (extorsion)
*innervated by CN III

Question 22

anatomical location of oculomotor nucleus

Answer 22

*at the level of the superior colliculus (midbrain)

Question 23

anatomical location of trochlear nucleus

Answer 23

*at the level of the inferior colliculus (midbrain)

Question 24

anatomical location of abducens nucleus

Answer 24

*at the level of the pons

Question 25

gaze palsy

Answer 25

*caused by compromised innervation (weakness) of one or more of the extraocular muscles
*symptoms = disconjugate gaze + diplopia (double vision)
*common causes: diabetic neuropathy, head trauma, and aneurysm

Question 26

abducens nerve palsy

Answer 26

*affected eye deviates in the nasal direction (unopposed medial rectus action b/c lateral rectus is not working)

Question 27

trochlear nerve palsy

Answer 27

*affected eye deviates upward and extorts (unopposed inferior oblique action b/c superior oblique is not working)

Question 28

oculomotor nerve palsy

Answer 28

*affected eye is DOWN and OUT (b/c only CN IV and VI are working)

Question 29

how does the abducens nucleus influence the action of the contralateral MEDIAL rectus?

Answer 29

*some projections (internuclear neurons) from the abducens nucleus travel in the medial longitudinal fasciculus (MLF) to the oculomotor nucleus on the contralateral side
*result = when one eye abducts (contraction of lateral rectus), the opposite eye adducts (contraction of contralateral medial rectus)

Question 30

paramedian pontine reticular formation

Answer 30

a structure that provides input to the abducens nucleus on the ipsilateral side (prevents disconjugate abduction of the eyes)

Question 31

lesion to abducens NERVE

Answer 31

*CN VI palsy
*impaired abduction of the eye on the ipsilateral side

Question 32

lesion to the abducens NUCLEUS

Answer 32

*impaired AB-duction of the eye on the ipsilateral side AND impaired AD-duction of the contralateral eye (neither eye moves when asked to look to the side of the lesion)

Question 33

lesion of the PPRF

Answer 33

*impaired AB-duction of the eye on the ipsilateral side AND impaired AD-duction of the contralateral eye (presents the same as a lesion to the abducens NUCLEUS)

Question 34

lesion to medial longitudinal fasciculus

Answer 34

*internuclear ophthalmoplegia
*the eye on the affected side cannot AD-duct when the contralateral eye AB-ducts
*results in nystagmus of contralateral eye (because the eye does not like the disconjugate gaze)

Question 35

lesion to MLF and abducens nucleus on the same side

Answer 35

*1 + 1/2 syndrome
*bilateral internuclear ophthalmoplegia AND unilateral deficit in abduction
*internuclear fibers to both eyes are affected, and lateral rectus of ipsilateral side is affected
(only horizontal movement patient would be able to make is abducting the contralateral eye)

Question 36

supranuclear gaze palsy

Answer 36

*occurs when the doll’s head rotation is intact (eyes counter-rotate with the movement of the head)