Anatomical Basis of Visual Reflexes

Question 1

outline the path of the simple visual pathway. note

• signals being carried
• stimulus
• any sites of deccusation
• any sites of synapse
• final destination

Answer 1

• an afferent tract
• path:
  
  • stimulus = light
  • light -> axons of the optic nerve
  • optic nerve -> optic chiasm: +/- dessucation site
    
    • axons from nasal half of eye: dessucate
    • axons from lateral half of eye: remain ipsilateral
  • optic nerve leaves chiasm as -> optic tract
  • optic tract > thalamus: lateral geniculate nucleus (synapse site)
  • after synapsing, exit as optic radiation
  • optic radiation -> occipital pole: primary visual cortex

Question 2

outline the path of the pupillary light reflex. note

• signals carried
• stimulus
• any sites of dessucation
• any sites of synapse
• final destination

Answer 2

“reflex” = aka afferent and efferent

• afferent:
  
  1. stimulus = light
  2. light -> optic nerve -> optic chiasm -> optic tract [simple visual pathway]
  3. optic tract -> prectectal nucleus (site of synapse)
  4. pretectual nucleus -> edinger-westphal nucleus
  5. edinger-westphal nucleus -> parasympathetic preganlgions
• efferent (parasympathetics)
  
  1. preganglions course in the oculomotor nerve, then
  2. synapse in the ciliary ganglion, producing post-ganlgions that
  3. innervate the pupillary sphincter muscle

Question 3

pupillary light reflex afferents - the optic tract axons synapse at what ganglion?

where is it located?

Answer 3

pretectal nucleus

on tectum, immediately superior to superior nucleus (immediately below pineal gland)

Question 4

pupillary light reflex efferents

carry what signals?

by what fibers?

Answer 4

parasympathetic, by:

• pre-ganglions that travel through ocular motor nerve
• post-ganlgions that travel through short ciliary nerves

Question 5

pupillary light reflex efferents - synapse at what ganglion

Answer 5

ciliary

Question 6

pupillary light reflex efferents - innervate what structure?

Answer 6

pupillary sphincter muscle (pupillary constriction)

Question 7

the simple visual pathway - carries afferents to what destination?

Answer 7

primary visual cortex

Question 8

how does the pupillary light reflex induce a response?

why is this the case?

Answer 8

• light stimulation - even in just one eye - will cause pupillary constriction in both eyes.
  
  • direct reflex: constriction in stimulated eye
  • consensual light reflex: constriction in non-stimulated eye
• this is because the prectecal nucleus [synapse site for optic nerve axons], projects bilaterally

Question 9

pupillary response reflect: if a light stimulation is removed, what happens?

Answer 9

1. inactivation of pupillary constriction:
   
   • excitation of pretectal nucleus stops
   • thus parasympathetics from edinger-westphal nucleus stops
2. activation of of pupillary dilator muscle

Question 10

outline the pathway of _pupillary dilator muscl_e excitation. note

• signals that are carried
• stimulus
• any sites of synapse / dessucation
• destination

Answer 10

• afferents:
  
  1. stimulus = discontinuous of light signal
  2. stimulus -> optic nerve -> optic chiasm -> optic tract [same as simple visual]
  3. optic tract -> suprachiasmatic nucleus [hypothalamus]
• efferent [sympathetic]
  
  • suprachiasmatic nucleus projects to -> paraventricular nucleus
  • paraventricular nucleus -> lateral horn (T1-T3), which
  • sends pre-ganlgions -> super cervical ganglion
  • sends post-ganglions (trigeminal nerve) to pupillary dilator muscle

Question 11

constrast pupillary constriction and pupillary dilation based on

• recipient of afferents
• source of efferents
• pre-ganlgion efferents
• efferent galgion
• post-ganlgion efferents

Answer 11

constriction (pupillary light reflex), pupillary dilation

• afferent recipient: prectectal nucleus, suprachiasmatic nucleus (hypothalamus)
• efferent source: edinger westphal nucleus (tegmentum), paraventricular nucleus
• pre-ganglionics: in oculmotor nerve, lateral horn of T1-T3
• ganglions: ciliary ganlgion, superior cervical ganlgion
• post-ganglionics: post-ciliary fibers, trigeminal nerve (V)

Question 12

oculomotor nucleus

• location
• anatomic relations
• role in visual reflexes

Answer 12

• location: tegmentum
• relations:
  
  • just ventral to periaqueductal gray matter
  • just lateral to edinger-westphal nucleus (midline)
  • in coronal plane with superior colliculi
• is an efferent source:
  
  • accomodation pathway (medial rectus)
  • conjugate gaze (contralateral medial rectus)

Question 13

edinger-westphal nucleus

• location
• anatomic relations
• role in visual reflexes

Answer 13

• location: tegmentum
• relations: medial to (“within”) oculomotor nerve
• role: source of efferents in
  
  • pupillary constriction (pupillary sphincter muscle)
  • accomation (ciliary muscle)

Question 14

prectectal nucleus

• location
• relations
• role in visual reflexes

Answer 14

• location: tectum
• relations:
  
  • just above superior colliculi
  • thus, right below the pineal gland
• role: afferent recipient for pupillary constriction

Question 15

outline with path of accomodation: note

• signals carried
• stimulus
• origin any sites of desuccation
• destination

Answer 15

afferent & efferent

• afferent:
  
  1. stimulus = an “unfocused” image
  2. unfocused image -> simple visual pathway
  3. occipital lobe:
     
     • primary visual cortex first receive signals, then synapses with
     • association visual cortex, which assesses image then stimulates frontal eyefield
• efferent [parasympathetic]:
  
  • frontal eyefield [frontal cerebral cortex] projects to:
    
    • _two tegmental nucle_i that are PNS & ventral to periaqeductal gray matter:
      
      • oculomotor: medial rectus - adducts eye for near vision
      • edinger-westphal: ciliary muscle - makes lens convex

Question 16

which component of the accomodation pathway designates the perceived image as “not sharp?

Answer 16

the association motor cortex

(occipital lobe)

Question 17

what are the targets of the the accomodation pathway?

what are their actions?

Answer 17

• medial rectus: narrows the field of vision
• ciliary muscle: contracts the ciliary body, which makes the lens more convex

both serve to enhance the focus of the eye

Question 18

categorize the pupillary light reflex, pupillary dilation, and accomodation as

• sympathetic or parasympathetic
• by the muscles they target
• by the direct motor nuclei of that target

Answer 18

• pupillary light reflex: PNS - edinger-westphal -> pupillary sphincter muscle
• pupillary dilation: SNS - LMNS of T1-T3 -> pupillary dilator muscle
• accomodation: PNS - oculomotor -> medial rectus, edinger-westphal -> ciliary muscle

Question 19

what innervates the ciliary muscle?

what does this do?

Answer 19

the oculmotor nucleus

it contracts the ciliary body to make the lens more convex

Question 20

what innervates the medial rectus?

what does this lead to?

Answer 20

the oculomotor nucleus

eye adduction, lens accomodation

Question 21

review how the superior & inferior colloculi serve as relay centers

Answer 21

both serve as sensory processing centers that relay information to thalamic nucleus

• inferior colliculi:
  
  • stimuli: auditory
  • pathway: lateral lemniscus (ascending)
  • thalamic nuclei: medial geniculate nucleus
• superior colliculi (relays minority of optic nerve fibers - most go directly to lateral geniculate)
  
  • stimuli: visual
  • pathway: optic tract
  • thalamic nuclei: lateral geniculate nucleus

Question 22

what roles does the superior colliculi play in vision?

Answer 22

1. relay center: relays a minority of optic tract fibers to the lateral geniculate (thalamic nuclei), which have recipricol centers in the visual cortex (occipital)
2. efferent source: reflex movements to light
   
   • tectobulbar tract
   • tectuspinal tract

Question 23

in response to light, the superior colliculus intiates reflexive movement via

• via what tracts?
• that dessucate where?
• that descend how?
• an innervate what targets?

Answer 23

• both: originate in tectum & dessucate in tegmentum
• tectospinal
  
  • descends in ventral funiculus, near ventral aspect of median fissure
  • innervates: cervical & superior axial muscle
• tectrorubral
  
  • descents only to brainstem
  • innervates: cranial nerve nuclei

Question 24

outline the path of the tectospinal tract

Answer 24

• originate in superior colliculus (tectum)
• curve around periaqeductal gray matter, then
• extend ventromedially -> dessucating immediately once in the tegumtum:
  
  • i.e., the dorsal tegmental decussation
• descend in ventral funiculus - near ventral-most medial fissure
• terminate in cervical segments: response of head/neck to light

Question 25

the horizantal motor gaze

Question 26

what is conjugate gaze?

Answer 26

eyes moving in conert

Question 27

horizantal conjugate gaze involves

• what motor nuclei?
• that innervate what targets? how?

Answer 27

• initiating motor nuclei: abducens (pons)
  
  • ​which stimulate the ipsilateral lateral rectus muscle + contralateral medial rectus muscle by sending:
    
    1. motor axons: directly to ​ipsilateral lateral rectus
    2. interneurons: to
       
       1. ascend through MLF
       2. dessucate / synapse at ocululmotor nucleus, which
          
          • innervates the medial rectus (now contralateral)

Question 28

where axons from abducents motor nucleus interneurons dessucate?

why is this important?

Answer 28

• in the oculmotor nuclei (tegmentum)
• exiting axons will thus innervate the medial rectus contralateral to the initating abducens nucleus

Question 29

vertical conjugate gaze involves

• what motor nuclei?
• that innervate what targets? how?

Answer 29

• initating motor nuclei:
  
  • both in MLF
    
    • interstitial nucleus of cajal
    • rostral interstitial nucleus
  • mediate vertical conjugate gaze via
    
    • trochlear nucleus (IV): superior oblique muscle - deivates contralateral eye upward & inward
    • oculomotor nucleus ?

Question 30

what are the higher regulators of horizantal conjugate gaze? in what situations do they reinforce conjugate gaze?

Answer 30

• when paired w/ rotation of the head: vestibular nuclei -> fixed gaze
• whith super rapid (saccadic) eye movements: PMPR formation -> saccades

both are reticular formation nuclei

Question 31

what is the vestibulo-reflex?

Answer 31

the path by which the vesitibular nuclei _reinforce & direct horizantal conjugate gaz_e in order to maintain a “fixed gaze” while the head rotates - i.e., eyes remain looking one direction while the head rotates another

Question 32

outline the path of the vestibulo-occular reflux

Answer 32

• head rotates
• the ipsilateral vestibular appartus will:
  
  • excite: ipsilateral vestibular nuclei, which
    
    1. ​excite the contralateral abducents (IV) nuclei, which:​
       
       • ​excite the:
         
         • ipsilateral lateral rectus
         • contralateral medial rectus
       • _drives basic conjugate gaz_e in opp direction of rotation
    2. directly excite the ipsilateral oculmotor (III) nucleus, which
       
       • ​reinforces conjugate gaze gaze in opp direction
  • inhibit: ipsilateral abducents (VI) nucleus, which ​
    
    • inhibits gaze in the same direction as head rotation
    • inhibits opposition of desired conjugate gaze (by ipsilateral vestiubular nucleus

Question 33

ascending & descending MLF (medial longitudinal fibers) come from / go where?

Answer 33

• ascending MLF: from abducens nucleus (pons) to oculomotor nucleus (tegmentum)
• descending MLF: from medial vestibular nuclei to cervical spinal cord neurons -> head position adjustment

Question 34

the lateral vestibular nucleus descents as the?

Answer 34

vestibulospinal tract

Question 35

what are saccades?

Answer 35

rapid eye movements (voluntary or involuntary) mediated by the paramedian pontine reticular, which ensures the gaze remains conjugate during quick movements

Question 36

outline the path of saccadic eye movement

Answer 36

1. superior colliculus
2. dorsal raphe nucleus (reticular formation): single midline nucleus that projects to both paramedian pontine reticular formations
3. PRRF / horizantal gaze center (midline pons)
   
   • excites: ipsilateral abducens nucleus
     
     • which
       
       • excites ipsilateral lateral rectus
       • excites contralateral median rectus
     • driving normal conjugate gaze
   • inhibits: contralateral abducents nucleus, which
     
     • ​prevents opposition of desired conjugate gaze

Question 37

the superior colliculus receives instructions from what sources to guide the saccadic eye movement?

Answer 37

• one of three cortical centers:
  
  • frontal eye fields:
    
    • to scan a familiar object
    • to search in a space from an anticipated objects
  • supplementary eye fields - superior to frontal eye fields
    
    • to coordinate with movement of head / body
  • parietal eye field - just before parietal occipital suture
    
    • ​in response to a _sudden appearanc_e for an object

Question 38

the superior colliculus receives refinement inputs from what sources to mediate the saccadic eye movement?

Answer 38

• vestibular nuclei
• cerebellum
• basal ganglia

Question 39

diplopia

• definition
• cause
• presentation

Answer 39

• definition: disruption of conjugate gaze
• cause: variety - opthalmoparesis: paralysis of extraocular muscles (i.e., lateral & medial rectus)
• presentation: dipolpia: two eyes form separate, irreconcilable images

Question 40

nystagmus

• definition
• presentation

Answer 40

• definition: disruption of vestibulo-ocular reflex
• presentation: slow displacement of conjugate gaze despite the absence of rotatory motion of the head