CRANIAL NERVE PATHWAYS & REFLEXES

Question 1

Protective reflexes

Answer 1

• Corneal blink reflex

* Pupillary light reflex

Question 2

Co-ordination of eye movement

Answer 2

• Gaze
• Vestibulo-ocular reflex (VOR)
• Accommodation

Question 3

Optic nerve CN II

Answer 3

Receptor = rods & cones
1st neuron = bipolar cell (special sense)
2nd neuron = ganglion cell
Axons of ganglion cells = optic nerve

Question 4

The visual pathway

Answer 4

• Ganglion cells synapse in the Lateral Geniculate Nucleus (thalamus)
• 3rd neuron travels via optic radiation to the
• Calcerine sulcus = primary visual cortex, occipital lobe
• Collaterals to midbrain
  > superior colliculus >pretectal area

Question 5

Lateralisation of the visual field at the optic chiasm

Answer 5

• The lateral (temporal) visual field projects onto the medial (nasal) retina
• Fibres from the nasal retinae decussate at the optic chiasm
• The optic tract carries information from the contralateral visual field
• lat. neurons of optic nerve remain ipsilateral while medial are contralateral

Question 6

Superior–inferior retinotopy

Answer 6

• Lower visual field travels through the parietal optic radiation
• Upper visual field travels via the Meyer loop through the temporal lobe
  left nasal retina > right visual cortex

Question 7

binocular vision

Answer 7

each eye sees different part of environment

Question 8

LESIONS OF THE OPTIC PATHWAY:

Answer 8

Named according to visual field lost! 
Circles = visual field
of each eye 
Dark = lost visual field
Test 1 eye at a time (other eye covered)

Question 9

Medial longitudinal fasciculus (MLF)

Answer 9

Interconnects nuclei of:
midbrain
1. Oculomotor 
2. Trochlear
pons
3. Abducens 
medulla oblongata
4. Vestibular
• Heavily myelinated
• Near midline tegmentum
• Anterior to 4th ventricle & PAG /cerebral aqueduct

Question 10

3 Cranial nerves control the muscles of the eyes

Answer 10

oculomotor CN3, trochlear 4, abducens 6

Question 11

oculomotor CN3,

Answer 11

1. Levator palpebrae superioris
2. Superior rectus
3. Inferior rectus
4. Medial rectus
5. Inferior oblique
6. Parasympathetic CN III fibres travel to the ciliary ganglion in the orbit
   – synapse with ciliary nerves that innervate the:
   * Constrictor pupillae: controls amount of light entering the eye
   * Ciliary muscle: rounding of lens for near vision

Question 12

trochlear CN4

Answer 12

Superior oblique

Question 13

abducens CN6

Answer 13

lateral rectus

Question 14

Oculomotor - somatic motor nucleus

Answer 14

• tegmentum of rostral midbrain
• near midline
• anterior to cerebral aqueduct & PAG
• rostral end of medial longitudinal fasciculus (MLF)
• fibres travel anteriorly to emerge in interpeduncular fossa

Question 15

Oculomotor - Parasympathetic nucleus = Edinger-Westphal nucleus

Answer 15

• posterior to somatic nucleus
• in periaqueductal grey (PAG)
• preganglionic axons travel with somatic axons, exit @ ciliary ganglion
The parasympathetic axons are located superficially in the CN III nerve - more susceptible to compression
- dilated pupil is an early sign of CN III compression
(Pretectal area = rostral to superior colliculus @ midbrain/diencephalon junction)

Question 16

Trochlear CN IV nucleus

Answer 16

• Midbrain @ level of the inferior colliculus
• near midline
• axons exit dorsally

Question 17

Abducens CN VI nucleus

Answer 17

• caudal pons @ level of the facial colliculus
• near midline
• Contains:
Somatic motor nucleus – neurons innervate lateral rectus Interneurons – coordination of horizontal eye movements
+ cell bodies of interneurons

Question 18

Trigeminal nerve CN V

Answer 18

The trigeminal (semilunar) ganglion is homologue to the dorsal root ganglia as it contains cell bodies of (pseudo)unipolar sensory neurons
3 major branches:
- ophthalmic nerve (V1) = sensory forehead, eyelids, eye upper nasal
- maxillary nerve (V2) = sensory midface, upper teeth, lower nasal cavity, paranasal sinuses - mandibular nerve (V3) = sensory lower face + motor muscles of mastication

Question 19

Trigeminal nucleus: 4 parts

Answer 19

mesencephalic
spinal trigeminal
chief sensory
motor trigeminal

Question 20

Facial - CN7

Answer 20

Large motor component to all muscles of facial expression •Small sensory component = taste anterior 2/3 tongue
•NOTE biting tongue pain = trigeminal nerve
•Into internal acoustic meatus, out through stylomastoid forame

Question 21

Facial - CN7 parasympathetic

Answer 21

• Lacrimal gland (tears)

* Submandibular & sublingual salivary glands

Question 22

CORNEAL BLINK REFLEX: (brainstem)

Answer 22

Protective reflex to remove foreign particles & lubricate the eye
1. Ophthalmic division of trigeminal nerve CN V
2. Synapse in spinal trigeminal (nociception) & chief sensory (touch)
nuclei
3. Interneurons synapse bilaterally with facial nucleus motor neurons
4. Facial nerve CN VII innervates orbicularis oculià“blink”

Question 23

pupillary light reflex

Answer 23

-Shine bright light in 1 eye (other eye shielded from light) -stimulates CN II
-Fibres travel in both optic tracts
-Collaterals through brachium of superior colliculus to pretectal area
-Pretectal neurons project bilaterally via posterior commissure to Edinger-Westphal nuclei
-CN III parasympathetic fibres to ciliary ganglion
-postganglionic neurons innervate constrictor pupillae
Should be direct & consensual (both eyes affected)

Question 24

gaze

Answer 24

• photoreceptors are sensitive but slow!
• binocular vision
  • coordinatedmovement of 2 eyes
  • each with 6 extrinsic eye muscles
  • controlled by 3 pairs of cranial nuclei
  1. Conjugate movements (eyes move together)
  2. Vergence movements (eyes opposite - both move in)

Question 25

Saccades

Answer 25

- types of gaze conjugate fast • Redirect gaze so a different image falls on the fovea • Use them when exploring the environment visually

Question 26

Smooth pursuit movements

Answer 26

conjugate gaze slower • Used to keep an image on the fovea • Due to either movement of the object or movement of self • Use cortical (visual), cerebellar & vestibular feedback • 2 eyes = 2 fovea ie. Both cerebral hemispheres involved. eg - looking at something and following it

Question 27

Coordination of horizontal conjugate eye movements

Answer 27

Left PPRF controls both eyes looking to left

Question 28

the initiation of saccades

Answer 28

``` • frontaleyefield(FEF) • supplementaryeyefield • parietaleyefield project to contralateral PPRF (or riMLF) +/- superior colliculus (which then projects to PPRF) PPRF = controls abducens and oculomotor ```

Question 29

Damage to the FEF

Answer 29

- Temporary inability to look voluntarily to the contralateral side (despite normal muscles)

Question 30

Damage to FEF & superior colliculus

Answer 30

- Longer & more severe deficit

Question 31

VESTIBULO-OCULAR REFLEX (VOR)

Answer 31

* photoreceptors are sensitive but slow * very fast reflex * generates fast eye movements to compensate for head movements in order to keep the fovea fixed on an object * adjust saccades * Cerebellar input can cancel VOR

Question 32

VESTIBULO-OCULAR REFLEX:

Answer 32

• Vestibular apparatus detects head movement (e.ghorizontal) • Vestibular nerve CN VIII • Ipsilateral vestibular nuclei at pons / medulla junction • Project to contralateral Abducens nucleus (pons) • Projects to • ipsilateral CN VI (lateral rectus) • contralateral oculomotor nucleus, midbrain, via the MLF • CN III (medial rectus) eg. moving at figure while moving head

Question 33

Smooth pursuit

Answer 33

Used to keep an image on the fovea when the object is moving relative to the background 1. interest 2. detect motion 3. VOR cancellation

Question 34

ACCOMMODATION: involves the cerebral cortex

Answer 34

Focusing on a near object 3 requirements: 1. Convergence so that the object falls on both foveae 2. Increase curvature of the lens to increase refractive power to focus the image on the fovea 3. Pupillary constriction – reduces blur & increases depth of field

Question 35

accomodation process

Answer 35

``` CN II afferents to bilateral LGN (thalamus) • Primary visual cortex • Visual association cortex • Project to pretectal area • Oculomotor & Edinger-Westphal nuclei • CNIII • somatic fibres to medial rectus • parasympathetic fibres to • ciliary muscle • constrictor pupillae ```