CRANIAL NERVE PATHWAYS & REFLEXES Flashcards
Protective reflexes
- Corneal blink reflex
* Pupillary light reflex
Co-ordination of eye movement
- Gaze
- Vestibulo-ocular reflex (VOR)
- Accommodation
Optic nerve CN II
Receptor = rods & cones
1st neuron = bipolar cell (special sense)
2nd neuron = ganglion cell
Axons of ganglion cells = optic nerve
The visual pathway
- 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
Lateralisation of the visual field at the optic chiasm
- 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
Superior–inferior retinotopy
- 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
binocular vision
each eye sees different part of environment
LESIONS OF THE OPTIC PATHWAY:
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)
Medial longitudinal fasciculus (MLF)
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
3 Cranial nerves control the muscles of the eyes
oculomotor CN3, trochlear 4, abducens 6
oculomotor CN3,
- Levator palpebrae superioris
- Superior rectus
- Inferior rectus
- Medial rectus
- Inferior oblique
- 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
trochlear CN4
Superior oblique
abducens CN6
lateral rectus
Oculomotor - somatic motor nucleus
- 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
Oculomotor - Parasympathetic nucleus = Edinger-Westphal nucleus
• 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)
Trochlear CN IV nucleus
- Midbrain @ level of the inferior colliculus
- near midline
- axons exit dorsally
Abducens CN VI nucleus
• 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
Trigeminal nerve CN V
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
Trigeminal nucleus: 4 parts
mesencephalic
spinal trigeminal
chief sensory
motor trigeminal
Facial - CN7
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
Facial - CN7 parasympathetic
- Lacrimal gland (tears)
* Submandibular & sublingual salivary glands
CORNEAL BLINK REFLEX: (brainstem)
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”
pupillary light reflex
-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)
gaze
- 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)
Saccades
- types of gaze
conjugate
fast
• Redirect gaze so a different image falls on the fovea
• Use them when exploring the environment visually
Smooth pursuit movements
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
Coordination of horizontal conjugate eye movements
Left PPRF controls both eyes looking to left
the initiation of saccades
• frontaleyefield(FEF) • supplementaryeyefield • parietaleyefield project to contralateral PPRF (or riMLF) \+/- superior colliculus (which then projects to PPRF) PPRF = controls abducens and oculomotor
Damage to the FEF
- Temporary inability to look voluntarily to the contralateral side (despite normal muscles)
Damage to FEF & superior colliculus
- Longer & more severe deficit
VESTIBULO-OCULAR REFLEX (VOR)
- 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
VESTIBULO-OCULAR REFLEX:
• 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
Smooth pursuit
Used to keep an image on the fovea when the object is moving relative to the background
- interest
- detect motion
- VOR cancellation
ACCOMMODATION: involves the cerebral cortex
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
accomodation process
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
