Cranial Nerves 1 Flashcards
What is the olfactory nerve formed by?
Axons of specialised olfactory hair cells in the olfactory epithelium of the nasal cavity.
Outline the pathway of the olfactory nerve.
Axons pass through olfactory foramina in cribriform plate of ethmoid bone –> enter olfactory bulb superior to ethmoid bone –> terminate
What is the optic nerve formed by?
Axons of retina ganglion cells (RGC)
What is the pathway of the optic nerve?
Enter the cranial cavity via optic canal –> run posteromedially to form optic chiasm
What is the function of the optic nerve?
Transmit visual impulses from the retina of the eye to the thalamus
Where does the occulomotor nerve (CNIII) originate from?
Midbrain (between the PCA and SCA)
Where does the trochlear nerve (CNIV) originate from?
Dorsal surface of the midbrain
Where does the abducens (CNIV) originate from?
Anteriorly between pons and medualla (pontomedullary junction)
Which extraoccular muscles does the occulomotor nerve innervate?
3 rectus (medial, superior, inferior) + inferior oblique
Which muscles does the trochlear nerve innervate?
Superior oblique
Which muscles does the abducens nerve innervate?
Lateral rectus
What are the three intrinsic muscles innervated by the occulomotor nerve?
Dilator pupillae, sphincter pupillae and ciliary muscle
What are the two types of occulomotor fibres?
1) Somatic motor fibres:
- arise from the oculomotor nucleus
- control all extraocular muscles except superior oblique and lateral rectus + levator palpebrae superioris
2) Preganglionic parasympathetic fibres
- arise from the Edinger-Westphal nucleus
- travel with branch supplying inferior rectus to terminate on postganglionic cells in the ciliary ganglion
- these cells enter the eye (intrinsic eye muscle) to supply
a. ciliary muscles makes the lens fatter (accommodation)
b. constrictor (sphincter) pupillae pupil constriction (bright light + accommodation)
What is the general path of the oculomotor nerve?
Emerges from brainstem in interpeduncular fossa –> pieces the dura –> runs along lateral wall of cavernous sinus –> enters orbit through the medial part of the SOF inside the common tendinous ring –> breaks into branches to supply each of the muscles
What are the key functions of the oculomotor nerve?
i. Eye movement – extraocular muscles
ii. Opening of eyelids – levator palpibrae superioris muscle
iii. Constriction of pupils – sphincter pupillae
iv. Focusing of vision – ciliary muscle + sphincter pupillae
v. Proprioception – extraocular proprioceptors
What are clinical manifestations of oculomotor nerve lesion?
i. Ptosis (droopy eyelid) – paralysis of levator palp. sup
ii. Eye deviation downwards and to affected side due to paralysis of MR + IO Diplopia (double vision)
iii. Impaired medial and vertical movements
iv. Mydriasis (dilated pupils) difficulty focusing – paralysis of sphincter pupillae
v. Loss of direct + consensual eye reflex in the affected side
What is the path of the trochlear nerve?
Arises from the trochlear motor nucleus –> exits brainstem at its dorsal surface –> two trochlear nerves cross eachother –> pierce the dura to enter lateral wall of cavernous sinus –> enter orbit through SOF inside common tendinous ring –> supplies superior oblique muscle
What will patients with a trochlear nerve lesion have?
i. Eye deviates upwards diplopia Patient tilts head towards affected side (compensatory head tilt)
What is the path of cranial nerve VI (abducens)?
Arises from abducens nucleus in caudal pons –> emerges at pontomedullary junction –> pierces the dura –> travels in the subarachnoid space –> passes through the cavernous sinus –> enters orbit through SOB inside common tendinous ring –> supply the lateral rectus muscle
What is the clinical test for injury of the abducens nerve?
i. ability to move the eye laterally
ii. read from top to bottom
What does a lesion of the abducens nerve look like?
i. Medial deviation at rest (unopposed MR) Strabismus diplopia
ii. Inability to move eye laterally
