Cranial Nerves : III,IV and VI

Occulomotor Nerve (Cranial Nerve III or CN III)

Functional Components:

• General Somatic Efferent (GSE)/ Somatic Efferent (SE)
• General Visceral Efferent (GVE)

Nuclei: situated at the midbrain

a. Main motor nuclei (GSE):

• At the level of superior colliculus
• Supplies all the extrinsic muscles of eye except the Superior oblique and the lateral rectus (Mnemonic: LR6SO4 rest 3)

b. Accesory Parasympathetic nucleus (GVE):

• Edinger-Westphal nucleus (E-W nucleus)
• Situated posterior to main motor nucleus
• Supplies intrinsic muscles of eye

E-W nucleus –> III nerve –> Nerve to inferior oblique –> Branch to ciliary ganglion –> Relay –> Short ciliary nerves supply ciliaris and constrictor pupillae muscles

Course:

• Emerges on the anterior surface of the midbrain
• Enters the cavernous sinus and is placed in its lateral wall most anteriorly
• Divides into 2 divisions both of which pass through the middle part of superior orbital fissure
• Superior division supplies superior rectus and levator palpebrae superioris
• Inferior division supplies medial rectus, inferior rectus and inferior oblique
• Nerve to inferior oblique gives a branch for ciliary ganglion which supplies constrictor pupillae and ciliaris muscle

Lesion:

• Down and out eye: involvement of elevators of eye (inferior oblique and superior rectus)
• Ptosis : involvement of levator palpebrae (elevator of eyelid)
• Dilation of pupil and loss of accomodation : involvement of intrinsic muscles of eye
• Diplopia
• Lateral squint

Trochlear nerve (Cranial Nerve IV or CN IV)

Functional components:

• GSE/SE

Nucleus: situated at midbrain

• Lies inferior to the occulomotor nucleus at the level of inferior colliculus
• Supplies superior oblique muscle of eyeball

Course:

• Emerges from the posterior surface of the midbrain and immediately decussates
• Cavernous sinus
• Superior orbital fissure
• Innervates uperior oblique muscle

Lesion:

• Affected eye is hypertropic than the unaffected eye (involvement of superior oblique muscle)
• Diplopia (double vision) : To counteract the nauseating effect of diplopia, patients tilt their head to bring the eyes in the same plane

Abducent Nerve (Cranial Nerve VI or CN VI)

Functional components:

• GSE/SE

Nucleus: situated at pons

• Lies in the floor of 4th ventricle and beneath the facial colliculus
• Supplies lateral rectus muscle of eyeball

Course:

• Fibres pass anteriorly through pons
• Cavernous sinus
• Superior orbital fissure
• Innervates lateral rectus muscle

Lesion:

• Medial squint
• Diplopia with attempted far vision

Pictures from: Neuroanatomy – Draw it to know it by Adam Fisch