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:

Course of occulomotor, trochlear and abducent nerve

Course of occulomotor, trochlear and abducent nerve

  • 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:

Functions of extrinsic muscles of eye

  • 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

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