Optic nerve

The optic nerve is the second cranial nerve which along with the olfactory nerve (CN I) is really an extension of the central nervous system, not surrounded by Schwann cells with first sensory bipolar cell body located peripherally in the retina. Their central processes synapse on ganglion cells on the vitreous surface of the retina and their central processes pass via the optic disc out of the globe and form the optic nerve proper.

The optic nerve is divided into four segments:

  1. intraocular segment: lies within the retina and emerges through a scleral opening (lamina cribrosa)
  2. intraorbital segment: passes posteriorly and centrally within the orbit and is surrounded by dural lining and CSF; hence it directly communicates with the subarachnoid space and therefore allows transmission of increased pressure from hydrocephalus to manifest as papilloedema; additionally, the dural covering can develop a meningioma
  3. intracanalicular segment: where the optic nerve exits through the tendinous ring and optic canal inferior to the ophthalmic artery
  4. intracranial or cisternal segment: enters the middle cranial fossa and passes within the suprasellar cistern with the anterior cerebral artery at its superolateral aspect to join the contralateral optic nerve at the optic chiasm

At the optic chiasm, the nasal fibres of each optic nerve (fibres carrying light impulses from the nasal side of the retina) decussate while the temporal fibres do not (partial decussation). From the optic chiasm arise two optic tracts, each one containing nasal fibres of the contralateral optic nerve and temporal fibres from the ipsilateral optic nerve. The optic tract courses around the cerebral peduncle to relay in the lateral geniculate body of the thalamus.

Arising from the lateral geniculate body of the thalamus, the optic radiations are divided into superior and inferior bundles. The superior bundle carries information from the superior retinal quadrant that represents the inferior visual field and ends at the superior aspect of calcarine sulcus (cuneus). The inferior bundle (Meyer's loop) carries information from the inferior retinal quadrant which represents the superior visual field loops anteriorly in the temporal lobe and forms the lateral wall of the inferior horn of lateral ventricle then pass posteriorly to end inferior to the calcarine sulcus (lingual gyrus).

Small branches of the ACA and the superior hypophyseal artery supply the chiasm and intracranial portion of the optic nerves whereas the optic tracts are supplied by small branches of the anterior choroidal and PCom arteries.

According to a study by Delano et al the course of the optic nerve in relation to the sphenoid sinus was classified according to four types 6:

  • type 1: most common (76%): the optic nerve is immediately adjacent to the lateral or superior wall of the sphenoidal sinus, without impression on the sinus wall
  • type 2: (15%): nerve causes an impression on the lateral sphenoidal sinus wall
  • type 3: (6%): nerve courses through the sphenoidal sinus rather than simply run adjacent to the sinus
  • type 4: (3%) nerve courses immediately lateral to the posterior ethmoidal and sphenoidal sinuses
Neuroanatomy
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Article Information

rID: 1779
Section: Anatomy
Tag: rewrite
Synonyms or Alternate Spellings:
  • Optic nerve (CN II)
  • Optic nerve (II)

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Cases and Figures

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    Figure 1: optic disc
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    Passage of the op...
    Optic nerves
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    Figure 2: lateral view of the orbit
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    Figure 3: optic nerves and chiasm (T2)
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    Human eye

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    Figure 4: orbit - diagram
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    Figure 5: CN II and others - MRI
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    Figure 6
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    Figure 7: orbital apex diagram
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