Optic Nerve

INTRODUCTION:
  • Optic nerve, also known as cranial nerve II, is a paired nerve that transmits visual information from the retina to the brain. 
  • It is derived embryologically from the neuroectoderm
  • Optic nerve is about 50mm (47-50mm) long from eye globe to chiasma
  • The optic nerve is derived from optic stalks during the seventh week of development and is composed of retinal ganglion cell axons and glial cells.
  • Optic nerve extends from the optic disc to the optic chiasm and continues as the optic tract to the lateral geniculate nucleus, pretectal nuclei, and superior colliculus
  • The optic nerve is composed of retinal ganglion cell axons and glial cells.
STRUCTURE:
  • The optic nerve is the second of twelve paired cranial nerves and is technically part of the central nervous system
  • Derived from an out-pouching of the diencephalon (optic stalks) during embryonic development.
  • Fibers covered with myelin produced by oligodendrocytes, and are encased within the meninges. 
  • The fibres from the retina run along the optic nerve to nine primary visual nuclei in the brain, from which a major relay inputs into the primary visual cortex.
  • Leaves the orbit (eye socket) via the optic canal 
  • Runs postero-medially towards the optic chiasm, where there is a partial decussation (crossing) of fibres from the temporal visual fields (the nasal hemi-retina) of both eyes. Most of the axons of the optic nerve terminate in the lateral geniculate nucleus
  • From here information is relayed to the visual cortex, while other axons terminate in the pretectal nucleus and are involved in reflexive eye movements. 
  • Other axons terminate in the suprachiasmatic nucleus and are involved in regulating the sleep-wake cycle.
  • Internal Carotid Artery is the nearest relation of the optic nerve
  • Onodi cells are seen in respect to optic nerve
  • Most of these fibres terminate in the lateral geniculate body.
  • The optic nerve may be divided in the four parts :
  1. The optic head (which is where it begins in the eyeball (globe) with fibers from the retina;
  2. Orbital part (which is the part within the orbit).
  3. Intracanicular part (which is the part within a bony canal known as the optic canal)
  4. Cranial part (the part within the cranial cavity, which ends at the optic chiasm).
  • From the lateral geniculate body, fibers of the optic radiation pass to the visual cortex in the occipital lobe of the brain.
  • Fibers carrying information from the contralateral superior visual field traverse Meyer's loop to terminate in the lingual gyrus below the calcarine fissure in the occipital lobe, and fibers carrying information from the contralateral inferior visual field terminate more superiorly, to the cuneus
FUNCTIONS:
  • The optic nerve transmits all visual information including brightness perception, color perception and contrast (visual acuity). 
  • It also conducts the visual impulses that are responsible for two important neurological reflexes: the light reflex and the accommodation reflex.
  • The light reflex refers to the constriction of both pupils that occurs when light is shone into either eye; the accommodation reflex refers to the swelling of the lens of eye that occurs when one looks at a near object as in reading (lens adjusts to near vision).
  • The eye's blind spot is a result of the absence of photoreceptors in the area of the retina where the optic nerve leaves the eye
Exam Question
  • Onodi cells are seen in respect to optic nerve
  • Optic nerve serves as the afferent pathway for light pupillary reflex
  • Internal Carotid Artery is the nearest relation of the optic nerve
  • Optic nerve Arises from axons of bipolar neurons
  • Optic nerve consists of axons of ganglionic cellso
  • Optic nerve is derived embryologically from the neuroectoderm

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