the optic nerve enters the eye through the optic disc.
the optic nerve enters the eye through the optic disc.
through what does the optic nerve enter the eye?
optic disc
the optic nerve leaves the orbit (eye socket) via the optic canal, running postero-medially towards the optic chiasm, where there is a partial decussation (crossing) of fibers from the temporal visual fields (the nasal hemi-retina) of both eyes. the proportion of decussating fibers varies between species, and is correlated with the degree of binocular vision enjoyed by a species. most of the axons of the optic nerve terminate in the lateral geniculate nucleus from where 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. its diameter increases from about 1.6 mm within the eye to 3.5 mm in the orbit to 4.5 mm within the cranial space. the optic nerve component lengths are 1 mm in the globe, 24 mm in the orbit, 9 mm in the optic canal, and 16 mm in the cranial space before joining the optic chiasm. there, partial decussation occurs, and about 53% of the fibers cross to form the optic tracts. most of these fibers terminate in the lateral geniculate body.
optic nerve
the optic nerve can be damaged when exposed to direct or indirect injury. direct optic nerve injuries are caused by trauma to the head or orbit that crosses normal tissue planes and disrupts the anatomy and function of the optic nerve; e.g., a bullet or forceps that physically injures the optic nerve. indirect injuries, like blunt trauma to the forehead during a motor vehicle accident, transmit force to the optic nerve without transgressing tissue planes. this type of force causes the optic nerve to absorb excess energy at the time of impact. the most common site of injury of the optic nerve is the intracanalicular portion of the nerve. deceleration injuries from motor vehicle or bicycle accidents account for 17 to 63 percent of cases. falls are also a common cause, and optic neuropathy most commonly occurs when there is a loss of consciousness associated with multi-system trauma and serious brain injury. in less than three percent of patients, an orbital hemorrhage after an injection behind the eye (retrobulbar block) can cause injury to the optic nerve, but this is readily manageable if it does not involve direct optic nerve injury and is caught early. the role of high-dose steroids and orbital decompression in treating these patients is controversial and, if administered, must be done very soon after injury with minimal effects. in patients with an orbital fracture, vomiting or nose blowing can force air into the orbit and possibly compromise the integrity of the optic nerve.
optic neuropathy