Neuro-ophthalmology: Neuromuscular Control of the Eyeball
The word optic nerve (ON) is a misnomer because this entity is not a nerve in the classic sense. Embryologically, it is a part of the brain, with which it has many common denominators. Both have a double blood supply, a peripheral and a central one, and i
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Neuro-ophthalmology: Neuromuscular Control of the Eyeball
Keywords
Optic nerve Axons of ganglion cells of retina Posterior scleral foramen Optic chiasma Optic tract Lateral geniculate nucleus Pupillary fibers Pretectal nucleus Neuromuscular control Optic radiation Visual center in occipital lobe Nucleus of third, fourth, fifth, sixth, and seventh cranial nerves Origin in posterior cranial fossa
The word optic nerve (ON) is a misnomer because this entity is not a nerve in the classic sense. Embryologically, it is a part of the brain, with which it has many common denominators. Both have a double blood supply, a peripheral and a central one, and it does not have a neurilemma; therefore after degeneration its fibers do not regenerate. It is likely to suffer from a degenerative disorder of the brain. Actually, it is a tract that consists of the axons of the ganglion cells of the retina that end in the lateral geniculate nucleus, where a new neuron takes over from its six layers. This new neuron’s axons continue as the optic radiation, which ends in the cells of the visual center in the occipital lobe. The ON nerve has various parts: (1) a canalicular part inside the optic canal that measures 6 mm, (2) the cranial part (1 cm), (3) the orbital part (3 cm), and (4) the intraocular part (0.7 mm). Roughly, the total length of the ON is about 5 cm. The intraocular part anterior to the lamina cribrosa is
about 1.5 mm wide, but as soon as it comes out, its fibers become medullated so that its diameter just behind the lamina cribrosa is about 3 mm. The orbital part of the nerve is continuous with the part inside the optic canal called the canalicular part. The canalicular part is accompanied by the ophthalmic artery, which crosses the undersurface of the ON from the medial to the lateral side. After this crossing the ophthalmic artery reaches the orbital opening of the optic canal, where it releases a branch called the central retinal artery. This artery travels on the undersurface of the sheaths of the ON before it pierces its three coverings to enter its center about 15 mm behind the eyeball. From there it travels forward inside the ON, giving branches to its pial network and some collateral branches that supply its posterior part in the orbit. After giving the central retinal artery branches to the ON, the ophthalmic artery enters the cone of muscles from its lateral side. After supplying
© Springer International Publishing Switzerland 2016 M.W. Ansari and A. Nadeem, Atlas of Ocular Anatomy, DOI 10.1007/978-3-319-42781-2_8
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Neuro-ophthalmology: Neuromuscular Control of the Eyeball
many branches, the artery crosses the upper surface of the ON from the lateral to the medial end. Thus the ophthalmic artery crosses the ON twice: once on its undersurface from the medial to the lateral side inside the optic canal, and once inside the cone of muscles. It proceeds forward inside the cone of muscles to ultimately end by dividing into the dorsal nasal and supratrochlear branches. The oph
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