The optic nerve and its disorders

Published on April 27, 2016   24 min
0:00
The subject of this talk is The Optic Nerve and Its Disorders. And over the course of this talk, we will explain the anatomy of the optic nerve, the pathology that can affect this nerve, and the consequences in terms of disease. The talk will be presented by Dr. Simon Skalicky from the University of Melbourne and myself. My name is Keith Martin. I'm Professor of Ophthalmology at the University of Cambridge.
0:24
Thinking embryologically, the optic vesicle and cup develop as an out-pouch of the developing forebrain. These develop into the oculus structures, including retina and optic nerve. And this is why the optic nerve is part of the central nervous system. Its histology and pathological responses and behavior resemble a central nervous system white matter tract. This is in contrast to all other cranial nerves, which are peripheral nerves. Peripheral and central nerves differ in terms of their glial cells. Like the rest of the central nervous system, the optic nerve is surrounded by three meningeal layers. Finally, unlike peripheral nerves, the optic nerve has a limited capacity to regenerate axons after injury, which is similar to other central nervous system white matter tracts.
1:10
The optic nerve consists of the axons of 1.2 million ganglion cells. These are initially found in the inner most retinal layer, nerve fiber layer, and converge on the optic nerve head. At the optic nerve head, the axons form neuroretinal rim around the empty cup. The neuro tissue is supported by the lamina cribrosa, a thin, perforated area of sclera that allows the extra ocular passage of axons. This may be the reasons why it is more susceptible to high intraocular pressure than the rest of the sclera. The optic nerve travels through the orbit from the lamina cribrosa to the optic canal. Its length is somewhat longer than the orbit providing some laxity to prevent tethering of the globe on movement and some protection against mechanical stretch during proptosis. The optic nerve then passes through the sphenoid bone in the optic canal where its dura is firmly attached to the bone. Consequently, this portion of the optic nerve is more susceptible to sharing forces in trauma. As it enters the cranial cavity, the optic nerve travels posteriorly, dorsally, and medially to meet the contralateral optic nerve at the optic chiasm.
2:25
An understanding of the topographical organization of retinal ganglion cell axons along the visual pathway helps to localize various patterns of visual field defect. In the retinal nerve fiber layer, they are strictly segregated into superior and inferior hubs by the horizontal raphe. For this reason, damage to the retinal nerve fiber layer and anterior optic nerve head result in visual defects that obey the horizontal mid line. The axons avoid the fovea, those that arise temporal to the fovea cause superiorly or inferiorly around it and to the disc on its superior or inferior rim where the nerve fiber layer is thickest. Nasal foveal fibers travel directly to the disc along the papillomacular bundle. At the optic nerve head, peripheral retinal fibers are located in the peripheral neuroretinal rim, while the macular fibers are more central and superficial. As the fibers travel down the optic nerve, the organization changes again. Temporal fibers gather temporally, while nasal fibers gather nasally in preparation for decussation at the chiasm.
3:31
The optic nerve projects to several central nervous system targets. Most of its axons synapse at the lateral geniculate nucleus from where visual information is relayed to the visual cortex via the optic radiation. This is for conscious perception of vision. Some fibers synapse at the pretectal nuclei. These are involved in the papillary light reflex and project to the Edinger-Westphal nuclei that control parasympathetic output to the iris and ciliary body. Some fibers synapse at the superior colliculi, which have numerous subcortical functions, including generating saccades and maintaining visual attention. The pulvinar nucleus found in the dorsal thalamus receives fibers in the parallel pathway to the lateral geniculate nucleus and projects widely to the visual and visual association cortex. This codes salience, that is, visual importance of the information relayed by the lateral geniculate nucleus. The suprachiasmatic nucleus also receives direct optic nerve afference and uses this to control the circadian rhythms.
4:38
Glial cells including oligodendrocytes, astrocytes, and microglia make up the bulk of the optic nerve parenchyma together with the retinal ganglion cell axons. These glial cells provide structural and metabolic support to the axons as well as other important tasks for optic nerve physiology.
4:60
Oligodendrocytes are characteristic central nervous system glial cells that have numerous processes, each wrapping multiple layers of myelin around a segment of axon. Myelin electrically insulates the axon and greatly enhances the speed of action potential transmission. In myelinated nerves, the action potential jumps from inter-myelin space to another. These spaces are known as the nodes of Ranvier and the process is called saltatory conduction, much faster than action potential propagation in non myelinated axons. Oligodendrocytes are derived from precursor cells that migrate from the brain during embryogenesis and possibly throughout life. The differentiation and renewal is controlled by neurotrophic factors including platelet-derived growth factor and basic fibroblast growth factor.
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The optic nerve and its disorders

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