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^ OFF-center Cell A type of bipolar cell or retinal ganglion cell synapsed with it as described: A retinal bipolar neuron which fires (depolarizes) in the dark and is inhibited from firing (hyperpolarizes) in the light. The mechanism for this behaviour lies with the bipolar cell's synapse with the photoreceptor cell. The OFF bipolar makes contact with the photoreceptor at a basal junction which has ionotropic AMPA-kainate glutamate channels as a post synaptic receptor. These are stimulated by glutamate released by depolarizing photoreceptors in the dark. So, it hyperpolarizes when the photoreceptors hyperpolarize, in the light. It synapses with its Off retinal ganglion cell in the external sublamina a of the inner plexiform layer.
Ralph Nelson. Visual Responses of Ganglion Cells Webvision.
ON and OFF center pathways and center surround organization of the retina are initiated at the photoreceptor to bipolar and horizontal cell contacts in the outer plexiform layer. Webvision.
^ Oligodendrocyte A variety of neuroglia. Their main function is the myelin insulation of the axons exclusively in the central nervous system including the optic nerve.
^ ON-center Cell A type of bipolar cell or retinal ganglion cell synapsed with it as described: A retinal bipolar neuron which fires (depolarizes) in the light and is inhibited from firing (hyperpolarizes) in the dark. The ON bipolar makes contact with the photoreceptor at invaginated junctions which have APB sensitive metabotropic glutamate channels as post synaptic receptors. These use a cGMP cascade to initiate a depolarization. Glutamate, released by all photoreceptors in the dark when they are depolarizing, down regulates cyclic nucleotide synthesis at the metabotropic receptor. So, the On cell hyperpolarizes when the photoreceptors depolarize, in the dark. It synapses with its On retinal ganglion cell in the internal sublamina b of the inner plexiform layer.
ON and OFF center pathways and center surround organization of the retina are initiated at the photoreceptor to bipolar and horizontal cell contacts in the outer plexiform layer. Webvision.
^ ON-OFF centre cell Wide field A19-20 amacrine cells with ON and OFF bipolar cell input.
Webvision. A19 and A20: ON-OFF wide-field amacrine cells.
^ Ophthalmoscope Instrument used to visualise the parts of the eye. Various optical configurations and technologies are available.
^ Opsin Opsins are a group of light sensitive proteins found in photoreceptor cells of the retina.
^ Optic Atrophy (see Atrophy, Optic)
^ Optic Chiasm
^ Optic Cup In embryology this refers to the cup shaped structure attached to the brain which later develops into the retina ciliary body and iris. In the developed eye this refers to the depression in the central optic nerve head as seen by the fundoscope.
Wikipedia optic cup embryology
Wikipedia optic cup ophthalmology
^ Optic Disk Where ganglion cell axons exit the eye to form the optic nerve.
^
Optic Nerve The 4 cm long cord joining the eyeball to the brain.
The second cranial nerve.
Normally, about 1.2 million retinal ganglion cell axons from the nerve fibre layer,
the inner most retinal layer,
converge on the optic disc 3 or 4mm superomedial
(above and inside) the posterior pole of the eye. They pass by the retina, the choroid
and the sclera,
which merges with the
lamina cribrosa.
Passing the lamina cribrosa the axons develop
myelin sheaths
from supporting oligodendrocytes and group into fascicles with septal
separations. Surrounding the optic nerve is its
pia mater containing a vascular plexus (see Artery for
description of blood supply).
This in turn is surrounded by the thin arachnoid mater. After a
narrow sudural space comes the dura mater which is continuous with the sclera.
The nerve takes a slight
s-bend in the orbital cavity then passes through the optic canal on its way to
the cranial cavity where
it joins its opposite at the optic chiasm.
The macular fibers or papillomacular bundle near the eye are lateral (temporal) in the nerve and graduate to
a medial position at the optic chiasm. The rest of the eye's fibers stay in in their respective positions
in the nerve.
The nasal fibres of each eye cross over in the
chiasm so that visual data from one side of each visual field goes to the opposite side of
the brain. (This is consistent with other sensory and motor innervation in the body e.g left hand supplied by
right brain). From here most of the fibres follow the right and left optic tracts
to the respective lateral geniculate nucleus in the thalamus. Though some go to the
pretectal nucleus, superior
colliculus and hypothalamic nuclei. Though most of the optic nerve
fibers are retinal ganglion cell axons, there are a small
number of efferent fibres from unknown origin in the brain to the eye.
~ head The anterior part of the optic nerve nearest the eye.
MedlinePlus optic nerve disorders
^ Optic Neuritis Any inflammatory disease of the optic nerve. In other words an optic nerve disease in which the immune system plays a major causative role. Examples include Multiple Sclerosis, Devic's disease (optic nerve involvement with transverse myelitis), complications of Mononucleosis, Syphilis, and Behcet's syndrome.
^ Optic Neuropathy or sometimes Optic Atrophy is any disease affecting the optic nerve, but usually excluding disease of an inflammatory cause (ie optic neuritis) or tumours of the nerve or congenital developmental anomalies like colobomas or hypoplasia. The major subgroupings are Glaucomatous Optic Neuropathy, Ischaemic Optic Neuropathy, Toxic Optic Neuropathy (including nutritional and metabolic causes) and Hereditary Optic Neuropathy (which includes Leber Hereditary Optic Neuropathy, the most prevalent of this group). Diseases which may overlap "neuritis" and "neuropathy" groups include some toxic causes, Arteritic Anterior Ischemic Optic Neuropathy, Dysthyroid Optic Neuropathy and HIV Optic Neuropathy.
^ Optic Tract Brain structure continuing from the optic nerve via the optic chiasm (where half of the information from each eye crosses sides, and half stays on the same side) to the lateral geniculate nucleus.
^ Optical Coherence Tomography
^ Ora serrata
^ Outer Plexiform Layer The most peripheral of two retina layers where neurons connect. [The other is the inner plexiform layer.] At the first synapse in the retina, in the outer plexiform layer, cone pedicles and rod spherules synapse with various bipolar cell and horizontal cell types.
Webvision outer plexiform layer
^ Oxidation In oxidation-reduction chemical reactions generally oxidation is an increase in oxidation number or a loss of electrons. This reaction is always accompanied by a reduction reaction of an electron receptor to keep the net change in oxidation number zero. In biochemistry oxidation is found in catabolic (energy releasing) processes as opposed to reductive reactions which are in biosynthetic (energy requiring) pathways. In the cell sometimes excessive harmful oxidation occurs due to the over production of free radicals which steal electrons from other molecules. Molecules internal to the cell and from the diet which neutralize these free radicals or stop cascading chain reactions of oxidation are called anti oxidants.
^ Oxidative Phosphorylation Adding a high energy phosphate group to ADP to form ATP using ATP Synthase (Complex V), the final step in the electron transport-oxidative phosphorylation chain of the inner mitochondrial membrane.
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Optic Nerve Disease
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