|
^ Action Potential Nerve impulse due to a moving wave of temporary changes in concentrations of ions inside and outside a cell caused by ion movement across the cell membrane. This changes the electrical transmembrane potential (voltage) difference. Resting potential is about -30 to -60mV, or, in the examples of photoreceptor cells and OFF-center bipolar cells in the light, hyperpolarization of about -80mV. As ions move across the membrane, a depolarization of up to +30mV occurs. In a nerve cell the action potential is started at the post synaptic membrane by neuroreceptor stimulus and propagated along the cell membrane and axon by a balance of actions of voltage-gated Sodium (Na+) and Potassium (K+) Channels and Na+, K+ ATPase ion transporter or pump. Speed of propagation may exceed 100 meters per second. Generally, the frequency, not the amplitude, of action potential firings carries information. Interestingly, photoreceptors, retinal horizontal cells and bipolar cells do not produce rapid action potentials at all. They only generate slow graded hyperpolarizations and depolarizations to various light stimuli.
Mathematical modelling of action potentials from the Science Asylum
^ Adenosine A nucleoside which may act as a retinal neuro transmitter in amacrine or ganglion cells. It is also involved with blood vessel dilation, heartbeat regulation, smooth muscle contraction and sleep regulation. Caffeine blocks adenosine neuronal receptors in the brain thereby promoting wakefulness.
^ Adenosine triphosphate (ATP) Energy transporting "currency". Made by substrate phosphorylation from ADP in glycolysis in the cytosol and the tricarboxylic acid (TCA, Krebbs) cycle of the mitochondrial matrix and by NADH dependant oxidative phosphorylation in the electron transport chain of the mitochondrial inner membrane. see Nucleotide
^ Advanced glycation end products (AGEs) Attachments of sugars onto amino acids of proteins causing cross linking of molecular structures. This is one likely mechanism of aging. This accelerated process in diabetes mellitus explains rapid tissue degeneration in this disease. Tobacco smoking also plays a role in AGE production. Experiments in animals have shown a reduction of retinal damage, amongst other degenerative changes, when AGE formation is prevented with aminoguanidine or reversed with dimethyl-3-phenacylthiazolium chloride or ALT-711. There are various potential therapies in glaucoma relating to neuroprotection and eye structure remodeling.
Asif M, Egan J, Vasan S, Jyothirmayi GN, Masurekar MR, Lopez S, Williams C, Torres RL, Wagle D, Ulrich P, Cerami A, Brines M, Regan TJ. An advanced glycation endproduct cross-link breaker can reverse age-related increases in myocardial stiffness. Proc Natl Acad Sci U S A. 2000 Mar 14;97(6):2809-13.PMID 10706607 see also PMID 11158613
Cerami C, Founds H, Nicholl I, Mitsuhashi T, Giordano D, Vanpatten S, Lee A, Al-Abed Y, Vlassara H, Bucala R, Cerami A. Tobacco smoke is a source of toxic reactive glycation products. Proc Natl Acad Sci U S A. 1997 Dec 9;94(25):13915-20.PMID 9391127
Tezel G. Oxidative stress in glaucomatous neurodegeneration: mechanisms and consequences. Prog Retin Eye Res. 2006 September; 25(5): 490-513 PMID 16962364
^ AION see Anterior Ischaemic Optic Neuropathy
^ AIDS (Acquired Immune Deficiency Syndrome) The clinical symptomatic stage of Human Immunodeficiency Virus (HIV) infection. Optic nerve disease can be demonstrated as early as the preclinical stage of HIV infection in the form of visual field defects. Pathology in the brain, Progressive Diffuse Leukoencephalopathy, is similar to the optic nerve. Retinal or optic nerve damage can occur with or without secondary eye infection [retinitis] and with or without Highly Active Anti Retroviral Therapy. Anti HIV drugs like suramin sodium may have optic nerve toxic effects or zidovudine which can cause retinal nerve damage independantly of the virus infection.
Mahadevan A, Satishchandra P, Prachet KK, Sidappa NB, Ranga U, Santosh V, Yasha TC, Desai A, Ravi V, Shankar SK. Optic nerve axonal pathology is related to abnormal visual evoked responses in AIDS. Acta Neuropathol. 2006 Oct;112(4):461-9. Epub 2006 Jun 21. PMID 16788820
Falkenstein IA, Bartsch DU, Azen SP, Dustin L, Sadun AA, Freeman WR. Multifocal Electroretinography in HIV-Positive Patients without Infectious Retinitis. Am J Ophthalmol. 2008 Feb 14 [Epub ahead of print] PMID 18280451
Freeman WR, Van Natta ML, Jabs D, Sample PA, Sadun AA, Thorne J, Shah KH, Holland GN; SOCA Research Group. Vision Function in HIV-Infected Individuals without Retinitis: Report of the Studies of Ocular Complications of AIDS Research Group. Am J Ophthalmol. 2008 Mar;145(3):453-462. Epub 2008 Jan 11. PMID 18191094
D A Jabs. Ocular manifestations of HIV infection.Trans Am Ophthalmol Soc. 1995; 93: 623-683. PMCID: PMC1312074
^ Allele A version of a gene responsible for a trait or traits.
^ Alstrom syndrome An autosomal recessively inherited syndrome of retinitis pigmentosa, deafness, obesity, and diabetes mellitus. The retinal lesion causes nystagmus and early loss of central vision in contrast to loss of peripheral vision first, as in other pigmentary retinopathies. So, it is in the differential diagnosis of optic nerve disease. The gene locus is 2p13.
^ Alzheimer's Disease Dementing brain disease in which visual field defects of optic nerve damage can be demonstrated. Chromosomal inheritance is implicated. Also, some studies suggest a preferential maternal inheritance implying a mitochondrial genetic role in Alzheimer's Disease.
^ Amacrine cell Retinal cell interneuron interacting at the Inner Plexiform Layer (IPL), the second synaptic retinal layer where bipolar and ganglion cells synapse. Most amacrine cell types lack an axon. There are about 40 different types. Classification is based on width of field of connection, and layer(s) of stratum/i in the IPL where the cell is located as well as neurotransmitter type. Most are inhibitory using either GABA or glycine as neurotransmitters. Some secrete dopamine.
^
Amaurosis A
general term for partial or total blindness especially referring to
retina or optic nerve degeneration.
~fugax Usually
temporary loss of vision due to temporary blood supply deficiency to
retina or optic nerve.
Leber's congenital ~
(see separate entry)
^
Amblyopia Poor visual acuity in one eye usually due
to perceptive system maldevelopment due to lack of stimulation usually from
esotropia (in turned or "lazy" eye) and non use of the affected eye (strabismic amblyopia).
This is traditionally treated with one or more of intermittent patching
and/or mydriatic eye drops in the good eye and
corrective lenses. Treatment is effective if started early,
usually under eight years of age.
The younger, the quicker the response.
May also refer to optic
neuropathy as in
Tobacco-alcohol amblyopia.
Procianoy E, Fuchs FD, Procianoy L, Procianoy F. The effect of increasing doses of levodopa on children with strabismic amblyopia. J AAPOS. 1999 Dec;3(6):337-40. doi: 10.1016/s1091-8531(99)70041-8. PMID: 10613576.
^
Amino Acid One of a class of organic
acids which are the building links in proteins and, also,
links of other sorts
in cell communication and metabolic processes. There are 20 "common"
amino acids which are used in proteins and some others which are not.
All have an alpha carbon to which are attached a
Hydrogen atom, an amino group (positive charge in neutral pH), a
carboxyl group (negative charge) and a side chain unique to each type
of amino acid. Some free amino acids such as Aspartic acid, Glutamate (glutamic
acid), Glycine, Serine, GABA (gamma-amino butyric acid) act as neurotransmitters in the eye and
others like Taurine are trophic (growth) factors in eye
development. Epinephrine (adrenaline) is a hormone. Each amino acid is coded for by a three
nuleotide base sequence on messenger RNA in the formation of proteins.
see Codon
Essential~ 10 of the 20 common amino acids which mammals cannot synthesize so must be
obtained from the diet: Arginine, histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine,
Threonine, Tryptophan and Valine. The non essential common amino acids are: Alanine, Asparagine,
Aspartate, Cysteine, Glutamate, Glutamine, Glycine, Proline, Serine and
Tyrosine (formed from phenylalanine).
^ Amiodarone Anti arrythmia heart drug associated with retinal toxicity.
^ AMPK 5' AMP-activated protein kinase or AMPK or 5' adenosine monophosphate-activated protein kinase is an enzyme that plays a role in cellular energy homeostasis.
^ Angle, ~of anterior chamber, or iridocorneal~, or filtration~The junction between the cornea and the iris in the anterior chamber where the aqueous humor drains through trabecular meshwork into the scleral venous sinus. Gonioscopy is used to view this angle.
^ Anterior Ischaemic Optic Neuropathy (AION) A grouping of optic nerve damage syndromes caused by poor blood supply. The blood deficit is to vessels supplying the front part of the nerve or Optic Nerve Head (as opposed to the rarer Posterior Ischemic Optic Neuropathy where the problem is at the back of the optic nerve). The causes of poor perfusion (flow) may be: (1)inflammatory artery occlusion (Arteritic) or (2) non inflammatory (Non-Arteritic) where pressure and resistance factors involved include reduced arterial pressure, increased intraocular pressure, narrowed arteries and/or increased blood viscosity as a result of various possible processes, diseases and drugs. Common predisposing factors are diabetes, high blood pressure, artery narrowing with atherosclerosis, and very low blood pressure. The syndromes are most common in middle age or later. Symptoms are typically sudden painless visual loss on waking in the morning which may fluctuate initially. The lower nasal visual field is most commonly lost first in non Arteritic AION. Arteritic AION may present early with headache, temple tenderness, flu-like symptoms. Early signs at the back of the eye include optic disc edema. Later the disc shows the paleness of optic nerve atrophy. It is important to present early because sight saving treatment is possible in some cases. See:
Ischemic Optic Neuropathy University of Iowa
What is Ischemic Optic Neuropathy? American Academy of Ophthalmology
Anterior Ischemic Optic Neuropathy Genetic and rare Diseases information Center
Ischemic Optic Neuropathy Merck Manual for 'Consumers' or 'Professionals'
Anterior Ischemic Optic Neuropathy Medscape [free registration required]
^ Antibody A specialized protein complex produced by the vertebrate immune system's B-lymphocytes in reaction to a specific antigen. The specificity of the antibody comes from variable and hypervariable regions of its protein amino acid code. The hypervariable region allows interaction with specific attachment sites on the antigen. A vast number of possible specific types of antibodies is gauranteed. A number of attachment sites on the antibody are each coded for by multiple possible genes each in turn varying through a somatic mutation mechanism of variable genetic recombination.
^ Antigen Any substance foreign to the body's immune system.
^ Anti oxidant In biochemistry either a scavenger of oxygen-derived free radicals (e.g. chain breaking antioxidant for lipid peroxidation) or preventer of their formation. Free radicals with their unpaired electron tend to oxidize other molecules i.e. take their electrons. Anti oxidant scavengers are actually oxidized to relatively stable compounds thus breaking the chain of free radical formation. Principle examples of chain breaking antioxidants are Superoxide Dismutase in the aqueous phase and Vitamin E (Tocopherols and Tocotrienols) in the lipid phase. When peroxides are formed from free radical attack the Selenium containing Glutathione peroxidase, by catalyzing the oxidation of reduced glutathione by peroxides, is another major defence against peroxidative damage to cellular components. Naturally occuring anti oxidants include Vitamin E, Vitamin C, urate, beta-carotene and flavonoids.
^ APB (2-amino-4-phosphobutyric acid, also called L-AP4) Glutamate receptor agonist (stimulator) which binds to the metabotropic receptor, probably mGlu6 on the post synaptic membrane of ON-centre bipolar cells and other metabotropic receptor sites.
^ Apoptosis A type of programmed cell death. Essential in tissue and organ development, immune mechanisms and likely involved in ageing and degenerative processes. Differentiated from other mechanisms of cell death such as necrosis and inflammatory processes by its organized manner, rapidity and lack of collateral damage to surrounding tissues. In neurons glutamate toxicity and mitochondrial calcium fluxes probably play a key role. A topic of intense study and guaranteed multihit with any Web search engine.
^ Aqueous Humour Fluid filling the anterior and posterior chambers of the eye in front of the lens. Produced by the ciliary processes in the posterior chamber it drains through the pupil into the anterior chamber and via the iridocorneal angle into the scleral venous sinus (Canal of Schlemm) and out through the ciliary veins. Reabsorption plays a major role in regulating intra ocular pressure and has large implications in the causes and treatment of glaucoma.
^ Arteritis Inflammation of artery(ies). Giant Cell Arteritis=Temporal Arteritis The most common inflammatory arterial disease of the nervous system. Blindness can occur from Arteritic Anterior Ischemic Optic Neuropathy in untreated cases. Prednisone is very effective treatment. Usually affecting the elderly, it is slightly more common in women. Early symptoms are low grade fever, fatigue, depression, and headache. Sometimes temple tenderness and jaw pain with chewing (claudication) are evident. The polymyalgia rheumatica syndrome of muscle and joint stiffness and pain predominantly in the shoulder girdle is sometimes associated. Measurement of the erythrocyte sedimentation rate (ESR) is the most sensitive test. With treatment, early vision loss is usually reversible. Temporal artery biopsy should be done after initiating emergency treatment to confirm diagnosis, as life long steriod treatment is indicated to prevent recurrence.
^ Artery Blood vessel taking blood away from the heart to a tissue or organ. The central retinal artery branches from the ophthalmic artery itself a branch of the internal carotid artery. It pierces the inferomedial aspect of the optic nerve to reach the retina. After branching into upper and lower temporal and upper and lower nasal branches it anastomoses with the ciliary arteries as well as having its own end arteries. The ciliary arteries are also branches of the ophthalmic artery. Two long posterior ciliary arteries breach the sclera to supply the ciliary body and the iris. Similarly, several short posterior ciliary arteries supply the choriod. Muscular branches of the opthalmic artery are the origins of the anterior ciliary which pierces the sclera, gives off anterior conjunctival arteries and supply the iris. Apart from its nerve fiber layer supplied by retinal arterioles, the blood supply to the Optic Nerve Head is from the short posterior ciliary circulation via the vessels in the choroid anteriorly and, behind the lamina cribrosa, via the surrounding plexus of the pia mater. There are varying patterns of detail due to variable short posterior ciliary artery numbers and positions. Sometimes the central retinal artery may send out arterioles from the center of the nerve anteriorly. The segmental supply to the nerve explains the patterns of visual loss in Anterior Ischaemic Optic Neuropathy. Toward the brain in the optic canal the nerve is nourished by the pial plexus supplied by recurrent ophthalmic artery branches. In the cranial cavity the nourishing pia mater is supplied by branches of the ophthalmic and of the superior hypophysial, another internal carotid branch.
^ Aspartate Amino acid acting as an excitatory neurotransmitter in the eye and brain.
^ Astroglia or Astrocyte Glial support cell in the central nervous system.
^ ATP Adenosine triphosphate see Adenosine triphosphate
^
ATPase Various membrane active transport proteins using ATP as
energy source. e.g.
Na+, K+ ATPase is a plasma membrane protein
responsible for the active extrusion of sodium ions and accumulation of potassium
ions in the cell. This maintains the concentration difference in side and outside the cell
which is essential for vital functions such as action potentials in neurons. It is a two subunit
transmembrane
complex. ATP binding occurs on the cytosol (inner) side. It is inhibited by cardiac glycosides
such as oabain and digitalis by binding on the extracellular surface of the protein.
Ca2+ ATPase Calcium transporter in muscle sarcoplasmic reticulum and mitochondria.
Gastric H+,K+ ATPase Responsible for maintaining acidity in the stomach.
Vacuolar H+ ATPase Bone remodeling osteoclast proton pump.
MDR ATPase Peptide and drug transporting protein responsible for
multidrug resistance in bacteria and cancer cells.
^
ATP Synthase Complex V of the mitochondrion (also F1F0-ATPase
or mitochondrial H+-ATPase).
The final step in the electron transport/oxidative phosphorylation chain.
Enzyme responsible for ATP production (phosphorylation of ADP) at the mitochondria
inner membrane. It uses the H+ gradient
between matrix and intermembrane space of mitochondria created by the preceding
electron transport
steps in the chain. A proton (H+) driven stator-rotor mechanism of protein subunits is postulated to
cause protein conformational changes in ADP binding subunits causing phosphorylation to ATP.
Inhibitors are oligomycin and dicyclohexylcarbodiimide (DCCD) both blocking proton flow through the
hydrogen channel, F0.
Two of its subunits labelled 6 and 8 by geneticists are coded by mitochondrial DNA.
Subunit 6 is coded for by mitochondrial DNA nucleotides 8527-9207.
Leigh syndrome
is associated with point mutations at mtDNA position 8993 affecting ATP synthase subunit 6.
This probably interferes with the hydrogen
channel of ATP synthase.
One LHON family has also been
described with a mutation at mtDNA nucleotide position 9101.
see OMIM 516060, also
Complexes of the electron transport chain
^ Atrophy, Optic (OA) or Optic Neuropathy includes various specific diseases causing degeneration of the optic nerve. The most common of the inherited forms is the mitochondrial inherited Leber Hereditary Optic Neuropathy (LHON) also named Leber Optic Atrophy typically affecting young adults, especially males . Other congenital (from birth) or early childhood forms are dominant OA, X-linked OA or others associated with syndromes of other organ defects e.g Wolfram, De Morsier septo optic dysplasia, Spastic Paraplegia Syndrome, PEHO, Optic Atrophy 3, Hallervorden-Spatz, etc.
Ahmad SS, Kanukollu VM. Optic Atrophy. 2020 Aug 11. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan–. PMID: 32644556.
^ Axon Cellular process carrying nerve signals (action potentials) from the cell body to the synapse.
^ Axon Transport or Axonal transport or Axoplasmic_transport System of moving organelles like mitochondria and vesicles along axons using the microtubule cell endoskeleton. Transport speeds of 2 to 5 micrometers/second are possible. Cytosolic dyenin proteins move organelles and vesicles from the plus end of a microtubule to the minus end - in an axon from the synaptic terminus to the cell body. Kinesins effect movement the other way from minus to plus resulting in outward movement. Since the axons of the optic nerve are relatively long as they pass from the ganglion cell layer to the lateral geniculate nucleus any problem affecting axon transport is potentially serious to the well being of the optic nerve. Defects in this mechanism are one speculated pathway of optic nerve degeneration in some diseases.
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