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^ P- 450 system See Cytochrome P450
^ P, ~Pathway, ~Cell Retinal ganglion midget cell projecting to the parvocellular layer of the lateral geniculate nucleus or corresponding lateral geniculate cells.
Wikipedia retinal ganglion midget cells
Wikipedia lateral geniculate M K P cells
^ Papilla, optic=Optic disc. The site of connection of the optic nerve to the eye is given this name due to the impression of a raised projection it sometimes gives. adj papillary.
^ Parallel Processing referring to M and P pathway. Multiple streams of data are dealt with at the same time by the brain.
^ Parkinson's Disease A progressive disease of the brain's basal ganglia resulting in poor muscle coordination, slowness of movement, gait abnormalities, tremor, rigidity, flat facial appearance, and depression of mood. It usually occurs in middle age to elderly persons. There is degeneration of the substantia nigra (a basal ganglia nucleus) resulting in a deficiency of dopamine neurotransmitter to its projections in the putamen and caudate nucleus (corpus striatum). An almost identical syndrome may be induced by a metabolite of MPTP which inhibits Complex I. This points the finger at a mitochondrial defect as the cause of Parkinson's disease. Indeed mitochondrial gene loci have been found for Parkinson's disease. This metabolic pathway is also involved in Leber's Hereditary Optic Neuropathy. It is hoped that study of both diseases may have mutually beneficial results.
^ Parvocellular layer One of four layers of the lateral geniculate nucleus in the thalamus where midget system retinal ganglion cell axons terminate. cf. Magnocellular layer
^ Patch Clamp
^ Pathway,Neural~(M~,P~,K~),Metabolic~
Wikipedia lateral geniculate M K P cells
^ Penetrance In genetics the proportion of phenotypic expression of a mutant gene. e.g. in LHON the frequency of expression in the form of optic neuropathy of the 11778 mutation in carriers, i.e. penetrance, varies widely amongst and within families and population groups-still a mystery as to why. This mystery along with the issue of tissue specificity (ie Why should a certain defect in the mitochondria which are present in every tissue show up only in select tissue?) is common with other mitochondrial genetic diseases. Search for a single secondary chromosomal factor has tended to be unsuccessful in general so far. However, mitochondrial haplotype, multiple chromosomal factors and environment exposure probably all contribute.
See a discussion of these issues in relation to mitochondrial inherited aminoglycoside-induced deafness: Nathan Fischel-Ghodsian. Mitochondrial Mutations and Hearing Loss: Paradigm for Mitochondrial Genetics. Am. J. Hum. Genet.,1998; 62:15-19.PMID 9443888
^ Penumbra, ischaemic
Matthias Endres, Ulrich Dirnagl. Neuroprotective Strategies in Animal and in Vitro Models of Neuronal Damage: Ischemia and Stroke Eurekah Bioscience Collection Neurodegenerative Disease
^ Perception
^
Perimetry, Visual Field Measurement of visual field using
various devices
see Scotoma
Contrast~
Goldman~
Humphrey~
See International Perimetric Society for standards of perimetry.
^ PEHO syndrome Inherited syndrome infantile progressive encephalopathy with severe hypotonia, convulsions with hypsarrhythmia, profound mental retardation, hyperreflexia, transient or persistent edema, and optic atrophy. see OMIM 260565
^ PET Positron Emmision Tomography
^ PGC-1α PGC-1α is a transcriptional coactivator that regulates the genes involved in energy metabolism. It is the master regulator of mitochondrial biogenesis.
^ Phagocytosis The cellular mechanism by which cells consume other cells or parts of other cells. The outer discs of photoreceptors are phagocytosed regularly by the pigment epithelial cells with a burst of activity in the morning light.
^ Phenotype Expressed result or form of an organism as opposed to genotype which is specific genetic code. Phenotype depends on an interaction of genes and environment over time.
^ PhospheneVisual perceptions, patterns or images not evoked directly through normal visual pathways from eye to brain.
^ Photon Unit of light energy having qualities of wave and particle in quantum physics. Click here for the number of photons a rod is sensitive to.
^ Photoreceptor Nerve cell in the retina which is light sensitive. Either rod or cone or melanopsin-containing retinal ganglion cells (mRGCs).
Wikipedia intrinsically photosensitive retinal ganglion cells
^ Pigment cell
^ Pia Mater The membranous vascular plexus surrounding the brain, spinal cord, and optic nerve. In the optic nerve the pia is supplied by the short posterior ciliary arteries.
^ Plasmid
^ Plasticity, neuronal The ability for neurons or groups of neurons to adapt to their input and change their numbers and/or signaling patterns. This process is important for short and long term memory in the brain and pain pathways in the spinal cord. Neuron adaptations in the visual system occur to light level, movement and pattern changes. The visual cortex is known to be very adaptive to loss of data and is well versed in "filling in the blanks" as it does so well with the visual blindspot. Another classic example is the typical lack of patient awareness of progressive visual field defects which occurs in glaucoma until the central vision is lost at end stage. More detailed knowledge of this process is the subject of research as it may lead to practical applications where sensory input is damaged in retinal and optic nerve disease.
Castaldi E, Lunghi C, Morrone MC. Neuroplasticity in adult human visual cortex. Neurosci Biobehav Rev. 2020 May;112:542-552. doi: 10.1016/j.neubiorev.2020.02.028. Epub 2020 Feb 21. PMID: 32092315.
Sabel BA, Gao Y, Antal A. Reversibility of visual field defects through induction of brain plasticity: vision restoration, recovery and rehabilitation using alternating current stimulation. Neural Regen Res. 2020 Oct;15(10):1799-1806. doi: 10.4103/1673-5374.280302. PMID: 32246620.
^ POAG (primary open angle glaucoma)The commonest type of glaucoma where the iridocorneal angle is not abnormaly narrowed.
^ Polymerase Chain Reaction (PCR) A DNA amplification technique used to detect specific DNA sequences in a small DNA sample. Useful for detecting mutant genes, identifying individuals and diagnosing infectious diseases. Care has to be taken when looking for mitochondrial DNA (mtDNA) sequences that chromosomal DNA with similar sequences are not confused with mtDNA. See:
Herrnstadt C, Clevenger W, Ghosh SS, Anderson C, Fahy E, Miller S, Howell N, Davis RE. A novel mitochondrial DNA-like sequence in the human nuclear genome. Genomics 1999 Aug 15;60(1):67-77 PMID 10458912
^ Pore
^ Potential, Membrane
^ Pressure, Intraocular, Blood
^ Pretectal area Or pretectum is a midbrain structure composed of seven nuclei and comprises part of the subcortical visual system. Nuclei, especially the olivary nucleus, include part of the afferent pathway of the pupillary light reflex.
^ Progressive Diffuse Leukoencephalopathy (PDL) Brain pathology in AIDS. Similar findings are seen in the optic nerves of AIDS cases.
^ Prion A non bacterial non viral infectious agent which causes central nervous system disease such as scrapie in sheep, mad cow disease, and Kuru and Jacob Creutzfeld dementias in humans. Inoculation of scrapie in rodent conjunctiva causes nerve damage. See:
Hogan RN, Brown P, Heck E, Cavanagh HD. Risk of prion disease transmission from ocular donor tissue transplantation. Cornea.1999;18:2-11. PMID: 9894930
Scott JR, Foster JD, Fraser H. Conjunctival instillation of scrapie in mice can produce disease. Vet Microbiol.1993;34:305-309.PMID 8506606
Buyukmihci NC, Goehring-Harmon F, Marsh RF. Photoreceptor degeneration during infection with various strains of the scrapie agent in hamsters. Exp Neurol.1987;97:201-206. PMID 3108023
^ Progesterone A female sex hormone with neuroprotective properties in the central nervous system and eye.
Hernández-Rabaza V, López-Pedrajas R, Almansa I. Progesterone, Lipoic Acid, and Sulforaphane as Promising Antioxidants for Retinal Diseases: A Review. Antioxidants (Basel). 2019 Mar 2;8(3):53. doi: 10.3390/antiox8030053. PMID: 30832304; PMCID: PMC6466531.
Wikipedia lateral geniculate nucleus
^ Projection, of optic nerve axons
Wikipedia lateral geniculate nucleus
^ Protein A string or polymer of amino acids. Also, polypeptide. Proteins make up a major portion of cell enzymes and mechanical structure. RNA holds the codes for protein amino acid sequence (primary structure) which is formed in the translation process. Proteins' secondary structure includes alpha-helixes and beta-strands. The bending and folding of the secondary structure into compact shapes is the tertiary protein structure. Quaternary structure refers to the association of multiple polypeptide strand subunits.
^ Proteome The protein make up of an organism. With automated machinery, e.g mass spectrometers using inkjet technology, it is possible to rapidly and accurately detail the amino acid structure of the numerous proteins in the cell apparatus. It is hoped that this detailed knowledge may allow tailored therapies for individuals' diseases. cf genome.
^ Pseudopapilledemasee Optic Nerve Drusen
^ Psychophysics
Michael Kalloniatis OD, PhD and Charles Luu OD. Psychophysics of Vision. Webvision
^ Pupil The changable (see IFOND logo) round (in humans) black circle in the middle of the coloured part (iris) of the eye. The space where the anterior and posterior chambers of the eye communicate. Isolated enlargement of pupil or pupils with no other eye inflammation or motor nerve signs may be present in optic nerve disease or may signify a benign abnormality of the ciliary ganglion (Adie's pupil).
^ Pupil reflexes The normal iris responds to changes in light intensity and to accommodation (near focusing) by constricting and opening thus changing pupil size. This is a significant sign of health of the nerve pathways going to (afferent) and coming from (efferent) the central nervous system. The afferent pathway for the light reflex is via the melanopsin neurons of the retina, optic nerve, optic chiasm and optic tract to mid brain pretectal nucleus then to the nucleus of the third cranial nerve(oculomotor nerve). The efferent pathway is from the 3rd nerve nucleus to the oculomotor nerve, ciliary ganglion and its short ciliary nerve branches. The accomodative afferent pathway is different, through the lateral geniculate nucleus and cortex. The direct light reflex is the constriction of the pupil to light shone on it. The consensual light reflex, the constriction of the pupil opposite to the one light is shone in, confirms an intact efferent pathway to the observed pupil and an intact afferent path for the eye the light shines in. When a pupil only constricts to light shone in its opposite and not (as much) to direct light this is called a relative afferent pupillary defect. This can be seen in optic nerve disease. The accommodative reflex is the pupil constricting in response to focusing on near objects. Light Near Dissociation is when the pupil responds normally to accommodation but not to light. This response may be seen in retinal and optic nerve disease. It may also occur in diabetes, syphllis (Argyl Robertson pupil), and with dorsal midbrain lesions such as hydrocephalus and pineal tumours.
^ PVECP (pattern visually evoked cortical potentials)
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