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Sensory System
Published in Peter Kam, Ian Power, Michael J. Cousins, Philip J. Siddal, Principles of Physiology for the Anaesthetist, 2020
Peter Kam, Ian Power, Michael J. Cousins, Philip J. Siddal
The visual pathway is from the retina to the cerebral cortex. Rods and cones are the receptors in the retina. The rods are found throughout the retina (except for the fovea) and contain the photopigment rhodopsin. They are used in night vision and have high sensitivity but do not transmit colour vision. The cones contain three pigments: erythrolabe (red sensitive), chlorolabe (green sensitive) and cyanolabe (blue sensitive). They are concentrated in the fovea and are used in day and colour vision. The rods and cones are connected to ganglion cells, the axons of which are carried in the optic nerve and in the lateral geniculate body to the primary visual cortex. Fibres from the nasal halves of the retinas cross, whereas those of the temporal sides remain ipsilateral. Optic nerve fibres also pass to the midbrain pretectal areas (pupillary reflexes) and to the superior colliculus (eye movements).
Physiology of the nervous system
Published in Peter Kam, Ian Power, Michael J. Cousins, Philip J. Siddal, Principles of Physiology for the Anaesthetist, 2015
Peter Kam, Ian Power, Michael J. Cousins, Philip J. Siddal
The visual pathway is from the retina to the cerebral cortex. Rods and cones are the receptors in the retina. The rods are found throughout the retina (except for the fovea) and contain the photopigment rhodopsin. They are used in night vision and have high sensitivity but do not transmit colour vision. The cones contain three pigments: erythrolabe (red sensitive), chlorolabe (green sensitive) and cyanolabe (blue sensitive). They are concentrated in the fovea and are used in day and colour vision. The rods and cones are connected to ganglion cells, the axons of which are carried in the optic nerve and in the lateral geniculate body to the primary visual cortex. Fibres from the nasal halves of the retinas cross, whereas those of the temporal sides remain ipsilateral. Optic nerve fibres also pass to the midbrain pretectal areas (pupillary reflexes) and to the superior colliculus (eye movements).
Specializations of the Human Visual System: The Monkey Model Meets Human Reality
Published in Jon H. Kaas, Christine E. Collins, The Primate Visual System, 2003
Most mammals possess two types of cones, with photopigments that respond maximally to short (S) and medium-long (M/L) wavelength light, respectively. In primate evolution, the gene coding the M/L photopigment underwent a duplication, with subsequent divergence of wavelength sensitivity, such that the catarrhine primates (i.e., the Old World monkeys and hominoids; see Figure 10.1) possess distinct S, M, and L photopigments and cones.1516× In most Old World monkeys that have been examined, the ratio of L:M cones is approximately 1:1,17 and studies of chimpanzees suggest a ratio of about 1.3:1.18 In humans, by contrast, the L:M ratio averages approximately 2:1. This conclusion is supported by evidence obtained with a variety of techniques.14,17 Humans also exhibit considerable individual variation in color sensitivity. Some of the variation in human color vision reflects polymorphisms in the alleles coding for L and M photopigments,19 but some is probably attributable to the remarkably wide range of individual variation in the L:M cone ratio.20,21 By contrast, genetic and spectral sensitivity studies of chimpanzees and macaques sug-gest that these taxa are nearly monomorphic at the L and M opsin loci and exhibit little variation in L:M ratio.17,18,22,23
Reduced Photoreceptor Outer Segment Layer Thickness and Association with Vision in Amblyopic Children and Adolescents with Unilateral High Myopia
Published in Current Eye Research, 2021
Tingkun Shi, Wenli Zhang, Shirong Chen, Honghe Xia, Haoyu Chen
Photoreceptor outer segment thickness decreased significantly and was associated with visual acuity in this study. Similar results were found in a previous comparative cross-sectional interventional study of 21 anisohypermetropic amblyopia and 25 age-matched controls.12 The anisohypermetropic amblyopic eyes had significant differences in OS length, and the higher OS length was associated with better BCVA. Experimental studies have revealed that photopigments are present in very high concentrations within the membranes of the photoreceptor OS discs, and a decreased number of discs reduced the magnitude of the outflow signal from the photoreceptors proportionately.27 The old discs are continually phagocytosed by the RPE and replaced by renewed discs. Therefore, the OS is believed to reflect the function of the outer retina to a certain extent.
Prolonged Melanopsin-based Photoresponses Depend in Part on RPE65 and Cellular Retinaldehyde-binding Protein (CRALBP)
Published in Current Eye Research, 2021
Krystal R. Harrison, Aaron N. Reifler, Andrew P. Chervenak, Kwoon Y. Wong
Intrinsically photosensitive retinal ganglion cells (ipRGCs) are output neurons of the mammalian retina that drive irradiance-dependent, nonimage-forming visual responses including pupil constriction, melatonin suppression, and circadian photoentrainment. These ganglion-cell photoreceptors contain the photopigment melanopsin and can signal light continuously for many hours.1 Like the photopigments of rods and cones, melanopsin uses the chromophore 11-cis-retinal to sense light: photon absorption isomerizes 11-cis-retinal to all-trans-retinal, which then activates the phototransduction cascade.2,3 To restore photosensitivity, all-trans-retinal must be reisomerized to 11-cis-retinal. For rods and cones, such reisomerization involves the nearby retinal pigment epithelium (RPE) which contains enzymes mediating the retinoid cycle. Located far from the RPE, ipRGCs possess intrinsic mechanisms for regenerating 11-cis-retinal.4–6 Surprisingly, when we used drugs to inhibit the RPE retinoid cycle or poison Müller glia, melanopsin-based ipRGC and pupil responses to prolonged illumination became less sustained, and exogenous cis-retinal rescued the deficit, suggesting that the continuous regeneration of light-sensitive melanopsin during extended photostimulation depends partly on retinoids imported from the RPE to ipRGCs via Müller cells,7 perhaps because ipRGCs’ endogenous regenerative mechanisms are insufficient.
Erythropsia and Chromatopsia: Case Study and Brief Review
Published in Neuro-Ophthalmology, 2021
Michael S. Vaphiades, Brendan D. Grondines, Christine A. Curcio
Congenital dyschromatopsia occurs in approximately 8.0%–8.7% in men and approximately 1% in women.2 It arises from disorders in the genes coding, expressing or involved in the operation of cone photopigments.2 It is bilateral, symmetrical and generally affects the entire visual field. Rod monochromatism causing achromatopsia is a congenital cone photoreceptor disorder affecting about 1 in 30,000 individuals. These patients have normal rod function but no detectable cone function; therefore, everything they see is in shades of grey (total colour blindness). Patients usually present in infancy with nystagmus and photophobia.3 Although congenital dyschromatopsia usually affects individual cones and thereby a single subsystem of colour vision, such characteristics are not frequently encountered in acquired dyschromatopsia.4