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Assessing Paediatric Development in Psychiatry
Published in Cathy Laver-Bradbury, Margaret J.J. Thompson, Christopher Gale, Christine M. Hooper, Child and Adolescent Mental Health, 2021
Functional ability and diversity are achieved via several mechanisms. The addition of new synapses between neurones is the first stage of maturation. The creation of synaptic linking of different neuronal pathways by bridging/inter-neurones allows regulation, co-ordination and feedback of information, e.g. the retina, amacrine and horizontal cells collate information from several cone/rod cells and interact with bipolar and retinal ganglia cells in the eye.
Describe the mechanisms of visual adaptation
Published in Nathaniel Knox Cartwright, Petros Carvounis, Short Answer Questions for the MRCOphth Part 1, 2018
Nathaniel Knox Cartwright, Petros Carvounis
Light adaptation occurs when a dark-adapted person moves into the light. Light adaptation is rapid and gets faster when the light is brighter. Three processes are involved: the pupil constricts from a maximum of 9 mm to a minimum of 1 mm, reducing light entry by about 1.3 log unit. This takes less than 1 secondretinal neuronal light sensitivity is altered within milliseconds by neural factors and can change by about 3 log units. It is thought that horizontal cells feed back on cones to reduce their light sensitivity. This effect is most pronounced with large diffuse stimuliphotochemical changes lead to the greatest change in visual sensitivity (8 log units) but take seconds to minutes to occur. Light exposure leads to photopigment bleaching: the molecular basis of this process is thought to involve changes in intracellular calcium levels. At normal levels of ambient lighting approximately 5% of rhodopsin is bleached.
Discussions (D)
Published in Terence R. Anthoney, Neuroanatomy and the Neurologic Exam, 2017
Another relevant comparison involves two other classes of retinal neurons—horizontal cells and amacrine cells—usually described as local interneurons. Although recent authors apparently agree that the amacrine cells have no axons (e.g., B&K, p. 301; C&S, p. 540), these cells can generate action potentials (e.g., W&G, p. 327; A&B, p. 360; Ruch and Patton, 1979, p. 486). Given the usual criteria for distinguishing between dendrites and axons, this presents a clear dilemma. Nevertheless, most authors who label the amacrine cell’s processes simply consider them all to be dendrites (e.g., B&K, p. 301; K&S, p. 354). Williams and Warwick are refreshingly equivocal in this regard, however. They state that: “All their [amacrine neurons’] processes resemble dendrites, but are said to show the cytoplasmic features of both axons and dendrites; the direction of conduction in any process at a particular time will be determined by the polarization of the synapses which are active.” (1980, p. 1171)
Paracentral Acute Middle Maculopathy Revealing a Giant Aortic Aneurysm
Published in Ocular Immunology and Inflammation, 2022
Furkan Fatih Gulyesil, Mustafa Dogan, Mehmet Cem Sabaner, Hamidu Hamisi Gobeka, Hayri Demirbas, Ayse Kubra Sap Kinar
Paracentral acute middle maculopathy has been linked to changes in the ICP and DCP vasculature.3 The DCP in PAMM eyes has been found to have lower vessel density, suggesting that PAMM could be caused by ischemia of the deep retinal circulation.2 In the current report, the patient’s LE had a significantly reduced vascular density in the DCP, which was consistent with previous findings. Reduced oxygen content raises oxygen stress near the superficial retinal layers and choroid, rendering this area more susceptible to ischemia. A high-oxygen intake of horizontal cells also makes the middle retina highly vulnerable to ischemic damage. Paracentral acute middle maculopathy pathogenesis may thus start at the ICP and progress to the DCP via secondary downstream modifications.1
Effect of vitamin D deficiency on spatial contrast sensitivity function
Published in Clinical and Experimental Optometry, 2022
Other important retinal cells that assist in the regulation of visual processing are horizontal cells and amacrine cells. The lateral coupling of these cells plays an important role in visual acuity and contrast sensitivity formation by adjusting the receptive field sizes.20,21 The centre-surround organisation of receptive fields, in which the object (centre) and its background (surround) have different illumination signals is primarily regulated by bipolar cells.22 The large receptive fields of horizontal cells have been considered to form the surround of bipolar cells,23 and as a result of a light-induced increase in dopamine, uncoupling of horizontal cells leads to a reduced surround receptive field that is related with increased local contrast detection.24
La Retina de los Vertebrados
Published in Journal of the History of the Neurosciences, 2023
The prologues are followed immediately by the text of Los problemas histofisiológicos de la retina (Ramón y Cajal 1933a). It comprises six sections: (1) an historical review of theories about vision and retinal structure and function; (2) speculations concerning the roles of multiple parallel pathways of similar function, specifically, modules of cells for transmitting rod signals; (3) the “paradox” of horizontal cells in vertebrate retinas—that is, that they are not part of the vertical (photoreceptor-bipolar-ganglion cell) pathway but only interconnect photoreceptors; (4) the enigma of amacrine cells, which lack axons (seemingly in contradiction to the law of dynamic polarization), except for the axon-bearing “association amacrines,” which have been identified (now, as then) only in avian retinas (enigmatic, indeed!); (5) discussion of whether neurofibrils play an exclusively conductive role in the retina, concluding that this is unlikely because they were not detected in most retinal cell types; and (6) noting that decussations of optic (retinofugal) fibers within the eye, which Cajal observed in several invertebrates, had not been found in any vertebrate, and discussing the possible roles of such structures in relation to binocular and panoramic vision. Although much of this section is of primarily historical interest, having been superseded by an explosion of retinal investigations in the past 90 years, it nevertheless provides documentation of how the retina was understood at that stage in the history of neuroscience, as further insight into the mind of Cajal as “father of neuroscience,” and as a perfect introduction to the centerpiece of this volume.