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Hypothalamic Neuroendocrine Regulation
Published in George H. Gass, Harold M. Kaplan, Handbook of Endocrinology, 2020
The neurohypophysis contains neurosecretory nerve terminals that originate in the hypothalamus from magnocellular neurons.57 Most of the magnocellular cells are concentrated in two paired structures, the supraopotic nuclei, located at the ventral surface of the brain near the optic chiasma, and the paraventricular nuclei, adjacent to the third ventricle. The neurosecretory terminals contain large amounts of oxytocin and vasopressin, which are synthesized in the cell somata and are transported down the axons into swellings in their nerve endings.
ENTRIES A–Z
Published in Philip Winn, Dictionary of Biological Psychology, 2003
A nucleus of the THALAMUS devoted to vision, the lateral geniculate nucleus (LGN) relays information from retinal ganglion cells to the PRIMARY VISUAL CORTEX. The nucleus contains six layers, three of which receive input from each eye. Each layer contains a RETINOTOPIC map of one half of the VISUAL FIELD, the left LGN representing the right visual hemifield, and vice versa. Each layer contains relay cells, which project to the visual cortex, and local inhibitory INTERNEURONS. In primates, the two ventral layers (the MAGNOCELLULAR layers) contain large cells that receive their input from the large M-type retinal ganglion cells. The four dorsal layers (the PARVOCELLULAR layers) contain relatively small cells that receive their input from the small P cells of the RETINA. The RECEPTIVE FIELD of LGN relay cells largely reflects the properties of their retinal inputs. All are roughly circularly symmetric, and many have a centre surround organization. ON-centre/ OFF-surround cells are excited by increased illumination of the receptive field centre and by decreased illumination of the receptive field surround. OFF-centre/ON-surround cells respond oppositely. Cells in the parvocellular layers have small receptive fields and so represent the visual image with high resolution. They are often colour-selective, with centre and surround regions of the receptive field being most sensitive to different wavelengths of light, for example, red and green, or blue and yellow. Magnocellular cells are much less sensitive to colour than parvocellular cells and have much larger receptive fields. They are, however, more sensitive than parvocellular cells to rapidly changing stimuli such as flickering or moving objects. In addition they may receive stronger input from RODS than do parvocellular cells, and so may be important for night or scotopic vision. Finally, magnocellular cells are more sensitive than parvocellular cells to very small changes in illumination and so are more sensitive to faint (low-contrast) image features. In line with these properties, lesions of the parvocellular layers result in a reduction of VISUAL ACUITY and loss of colour vision. Lesions of the magnocellular layers leave acuity and colour sensitivity intact, but reduce sensitivity to motion. Magno- and parvocellular relay cells project to different layers of the visual cortex (4ca and 4cb), and so the form and colour pathway is kept separate from the motion pathway at several further stages of vision processing. The LGN receives what are thought to be modulatory synaptic inputs from the RETICULAR FORMATION of the BRAINSTEM. Electrical activation of these inputs alters the responsiveness of geniculate neurons, and so the likely function of these inputs is to modulate geniculate function during changes in attentional state, but the exact function of these inputs is not well-understood.
Multiple Sclerosis: What Methods are Available for the Assessment of Subclinical Visual System Damage?
Published in Neuro-Ophthalmology, 2022
Demet Yabanoglu, Pinar Topcu-Yilmaz, Murat Irkec, Belgin Kocer, Berna Arli, Ceyla Irkec, Sevilay Karahan
The current study sought to determine which ophthalmological approach was more sensitive in detecting subclinical visual involvement in MS. Based on the results of this study, it seems that FDTP may be more sensitive than SAP in detecting initial visual field (VF) damage in visually asymptomatic MS patients regardless of ON history. There are conflicting results in the literature on the appraisal of neuro-ophthalmological diseases such as MS with FDTP. Sisto et al. claimed that the magnocellular subgroup of retinal ganglion cells was not impaired in MS patients, but even if it was, they provided limited evidence in MS, as FDTP was unable to isolate the function of these cells due to the patients’ substantial VF deficits.5 The magnocellular cell subgroup is known to be present in 3–5% of the retina, and FDTP is a specific test that mainly isolates this group of cells.15 Corallo et al. highlighted how the fewer fibres in this ganglion cell system meant the VF loss manifested earlier.10 Additionally, they contended that FDTP was more sensitive than SAP in detecting early VF defects. The contribution of the present study to the literature is consistent with this view. Merle et al. also maintained that FDTP is equally sensitive as SAP in MS patients with subclinical optic nerve damage, but not more so.16 The authors of this study believed that when used in conjunction with other conventional tests, FDTP may be effective in diagnosing subclinical visual involvement in MS patients without a history of ON, but not alone.
Focus on eye care in schizophrenia
Published in Clinical and Experimental Optometry, 2019
Clinically, the predilection toward impaired magnocellular function associated with some patients with schizophrenia may present during frequency doubling perimetry. Frequency doubling technology is commonly used in eye clinics for visual field screening as well as threshold testing. This technology differs from white‐on‐white perimetry in that the target image has features of low spatial and high temporal frequency, creating a doubling effect that aims to selectively stimulate the magnocellular pathway. The magnocellular pathway specifically drives visual perception of low spatial frequency and high temporal frequency stimuli.2013 Frequency doubling technology is thought to highlight visual field loss earlier in glaucoma disease due to reduced redundancy of the magnocellular cell type.
The therapeutic potential of a calorie-restricted ketogenic diet for the management of Leber hereditary optic neuropathy
Published in Nutritional Neuroscience, 2019
Mithu Storoni, Matthieu P. Robert, Gordon T. Plant
Mitochondrial bioenergetic capacity may partly account for this variation in susceptibility. An axon’s volume determines its capacity for mitochondria, whereas its surface area dictates its energy needs. Small diameter parvocellular type (P cells) have the smallest ratio of volume to surface area, which places them at greater risk from a bioenergetic imbalance. Among non-MCGC cells, P cells appear to be more vulnerable to damage in LHON than magnocellular cells (M cells) that generally have larger diameters.17,18 Impaired axonal transport precedes the death of retinal ganglion cells in LHON. Axoplasmic transport, which shuttles both mitochondria and trophic factors along ganglion cell axons, is ‘energy-expensive’ and becomes compromised in the presence of an energy constraint, which may further predispose small diameter retinal ganglion cells to maximal damage.19,20