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Neurohypophysis
Published in Paul V. Malven, Mammalian Neuroendocrinology, 2019
Each magnocellular neuron receives synaptic input on its perikaryon and dendrites from approximately 1000 other neurons by way of about 5000 synapses per magnocellular neuron. About 60 % of these synapses are associated with axons that originate inside the individual nucleus containing the magnocellular perikaryon. Control of magnocellular neurons by neurotransmitters from the large number of different synaptic inputs is only partly understood (Crowley and Armstrong, 1992). Acetylcholine as well as glutamate and other excitatory amino acids are stimulatory to magnocellular neurons. Exogenous angiotensin II is also consistently stimulatory to oxytocin release, but the physiological role of endogenous angiotensin II is unclear. Norepinephrine appears to have dual effects on oxytocin perikarya with stimulation caused by α-adrenergic receptors and inhibition caused by β-adrenergic receptors. There is also evidence that oxytocin, acting as a neurotransmitter, can modulate the firing of oxytocin-containing magnocellular neurons. Although vasopressinergic magnocellular neurons are profoundly affected by osmotic stimuli, the vasopressin neuron is probably not especially osmosensitive. Rather, parvocellular neurons, located nearby or in distant sites, are osmosensitive and communicate with the magnocellular neuron using acetylcholine. Structures of the limbic system and the brain stem also communicate with magnocellular vasopressinergic neurons.
Developmental Dyslexia
Published in Ivanka V. Asenova, Brain Lateralization and Developmental Disorders, 2018
At the biological level, the cause of reading disorders is viewed in the division of the visual system into magnocellular and parvocellular pathways that have different roles and properties. Since the visual magnocellular system is responsible for timing visual events when reading, it is hypothesized that the magnocellular pathway is selectively disrupted in certain dyslexic individuals, which leads to deficiencies in visual processing, and, via the posterior parietal cortex, to abnormal binocular control and visuospatial attention (for details, see [206]).
ENTRIES A–Z
Published in Philip Winn, Dictionary of Biological Psychology, 2003
(from Latin, parvus: little) Literally, small cells. In many paces in the CENTRAL NERVOUS SYSTEM there are groups of neurons (see NEURON) described as MAGNOCELLULAR—big cells. The neighbouring, smaller, neurons are often referred to as parvocellular. Both the PARAVENTRICULAR NUCLEUS OF THE HYPOTHALAMUS and LATERAL GENICULATE NUCLEUS for example have parvocellular and magnocellular components.
Diagnostic performance of a modified visual perception test
Published in Hearing, Balance and Communication, 2023
Mayada Elsherif, Mona Mourad, Nesrine Hamouda, Rania Abdou, Taima Salem
In our study, dyslexic children showed significantly lower scores from non-dyslexic in the identification of the colour of the optotype presented to them in saccadic and distractor tasks. This may be urged to the interaction between the parvocellular and magnocellular systems. In dyslexic children, Ahmadi et al. [26] revealed a parvocellular system deficit. They did so by presenting colourful sensory images in the form of natural environment photographs and utilizing the psycho-physical technique (subjectively) to determine the red-green isoluminant point. Bonfiglio et al. [27] discovered that dyslexic children displayed delayed visual evoked potential to achromatic stimulus (magnocellular-dorsal stream) along with isoluminant red/green and blue/yellow stimulus (parvocellular-ventral and koniocellular streams). They were able in demonstration such weakness objectively.
Heavy and light smokers have slight differences in chromatic discrimination
Published in Journal of Addictive Diseases, 2022
Gabriella M. Silva, Thiago P. Fernandes, Fatima M. Felisberti, Milena E. Oliveira, Natalia L. Almeida, Jandirlly J. Souto, Natanael A. Santos
The parvocellular pathway in the early visual system carries both luminous contrast information for fine detail along with chromatic information based on the long and middle-wavelength sensitive cones. Conversely, the magnocellular pathway relays luminous contrast information for low contrast in their cells. The koniocellular pathway carries chromatic information based on the short (S) wavelength sensitive cone and both the M- and L-cone responses.22 These results do not support the notion of pathway selectivity deficits, despite the existence of nicotinic acetylcholine receptors (nAChRs) in the primary visual cortex (i.e., there are more nAChRs in the parvocellular pathway).22 It is difficult to segregate the signals in visual pathways23 because it contains a combination of additive and opponent cone signals (L + M, L − M, and S − [L + M]) and it is not clear which of the parvo-, magno-, and koniocellular pathways are affected by smoking.19 We can only speculate that diffuse impairment may involve one, two, or all three pathways and hypothesize the existence of a diffuse deficit in color processing for both heavy and light smokers.
Focus on eye care in schizophrenia
Published in Clinical and Experimental Optometry, 2019
The ability to negotiate the visual world, or visual processing, has been shown to be markedly affected in schizophrenia, resulting in multiple visual sensory deficiencies with accompanying functional impairments. The two major pathways which drive visual perception are the parvocellular and magnocellular neuronal routes, which process colour and detail, as well as motion, respectively.1999 However, it is well established that schizophrenia results in greater irregularities in the pre‐geniculate retinal ganglion cells and post‐geniculate retinocortical neurons that compose the magnocellular pathway, compared to the parvocellular pathway, although both are thought to be involved.2017 Specifically, when patients with schizophrenia process the environmental visual field (faces, objects, light variance), they exhibit a reduced contrast sensitivity with combined impairment of orientation discrimination, motion processing, and global processing. It is thought that the neural connections and synapses are compromised in the visual cortex of these patients.2017 The combined effect of these visual perception impairments has been described as processing the world in fragmented parts rather than the continuous streaming of visual information that is typically experienced.2017