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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.
Hypothalamic-pituitary-adrenal axis activity in post-traumatic stress disorder and cocaine use disorder
Published in Stress, 2020
Natalie A. Hadad, Marek Schwendt, Lori A. Knackstedt
The HPA axis, a neuroendocrine system, responds to a variety of stimuli. One such stimulus is stress, which is defined as a circumstance that threatens homeostasis (de Kloet et al., 1998). Upon activation of the HPA axis, the parvocellular cells of the hypothalamic paraventricular nucleus synthesize and release corticotropin-releasing hormone (CRH; also known as corticotropin-releasing factor or CRF) and its co-secretagogue vasopressin into the primary plexus of the median eminence. From there, CRH and vasopressin travel along the hypophyseal portal system, to reach the secondary plexus of the anterior pituitary where they act synergistically to stimulate corticotroph cells to produce the polypeptide precursor proopiomelanocortin (POMC). POMC is then cleaved into several active peptides, including adrenocorticotropic hormone (ACTH; also known as corticotropin). In turn, ACTH circulates in the blood of the secondary plexus and is carried to the adrenal cortex. Binding of ACTH to its receptors within the zona fasciculata of the adrenal cortex promotes the local synthesis and release of glucocorticoids into the bloodstream. In humans and other primates, the main glucocorticoid secreted by the adrenal cortex is cortisol, while in many other species (including rodents and birds) it is corticosterone. Both glucocorticoids are abbreviated here and elsewhere as “CORT”. CRH and ACTH are secreted in bursts, with the frequency of bursts showing a circadian rhythm (Baum & Grunberg, 1995). As a result, under basal conditions, human CORT levels peak at approximately 8AM and decline throughout the day to reach the lowest levels around midnight (Baum & Grunberg, 1995). Under conditions of stress, CORT reaches its peak levels in the blood 15–30 min after the onset of the stressor and then declines to pre-stress levels after 60–90 min (Baum & Grunberg, 1995; de Kloet et al., 1998, 2005).