SBA Answers and Explanations
Vivian A. Elwell, Jonathan M. Fishman, Rajat Chowdhury in SBAs for the MRCS Part A, 2018
The visual pathway may be summarized as follows: Photoreceptors (rods, cones) within the retina convert light energy into electrical impulses (phototransduction)This is transmitted to ganglion cells, directly via bipolar cells, or indirectly via horizontal and amacrine cellsGanglion cells are the output cells of the retina. Axons from ganglion cells converge at the optic disc (blind spot) and travel in the optic nerveIncomplete decussation occurs at the optic chiasm; those from the nasal half of each retina (corresponding to the temporal halves of the visual field) cross over (decussate), while those from the temporal halves of each retina stay on the same sideThe optic tracts synapse in the various layers of the lateral geniculate nucleus of the thalamus before being relayed to the primary visual cortex in the occipital lobe via the optic radiation
The Pineal Gland Energy Transducer
Len Wisneski in The Scientific Basis of Integrative Health, 2017
As we have indicated, light has something to do with how our biological clock adjusts itself, that is, how it makes the necessary corrections as days lengthen or shorten with seasonal changes. So, naturally, scientists want to locate the photoreceptors that pass this information from the environment to the SCN. The obvious place to look would be the light-sensitive rods and cones in the retina that provide us with our visual information. However, research on people who are blind gives us cause to question the role of rods and cones as primary phototransducers. Corroborating this supposition is a study that found that cone degeneration in aged mice did not render them incapable of circadian phase shifts and that their responses to light were similar to that of controls (Provencio et al., 1994). Following this study, two experiments established that mutant mice, lacking both rods and cones, still exhibited melatonin suppression when exposed to light (Freedman et al., 1999; Lucas et al., 1999). This finding conclusively demonstrates that something other than rods and cones are conveying the light information; in other words, they are not the sought-after photoreceptors. Research on humans is similar and shows that there is a unique short wavelength-sensitive photopigment involved in light-induced melatonin suppression, providing the first direct evidence of a nonrod, noncone photoreceptive system in humans (Thapan et al., 2001). So, if not rods and cones, what might these photoreceptors be?
Medical and Mathematical Background
Arwa Ahmed Gasm Elseid, Alnazier Osman Mohammed Hamza in Computer-Aided Glaucoma Diagnosis System, 2020
The retina is a multi-layered sensory tissue that lies at the back of the eye, and contains millions of photoreceptors that capture light rays and convert them into electrical impulses that will in turn convert them into images in the brain. The main photoreceptors in the retina are rods and cones. Rod cells are very sensitive to changes in contrast, even at low light levels, hence they are able to detect movement, but they are imprecise and insensitive to color located in the periphery of the retina and are used for scotopic vision (night vision). Cones are high precision cells capable of detecting the colors that are concentrated in the macula, the area responsible for photopic vision (day vision). The very central portion of the macula is called the fovea, which is where the human eye is able to best distinguish visual details. The loss of peripheral vision may cause damage to the macula and can result in the loss of central vision.
CD133: beyond a cancer stem cell biomarker
Published in Journal of Drug Targeting, 2019
Amir Barzegar Behrooz, Amir Syahir, Syahida Ahmad
CD133 is also found in non-epithelial cells, such as rod photoreceptor cells and bone marrow cells [9,35]. In this regard, it appears that CD133 plays a role in the formation of photoreceptor discs. Photoreceptors are specialised cells that are located in the retina and are involved in the process of visual phototransduction. These cells are divided into either cones or rods and are generally made up of the following parts: synaptic area, cell body, inner segment, transition zone and outer segment [39]. The discs are made from the base of the outer segment of these cells (membrane evaginations are located in the transition zone area and nascent discs are created from this region and from the connecting cilium) [39]. The precise mechanism of action of CD133 in disc morphogenesis is not clear but it appears to be involved in disc synthesis due to the presence of sufficient amounts of proteins and lipids, particularly, cholesterol [39]. Dystrophy of photoreceptors cells is one of the most common causes of blindness in the world and Stargardt (STGD) disease (macular degeneration) is caused by mutation of the CD133 gene, proving the important role of this protein in photoreceptors [44] (Figure 3).
Circadian genes in major depressive disorder
Published in The World Journal of Biological Psychiatry, 2020
Lindsay Melhuish Beaupre, Gregory M. Brown, James L. Kennedy
Sleep–wake cycles are largely determined by circadian rhythms (Mongrain et al. 2004). These rhythms are controlled by a set of genes known as the circadian or clock genes (Reppert and Weaver 2002). In brief, there is a transcriptional-translational feedback loop in mammals that is controlled by two transcription factors, Circadian Locomotor Output Cycles Kaput (CLOCK) and Aryl Hydrocarbon Receptor Nuclear Translocator Like (BMAL1/ARNTL). CLOCK and BMAL1 act on other clock genes, such as Cryptochrome 1 and 2 and Period 1, 2 and 3 to increase their expression, which, in turn, will lead to a feedback mechanism whereby cryptochrome and period regulate their own expression (Jagannath, Peirson and Foster 2013). In its entirety, this cycle takes approximately 24 h (King and Takahashi 2000). The majority of the cells in the body follow this same pattern of oscillation because of the suprachiasmatic nucleus (SCN), the pacemaker of the brain, which is located in the hypothalamus (for a complete review see Partch et al. 2014). Light is perceived by three types of photoreceptors: rods, cones and a tiny group of retinal ganglion cells that are uniquely photosensitive (ipRGCs). Only the SCN receives input from the ipRGCs, while the rods and cones provide visual input for perception of images. The ipRGCs contain melanopsin, a photopigment that is especially sensitive to blue light which recently has been shown to be the wavelength effective in regulating SCN activity (Bailes and Lucas 2013; Jagannath, Peirson and Foster 2013).
Effect of monochromatic light on the temporal expression of N-acetyltransferase in chick pineal gland
Published in Chronobiology International, 2020
Nan Jiang, Jing Cao, Zixu Wang, Yulan Dong, Yaoxing Chen
In birds, the photoreceptors are located in multiple organs, not only in the eye but also in the deep brain and pineal gland (Surbhi 2015). The photoreceptors express the genes of photopigments, such as pinopsin and melanopsin, that initiate a cascade to submit light information to the chick pineal gland (Holthues et al. 2005). Pinopsin (Opnp) is an opsin-like protein whose gene is exclusively expressed in the chick pineal gland (Okano et al. 1994). The mRNA levels of pinopsin in chicks kept under 12L:12D (12 h light/12 h dark) showed diurnal fluctuations with peaks in the second half of the day and troughs during the dark (Takanaka et al. 1998), and birds exposure to light at night increased the amount of pinopsin mRNA. Melanopsin (Opn4) was first isolated from the photosensitive melanophores of Xenopus laevis (Provencio et al. 1998). An avian homolog of melanopsin identified in the chick pineal gland had a 72% amino acid identity to Xenopus melanopsin and was exclusively expressed in the parafollicular cells of the pineal gland (Natesan et al. 2002). Melanopsin in the pineal gland has been shown to exhibit circadian oscillations in chicks reared under 12L:12D (Holthues and Vollrath 2004); while the level of melanopsin mRNA in the pineal gland was suppressed when exposure to light at night but not in the retina of male white leghorn chickens (Chaurasia et al. 2005). However, the mechanism underlying the relative importance of opsins to convey the spectral light in the chick pineal gland is still poorly understood.
Related Knowledge Centers
- Cone Cell
- Membrane Potential
- Retina
- Rod Cell
- Visual Perception
- Visual System
- Neuroepithelial Cell
- Visual Phototransduction
- Photoreceptor Protein
- Intrinsically Photosensitive Retinal Ganglion Cell