China
Africa
Explore chapters and articles related to this topic
Comparative Anatomy and Physiology of the Mammalian Eye
Published in David W. Hobson, Dermal and Ocular Toxicology, 2020
There is a steady dark current that flows along the length of the photoreceptors. This current is maintained by Na+ which is extruded from the inner segment by a Na+-K+-ATPase pump and then enters passively through Na+ channels in the outer segment.129,135 Thus, during darkness, the photoreceptors are continuously discharging. Exposure to light results in the closure of the Na+ channels in the outer segment and a resultant hyperpolarization and a decrease in the release of neurotransmitter. This process is termed transduction and involves the absorption of a photon by rhodopsin. This, in turn, communicates via a molecule termed transducin to activate the enzyme phosphodiesterase which will hydrolyze many cyclic GMP molecules which are responsible for maintaining the Na+ channels.129 It is this phosphodiesterase enzyme that has been found to be abnormal in animals, such as the Irish Setter and Collie dogs and the Rd mouse, that undergo hereditary retinal degeneration. The signal from the photoreceptor cell is transmitted inwards to the ganglion cells via the bipolar cell with input from the horizontal and amacrine cell. The neurotransmitters involved in this process may include GABA, glycine, acetylcholine, monoamines, peptides, substance-P, aspartate, and glutamate.136,137
Impact of Retinal Stimulation on Neuromodulation
Published in Yu Chen, Babak Kateb, Neurophotonics and Brain Mapping, 2017
There are more than 50 cell types in the retina (Raviola 2002), each contributing to a different aspect of retinal processing before signals leave the eye to the brain’s subcortical and cortical pathways. The main cells in the retina include the straightforward pathway—from photoreceptors to bipolar cells to ganglion cells, with horizontal and amacrine cells interspersed in between to inhibit the overflow of incoming information. There are also Mueller glial cells, which run the entire depth of the retina. This star-shaped type of glial cell (astrocyte) is found in the brain and the spinal cord and once was able to help in maintaining support structure. However, as of 2000, research has demonstrated that astrocytes are involved in metabolic transfers between extracellular environments. Disruptions in their signaling may play a possible role in neuropsychiatric disorders (Molofsky et al. 2012) and brain plasticity (Araque and Navarrete 2010, Perea and Araque 2010).
Retina
Published in Fiona Rowe, Visual Fields via the Visual Pathway, 2016
The fovea and macula pertain to the highest visual function and are located at the posterior pole of the eye. The photoreceptors synapse with bipolar cells which transmit to ganglion cells. These retinal axons converge on the optic disc (optic nerve head). The optic disc is an oval structure with a measurement of approximately 1.5 mm in diameter.
Retinal Ganglion Cell Death in Glaucoma: Advances and Caveats
Published in Current Eye Research, 2023
Glaucoma is the most frequent cause of irreversible blindness across the world and is characterized by progressive retinal ganglion cell (RGC) dysfunction and visual field defects.1 RGCs are a type of neuron located in the inner layer of the retina. They receive visual information from photoreceptors via bipolar cells and amacrine cells and further convey messages to the brain.2 Given that the optical nerve is part of the central nervous system (CNS), we expect that an understanding of the underlying mechanisms of RGC death might help to provide the foundations for treatments that ameliorate cell death in retinopathy and aid in promoting and restoring neuronal health. This review highlights some recent advances in exploring the mechanisms of RGC death in glaucoma-like animal models and the caveats that lead to hampered RGC survival.
Visual Pathway Degeneration in Chemotherapy-Related Neurotoxicity: A Review and Directions for Future Research
Published in Neuro-Ophthalmology, 2020
David E. Anderson, Sarah A. Holstein, Sachin Kedar
The neurosensory retina, being an outpost of the brain,22 offers an elegant model for studying neural circuits in health and disease.23 The retina is comprised of three nuclear layers containing cell bodies of distinct neuronal populations and two layers containing synaptic processes between each nuclear layer. The outer nuclear layer contains cell bodies of rods and cones, which selectively respond to varying degrees of light intensity and wavelengths (i.e. colours). The inner nuclear layer contains cell bodies of bipolar, horizontal, and amacrine cells, which selectively modulate neural activity transmitted by photoreceptors through excitatory and inhibitory interactions. The ganglion cell layer contains cell bodies of ganglion cells, which relay neural activity from the retina to the brain via ganglion cell axons within the retinal nerve fibre layer (RNFL) through the optic nerve. Synaptic processes between the photoreceptors and bipolar cells make up the outer plexiform layer, and synaptic processes between bipolar cells and ganglion cells make up the inner plexiform layer. Ophthalmic biomarkers have been developed and incorporated into clinical practice to evaluate the structure and function of these distributed retinal populations.
Progress in gene and cell therapies for the neuronal ceroid lipofuscinoses
Published in Expert Opinion on Biological Therapy, 2018
Anthony Donsante, Nicholas M Boulis
Kleine Holthaus et al. have made a surprising discovery while studying eye-directed gene therapy in CLN6-deficient mice. In this model, photoreceptor death occurs early in the disease, suggesting that these cells, which express CLN6, should be the primary target for therapy. However, treating these mice with an AAV8-CLN6 vector that provided substantial photoreceptor transduction had no effect on retinal function loss. Examining eyes from humans and mice, the authors discovered that bipolar cells in the inner nuclear layer (INL) exhibited higher levels of CLN6 expression, even though they do not begin to die until late in the disease. Intravitreal injection of a newer AAV2-derived vector, 7m8, which efficiently transduces the INL, substantially delayed the loss of the scotopic a-wave of the electroretinogram. Furthermore, restricting expression of the transgene to bipolar cells was nearly as effective as the broader approach, highlighting the importance of these cells in disease progression. The authors suggest that this phenomenon, where the earliest cells to die are not necessarily the target for therapy, may not be limited to the eye. Researchers should undertake careful studies to determine which targets are the most appropriate [60].