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Human and Biomimetic Sensors
Published in Patrick F. Dunn, Fundamentals of Sensors for Engineering and Science, 2019
The visual pathway through which electromagnetic energy is converted into an electrical signal is shown in Figure 3.2. Light encounters the surface of the retina after passing through the lens. It then travels through most of the retinal tissue to the outer segment where the membrane disks of the rods and cones are located. These disks contain the visual pigments. In rods, the pigment is rhodopsin (retinal, a form of vitamin A, bound to opsin, a protein). In cones, the process is similar, but with other visual pigments. When retinal is activated by light, it changes shape, causing it to unbind from opsin. One photon (an energy of only 358 zJ) activates one rhodopsin molecule. The conformational change in retinal causes the receptor to hyperpolarize and release a neurotransmitter across synapses to neighboring bipolar cells. These cells subsequently transmit to ganglion cells, each of which receives signals from bipolar cells covering an area of the retina.
Impact of Retinal Stimulation on Neuromodulation
Published in Yu Chen, Babak Kateb, Neurophotonics and Brain Mapping, 2017
Another recent research has demonstrated that many subtypes of each kind of retinal cell exist. For instance, three types of cones, L, M, and S, respond to long, medium, and short wavelengths of light, respectively. Bipolar cells are separated into midget and diffuse general groupings, with many subtypes of those, but, functionally, they are considered to be activated by central or surrounding targets. There are ON cone bipolar cells (for each type of cone cell grouping) and OFF cone bipolar cells as well as ON and OFF rod bipolar cells. As for the inhibitory horizontal and amacrine cells, three types of horizontal cells have been identified in the human retina as of 1994 (Ahnelt and Kolb 1994), and more than 20 kinds of amacrine cells are separated into wide and narrow field classifications. The most commonly researched of these cells are the starburst amacrine and the AII cells. The AII cells link some rod and cone information before signals exit the eye, and the starburst cells are involved in directional sensitivity involved by optokinetic reflexes (Yoshida et al. 2001).
The Biological Bases of Photoreception in the Process of Image Vision
Published in Agnieszka Wolska, Dariusz Sawicki, Małgorzata Tafil-Klawe, Visual and Non-Visual Effects of Light, 2020
Agnieszka Wolska, Dariusz Sawicki, Małgorzata Tafil-Klawe
The human retina processes image-forming visual information from cone and rod photoreceptors through parallel ON and OFF pathways [Schiller 1992; Schiller 2010]. The photoreceptors make synaptic connections with two types of bipolar cells, transmitting signals into parallel ON- and/or OFF-bipolar cells. The presynaptic terminals of photoreceptors release the neurotransmitter glutamate. The rate of glutamate release is high in darkness (depolarization of the photoreceptors) and reduced in response to light (hyperpolarization of the photoreceptors). The first step is the segregation of the cone signals to the ON and OFF pathways: the cone information diverges to ON- and OFF-bipolar cells. Thus, both types of cone bipolar cells (ON and OFF) receive inputs from cone photoreceptors and transmit directly to retinal ganglion cell dendrites in the inner plexiform layer (see Figure 2.1B). There are only ON-bipolar rod cells whose axon terminals extend to the deeper region of the inner layer. Thereby, rod photoreceptors transmit signals only through the ON pathway. Dendrites of ON-bipolar cells express the metabotropic glutamate receptor 6 (mGluR6, Grm6). The ON-bipolar dendrite has a single invaginating contact with a photoreceptor presynaptic axon terminal [D’Orazi et al. 2014; Ueno et al. 2018]. The OFF-bipolar cells present inotropic GluRs (AMPA/kainate receptors), glutamate-gated cation channels on their dendrites. The OFF-bipolar dendrite forms multiple flat contacts with a cone photoreceptor presynaptic axon terminal [D’Orazi et al. 2014; Ueno et al. 2018]. In the dark, glutamate released from cone presynaptic axon terminals depolarizes OFF-bipolar cells, acting through GluRs and hyperpolarizes ON-bipolar cells, acting through mGluR6, the activation of which leads to the closure of TRPM1 channels (transient receptor potential cation channel subfamily M member1). The TRPM1 channel in ON-bipolar cells is gated by both the α and the β gamma subunits of the G-protein Go [Xu et al. 2016]. Consistently, OFF-bipolar cells become more negative in the presence of light (inhibitory hyperpolarizing signal), whereas ON-bipolar cells show a depolarizing light response (excitatory depolarizing signal). The importance of the phenomenon of the two types of bipolar responses lies in the fact that it provides a mechanism for lateral inhibition. Depolarized and hyperpolarized bipolar cells lie immediately against each other, which provides a mechanism for separating contrast borders in the visual image. Thus, lateral inhibition is considered to be the first stage in vision processing.
Recent developments in imaging and surgical vision technologies currently available for improving vitreoretinal surgery: a narrative review
Published in Expert Review of Medical Devices, 2023
Elham Sadeghi, Sashwanthi Mohan, Danilo Iannetta, Jay Chhablani
Degenerative retinal diseases and hereditary conditions, such as age-related macular degeneration and retinitis pigmentosa, involve the photoreceptors and may preserve the inner retina, which contains electrically activated ganglion and bipolar cells. Optic nerve prosthesis and thalamic and cortical intracranial stimulation devices were designed to bypass the retina in profound vision loss and provide artificial images. Recently, the retinal prosthesis, which inserts intraocularly, has received lots of attention. This device consists of an imager that converts the light wavelengths to electrical voltage, electronics that process the image and make an electrical pulse, and multiple microelectrodes that transfer the signal to the inner retina [167,168].