Neurons
Nassir H. Sabah in Neuromuscular Fundamentals, 2020
Neurons can be classified in a number of ways. One of the earliest classifications is anatomical, according to which neurons can be unipolar, bipolar, or multipolar. Unipolar neurons have only one process emanating from the cell body. In the case of sensory neurons of the dorsal root ganglia (Figure 11.2), this single process divides at some distance from the cell body into two main branches that conduct APs from peripheral sensory receptors, past this branch point and into the spinal cord. On the other hand, most amacrine cells of the retina (Figure 7.1a) have dendrites and no axons. They neither generate nor conduct action potentials; instead they actively conduct synaptic signals through their dendrites to their target cells. Figure 7.1b illustrates a bipolar cell of the retina having an axon and a single dendrite on opposite ends of the soma. The axon of the dopamine-releasing neuron of the substantia nigra, which is a nucleus of the basal ganglia (Section 12.2.3), emerges from a dendrite up to 240 µm from the soma.
Discussions (D)
Terence R. Anthoney in Neuroanatomy and the Neurologic Exam, 2017
Another relevant comparison involves two other classes of retinal neurons—horizontal cells and amacrine cells—usually described as local interneurons. Although recent authors apparently agree that the amacrine cells have no axons (e.g., B&K, p. 301; C&S, p. 540), these cells can generate action potentials (e.g., W&G, p. 327; A&B, p. 360; Ruch and Patton, 1979, p. 486). Given the usual criteria for distinguishing between dendrites and axons, this presents a clear dilemma. Nevertheless, most authors who label the amacrine cell’s processes simply consider them all to be dendrites (e.g., B&K, p. 301; K&S, p. 354). Williams and Warwick are refreshingly equivocal in this regard, however. They state that: “All their [amacrine neurons’] processes resemble dendrites, but are said to show the cytoplasmic features of both axons and dendrites; the direction of conduction in any process at a particular time will be determined by the polarization of the synapses which are active.” (1980, p. 1171)
Glutamine and its Neuroactive Derivatives in the Retina
Elling Kvamme in Glutamine and Glutamate in Mammals, 1988
In marked contrast to the situation in the rat retina, GABA turnover and concentration are significantly increased in goldfish and frog tissue in the light.95-98 It is known that horizontal cells are involved in goldfish,99,100 but the changes in the frog retina need to be more precisely localized than heretofore.97 Apparent similarities between the species may be superficial as autoradiography has shown that whereas exposed grains are present over horizontal cells of both light- and dark-adapted frog retina, preloaded with 3H-GABA,67 it is only cells of the light-adapted goldfish that are clearly labeled.64 Goldfish horizontal cells possess a voltage-dependent, Ca-independent release mechanism for GABA that is thought to be involved at feedback synapses in photoreceptor terminals. This has been implicated in the light/dark differences in GABA accumulation4,99 and may not be present in frog horizontal cells.4,101 In the frog retina, it is labeling of the amacrine cells that is variable, being greatly diminished in the light as compared with the dark.67 The cause is not known.
Advances in Neuroscience, Not Devices, Will Determine the Effectiveness of Visual Prostheses
Published in Seminars in Ophthalmology, 2021
Bardia Abbasi, Joseph F. Rizzo
In the retina, visual signals emerging from over 90 million photoreceptors are processed by horizontal, bipolar, and amacrine cell circuits prior to activation of some complement of the 1.2 million RGCs whose axons comprise the optic nerve.65–67 Inner retinal processing by pre-RGC circuits provides spatial resolution, detection of color, motion and texture detection, light adaptation (across 8 log units of intensity), control of circadian rhythms, and saccadic suppression (to counter image degradation by larger eye movements).68–70 These processed data are then encoded among at least 15 subtypes of RGCs, each tiled across the retina with relatively little overlap, conveying distinct visual representations in parallel to the brain.70 The best-characterized RGC populations include midget cells (with small, center-surround receptive fields with high spatial sensitivity), parasol cells (with large, center-surround achromatic receptive fields with high temporal sensitivity), and bistratified cells (with blue-yellow center-surround receptive fields), which collectively comprise around 90% of RGCs in the primate retina.67,70,71 The major RGC classes are further divided equally between “ON” and “OFF” cells that respond antagonistically to the intensity of light.64
Diagnostic Electron Microscopy of Retina
Published in Seminars in Ophthalmology, 2018
Rishikesh Kumar Gupta, Inderjeet Kaur, Tapas C. Nag, Jay Chhablani
A study was performed to characterize the organ culture of human retina in a 9-day experiment and also checked the persistence and neurodegeneration pattern of the neuroretinal cells. In this study, the authors showed that, in retinal tissue culture, starting from day 0 to day 9, there was a progressive loss of the bipolar rod cells and the axon terminals in the inner plexiform layer.55 After day 9, the processes of the horizontal cells and the dendrites of the bipolar cells were complicated to distinguish in the EM image (Figure 4). Also, there was a significant degeneration had shown in the outer plexiform layer (OPL), which could give the predictive analysis for the patients in the case of retinal detachment and those who are undergoing the reattachment surgery, i.e., there could be a weak recovery of the cells.55 However, electron microscopy (EM) study of the interneurons (bipolar cells and amacrine cells) is still very poorly understood in the human retina.
Advances in understanding the mechanisms of retinal degenerations
Published in Clinical and Experimental Optometry, 2020
In contrast, the localisation of P2X receptors in rod mediated pathways are more complex. The neural pathway important for mediating scotopic vision involves rod photoreceptors communicating with rod bipolar cells that in turn pass information to two highly specific amacrine cell types called AII and A17 amacrine cells.200422,23 The AII amacrine cells then pass scotopic information to ganglion cells by communicating with ON and OFF cone bipolar cells.200422,23 A critical step in shaping vision processing in the scotopic pathway is the modulation that occurs between rod bipolar cells and AII and A17 amacrine cells. An example of the synapse between a rod bipolar cell and an A17 and AII amacrine cell is shown in Figure 1B. Importantly, A17 amacrine cells which release the inhibitory neurotransmitter GABA, feedback directly onto rod bipolar cell terminals via reciprocal synapses to shape signalling by rod bipolar cells.24
Related Knowledge Centers
- Anatomy
- Dendrite
- Eye
- Inner Plexiform Layer
- Interneuron
- Neurite
- Retina
- Synapse
- Retinal Ganglion Cell
- Retina Bipolar Cell