Abstract
Visual experience is mediated through a relay of finely-tuned neural circuits extending from the retina, to retinorecipient nuclei in the midbrain and thalamus, to the cortex which work together to translate light information entering our eyes into a complex and dynamic spatio-temporal representation of the world. While the experience-dependent developmental refinement and mature function of neurons in each major stage of the vertebrate visual system have been extensively characterized, the contributions of the glial cells populating each region are comparatively understudied despite important findings demonstrating that they mediate crucial processes related to the development, function, and plasticity of the system. In this article we review the mechanisms for neuron-glia communication throughout the vertebrate visual system, as well as functional roles attributed to astrocytes and microglia in visual system development and processing. We will also discuss important aspects of glial function that remain unclear, integrating the knowns and unknowns about glia in the visual system to advance new hypotheses to guide future experimental work.
Highlights
OF THE VERTEBRATE VISUAL SYSTEMIn the back of each eye, specialized photoreceptive retinal tissue senses and responds to distinct features of light energy relaying detailed information about wavelength, intensity, spatial location, and motion to the brain (Figure 1A)
The first demonstration that calcium transients occur in Müller glia under physiological conditions in vivo observed that they were mediated through the propagation of extracellular ATP, as treatment with apyrase reduced their occurrence by 95%, confirming that the isolated retina was an informative preparation for investigating the mechanisms mediating the activity of Müller glia (Kurth-Nelson et al, 2009)
Despite the extensive heterogeneity in the morphologies of glia (Figures 1B–E) and the host of molecular signaling pathways mediating neuron-glia communication across brain regions and development (Figure 2), there appear to be important evolutionarily conserved functions mediated by glia throughout the visual system which are likely to be of general relevance to other sensory systems in the vertebrate brain
Summary
The vertebrate visual system has long been used as a highly tractable system in which to study how sensory experience influences the development, function, and plasticity of neurons and neural circuits in the brain. Throughout all areas of the developing and adult brain, including the visual system, glial cells are found in close apposition to neurons where they contact synapses, influencing their formation, maturation, and function (Figure 1). We will discuss what is known (and unknown) about the roles of Müller glia, astrocytes, and microglial cells in shaping the development, function, and plasticity of the vertebrate visual system, focusing on mechanisms mediating neuron-glia communication and their functional roles in each stage of the system from the retina to the primary visual cortex
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