Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 127

RGC axons constitute the optic nerve fibers. These axons converge at the optic disc, where they are joined in bundles to form the nerve. As the nerve contains RGC axons but no other neuronal cell body, it can be considered a pure white matter tract. However, the nerve does contain several important glial cell populations. These include oligodendrocytes, which contribute to its myelin sheath and formation of nodes of Ranvier, and astrocytes, which have several functions including K⁺ homeostasis and transportation and storage of metabolites (mainly glycogen) used by the axons. RGCs (and some subtypes of amacrine cells) are the only retinal neurons that generate action potentials. Unlike the graded hyperpolarizing or depolarizing responses of other retinal neurons, action potentials are all‐or‐nothing spikes of electrical activity. This means that all of the neuronal processing of the visual signal that has taken place in the retina so far, including information about stimulus size, contrast, color, movement, and location, is coded as alterations in the firing pattern (e.g., short bursts or sustained episodes of firing) and firing rates of the RGCs.

Figure 2.13 Schematic drawing of the mammalian retina with part of the choroid (Ch) on top. Below the retinal pigment epithelium (PE) are the layers of the neuroretina: the outer segments of the photoreceptors (OS), the outer limiting membrane (OLM), the outer nuclear layer with nuclei of cones (C) and rods (R), the neuropil of the OPL, the INL with nuclei of horizontal (H), bipolar (B), and amacrine (A) cells, the inner plexiform layer with synapses in strata, the ganglion cell layer (G), their axons in the nerve fiber layer, and finally the inner limiting membrane (ILM) facing the vitreous. The glial elements of the retina, Müller cells (M) and microglia (Mi), as well as astrocytes (As) embracing retinal blood vessels, are shown.