Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 58

The retina and optic nerve are derivatives of the forebrain; consequently, their morphology and physiology are similar to those of the brain. The nine layers of the neurosensory retina are connected to the brain by the optic nerve and the optic tracts (Figure 1.52). The rods and cones, the primary retinal photoreceptors, comprise a complex layer of specialized cells, which contain photopigments that convert light energy into a series of biochemical events. The RPE furnishes important metabolites to the photoreceptors; it also actively phagocytizes the outermost photoreceptor segments as they are shed during normal outer segment renewal. The retina has one of the highest rates of metabolism of any tissue in the body and receives almost all its nutrition from the retinal and choroidal capillaries.

The function of the retina is to turn light stimuli from the external environment into nervous impulses and transmit this information accurately to the brain, where it is then interpreted as vision. Once photoreceptors are stimulated by light, their release of a neurotransmitter is altered and this response is then received and modified by cells whose nuclei are in the inner nuclear layer (i.e., amacrine cells, bipolar cells, and horizontal cells). The modified message is then transferred to ganglion cells, whose axons form the nerve fiber layer and extend through the optic nerve to targets in brain (including the lateral geniculate nucleus and occipital cortex) (see Chapter 2). Recent studies indicate that a considerable amount of processing of visual impulses occurs within the retina. Classically, 10 layers are described in retinal histology. The neurosensory retina contains nine, and the supportive pigmented epithelium is the tenth layer. Remember that the retina develops from both inner (which invaginates) and outer optic cups. Hence, light and images must pass through the entire neurosensory retina to reach the photoreceptors. The 10 identifiable layers are considered, sclerad to vitread, in the following order: (i) RPE; (ii) photoreceptor layer (rod and cone layer); (iii) outer limiting membrane; (iv) outer nuclear layer; (v) outer plexiform layer; (vi) inner nuclear layer; (vii) inner plexiform layer; (viii) ganglion cell layer; (ix) nerve fiber layer; and (x) inner limiting membrane (Figure 1.53).

Figure 1.52 Relationship between different neuronal cells within the retina. The amacrine cell has a reciprocal inhibitory response onto the bipolar cell from which the information originated and acts to adjust the sensitivity of the ganglion cell synapse after receiving a signal. Horizontal cells interconnect laterally to integrate and regulate input from multiple photoreceptors.