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Ganglion cells

The cell bodies of most ganglion cells are located in the innermost nucleated layer of the retina (ganglion cell layer) situated between the nerve fibre layer and the inner plexiform layer (Figs 1-29 and 1-32); however, ‘displaced’ ganglion cells have been identified in the inner nuclear layer. Ganglion cells are the last neuronal link in the retinal component of the visual pathway (Fig. 1-32B). Their axons form the nerve fibre layer on the innermost surface of the retina and synapse with cells in the lateral geniculate nucleus of the thalamus. The axons form bundles separated and ensheathed by glial cells (Fig. 1-34E). The bundles leave the eye to form the optic nerve. Upon exiting through the lamina cribrosa, the axons become myelinated with oligodendrocytes. There are up to seven layers of ganglion cell bodies in the central retina or fovea (ganglion cell layer is 60–80 µm thick) and as few as one cell layer in the peripheral retina (10–20 µm thick). There are approximately 1.2 million ganglion cells per retina; thus, theoretically, there are approximately 100 rods and four to six cones per ganglion cell. While they are functionally diverse, ganglion cells are characterized morphologically by a large cell body, abundant Nissl substance (arrays of rough endoplasmic reticulum) and a large Golgi apparatus (Fig. 1-34E). They are classified into different types on the basis of cell body size, dendritic tree spread, branching pattern and branching level in the five strata of the inner plexiform layer (Fig. 1-32B,C). Some ganglion cells in the macular area may contain yellow (xanthophyll carotenoid) pigment in the cytoplasm, although the cone axons and Müller cells are thought to also contain these pigments in the macular region. Impulses are received primarily from bipolar cells and amacrine cells via axodendritic and axosomatic synapses, the former occurring predominantly in the inner plexiform layer (Fig. 1-32B) where their dendrites repeatedly branch to form the ‘dendritic tree’, whose form and size varies considerably and may be correlated with location in the retina and therefore function (receptive field size) (Fig. 1-34B,C). The morphological diversity of ganglion cells (up to 25 types in mammalian and 18 types in human retinas) has prompted classification of these cells into categories, α, β and γ, or X, Y and W types, predominantly based on research in the cat.

Recently a non-rod, non-cone photoreceptive pathway, arising from a population of retinal ganglion cells, was discovered first in nocturnal rodents and then in primates. These ganglion cells express the putative photopigment melanopsin and by signalling gross changes in light intensity serve the subconscious, ‘non-image-forming’ functions of circadian photoentrainment and pupil constriction. The primate retina, in addition to being intrinsically photosensitive, is strongly activated by rods and cones to signal irradiance over the full dynamic range of human vision. Thus, in the diurnal trichromatic primate, ‘non-image-forming’ and conventional ‘image-forming’ retinal pathways are merged, and the melanopsin-based signal might contribute to conscious visual perception.

Association neurones (amacrine and horizontal cells) (Figs 1-32B and 1-34A,B)

Horizontal cells.

These cells derive their name from the extensive horizontal extensions of their cell processes. There are two distinct morphological varieties in the retina of most species, of which the cat is the most extensively studied: type A is a large sturdy axonless cell with stout dendrites that contact only cones; type B has a smaller bushier dendritic tree that contacts cones exclusively but, in addition, has an axon up to 300 µm in length that ends in extensive arborization that is postsynaptic only to rods (Figs 1-32B and 1-34B). Type A cells have much larger receptive fields than type B. In primates it appears that the two types of horizontal cell, HI (approximates to type B) and HII (approximates to type A), both possess axons. A third type (HIII) has been described in the human retina. Each rod has connections with at least two horizontal cells and each cone with three or four horizontal cells of each type. In primates the stout dendrites of HI cell soma processes contact around seven cones near the fovea (dendritic tree covering 15 µm); this number increases to as many as 18 further from the fovea (dendritic tree covering 80–100 µm). The axon from HI cells passes laterally and terminates up to 1 mm away in a thickened axon terminal bearing a fan-shaped protrusion of lollipop-like endings in rod spherules (up to 100) (Fig. 1-34B). HII dendritic trees are more spidery and contact about twice as many cones. Their axons are generally shorter (100–200 µm) and contact cone pedicles by small wispy terminals. The manner of their insertion is depicted in Figure 1-32B. Their cell bodies are located primarily in the outer part of the inner nuclear layer. They have few distinctive cytoplasmic organelles except the crystalloids, a series of densely stacked tubules with associated ribosomes. Their processes ramify in the outer plexiform layer close to the cone pedicles. The overlap between horizontal cells is considerable and any one area of retina may be served by up to 20 horizontal cells. Horizontal cells have an integrative role in retinal processing and release inhibitory neurotransmitters, mainly γ-aminobutyric acid (GABA). Recent evidence suggests that there is some colour-specific wiring for the three types of horizontal cells in the human retina.

Retinal neuroglia

Astrocytes.

Astrocytes are not the principal or predominant glial cell in the retina. This role is fulfilled by Müller cells, which are analogous to central nervous system oligodendrocytes. Astrocytes are predominantly located in the nerve fibre layer, ganglion cell layer, inner plexiform layer (site of cell bodies), and their outer limit is the vitread aspect of the inner nuclear layer in humans. They...