The Ganglion Cell Layer

The cell bodies o f ganglion cells and displaced amacrine cells are located within the innermost nuclear layer of the retina, the ganglion cell layer. It is generally a single cell thick,

Chapter 2 The Retina

except around the macula, where the cells displaced from the central fovea reside. Some displaced Muller cell bodies and astroglial cells may also be found in the ganglion cell layer

{Cohen, 1992).

2.10.1 Ganglion Cells

Ganglion cells are the final mediator of the retinal part of the visual pathway. By the time the neural signal is passed to the ganglion cells, via synapses in the inner plexiform layer, it has already been modified by the processing which occurs in the vertical and lateral pathways within the outer layers o f the retina.

Cajal {1892) first used a Golgi staining technique to identify various types o f ganglion cell in the vertebrate retina. He classified them according to dendritic morphology, dendritic tree and cell body size, and stratification characteristics in the inner plexiform layer. In 1941 Polyak extended this classification to primate retinas, and identified 6 types of ganglion cells according to shape and size. He named them midget, parasol, shrub, small diffuse, garland and giant. Of these, midget and parasol are most prevalent in the human retina.

Ganglion cells in the cat retina have been described according to their physiological behaviour as X-, Y- and W- cells {Enroth-Cugell et al, 1966; Cleland et al 1971; Fukada,

1971). X- cells predominate in the central retina, and show strong, sustained responses to

stationary stimuli, whilst being unresponsive to fast-moving stimuli. The X-cell axons have a slower conduction velocity than Y- and W- cells, and thus have longer response latency. Y- cells, by comparison, are more prevalent in the retinal periphery, show transient, weak responses to stationary stimuli, whilst responding strongly to fast-moving stimuli. W- cells have a conduction velocity faster than both X- and Y- ganglion cells {Stone & Fukuda, 1974). Boycott & Wassle {1974) found the morphological correlates o f the X- Y- and W- type cells. X- type cells were labelled as beta ganglion cells, and Y- type as alpha ganglion cells. They also found two more classes o f ganglion cells, which they labelled gamma, and delta. Approximately 3% of cat ganglion cells are alpha cells, whilst 45-50% are beta cells {Fukuda

& Stone, 1974; Stone <£ Fukuda, 1974). Both can be further subclassified according to their

level o f stratification in the inner plexiform layer {Famiglietti & Kolb, 1976). Cells which stratify in the outer sublamina, a, are cells with OFF- centre receptive fields, whilst those forming synapses in the inner sublamina, b, have ON- centre receptive fields. ON- and OFF- ganglion cells are always paired, and cover a similar receptive field area. Alpha and beta cells

Chapter 2 The Retina

are arranged in a closely packed pattern across the retina, but there is very little overlapping o f dendritic trees o f adjacent ganglion cells. This means that, towards the periphery, where the receptive field size is greater, the density o f ganglion cells decreases.

In addition to the alpha and beta cells there are 21 other types of ganglion cell, which have been identified in the cat retina (Kolb, 1981). These cells constitute 50-60% of the cat ganglion cell population (Fukuda & Stone, 1974). These have been named G3-G23, according to cell body size and dendritic form. G3 is equivalent to the gamma cell of Boycott and Wassle, whilst G19 is equivalent to the delta ganglion cell.

At least 18 types o f ganglion cell have been identified in the primate retina {Kolb et al, 1992). The tonic cells o f the primate retina are equivalent in many ways to the cat X- type or beta ganglion cells, showing the same small, concentric receptive fields, predominating in the central retina. The main difference between these and the X-cells of cat, is the colour antagonistic receptive field in the tonic cells o f the primate retina. The midget cells identified in the primate retina by Polyak {1941) are the morphological correlate of these tonic cells. They project to the parvocellular layers o f the lateral geniculate nucleus, and are therefore labelled as P-type. The midget ganglion cell has only a single dendrite, and may be connected to only one midget bipolar, which in turn synapses with only one cone photoreceptor {Kolb &

Dekorvar, 1991). This leads to a high definition channel, which is also capable of colour

resolution. Studies have suggested that up to 80% o f the monkey ganglion cell population is composed of midget cells {Perry, 1984).

The Y-type or alpha cells o f the cat are similar to the phasic ganglion cells of the primate retina, which also respond maximally to transient, fast-moving stimuli. The parasol ganglion cell {Polyak, 1941) shows phasic type activity. Parasol cells project to the magnocellular layers o f the lateral geniculate nucleus, and are thus described as M-type.