Small-tufted (ST) ganglion cells of rabbit retina are divided into
eight types based upon morphology, branching pattern, level of dendritic
stratification, and quantitative dimensional analysis. Only one of these
types has been previously characterized in Golgi preparations, and four
may be discerned in the work of others. Given their small dendritic-field
size, and assuming uniform mosaics of each across the retina, ST cells
comprise about 45% of all rabbit ganglion cells, and are therefore of
major functional significance. Four ST cells occur as two paramorphic
(a/b) pairs, and thus belong to class III, as previously defined. Four
branch in sublaminae a and b of the inner plexiform
layer (IPL) and therefore belong to class IV. ST cells have small cell
bodies 10–15 μm in diameter, small axons 0.7–1.3 μm in
diameter, and small dendritic-field diameters, 40–110 μm in
mid-visual streak. The dendrites of ST cells are highly branched, and bear
spines and appendages of varying length, but vary from type to type. Class
III.2 cells and class III.3 cells are partly bistratified. Class IV
small-tufted cells differ characteristically in multiple features of
dendritic branching and stratification. Class III small-tufted cells
apparently have concentric (ON-center and OFF-center) receptive fields and
may have “sluggish-transient” (class III.2) and
“sluggish-sustained” (class III.3) physiology. Class IV cells
include the “local-edge-detector” (LED) (class IVst1), and are
all expected to give ON–OFF responses to small, centered, slowly
moving visual stimuli. Based upon systematic variation in dendritic-field
size across the retina, ST cells may be divided into two groups. In this
“universal prey” species, they may belong to two systems of
motion detection, typified by ON–OFF directionally selective and LED
ganglion cells, respectively, specialized for detection of rapid motion at
the horizon for land-based predators, and slow motion for airborne
predators.