AMACRINE CELLS

    These cells come in an 
astonishing variety of shapes 
and use an impressive number 
of neurotransmitters. There 
may be well over twenty 
different types. They all have in 
common, first, their location, 
with their cell bodies in the 
middle retinal layer and their 
processes in the synaptic zone 
between that layer and the 
ganglion cell layer; second, 
their connections, linking 
bipolar cells and retinal 
ganglion cells and thus forming 
an alternative, indirect route 
between them; and, finally, 
their lack of axons, 
compensated for by the ability 
of their dendrites to end 
presynaptically on other cells.

    Amacrine cells seem to have 
several different functions, 
many of them unknown: one 
type of amacrine seems to play a 
part in specific responses to 
moving objects found in retinas 
of frogs and rabbits; another 
type is interposed in the path 
that links ganglion cells to 
those bipolar cells that receive 
rod input. Amacrines are not 
known to be involved in the 
center-surround organization 
of ganglion-cell receptive 
fields, but we cannot rule out 
the possibility. This leaves most 
of the shapes unaccounted for, 
and it is probably fair to say, for 
amacrine cells in general, that 
our knowledge of their anatomy 
far outweighs our 
understanding of their 
function.