Bipolar cells, like receptors 
and horizontal cells, do not fire 
impulses, but we still speak of 
an on response, meaning a 
depolarization to light and 
therefore increased transmitter 
release from the cell's 
terminals, and an off response, 
to imply hyperpolarization and 
decreased release. For the off-
center bipolars the synapses 
from the receptors must be 
excitatory, because the 
receptors themselves are 
turned off (hyperpolarized) by 
light; for the on-center 
bipolars the synapses must be 
inhibitory. To see why (if you, 
like me, find this confusing), 
you need only think about the 
effects of a small spot of light. 
Receptors are active in the 
dark: light hyperpolarizes 
them, turning them off. If the 
synapse is excitatory, the 
bipolar will have been activated 
in the dark, and will likewise 
be turned off by the stimulus. If 
the synapse is inhibitory, the 
bipolar will have been 
suppressed in the dark, and the 
light, by turning off the 
receptor, will relieve the 
suppression of the bipolar cell-
-that is, the bipolar cell will be 
activated. (No one said this 
would be easy.)

    Whether the receptor-to-
bipolar synapse is excitatory or 
inhibitory could depend on 
either the transmitter the 
receptor releases or the nature 
of the channels in the bipolar 
cell's postsynaptic membrane. 
At present no one thinks that 
one receptor releases two 
transmitters, and much 
evidence favors the idea that 
the two biolar types have 
different receptor molecules.

    Before we discuss where the 
receptive-field surrounds of the 
bipolar cells come from, we 
have to consider the horizontal 
cells.

    Horizontal cells are 
important because they are 
probably at least in part 
responsible for the receptive-
field surrounds of retinal 
ganglion cells; they represent 
the part of the indirect pathway 
about which we know the most. 
They are large cells, and among 
the strangest in the nervous 
system. Their processes make 
close contact with the 
terminals of many 
photoreceptors distributed over 
an area that is wide compared 
with the area directly feeding a 
single bipolar cell. Every 
receptor contacts both types of 
second-order cell, bipolar and 
horizontal.

    Horizontal cells come in 
several subtypes and can differ 
greatly from species to species; 
their most unusual feature, 
which they share with 
amacrine cells, is their lack of 
anything that looks like an 
ordinary axon. From the 
slightly simplified account of 
nerve cells given in the last 
chapter you may rightly wonder 
how a nerve without an axon 
could transmit information to 
other neurons. When the 
electron microscope began to be 
used in neuroanatomy, we soon 
realized that dendrites can, in 
some cases, be presynaptic, 
making synapses onto other 
neurons, usually onto their 
dendrites. (For that matter, 
axon terminals can sometimes 
be postsynaptic, with other 
axons ending on them.) The 
processes that come off the cell 
bodies of horizontal cells and 
amacrine cells apparently serve 
the functions of both axons and 
dendrites.