BIPOLAR CELLS AND HORIZONTAL 
CELLS
To record from a cell in the 
nervous system is one thing: it 
is another to record from a cell 
and know exactly what kind of 
cell it is. This microscopic 
picture shows a single bipolar 
cell in the retina of a goldfish, 
recorded in 1971 by Akimichi 
Kaneko, then at Harvard 
Medical School. The fact that it 
is a bipolar cell and not an 
amacrine or horizontal cell was 
proven by injecting a 
fluorescent dye, procyon 
yellow, through the 
microelectrode. The dye spread 
throughout the cell, revealing 
its shape. In this cross section, 
receptors are on top.

    Horizontal cells and bipolar 
cells occur, along with 
amacrine cells, in the middle 
layer of the retina. The bipolar 
cells occupy a strategic position 
in the retina, since all the 
signals originating in the 
receptors and arriving at the 
ganglion cells must pass 
through them. This means that 
they are a part of both the 
direct and indirect paths. In 
contrast, horizontal cells are a 
part of the indirect path only. 
As you can see from the diagram 
later in this section, 
horizontal cells are much less 
numerous than bipolar cells, 
which tend to dominate the 
middle layer.

    Before anyone had recorded 
from bipolar cells, the big 
unknown was whether they 
would prove to have center-
surround receptive fields, as 
ganglion cells do, and come in 
two types, on center and off 
center. If the answer was yes, it 
would almost certainly mean 
that the organization 
discovered by Kuffler for 
ganglion cells was a passive 
reflection of bipolar-cell 
organization. The knowledge 
that the receptive fields of 
bipolar cells were indeed 
center-surround and were of 
two types came from 
intracellular recordings first 
made by John Dowling and 
Frank Werblin at Harvard 
Biological Laboratories and by 
Akimichi Kaneko at Harvard 
Medical School. The next 
question is how these receptive 
fields are built up. To answer it 
we have to begin by examining 
the connections of receptors, 
bipolar cells, and horizontal 
cells.