Biology Reference
In-Depth Information
types of bipolar cell might be absent or somehow impaired, or they
might be incompletely differentiated into clear-cut types. Without
bipolar cells the visual impulses from cones would be blocked and
complete blindness would result.
6.5 Common pathways of rods and cones
Polyak's histological investigation of the primate retina also resulted
in profound new insights into the function of the rod system.
Schultze (
1866
) had presumed that the rod impulses were transmitted
independently of the cone impulses to the brain. Strong supporting
evidence had been provided by Ramon y Cajal (
1937
). Presuming,
like Schultze, that there were two fundamentally different receptor
types, rods and cones, that functioned under quite different conditions
mediating qualitatively different colour sensations, Cajal predicted that
there had to be two different types of bipolar cell in the retina: those
connected solely to rods and those connected solely to cones. Indeed,
he assumed that there had to be separate rod and cone pathways all
the way from the photoreceptors to the brain (see Cajal,
1937
, p. 384,
Fig. 47 and pp. 393-395). In accord with his prediction, Cajal, in a series
of histological studies on both fish and mammals, found that the rod
and cone receptors were connected to different types of bipolar cell.
Polyak (
1948
), however, strongly rejected Cajal's idea that the
rod and cone impulses travelled along separate routes to the brain. In
fact, he found that all the bipolar and ganglion cell varieties, except
the midget bipolar variety that was exclusively connected to the
cones, were common to both the rod and cone systems. By examining
Cajal's own histological results, he also pointed out that these results
indicated common rod and cone pathways, since the rod bipolar cells
shown could scarcely avoid contact with cone pedicles, at least in
the central region of the retina. Indeed, Cajal, in an earlier paper, had
suggested that the rod and cone pathways to the bipolar cells were
not completely separated from each other, but that each pathway was
dominated by either rod or cone impulses (see Cajal,
1894
, p. 129).
