Page 247 - Biology_F5

Biology for Advanced Level Secondary Schools

on the same ganglion cells; that is, bipolar and ganglion cells is, of course,
individual ganglion cells respond to both required to maximise acuity.
rod and cone inputs, depending on the
level of illumination. The early stages Mechanism of photoreception
of the pathways that link rods and cones
to ganglion cells, however, are largely Task 4.8
FOR ONLINE READING ONLY
independent. For example, the pathway From the internet sources, search
from rods to ganglion cells involves a simulations or videos on the mechanism
distinct class of bipolar cells called rod of photoreception and physiology
bipolar that, unlike cone bipolar cells, of seeing. Write short notes on the
does not contact retinal ganglion cells. searched information
Instead, rod bipolar cells synapse with the
dendritic processes of a specific class of Rods contain light sensitive pigment
amacrine cells that makes gap junctions rhodopsin, which is attached to the
and chemical synapses with the terminals outer surface of vesicles. Rhodopsin is a
of cone bipolars. These processes, in turn, molecule formed by the combination of a
make synaptic contacts on the dendrites of protein called scotopsin with a small light-
ganglion cells in the inner surface layer. absorbing molecule called retinene, which
is a carotenoid molecule derived from
Moreover, the rod and cone systems vitamin A. When the rhodopsin molecule
differ dramatically in their degree of is exposed to bright light, it breaks down
convergence; a factor which contributes into retinene and scotopsin. This process
greatly to their distinct properties. Each rod is called bleaching.
bipolar cell is contracted by a number of Light
rods, and many rod bipolar cells contract a Rhodopsin Retinene + Scotopsin
given amacrine cell. In contrast, the cone (Purple) (Bleached)
system is much less convergent. Thus, Rhodopsin is reformed immediately when
each retinal ganglion cell that dominates light stimulation decreases. Trans retinene

central vision (called midget ganglion is first converted into cis retinene and then
cells) receives input from only one cone recombined with scotopsin. This process
bipolar cell, which in turn is contacted by is called dark adaptation. Similary, the
a single cone. Convergence makes the rod cone system has a very high spatial
system a better detector of light, because resolution but it is relatively insensitive to
small signals from many rods are pooled light. It is therefore specialised for acuity
to generate a large response in the bipolar at the expense of sensitivity. This property
cell. At the same time, convergence of the cone system allows us to see colour.
reduces the spatial resolution of the rod
system; since the source of a signal in a Physiology of seeing
rod bipolar cell or retinal ganglion cell The eye works on the same principle as
could have come from anywhere within a that of the camera. Light rays from the
relatively large area of the retinal surface. object pass from the external part of the
The one-to-one relationship of cones to eye to the retina through the conjunctiva,
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