Page 246 - Biology_F5

Coordination and Irritability

Structural differences between rods to vision at twilight. Thus, from these
and cones considerations, it should be clear that
most of what we think of as “seeing” is
Task 4.7 mediated by the cone system. Thus, the
From the library and internet sources loss of cone function is devastating, as
search for information on the structure of it occurs in elderly individuals suffering
retina including the structural differences
FOR ONLINE READING ONLY
between rods and cones. Write short from macular degeneration. Individuals
notes on the searched information. who have lost cone function are blind,
whereas those who have lost rod function

Rods and cones are generally similar only experience difficulty seeing at low
in structure, but they differ in size and levels of illumination (night blindness).

shape, as well as in the arrangement of the Differences in the transduction
membranous discs in their outer segments. mechanisms between the two receptor
At the lowest level of light, only rods are types also contribute to the ability of rods
activated. Such rod mediated perception and cones to respond to different ranges of
is called scotopic vision; the difficult of light intensity. For example, rods produce
making visual discrimination under very a reliable response to a single photon of
low light conditions where only the rod light, whereas more than 100 photons
system is active. The problem is primarily are required to produce a comparable
the poor resolution of the rod system and, response in a cone. Another difference is
to a lesser degree, the lack of perception that, the response of an individual cone
of colour in dim light since such cones are does not saturate at high levels of steady
not involved to a significant degree. illumination, as does the rod response.

Although cones begin to contribute to Although both rods and cones adapt to
visual perception at about the level of operate over a range of luminance values,
starlight, spatial discrimination is still the adaptation mechanisms of the cones
very poor. As illumination increases, are more effective. This difference in
cones become increasingly dominant in adaptation is evident in the time course
determining what is seen. In addition, they of the response of rods and cones to light
are the major determinant of perception flashes. The response of a cone, even
under relatively bright conditions, such to a bright light flash that produces the
as normal indoor lighting or sunlight. The maximum change in photoreceptor current
contribution of rods to vision drops out pick up is about 200 ms; which is more
nearly entirely in the so called photopic than four times faster than rod recovery.
vision because their response to light
saturates, that is, the membrane potential The arrangement of the circuits that
of individual rods no longer varies as transmit rods and cones information to
a function of illumination because all retinal ganglion cells also contributes to
of the membrane channels are closed. the different characteristics of scotopic
Mesopic vision occurs at levels of light and photopic vision. In most parts of the
at which both rods and cones contribute retina, rods and cones signals converge

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