Neurosciences 11+13+15
HC 13
Chapter 11 – Vision
The eye consists of many subsets of structures, of which the retina is the most important
when it comes to processing and detecting light for vision purposes. It is formed from the
neural tube, hence ectodermic, but nevertheless contains many different kinds of layers
with various cell types. Being the neural portion of the eye, it is actually part of the CNS. The
five classes of cells, or neurons, in the retina part of the retinal circuitry are:
- Photoreceptors:
o Rods: these cells are light-sensitive for black vs. white coloring, so the
primary cell type you use at night.
o Cones: less sensitive than rods, these still are able to pick up photons of
different wavelengths and thus ensure color vision.
Both have an outer segment with a many disc-like structures which are the actual
light-detecting units. Because light detection does not go without damage, these
discs are continuously replenished by the pigment epithelium to which the rods &
cones are connected. The discs curl up, become spherical, separate from rod and
then are engulfed by the pigment epithelium. New discs are subsequently formed.
So, the pigment epithelium removes damaged receptor disks and it regenerates
photopigment molecules after they have been exposed to light.
In the dark, all channels are open, allowing calcium and sodium influx, and potassium
efflux. The influx of calcium/sodium is mediated by a cGMP-gated channel. Thus, in
the dark there are high cGMP levels – leading to depolarization because of the ion
movement. When photoreceptors are depolarized, the number of calcium channels
open is high. However, when light shines on the rod (or cone), cGMP levels decrease,
the calcium/sodium channels close, so there is no net influx. Because potassium
channels remain open, the cell hyperpolarizes.
The intricate mechanism when a rod is exposed to light: a G-protein coupled
receptor on the membrane of disks in rods is called rhodopsin. This contains a 11cis-
retinal molecule, connected to opsin proteins light changes 11cis-retinal into all-
trans retinal this changes the conformation of opsin proteins and activates a
special G-protein intracellular messenger called transducin this activates a
phosphodiesterase hydrolyzes cGMP at the disk membrane cGMP levels drop
less cGMP is present to bind to the channels and open them, so there is closure of
calcium/sodium channels. There are various steps of amplification in this cascade:
one photon activates many transducin proteins, one activated PDE hydrolyzes many
cGMPs,
In short: light hyperpolarizes photoreceptors, whereby they stop or decrease their
constitutive signaling. Hyperpolarization decreases or terminates the glutamate
release at the synaptic terminal.
The distribution of cones and rods in the retina is peculiar; all rods are located
throughout the entire retina, except for an area called the fovea. The cones in turn
are exclusively located in the fovea. This means of distribution leads to the fact that
best color vision is in the fovea, which is thus the area you use mostly during the day.
This component of the cone system, the fovea, has the highest level of visual acuity,
, Also the reason you move your eyes – to let light shine upon the fovea. The fovea is
part of the macula lutea, also known as the region that supports highest visual
acuity. You have neither rods nor cones at the optic disk, was this is the site where
the optic nerve exits the eye.
- Bipolar cells: these cells form the connection between the photoreceptors and
ganglion cells. One bipolar cell receives information from 15-30 rods (large sensitive
receptive field) while one bipolar cell receives information from one cone (small
receptive field, limited sensitivity). So, the degree of convergence between rods and
cones is also significant. For rods, the convergence increases light detection, but
decreases spatial resolution. There are two types of bipolar cells:
o ON-center bipolar cells: contain metabotropic mGluR6 receptors at the
synaptic terminal, which are activated when glutamate is released by
photoreceptors, which is the case during the dark. Glutamate in combination
with mGluR6 is an inhibitory response, so when light shines, it reduces
glutamate secretion (hyperpolarized photoreceptor), the suppression is
relieved, the bipolar cell is turned on and relays the ‘on’ signal to the
connected ganglion cell; depolarizes it. Inactivating ON-center ganglion cells
resulted in the inability to detect objects brighter than the background, while
objects darker than the background could be detected. ON-cells increase
their discharge rate if luminance increases.
o OFF-center bipolar cells: contain ionotropic AMPA and kainite receptors at
the synaptic terminal, which are activated when glutamate is released by
photoreceptors, which is the case during the dark. Glutamate in combination
with AMPA/kainite is an excitatory response, so when light shines, it reduces
glutamate secretion (hyperpolarized photoreceptor), the excitation is
reduced, the bipolar cell is turned off and relays the ‘off’ signal to the
connected ganglion cell; hyperpolarize it. OFF-cells increase their discharge
rate if luminance decreases.
In short: ON-bipolar = sign-inverting, change in membrane potential
of the bipolar cell is opposite to that of the photoreceptor. If the one
is depolarized, the other hyperpolarized.
OFF-bipolar = sign-conserving, change in membrane potential of the
bipolar cell is the same as that of the photoreceptor. If the one is
depolarized, the other is also depolarized.
- Ganglion cells: these are the actual nerve cells in the retina which aggregate
together to form the optic nerve, which passes information on to the CNS.
- Horizontal cells: these regulate and integrate information from photoreceptor to
bipolar cells. They enable lateral interactions between photoreceptors and bipolar
cells that maintain the visual system’s sensitivity to contrast. If light shines on cone
B, this is the central cone. Cone A and C lie adjacent to B and are termed surrounding
cones. Thus, the center of a ganglion cell receptive field is surrounded by a
concentric region. Glutamate from photoreceptors has a depolarizing effect on
horizontal cells (sign-conserving), but horizontal cells have a hyperpolarizing effect
on photoreceptors (sign-inverting). So, horizontal cells tend to oppose changes in the
membrane potential of the photoreceptor.
If horizontal cells of surrounding cones A and C become hyperpolarized, they feed
back to the central cone B and reduce its suppressive signals. So, the concentric
