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DR. DAVID COX: The most direct path of information flow in the retina
is from an input photoreceptor cell through a bipolar cell
to an output retinal ganglion cell.
This is also known as the direct pathway.
At each synaptic connection between these cell layers,
however, the neuronal responses are modulated by the lateral connections
of horizontal and amacrine cells.
This is known as the lateral pathway.
To understand in detail how this information processing occurs,
let's first focus on the information flow from photoreceptor to bipolar
cells.
And then focus our understanding on retinal ganglion cell
output to the optic nerve.
Photoreceptors, like other neurons, release
neurotransmitter when depolarized.
Since the retina is part of the central nervous system,
the main neurotransmitter is the amino acid, glutamate.
As we've just learned, photoreceptors are depolarized in the dark
and hyperpolarized in the light.
While this might seem counterintuitive at first,
it makes sense when considering that dark, rather than light,
is the preferred stimulus for a photoreceptor.
To illustrate, when a shadow cast by an object
in our surrounding passes across the retina,
it responds by depolarizing and releasing neurotransmitter, thus
registering the presence of the object.
Since each photo cell is in a synaptic contact with both bipolar cells
and horizontal cells, this information can
be passed on both in the direct and lateral pathways.
Let's consider the transmission to bipolar cells
through the direct pathway first.
Based on the responses to glutamate released by photoreceptors,
bipolar cells can be classified as either ON or OFF cells.
In off bipolar cells, glutamate gated cation channels
mediate a depolarizing excitatory postsynaptic potential, our old friend,
the EPSP, after sodium influx.
In on bipolar cells, G protein-coupled receptors
respond to glutamate released by the photoreceptors by hyperpolarizing.
It's worth pointing out that the labels, Off and ON, for bipolar cells
refer to the fact that these cells depolarize in response
to light off, more glutamate, or light on, less glutamate.
Each bipolar cell receives direct synaptic input
from a cluster of photoreceptors ranging from one to thousands in number.
In addition, each bipolar cell is also connected via horizontal cells
to a ring of photoreceptors that surround the central direct cluster.
Thus the receptive field to which these cells respond,
consists of two parts, a circular central area providing direct input
from the photoreceptors and it's surrounding
area of retina providing indirect input by horizontal cells.
Consequently, the name center-surround.
Generally speaking, the response of bipolar cell's membrane potential
to light in the center of the receptive field
is opposite to the response in the surround
irrespective of whether the cell is classified as ON or OFF center cell.
This organization has an interesting effect
of opposing physiological responses when illuminating either the center
or surround with the same spot.
This antagonistic surround appears to come
from a complex interaction between horizontal cells, photorecptor,
and bipolar cells at their synapses, and is a very active area
of neuroscience research.
This center-surround organization is passed on from the bipolar cells
to retinal ganglion cells via synapses in the ganglion cell layer.
And in addition, modulated by the action of amacrine cells
and the lateral pathway in this layer coordinating the integrating rod
and cone inputs in the retinal ganglion cells.
As mentioned previously, retinal ganglion cells
are actually the only cells in the retina
that fire action potentials and convey their output to the rest of the brain.
There are about 1 million retinal ganglion cells in the human retina.
Like bipolar cells, most retinal ganglion cells
have a center-surrond receptive field organization,
and they receive their ON or OFF input from the corresponding bipolar cells.
Thus, an ON center retinal ganglion cell will be depolarized
and fire a range of action potentials if the center of its receptor field
is illuminated with a bright spot.
Conversely, an OFF center retinal ganglion cell
will respond to a small, dark spot in the center of its receptive field.
Some retinal ganglion cells have more complex response profiles
than simple center-surround receptive fields.
For instance, some RGCs respond selectively
to particular colors or movement of light patterns
across local regions of the retina.
This diversity of response properties derives from different connectivity
patterns among the different classes of amacrine and bipolar cells.
Since there are currently over 30 specific types of amacrine cells
and about one dozen types of bipolar cells known,
this allows for a large combinatorial space to respond to specific features.
The full parts list of different kinds of retinal ganglion cells
is not yet fully known, though great progress is being made in this area
today.
Likewise, we don't yet know what all of these different RGC types are for.
For instance, some RGCs in the retina respond selectively
to different kinds of motion, but we also
know that motion is processed in cortex starting from different retinal inputs.
In some cases, evidence suggests that most selective cells in the retina
may be involved in ocular motor reflexes,
such as the optic kinetic reflex, which moves the eye
to stabilize the image on the retina when whole field motion is detected.
In many cases, it's not yet clear how different RGCs participate
in different aspects of vision, and the answer
might be different in different species.
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