Neuroscience Online (ii,14,6)

Section II: Sensory Systems
14. Visual Processing: Eye and Retina
Part 6 of 7

Valentin Draogoi, Ph.D.
(Content adapted from material by Chiyeko Tsuchitani, Ph.D.).

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Visual Processing in the Retina (continued)

Horizontal Cells

Within the outer plexiform layer, the photoreceptor cells make both presynaptic and postsynaptic contact with horizontal cells.

The horizontal cells have large receptive fields involving
- presynaptic (axonal) contact with a small group of photoreceptors and
- postsynaptic (dendritic) contact with a larger group of surrounding photoreceptor cells.

By controlling the responses of their “center” photoreceptors (based on the responses of the surrounding photoreceptors), the horizontal cells indirectly produce the bipolar cell receptive field surround effect. The surround effect produced by the horizontal cell is weaker than the center effect.

Figure 14.26

The horizontal cells make presynaptic and postsynaptic contact with photoreceptor cells. The axon terminals of a horizontal cell receives synaptic contact from one group of photoreceptors (colored red) and its processes make synaptic contact with surrounding photoreceptor cells (colored green).

The surround effect, produced by the horizontal cells, enhances brightness contrasts to produce sharper images, to make an object appear brighter or darker depending on the background and to maintain these contrasts under different illumination levels.

Retinal Ganglion Cells

Within the inner plexiform layer, the axon terminals of bipolar cells (the 2° visual afferents) synapse on the dendritic processes of amacrine cells and ganglion cells. As in most neurons, depolarization results in neurotransmitter release by the bipolar cell at its axon terminals. Most bipolar cells release glutamate, which is excitatory to most ganglion cells (i.e., depolarizes ganglion cells). The amacrine cells may synapse with bipolar cells, other amacrine cells or ganglion cells. It is the axons of the retinal ganglion cells (the 3° visual afferents) that exit the eye to form the optic nerve and deliver visual information to the lateral geniculate nucleus of the thalamus and to other diencephalic and midbrain structures.

Ganglion Cell Response Properties. The retinal ganglion cells are the final retinal elements in the direct pathway from the eye to the brain. Because they must carry visual information some distance from the eye, they posses voltage-gated sodium channels in their axonal membranes and generate action potentials when they are depolarized by the glutamate released by the bipolar cells.

The off bipolar cell (Figure 14.27, Right) will depolarize when it is dark on its center cones and will therefore release glutamate when it is dark on the center of its receptive field. This will result in the depolarization of the retinal ganglion cells with which the off bipolar synapses and in the production of action potentials (i.e., discharges) by these ganglion cells (Figure 14.27, Right). Consequently, the retinal ganglion cells that synapse with off bipolar cells will have off-center/on-surround receptive fields and are called off ganglion cells.

The on bipolar cell (Figure 14.28, Left) will depolarize when there is light on its center cones and will therefore release glutamate when it is light on the center of its receptive field. This will result in the depolarization of the retinal ganglion cells with which the on bipolar synapses and in the production of action potentials (i.e., discharges) by these ganglion cells (Figure 14.28, Left). Consequently, the retinal ganglion cells that synapse with on bipolar cells will have on-center/off-surround receptive fields and are called on ganglion cells.

Figure 14.27

An off ganglion cell synapses with an off bipolar cell and produces action potentials (i.e., is excited) when the off bipolar cell is depolarized (i.e., when the light is off). In contrast, an on ganglion cell that synapses with an on bipolar cell reduces the rate at which it produces action potentials (i.e., is inhibited) when the on bipolar cell is hyperpolarized (when the light is off).

In short, the receptive fields of the bipolar cells with which the retinal ganglion cell synapses determine the receptive field configuration of a retinal ganglion cell.

The retinal ganglion cells provide information important for detecting the shape and movement of objects.

In the primate eye, there are two major types of retinal ganglion cells, Type M and Type P cells, that process information about different stimulus properties.

Type P retinal ganglion cells are color-sensitive object detectors.

. The P ganglion cell(s)

outnumber the M-ganglion cells, by approximately 100 to 1 in the primate retina
makes synaptic contact with one to a few cone bipolars that are innervated by cone receptors in the macula fovea
is color sensitive
has a small concentric receptive field
produces a sustained, slowly adapting response that lasts as long as a stimulus is centered on its receptive field.
produces weak responses to stimuli that move across its receptive field.

The slowly adapting response of the Type P retinal ganglion cell is best suited for signaling the presence, color and duration of a visual stimulus and is poor for signaling stimulus movement.

Type M retinal ganglion cells are color-insensitive motion detectors.

The M ganglion cell

is much larger than P ganglion cells
synapses with many bipolar cells
is color insensitive
has a large concentric receptive field
is more sensitive to small center-surround brightness differences
responds with a transient, rapidly adapting response to a maintained stimulus.
responds maximally, with high discharge rates, to stimuli moving across its receptive field.

Figure 14.28

Left: The on ganglion cell synapses with an on bipolar cell and produces action potentials (i.e., is excited) when the on bipolar cell is depolarized (i.e., when the light is on). Right: In contrast, an off ganglion cell that synapses with an off bipolar cell reduces the rate at which it produces action potentials (i.e., is inhibited) when the off bipolar cell is hyperpolarized (when the light is on).

The rapidly adapting responses of Type M ganglion cells are best suited for signaling temporal variations in, and the movement of, a stimulus.

The axons of the M and P retinal ganglion cells travel in the retina optic nerve fiber layer to the optic disc where they exit the eye. Most of the axons travel to and terminate in the lateral geniculate nucleus of the thalamus.

Amacrine Cells

Amacrine cells synapse with bipolar cells and ganglion cells and are similar to horizontal cells in providing lateral connections between similar types of neurons (e.g., they may connect bipolar cells to other bipolar cells)⁵. They differ from horizontal cells, however, in also providing ‘’vertical” links between bipolar and ganglion cells.

Amacrine cell types. There are 20 or more types of amacrine cells based on their morphology and neurochemistry. The roles of three types have been identified. One type

is responsible for producing the movement sensitive (rapidly adapting) response of the Type M ganglion cells.
enhances the center-surround effect in ganglion cell receptive fields.
connects rod bipolar cells to cone bipolar cells, thus allowing ganglion cells to respond to the entire range of light levels, from scotopic to photopic.

Convergence of Inputs and Visual Acuity

Low convergence of cones to cone bipolar cells and low convergence of cone bipolar cells to P-retinal ganglion cells produce high visual acuity in the central visual field.

Recall that

visual acuity and color vision are greatest in the central visual field.
the image of the central visual field is projected onto the fovea.
the cones are concentrated in the fovea, whereas the rods predominate in the peripheral retina.
there is low convergence of foveal cones onto macular bipolar cells, as low as one cone receptor to one bipolar cell.

In addition, the cones in the fovea are of smaller diameter than those in the periphery of the retina, which allows for a greater packing density of foveal cones. The high packing density of cones and the low convergence of cones onto bipolar cells in the macula support higher visual acuity in the central visual field. Consequently, the foveal cones, macular bipolar cells and the P-retinal ganglion cells are responsible for photopic, light-adapted vision in the central visual field. In contrast, the higher convergence of the rods onto peripherally located bipolar cells and of peripheral bipolar cells onto amacrine cells forms the basis for the poor visual acuity but high light sensitivity of scotopic vision.

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