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Visual Processing in the Retina

The photoreceptors exhibit a fairly high basal release of glutamate. When light strikes the photoreceptor cell, it initiates a biochemical process in the cell that reduces the release of glutamate from its axon terminal. The glutamate, in turn, affects the activity of the bipolar and horizontal cells, which synapse with the photoreceptor. The bipolar cells, in turn, synapse with amacrine and retinal ganglion cells. It is the axons of the retinal ganglion cells that exit the eye as the optic nerve and terminate in the brain. Notice that the direct pathway for the transmission of visual information from the eye to the brain includes only the receptor cell, bipolar cell and ganglion cell. The horizontal cells modulate the synaptic activity of receptor cells and, thereby, indirectly affect the transmission of visual information by bipolar cells. Similarly the amacrine cells modulate the synaptic activity of the retinal bipolar and ganglion cells, thereby affecting the transmission of visual information by the ganglion cells.

Bipolar Cells

Within the outer plexiform layer of the retina, approximately 125 million photoreceptor cells synapse with approximately 10 million bipolar cells. A smaller number of horizontal cells also synapse with the photoreceptor cells within the outer plexiform layer of the retina. The bipolar and horizontal cells respond to the glutamate released by the photoreceptor cells⁴.

• Bipolar cells
  
  • do not generate action potentials.
    
  • respond to the release of glutamate from photoreceptors with graded potentials (i.e., by hyperpolarizing or depolarizing).
    

Bipolar cells differ based on their responses to photoreceptor stimulation.

• There are at least two types of bipolar cells based on their responses to glutamate.
  
  • The off bipolar cells are depolarized by glutamate.
    
  • The on bipolar cells are hyperpolarized by glutamate.
    
• The two bipolar cell types have different functional properties.
  
  • The off bipolar cells function to detect dark objects in a lighter background.
    
  • The on bipolar cells function to detect light objects in a darker background.
    

The stimulus condition that produces a depolarizing response from a bipolar cell is used to name the bipolar cell type.

• An off bipolar cell depolarizes when the photoreceptors that synapse with it are in the dark (i.e., when the light is off, Figure 14.22).
  
• An on bipolar cell depolarizes when the photoreceptors that synapse with are in the light (i.e., when the light is on, Figure 14.22). Note that the depolarization of the on bipolar cell does not result from excitation of the presynaptic cell but rather from a reduction of the inhibitory action of glutamate produced by the light-induced decreased release of glutamate from the photoreceptor.

Figure 14.22 When the receptor cells with which an off bipolar cell synapses are in the dark, the off bipolar cell is depolarized and the on bipolar cell is hyperpolarized. In contrast, when the receptor cells with which an off bipolar cell synapses are in the light, the off bipolar cell is hyperpolarized and the on bipolar cell is depolarized.

Bipolar Cell Receptive Field: The receptive field of a bipolar cell is defined anatomically by the location and distribution of receptor cells with which it makes synaptic contact.

• Each cone-bipolar cell makes direct synaptic contact with a circumscribed patch of cone receptors, which may be as few as one foveal cone. Consequently, the receptive fields of bipolar cells synapsing with cones in the fovea are extremely small and are color sensitive. The cone-bipolars may be hyperpolarized or depolarized by glutamate and, consequently, may be on-type or off-type bipolar cells.
  
• Each rod-bipolar cell may make synaptic contact with a few to fifty or more of rod receptor cells. Consequently, the rod-bipolar cell receptive field is relatively large and color insensitive. All rod-bipolar cells are hyperpolarized by glutamate and, consequently, are on-type bipolar cells exclusively.
  

The bipolar cell receptive field is also defined physiologically as the retinal area which when exposed to light produces a response (i.e., depolarization or hyperpolarization) in the bipolar cell.

Bipolar cells have concentric receptive fields. Light directed on the photoreceptor(s) that synapse with a bipolar cell produces a response from the bipolar cell called the center response (Figure 14.23). In contrast, light directed on immediately surrounding receptors produce the opposite response (Figure 14.24).

Figure 14.23 Bipolar cells have concentric receptive fields. The on bipolar cell depolarizes when the receptor cells with which it synapses are illuminated ("Light On"). These center receptors (i.e., the ones making direct synaptic contact with the bipolar cell) produce the bipolar cell center response.		Figure 14.24 Bipolar cells have concentric receptive fields. When the receptors surrounding the center receptors of the on bipolar receptive field are illuminated ("Light On") and the center receptors kept in the dark, the on bipolar cell is hyperpolarized.

When both the center and surrounding receptor cells are illuminated with light, the on bipolar cell response to stimulation of the center receptors is reduced by stimulation of the surround receptors (Figure 14.25).

Figure 14.25 Bipolar cells have concentric receptive fields. When both the center and surrounding receptors of the on bipolar cell receptive field are illuminated, the on bipolar cell depolarizes. However, the magnitude of the depolarization is reduced to less than the depolarization to illumination of only the center receptors.

Consequently, the strongest on bipolar cell response is produced when the stimulus is a light spot encircled by a dark ring. For the off bipolar cell, a dark spot encircled by a light ring produces maximal depolarization.