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Marek Niewczas

RETINAL PROCESSING Transformations in the Outer Plexiform Layer The transmitter released by photoreceptors is glutamate. Photoreceptors are depolarized in the dark and are hyperpolarized by light and thus fewer transmitter molecules are released in the light than in the dark. In the out plexiform layer, each photoreceptor is in synaptic contact with bipolar and horizontal cells. Bipolar cells create the direct pathway from photoreceptors to ganglion cells; horizontal cells feed information laterally in the outer plexiform layer to influence the activity of neighbouring bipolar cells and photoreceptors. Bipolar Cell Receptive Fields Bipolar cells can be categorized into two classes: OFF bipolar cells (glutamate-gated channels mediating depolarizing EPSP from Na influx) and ON bipolar cells (G-protein-coupled receptors and respond to glutamate by hyperpolarizing). Note: OFF and ON refer to whether these cells respond to light off (more glutamate) or light on (less glutamate). Each bipolar cell receives direct synaptic input from a cluster of photoreceptors. The receptive field of a bipolar cell is the area of the retina that, when stimulated with light, changes the cell’s membrane potential. It contains two parts: a circular area providing direct photoreceptor input (receptive field center) and a surrounding area of the retina providing input via horizontal cells (receptive field surround) (see Fig. 9.22).  Bipolar cell receptive field diameters range from a fraction of a degree in the central retina to several degrees in the peripheral retina. The response of a bipolar cell’s membrane potential to light in the receptive field is opposite to that of light in the surround and these cells are said to have antagonistic center-surround receptive fields. RETINAL OUTPUT The sole source of output from the retina to the rest of the brain is the action potentials arising from million or so ganglion cells. Ganglion Cell Receptive Fields Most retinal ganglion cells have the concentric center-surround receptive field organization. An ON-center ganglion cell will be depolarized and respond with a barrage of action potential when a small spot of light is projected onto the middle of its receptive field. An OFF-center cell will respond to a small dark spot presented to the middle of its receptive field in the same way.  In both types of cells, the response to stimulation of the center is cancelled by the response to stimulation of the surround (see Fig. 9.23).  It appears ganglion cells are mainly response to differences in illumination that occur within the receptive fields. Consider the response generated by an OFF-center cell as a light-dark edge crosses its receptive field. In a uniform illumination:  The center and surround cancel to yield some low level of response.  When the edge enters the surround region without passing the center, the dark area will hyperpolarize the neuron and lead to a decrease in firing rate.  As the dark area includes the center, the
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