Excitation of the Ganglion Cells

Spontaneous, Continuous Action Potentials in the Ganglion Cells. It is from the ganglion cells that the long fibers of the optic nerve lead into the brain. Because of the distance involved, the electrotonic method of conduction employed in the rods, cones, and bipolar cells within the retina is no longer appropriate; therefore, ganglion cells transmit their signals by means of repetitive action potentials instead. Furthermore, even when unstimulated, they still transmit continuous impulses at rates varying between 5 and 40 per second. The visual signals, in turn, are superimposed onto this background ganglion cell firing.

Transmission of Changes in Light Intensity—The On-Off Response. As noted previously, many ganglion cells are specifically excited by changes in light intensity. This is demonstrated by the records of nerve impulses in Figure 50-13. The upper panel shows rapid impulses for a fraction of a second when a light is first turned on, but decreasing rapidity in the next fraction of a second. The lower tracing is from a ganglion cell located lateral to the spot of light; this cell is markedly inhibited when the light is turned on because of lateral inhibition. Then, when the light is turned off, opposite effects occur. Thus, these records are called "on-off" and "off-on" responses. The opposite directions of these responses to light are caused, respectively, by the depolarizing and hyperpolarizing bipolar cells, and the transient nature of the responses is probably at least partly generated by the amacrine cells, many of which have similar transient responses themselves.

This capability of the eyes to detect change in light intensity is strongly developed in both the peripheral

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Essentials of Human Physiology

Essentials of Human Physiology

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