1. The optic nerve of Tritonia contains axons of the five primary sensory cells. It joins a cerebral nerve about 2.0 mm from the eye and then travels another 2.5 mm to the central ganglia.

2. Large DC responses of positive polarity were recorded with suction electrodes in the presence of light. These graded responses are generator potentials passively conducted from a site of origin in or near the receptor somata. DC responses to light were not recorded at points central to the junction of the optic nerve with the cerebral nerve.

3. The shape of extracellular spike waveforms and the temporal relationship between soma and nerve spikes support the conclusion that action potentials are initiated in the optic nerve. In the,dark, spikes originate in portions of the nerve distant from the eye. When the eye is illuminated, the trigger zone shifts about 700 µm more proximal to the eye.

4. The shift in the spike trigger zone during illumination probably reflects an habitual accommodation of proximal portions of the nerve under the conditions of these experiments, and the prevalence of partially or completely silent optic nerves is probably due to more severe consequences of sustained depolarization. The sensitivity of the receptors, in combination with the passive properties of the nerve, makes the nerve susceptible to debilitating effects of maintained illumination.

5. The excitability of optic nerve fibres is extremely low. Absolute refractory periods are 25 msec, and relative refractory periods are as long as several hundred msec. When stimulated with just-suprathreshold voltages the nerve cannot support action potentials at frequencies greater than 1 Hz.

6. The Tritonia optic nerve appears to be transitional between transmission by graded responses and transmission by action potentials.

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