1. 1.

    Intracellular recordings were made from secondary neurones and photo-receptors of the Aplysia eye concurrently with extracellular recordings from the optic nerve. These cells were injected with Lucifer yellow to reveal their structure after they were typed according to electrical activity. Secondary neurones are described in this paper.

  2. 2.

    All secondary neurones injected with Lucifer yellow were in the outer, non-receptor layer of the retina. Each had an axon in the optic nerve, short dendritic processes on the soma, but no distinct photoreceptive apparatus. Dye coupling between secondary neurones and between secondary neurones and photoreceptors was observed.

  3. 3.

    Secondary neurones had pacemaker potentials and action potentials (APs) correlated 1:1 with the optic nerve compound action potentials (CAPs) during spontaneous dark and light evoked activity. It is deduced that the secondary neurones are the output neurones of the circadian clock system of the eye.

  4. 4.

    Secondary neurones appear to be electrically coupled to each other and to some photoreceptors, since blocking chemical synapses with high Mg2+ saline did not block the spontaneous or light evoked activities, and antidromic activation of the secondary neurones produced a compound input dependent in amplitude on stimulus voltage.

  5. 5.

    Backfilling the optic nerve with cobalt revealed filled secondary neurones, 2 photoreceptor types and a small non-receptor cell type suggesting that most of these retinal cells have axons in the optic nerve.

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