The Initiation and Conduction of Action Potentials in the Optic Nerve of Tritonia

As noted in the preceding article (Chase, 1974), an optic nerve origin for photoreceptor spikes in the mollusc Tritonia diomedia is indicated by the small size and frequent absence of somatic action potentials. Axonal trigger sites are the rule for invertebrate ganglion cells (Tauc, 1962) and most peripheral sensory neurones (Edwards & Ottoson, 1958), but there are exceptions (Mellon & Kennedy, 1964) and the issue has not been well investigated in molluscan sensory neurones. Recently Alkon & Fuortes (1972) found evidence for axonal spike initiation in photoreceptors of the nudibranch Hermissenda. In Tritonia, also a nudibranch, the receptor fibres are several millimetres longer than in Hermissenda, and the optic nerve itself, apparently composed exclusively of these fibres, measures about 2·0 mm. These experiments take advantage of the long optic nerve in Tritonia to examine in detail the origin of photoreceptor spikes.