Electrotonic conduction in the optic nerves of planorbid snails.

While action potentials are the usual means of transmission in axons, recent work has established that decrementally conducted potentials are also utilized. Decremental conduction has been demonstrated to be the exclusive mode of transmission in certain sensory axons of vertebrates (Werblin & Dowling, 1969; Kaneko, 1967, 1970) and invertebrates (Gwilliam, 1965, Ripley, Bush & Roberts, 1968; Ioannides & Walcott, 1971). Although decremental conduction had long been suspected in gastropod nerves, there was no compelling evidence for it (Bullock & Horridge, 1965). Recently, however, a DC response has been recorded from the optic nerve of Helix (Gillary, 1970). Since the axons of the primary receptors of the gastropod eye usually leave via the optic nerve and pass directly to the central nervous system (Smith, 1906; Clark, 1963; Eakin & Brandenburger, 1967; Newell & Newell, 1968; Jacklet, 1969), the DC shift observed by Gillary could be the decrementally conducted receptor potential as well as a manifestation of an active process. These alternatives might be distinguished by recordings obtained under experimental conditions that block active processes. Employing this approach, the present communication provides evidence for decremental conduction of receptor potentials along the length of the axons of the optic nerves of planorbid snails.