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Barnacles respond to decreases in light intensity (shadows). Previous evidence indicates that the first stages of the visual pathway within the supraoesophageal ganglion comprise a limited number of neurones: a single pair of second-order neurones (I-cells), a pair of third-order projection neurones (A-cells), and at least five other projection neurones (four A-like shadow-excited cells and one shadow-inhibited cell) whose spikes may be recorded extracellularly from the circum-oesophageal connective nerve. Here we present evidence that information in this pathway diverges at the A-cell stage. Destruction of both A-cells by intense illumination of cells injected with carboxyfluorescein abolishes all shadow-evoked activity in the connectives and also the ability of shadows to influence motor neurones in the ventral ganglion. In addition, we have located the somata of these previously unidentified projection neurones, injected them with carboxyfluorescein, and observed that they have distinct branching patterns different from that of the A-cell. We name these cells ‘alpha’ (alpha), ‘beta’ (beta) and ‘gamma’ (gamma) cells. Simultaneous impalements show that A-cells drive at least one of the beta-cells but not vice versa. The same observation has not yet been made with regard to alpha- or gamma-cells. Depolarizing an I-cell does not readily drive a beta-cell impaled simultaneously. These observations support our conclusion that information diverges from the A-cells and not from the second-order I-cells to the other projection neurones in this ganglion.

Characteristics of neurones projecting from the supraoesophageal ganglion in the shadow reflex pathway of the barnacle