The central pattern generator controlling ventilation in the crab can generate two distinct motor programmes, which determine the direction of water flow during irrigation of the gills. An interneurone has been identified that depolarizes when the ventilatory motor output switches from forward to reverse ventilation and remains depolarized for the duration of the reverse motor programme. Depolarization of this neurone by intracellular current injection causes a switch in the motor programme from forward to reverse ventilation, which persists for the duration of the current step. Hyperpolarization of this cell during reverse ventilation terminates the reverse motor programme. The possible role of this reversal switch interneurone is considered in the context of the observed changes in the activity of other ventilatory interneurones and motor neurones during reverse ventilation.

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