1. 1.

    Neuronal mechanisms underlying the directional sensitivity of the crayfish equilibrium system were studied in the brain by intracellular recording combined with mechanical statocyst hair deflection.

  2. 2.

    Five primary afferents were successfully characterized. Three of them showed a decrease in response to inward hair deflection. The remaining two showed the opposite directional response.

  3. 3.

    Directional sensitivity was found in six interneurones. Two of them were excited during inward hair deflection while the other four were excited during outward deflection. Both groups exhibited active inhibition during hair deflections in the opposite direction.

  4. 4.

    This ‘null-phase inhibition’ appeared to arise from the convergence of the two classes of afferents onto an interneurone with the opposite sign.

  5. 5.

    Three identified descending statocyst interneurones, S3, S6 and S7, were found to receive excitatory input from one statocyst and inhibitory input from the other.

  6. 6.

    The results thus indicated that the directional sensitivity of the crayfish equilibrium system was achieved by selective excitatory connections between the interneurone and the directionally arranged receptor and sharpened by inhibitory mechanisms.

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