1. 1.

    When the crayfish Procambarus is rolled with legs not upon a substratum, uropod opener muscles on the lifted side are activated in co-contraction whereas antagonistic closer muscles on the same side are all relaxed simultaneously. The closers are activated and the openers are relaxed on the lowered side.

  2. 2.

    This reciprocal pattern is also observed in the motor neurone activity: the contraction of opener muscles on the lifted side and closer muscles on the lowered side is caused by an increase in the activity of excitatory motor neurones innervating these muscles, whereas the relaxation of their antagonists on each side is caused by a decrease in the activity of excitatory motor neurones innervating them. Deafferentation by cutting all roots of the terminal ganglion has no significant effect on the steering pattern.

  3. 3.

    The decrease in the excitatory motor neurone activity during steering was found to be due to an increase in the inhibitory input to the motor neurones.

  4. 4.

    During body rolling, the statocyst receptors on the lifted side increase their activity while those on the lowered side decrease it (Takahata & Hisada, 1979). We conclude that the opener motor neurones receive excitation and inhibition respectively from the ipsilateral and the contralateral statocyst, whereas the closer motor neurones receive excitation and inhibition respectively from the contralateral and ipsilateral statocyst. From these results, the connections between the motor neurones and the identified statocyst interneurones were deduced.

  5. 5.

    The normal, bilaterally organized steering pattern of the uropod muscle activity seems to be produced by the statocysts of both sides, whose information is mediated by a bilateral set of interneurones having different connections to individual motor neurones.

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