Morphological and physiological bases of crayfish local circuit neurones.

As the subject of neuroethological studies, arthropods offer several advantages to elucidate the neural processes that generate and control behavioural acts. The relative simplicity of their neural organization and their identifiable neurones are particularly advantageous. The crayfish, Procambarus clarkii, uropods are paired appendages of the last abdominal segment that show bilateral closing movement in response to mechanical stimulation. This movement, the avoidance "dart" response, is mediated by a cascade and parallel organization of local circuit neurones in the terminal abdominal ganglion. Thirty intersegmental ascending interneurones and 20 spiking local interneurones have been identified both morphologically and physiologically. They receive exteroceptive inputs directly from the sensory afferents. Two complementary groups of unilateral nonspiking local interneurones have also been functionally identified. They exert fine control over the wide range of activity of motor neurones in a graded and sustained manner. These nonspiking interneurones form opposing and parallel pathways that are essential in modulating the pattern of movement of the uropods.