Processing of proprioceptive signals by ascending interneurones in the terminal abdominal ganglion of the crayfish

Intersegmental interneurones are crucial for the appropriate coordination o f the activity of local circuits located in different body segments. We hav e analysed the synaptic inputs to ascending intersegmental interneurones fr om a proprioceptor in the tailfan of the crayfish. Twenty identified intern eurones responded during stimulation of the exopodite-endopodite chordotona l organ. Of these 20 interneurones, three were excited phase-tonically, nin e were excited phasically and eight were inhibited.,All received convergent exteroceptive inputs from water-motion- or touch-sensitive hairs on the ur opods, The effects of simultaneous exteroceptive and proprioceptive stimula tion depended upon the identity of an interneurone. For interneurones that were inhibited by proprioceptive stimulation, suprathreshold exteroceptive responses were reduced to a subthreshold level by simultaneous propriocepti ve stimulation. In contrast, for interneurones that were excited by proprio ceptive stimulation, the simultaneous application of subthreshold proprioce ptive and exteroceptive stimulation elicited action potentials. Two of the interneurones that receive proprioceptive input (NE-1 and RC-8) are known to be presynaptic to giant interneurones that mediate and coordin ate the tailhip, Many of the other interneurones that receive proprioceptiv e inputs in the tailfan are known to excite abdominal extensor motor neuron es. Thus, proprioceptive input to these intersegmental interneurones could serve two roles: first, to extend the abdomen during postural movements or prior to escape and, second, to drive the tailhip escape response.