Physiology and Structure of Three New Uropod Proprioceptors in the Crayfish Procambarus Clarkii

Abstract

Three new chordotonal organs are described: APCO for the abdominal-protopodite joint, PExCO for the protopodite-exopodite joint and PEnCO for the protopodite-endopodite joint of the crayfish uropod. The PEnCO is a distinct strand chordotonal organ, while the APCO and PExCO are webbed structures arising from peripheral nerve roots 2 and 3 of the terminal abdominal ganglion, in which some of their somata are located (‘root cells’). Physiological experiments showed that the organs monitor position and movement at all three joints. Position monitoring is limited to open angles of the exopodite and extended angles of the protopodite (no limits on endopodite position sensitivity). Distinct hysteresis is shown by most position-sensitive units. The abdomen-protopodite joint is monitored through four orders of velocity magnitude, while the exopodite and endopodite, which are involved in tailfan flare, are primarily monitored at high movement velocities. Large, phasic units in all three chordotonal organs (COs) can monitor continuous, high-velocity movement with little adaptation. These results are discussed in relation to known motor control mechanisms for crayfish uropods during swimming, walking, escape and righting behaviours.