The motor program and role of sensory feedback in claw extension of the crayfish

The motor program and role of the meropodite sensory organs (the chordotonal organs MC-1, MC-2 and the myochordotonal organ MCO) in extension of the merocarpodite (M-C) joint of the claw was examined during an antennal-evoked defense response. Recordings from the meropodite extensor muscle indicated that the tonic extensor motoneuron was primarily responsible for M-C joint extension. The phasic extensor motoneuron was co-activated with the tonic motoneuron in only 48% of the responses. Tonic motoneuron discharge was elevated in those responses in which the phasic motoneuron was active. Inactivation of individual meropodite sensory organs did not alter the tonic motoneuron response. Simultaneous inactivation of all 3 organs reduced tonic motoneuron discharge by 47%. Phasic motoneuron discharge was reduced following inactivation of the individual MC-1, MC-2 and MCO organs as well as in the sham operated group. Following inactivation of all 3 organs phasic motoneuron activity ceased. These results suggest that feedback from the M-C sensory organs during an active extension response is positive and redundant.