MONOSYNAPTIC INTERJOINT REFLEXES AND THEIR CENTRAL MODULATION DURING FICTIVE LOCOMOTION IN CRAYFISH

An in vitro preparation of the crayfish nervous system has been utilized to study an interjoint reflex pathway and its variability during rhythmic locomotor activity. The coxo-basal chordotonal organ (CBCO) is a joint stretch receptor spanning the second joint of walking legs in crayfish, where it encodes joint movements and position. Mechanical stimulation (stretch and release) of the CBCO and electrical stimulation of the CBCO nerve elicits reflex responses in promotor and remotor motor neurons innervating muscles moving the basal thoraco-coxal (TC) leg joint. Promotor and remotor motor neurons receive monosynaptic excitatory inputs from at least four CBCO afferents, including both stretch- and release-sensitive CBCO afferents. In a tonic preparation, in which there is no tendency to produce alternating bursts of activity in antagonistic motor neurons, the reflex responses were evoked during each cycle of imposed movement. However, when the preparation became rhythmic and produced bouts of fictive locomotion, the reflex responses were unstable and their gain was phasically modulated. Paired recordings indicate that such a modulation of the monosynaptic interjoint reflex could be due to both a phasic change in the excitability of the motor neurons and presynaptic inhibition that reduces the excitatory input from CBCO primary afferents.