NEURAL MECHANISMS OF REFLEX REVERSAL IN COXO-BASIPODITE DEPRESSOR MOTOR NEURONS OF THE CRAYFISH. Didier LE RAY, and Daniel CATTAERT. Laboratoire de Neurobiologie et Mouvements, CNRS, 31 chemin Joseph Aiguier, Marseille, France.
APStracts 4:0004N, 1997.
ABSTRACT
The in vitro preparation of the 5th thoracic ganglion of the crayfish was used to investigate the mechanisms underlying the reflex reversal in a sensory- motor pathway. Sensory afferent neurons from the coxo-basipodite chordotonal organ (CBCO), that senses vertical movements of the limb, connect monosynaptically with basal limb motor neurons (MNs). In tonically active preparation, stretching the CBCO (corresponding to downward movements of the leg) stimulates the levator MNs, whereas releasing the CBCO activates the depressor (Dep) MNs. These reflexes, opposed to the imposed movement, are termed resistance reflexes. By contrast during fictive locomotion, the reflexes are reversed and termed assistance reflexes. Intracellular recordings from all twelve Dep MNs were performed in single experiments. It allowed us to characterize three types of Dep MNs according to their response to CBCO imposed step-and-ramp movements : 1) 8 of the 12 Dep MNs are resistance MNs that are depolarized during release of the CBCO and are monosynaptically connected to release sensitive CBCO neurons; 2) one Dep MN is an assistance MN that is depolarized during stretching of the CBCO and is monosynaptically connected to exclusively velocity-coding stretch sensitive CBCO neurons; 3) in our experimental conditions, three Dep MNs do not display any response to CBCO stimulation. Assistance reflex interneurons (ARINs), involved in polysynaptic assistance reflexes recorded from depressor MNs, are presented. During low velocity (0.05 mm.s-1) stretching ramps imposed on the CBCO, ARINs display compound EPSPs, while during high velocity (0.25 mm.s-1) ramps, they display a mixed excitatory and inhibitory response. Whereas a single MN generally receives monosynaptic EPSPs from 3 to 6 CBCO neurons, ARINs receive monosynaptic EPSPs from up to eight velocity-coding stretch sensitive CBCO neurons. In addition, ARINs receive disynaptic inhibitory phasic inputs from stretch sensitive CBCO afferents. Injection of a depolarizing current pulse into ARINs elicits a fast transient voltage-dependent depolarization. Its time to peak decreases and its peak amplitude increases with increasing current intensity. ARINs are likely to be directly connected to Dep MNs. The synaptic delay between these nonspiking ARINs and Dep MNs is short (< 2 ms) and constant. The post-synaptic EPSP amplitude increases with increasing current pulse intensity injected into ARIN. The dual sensory control (excitatory and inhibitory) makes it likely that ARIN represents a key element in reflex reversal control. 

Received 18 December 1996; accepted in final form 19 December 1996.
APS Manuscript Number J515-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 21 January 1997