FUNCTIONAL ANALYSIS OF THE SENSORY MOTOR PATHWAY OF RESISTANCE REFLEX IN CRAYFISH. I- MULTI-SENSORY CODING AND MOTOR NEURON MONOSYNAPTIC RESPONSES. Didier LE RAY, François CLARAC, and Daniel CATTAERT. Laboratoire de Neurobiologie et Mouvements, CNRS, 31 chemin Joseph Aiguier, Marseille, France..
APStracts 4:161N, 1997.
ABSTRACT
An in vitro preparation of the fifth thoracic ganglion of the crayfish was used to study in detail the negative feedback loop involved in the control of passive movements of the leg. Release-sensitive primary afferents of from the coxo-basipodite chordotonal organ (CBCO), a proprioceptor whose strand is released by upward movement of the leg, monosynaptically connect to depressor motor neurons (Dep MNs). Extracellular identification of sensory units from the CBCO neurogram allowed us to determine the global coding of a sine wave movement, imposed from the most released position of the CBCO strand. Intracellular recordings from sensory terminals (CBTs) and ramp movement stimulations applied to the CBCO strand allowed us to characterize two groups of release-sensitive CBCO fibers. The first group, divided into two subgroups (phasic and phaso-tonic), is characterized by discontinuous firing patterns: phasic CBTs fired exclusively during release movements; phaso- tonic CBTs displayed both a phasic firing and a tonic discharge during the more released plateaus. The second group was continuously firing whatever the movement, with higher frequencies during the release phase of the movement stimulation. All CBTs displayed a marked sensitivity for release movements while only the phaso-tonic ones showed a clear sensitivity to maintained positions. Surprisingly, no pure tonic sensory fibers were encountered. Systematic intracellular recordings from all resistant Dep MNs, performed in high divalent cation saline, allowed us to describe two shapes of monosynaptic resistance reflex responses. A phasic response was characterized by bursts of EPSPs occurring exclusively during CBCO strand release movements. A phaso- tonic response was characterized by a progressive depolarization occurring all along the release phase of the stimulation: during maintained released positions, the amplitude of the sustained depolarization was position- dependent; in addition, each release movement produced a phasic burst of EPSPs in the MN. The parallel study of the Dep MN properties failed to point out any correlation between the type of reflex response recorded from the MN and the MN intrinsic properties, which would indicate that the type of MN response is entirely determined by the afferent messages it receives. 

Received 14 April 1997; accepted in final form 22 July 1997.
APS Manuscript Number J300-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 28 August 1997