NEURAL MECHANISMS OF REFLEX REVERSAL IN COXO-BASIPODITE DEPRESSOR MOTOR-NEURONS OF THE CRAYFISH

The in vitro preparation of the fifth thoracic ganglion of the crayfis h was used to investigate the mechanisms underlying the reflex reversa l in a sensory-motor pathway. Sensory afferent neurons from the coxo-b asipodite chordotonal organ (CBCO), which 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 c ontrast, during effective locomotion, the reflexes are reversed and te rmed assistance reflexes. Intracellular recordings from all 12 Dep MNs were performed in single experiments. It allowed us to characterize t hr ee types of Dep MNs according to their response to CBCO imposed ste p-and-ramp movements: 8 of the 12 Dep MNs are resistance MNs that are depolarized during release of the CBCO and are connected monosynaptica lly to release-sensitive CBCO neurons; 1 Dep MN is an assistance MN th at is depolarized during stretching of the CBCO and is connected monos ynaptically to exclusively velocity-coding stretch-sensitive CBCO neur ons; in our experimental conditions, 3 Dep MNs do not display any resp onse to CBCO stimulation. Assistance reflex interneurons (ARINs), invo lved in polysynaptic assistance reflexes recorded from depressor MNs, are presented. During low-velocity (0.05 mm/s) stretching ramps impose d on the CBCO, ARINs display compound excitatory postsynaptic potentia ls (EPSPs), whereas during high-velocity (0.25 mm/s) ramps, they displ ay a mixed excitatory and inhibitory response. Whereas a single MN gen erally receives monosynaptic EPSPs from three to six CBCO neurons, ARI Ns receive monosynaptic EPSPs from up to eight velocity-coding stretch -sensitive CBCO neurons. Tn addition, ARINs receive disynaptic inhibit ory phasic inputs from stretch-sensitive CBCO afferents. Injection of a depolarizing current pulse into ARINs elicits a fast transient volta ge-dependent depolarization. Its time to peak decreases, and its peak amplitude increases with increasing current intensity. ARINs likely ar e to be connected directly to Dep MNs. The synaptic delay between thes e nonspiking ARINs and Dep MNs is short (<2 ms) and constant. The post synaptic EPSP amplitude increases with increasing current pulse intens ity injected into ARIN. The dual sensory control (excitatory and inhib itory) makes it likely that ARIN represents a key element in reflex re versal control.