RHYTHMIC PATTERNS IN THE THORACIC NERVE CORD OF THE STICK INSECT INDUCED BY PILOCARPINE

Bath application of the muscarinic agonist pilocarpine onto the deaffe rented stick insect thoracic nerve cord induced long-lasting rhythmic activity in leg motoneurones. Rhythmicity was induced at concentration s as low as 1X10(-4) mol l(-1) pilocarpine, The most stable rhythms we re reliably elicited at concentrations from 2x10(-3) mol l(-1) to 5X10 (-3) mol l(-1), Rhythmicity could be completely abolished by applicati on of atropine, The rhythm in antagonistic motoneurone pools of the th ree proximal leg joints, the subcoxal, the coxo-trochanteral (CT) and the femoro-tibial (FT), was strictly alternating, In the subcoxal moto neurones, the rhythm was characterised by the retractor burst duration being correlated with cycle period, whereas the protractor burst dura tion was almost independent of it, The cycle periods of the rhythms in the subcoxal and CT motoneurone pools were in a similar range for a g iven preparation, In contrast, the rhythm exhibited by motoneurones su pplying the FT joint often had about half the duration, The pilocarpin e-induced rhythm was generated independently in each hemiganglion, The re was no strict intersegmental coupling, although the protractor moto neurone pools of the three thoracic ganglia tended to be active in pha se. There was no stereotyped cycle-to-cycle coupling in the activities of the motoneurone pools of the subcoxal joint, the CT joint and the FT joint in an isolated mesothoracic ganglion, However, three distinct 'spontaneous, recurrent patterns' (SRPs) of motoneuronal activity wer e reliably generated, Within each pattern, there was strong coupling o f the activity of the motoneurone pools, The SRPs resembled the motor output during step-phase transitions in walking: for example, the most often generated SRP (SRP1) was exclusively exhibited coincident with a burst of the fast depressor trochanteris motoneurone. During this bu rst, there was a switch from subcoxal protractor to retractor activity after a constant latency, The activity of the FT joint extensor moton eurones was strongly decreased during SRP1, SRP1 thus qualitatively re sembled the motoneuronal activity during the transition from swing to stance of the middle legs in forward walking, Hence, we refer to SRPs as 'fictive step-phase transitions'. In intact, restrained animals, ap plication of pilocarpine also induced alternating activity in antagoni stic motoneurone pools supplying the proximal leg joints, However, the re were marked differences from the deafferented preparation, For exam ple, SRP1 was not generated in the latter situation, However, if the i psilateral main leg nerve was cut, SRP1s reliably occurred, Our result s on the rhythmicity in leg motoneurone pools of deafferented preparat ions demonstrate central coupling in the activity of the leg motoneuro nes that might be incorporated into the generation of locomotion in vi vo.