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Interjoint coordination in multi-jointed limbs is essential for the generation of functional locomotor patterns. Here we have focused on the role that sensory signals from the coxa-trochanteral (CT) joint play in patterning motoneuronal activity of the femur-tibia (FT) joint in the stick insect middle leg. This question is of interest because when the locomotor system is active, movement signals from the FT joint are known to contribute to patterning of activity of the central rhythm-generating networks governing the CT joint. We investigated the influence of femoral levation and depression on the activity of tibial motoneurons. When the locomotor system was active, levation of the femur often induced a decrease or inactivation of tibial extensor activity while flexor motoneurons were activated. Depression of the femur had no systematic influence on tibial motoneurons. Ablation experiments revealed that this interjoint influence was not mediated by signals from movement and/or position sensitive receptors at the CT joint, i.e., trochanteral hairplate, rhombal hairplate, or internal levator receptor organ. Instead the influence was initiated by sensory signals from a field of campaniform sensillae, situated on the proximal femur (fCS). Selective stimulation of these fCS produced barrages of inhibitory postsynaptic potentials (IPSPs) in tibial extensor motoneurons and activated tibial flexor motoneurons. During pharmacologically activated rhythmic activity of the otherwise isolated mesothoracic ganglion (pilocarpine, 5 x 10(-4) M), deafferented except for the CT joint, levation of the femur as well had an inhibitory influence on tibial extensor motoneurons. However, the influence of femoral levation on the rhythm generated was rather labile and only sometimes a reset of the rhythm was induced. In none of the preparations could entrainment of rhythmicity by femoral movement be achieved, suggesting that sensory signals from the CT joint only weakly affect central rhythm-generating networks of the FT joint. Finally, we analyzed the role of sensory signals from the fCS during walking by recording motoneuronal activity in the single middle leg preparation with fCS intact and after their removal. These experiments showed that fCS activity plays an important role in generating tibial motoneuron activity during the stance phase of walking.

Bath application of the muscarinic agonist pilocarpine onto the deafferented stick insect thoracic nerve cord induced long-lasting rhythmic activity in leg motoneurones. Rhythmicity was induced at concentrations as low as 1x10(-4) mol l-1 pilocarpine. The most stable rhythms were reliably elicited at concentrations from 2x10(-3) mol l-1 to 5x10(-3) mol l-1. Rhythmicity could be completely abolished by application of atropine. The rhythm in antagonistic motoneurone pools of the three proximal leg joints, the subcoxal, the coxo-trochanteral (CT) and the femoro-tibial (FT), was strictly alternating. In the subcoxal motoneurones, the rhythm was characterised by the retractor burst duration being correlated with cycle period, whereas the protractor burst duration 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 given preparation. In contrast, the rhythm exhibited by motoneurones supplying the FT joint often had about half the duration. The pilocarpine-induced rhythm was generated independently in each hemiganglion. There was no strict intersegmental coupling, although the protractor motoneurone pools of the three thoracic ganglia tended to be active in phase. 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 were reliably generated. Within each pattern, there was strong coupling of 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 burst, there was a switch from subcoxal protractor to retractor activity after a constant latency. The activity of the FT joint extensor motoneurones was strongly decreased during SRP1. SRP1 thus qualitatively resembled 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, application of pilocarpine also induced alternating activity in antagonistic motoneurone pools supplying the proximal leg joints. However, there were marked differences from the deafferented preparation. For example, SRP1 was not generated in the latter situation. However, if the ipsilateral main leg nerve was cut, SRP1s reliably occurred. Our results on the rhythmicity in leg motoneurone pools of deafferented preparations demonstrate central coupling in the activity of the leg motoneurones that might be incorporated into the generation of locomotion in vivo.