Mechanisms of motor control by plurisegmental interneurons in locusts

Summary

• 1. A glass microelectrode was used to stimulate plurisegmental fibres in the locust ventral nerve cord. Simultaneous intracellular recording from neck muscle motoneurons (to muscles 54 and 56) and some leg motoneurons in the prothoracic ganglion allowed testing of the connections of these fibres to the motoneurons.

• 2. Single pulse stimulation at many positions in the connective evoked both an epsp followed by an ipsp in the same motoneuron. In most cases the epsp was produced via monosynaptic connections (latency and high frequency stimulation tests) but sometimes a spiking or nonspiking interneuron was interposed. In one case a fast ipsp was followed by an epsp (Figs. 7–9). The effects attributable to each stimulating pulse were termed direct effects.

• 3. When the stimulation was applied as short trains (30–100 Hz for 200 ms) the direct epsp's added to produce excitation during the train; the ipsp's added to produce inhibition for up to 1 s after the train (Figs. 7, 8, 10 and 11). These direct effects may influence the timing of motor outputs.

• 4. Certain stimulating positions evoked not only direct effects but also quite different long lasting motor aftereffects; the responsible fibres were classified separately as ‘activating’ interneurons. They could be multimodal but, like the other plurisegmental fibres studied here, were predominantly mechanosensory and some responded to movement (Figs. 2–6).

• 5. Unlike the direct effects, the long lasting motor aftereffects could not be related to each stimulus pulse. They were often larger in amplitude than the direct effects and consisted of barrages of mixed excitation and inhibition, lasting several seconds after a 200 ms stimulus and dramatically altering the motor excitation state of the whole body. The effects varied in form with the stimulating site, ranging from regular to quite unpatterned (Figs. 4, 13, 14 and 17).

• 6. Intracellular recordings and experiments where the ganglion was isolated showed that the long lasting aftereffects were generated locally. The ‘activating’ interneurons initiate long lasting activity in locally active interneurons which in turn drive the motoneurons and produce the major part of the motor effects (Figs. 15 and 16).

• 7. While the direct effect psp's added with the epsp's from a monosynaptic sensory-motor pathway, the sensory epsps's were enhanced or diminished during the aftereffects. Thus, sensory inflow to motoneurons can be gated by local interneurons under the control of ‘activating’ fibres (Figs. 12 and 18).

• 8. ‘Activating’ fibres may play a significant role in selecting thoracic motor programmes and setting motor excitation using local interneurons as an interface. Their direct effects and those of the other plurisegmental interneurons described here may contribute to timing of motor outputs.