Cooperative mechanisms between leg joints of Carausius morosus II. Motor neuron activity and influence of conditional bursting interneuron.

The activity of the motor neuron pools of the protractor coxae muscle and of the thoracic part of the depressor trochanteris muscle during forward walking in the stick insect was investigated, and a spiking local interneuron, able to produce "endogenous bursting" and innervating both motor neuron pools, was identified. Extracellular recordings of the motor neurons innervating the protractor and the thoracic depressor of front, middle, and rear legs, respectively, were made with oil-hook electrodes from the peripheral nerves nl2c and nl4a while the animals were walking on a styrofoam treadwheel. The corresponding leg movements were registered and phase histograms were created with the software Spike2. Intracellular recordings were made in the neuropile of the metathoracic ganglion with glass electrodes filled with the dye Lucifer yellow. In all three legs measured (front, middle, and rear), both motor neuron pools increased their activity during the swing movement. The increase in the activity of the protractor motor neurons started at the end of the stance approximately 100 ms before reaching the posterior extreme position (PEP), and the activity of the large-sized depressor motor neurons increased as soon as the tarsus was lifted at the PEP. A local spiking interneuron was identified that excited both motor neuron pools. In 4 of 23 recordings the interneuron started to burst in synchrony with protractor and thoracic depressor motor neurons. During bursting a depolarizing stimulus reinforced and a hyperpolarizing stimulus inhibited the activity of both motor neuron pools. Thus we conclude that the thoracic part of the depressor trochanteris muscle might be a component of the neuromuscular system that shapes the swing movement. The two proximal joints, subcoxal and coxa-trochanter, connected mechanically via the thoracic part of the depressor trochanteris muscle, are also connected neurally by segmental and intersegmental spiking interneurons (this paper) and by nonspiking local interneurons (see companion paper).