Pattern generation for walking and searching movements of a stick insect leg. II. Control of motoneuronal activity

In the stick insect, Cuniculina impigra, intracellular recordings from meso thoracic motoneurons that control flexion and extension of the tibia and de pression and levation of the trochantero-femur were made while the leg perf ormed walking-like movements on a treadband or stereotyped rhythmic searchi ng movements. We were interested in how synaptic input and intrinsic proper ties contribute to form the activity pattern of motoneurons during rhythmic leg movements without sensory feedback from other legs. During searching a nd walking, motoneurons expressed a rhythmic bursting pattern that was form ed by a depolarizing input followed by a hyperpolarizing input in the inter -burst interval. This basic pattern was similar in all fast, semi-fast, and slow motoneurons that were recorded. Hyperpolarizations were in synchrony with activity in the antagonistic motoneurons. De- and hyperpolarizations w ere associated with a decrease in input resistance. All motoneurons showed spike frequency adaptation when depolarized by current injection to a membr ane potential similar to that observed during walking. In the hyperpolarizi ng phase of fast flexor motoneurons, the initial maximum hyperpolarization was followed by a sag in potential toward more depolarized values. Consiste nt with this observation, only fast flexor motoneurons developed a depolari zing sag potential when hyperpolarized by injection of constant negative cu rrent.