State-dependent long lasting modulation

The basis of locomotion is the rhythmic activity of locomotor organs. During walking, rhythmic alternating bursts occur in leg motoneurons which result from the integration of signals from central pattern generators, sense organs and coordinating signals from neighboring segments. The rhythmic bursting pattern is shaped by rhythmic excitatory and inhibitory drive, as well as a long lasting (tonic) depolarizing modulation. This dissertation investigates the mechanisms that underlie the tonic modulation of neuronal activity, which is the basis of the rhythmic activity. A single-legged preparation of the stick insect was used, that allows the analysis of neuronal activity in mesothoracic motoneurons during front leg stepping, without the influence of local sense organs. Intracellular recordings of the membrane potential in flexor motoneurons ipsilateral and contralateral to the stepping front leg revealed a mean tonic membrane depolarization of 1.8 ±1.1 mV that could outlast the stepping sequence by several seconds. Furthermore, a phasic modulation of membrane potential occurred on top of the tonic depolarization, which was variably coupled to front leg steps. The tonic depolarization was associated with a decrease in input resistance, its amplitude depended on membrane potential and its mean reversal potential was found to be −41 mV. These properties of the tonic depolarization indicate that it is based on a nonselective cation conductance or a mixed inward and outward current through different channels. Furthermore, the tonic depolarization increased the excitability of the membrane to depolarizing input and was found to have long repolarization time constants (τ̄=800 ms).