Guest Talk: Joachim Schmidt

10 December 2015
Begin time: 
Hauptgebäude, W0-135

Leg motor neurons in stick insects: input, output and modulation

During walking, leg motor neurons generate bursts of action potentials that control the activity of the innervated muscles. In my talk, I will introduce mechanisms that contribute to the generation of a motor neuron’s activity pattern. Throughout walking, motor neurons receive tonic excitatory drive, phasic excitatory drive and phasic inhibitory input.

Pharmacological experiments in situ and in vitro indicate that acetylcholine is involved in the generation of the tonic depolarization and a persistent inward current, respectively. It can further be inferred that the phasic inhibitory input is ultimately generated by a pattern-generating neural network that can be stimulated pharmacologically to generate rhythmic activity independent from sensory input. Input from sense organs appears to contribute to
phasic excitatory drive.

The neuromodulator octopamine is said to be a general promoter of motor activity in insects. In line with common knowledge, application of octopamine to thoracic ganglia during walking increases the tonic depolarization in motor neurons. However, in vitro octopamine decreases the persistent inward current in motor neurons evoked by acetylcholine. In search of octopaminergic neurons that modulate motor neuron activity in vivo we identified a group of
five to six descending neurons in the suboesophageal ganglion that contain octopamine as shown by MALDI-TOF mass spectrometry. Spike activity in some of these neurons decreases leg motor neuron activity. But spike activity in others increases leg motor neuron activity.

Thus, these findings match the results of our pharmacological experiments that revealed opposing effects of octopamine on motor neuron activity.