INHIBITORY SYNAPTIC DRIVE PATTERNS MOTONEURONAL ACTIVITY IN RHYTHMIC PREPARATIONS OF ISOLATED THORACIC GANGLIA IN THE STICK INSECT

During active leg movements of an insect leg, the activity of the moto neuron pools of each individual leg joint is generated by the interact ion between signals from central rhythm generating sources, peripheral signals as well as coordinating signals from other leg joints and leg s. The nature of the synaptic drive from the central rhythm generators onto the motoneuron pools of the individual leg joints during rhythmi c motor activity of the stick insect (Carausius morosus) middle leg ha s been investigated. in the isolated mesothoracic ganglion central rhy thm generators were activated pharmacologically by topical application of the muscarinic agonist pilocarpine. Motoneurons supplying the femu r-tibia (FT) joint were investigated in detail. Recordings from neurop il processes of these motoneurons revealed that patterning of their rh ythmic activity is based on cyclic hyperpolarizing synaptic inputs. Th ese inputs are in clear antiphase for extensor and flexor motoneurons. DCC (discontinuous current clamp) and dSEVC (discontinuous single ele ctrode voltage clamp) recordings showed reversal potentials of the inh ibitory inputs between -80 to -85 mV (FETi, N = 7; Flex MN, N = 3). Af ter intracellular injection of TEA rhythmic inhibition in FETi was dec reased by about 84% (N=4). Both findings indicate that the cyclic inhi bition is mediated by potassium ions. Thus, it appears that central rh ythm generators pattern motor activity in antagonistic tibial motoneur on pools by cyclic alternating inhibition. (C) 1998 Elsevier Science B .V.