Patterns of synaptic input to identified flight motoneurons in the locust

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1. We examined the pattern of synaptic input to identified flight motoneurons of the locust,Locusta migratoria. The preparation we used was one in which rhythmic motor activity resembling flight activity could be generated following removal of all sensory input from wing receptors. Intracellular recordings were made from the main neuropil processes of all the 17 different types of fast motoneurons in the meso- and metathoracic ganglia which innervate the main flight muscles. Motoneurons were identified anatomically by intracellular staining with Lucifer yellow following recording. 2. The pattern of synaptic input was similar in all elevator motoneurons (4 different types in each of the two ganglia). Three distinct components were observed in the profile of synaptic activity in these motoneurons. The first was a relatively slow depolarization beginning immediately after the end of a depolarization in depressor motoneurons, the second was a rapid depolarization which began at a variable interval after the beginning of the initial slow depolarization, and the third was a rapid repolarization occurring in-phase with depressor depolarizations. The similarity of synaptic input to elevator motoneurons prevented the use of physiological criteria for the identification of individual elevator motoneurons. 3. Unlike the elevators, the depressor motoneurons do not form a homogeneous group with regard to the pattern of centrally derived synaptic input. Different patterns of input were observed in different depressor motoneurons, including differences in input to homologous motoneurons in the meso- and metathoracic ganglia. The most obvious difference between the profiles of synaptic input to elevator and depressor motoneurons was that the depolarizations in the latter consisted of only a single component, the duration of which increased relatively slowly with increases in cycle time. Knowledge of the recording site within a ganglion and the pattern of synaptic input allowed depressor motoneurons to be identified individually. 4. Simultaneous recordings from an elevator and a depressor motoneuron within the same ganglion showed that the synaptic input to the two groups of motoneurons is not symmetrically timed. Each depolarization in depressor motoneurons began immediately following the end of a rapid depolarization in elevators, but the beginning of the rapid depolarizations in elevators was delayed following the end of each depressor depolarization by an amount depending on cycle time. A single depressor depolarization never occurred in the absence of a preceding rapid elevator depolarization. We conclude that the basic unit of activity in the central oscillator is an elevator followed by a depressor depolarization sequence. 5. Simultaneous recordings from homologous motoneurons in the meso- and metathoracic ganglia of deafferented preparations revealed that the depolarizations in elevator motoneurons in the two ganglia were in-phase. In depressors, however, the depolarizations in hindwing motoneurons led those in homologous forewing motoneurons by 5 to 15 ms indicating that central processes are partly, but not completely, responsible for establishing the relative timing of fore and hind wing movements in normal flight. This basic difference in the intersegmental driving of elevator and depressor motoneurons in deafferented preparations was confirmed in electromyographic recordings from homologous fore- and hindwing muscles.