ACTION OF LOCUST NEUROMODULATORY NEURONS IS COUPLED TO SPECIFIC MOTORPATTERNS

1. Many muscles of the locust are supplied by dorsal unpaired median n eurons (DUM neurons) that release octopamine and alter the contraction s caused by spikes in motor neurons. To determine when these neuromodu latory neurons are normally activated during behaviour, intracellular recordings were made simultaneously from them and from identified moto r neurons during the specific motor pattern that underlies kicking. A kick consists of a rapid and powerful extension of the tibia of one or both hind legs that is produced by a defined motor pattern. Only 3 id entified DUM neurons of the 20 in the metathoracic ganglion spike duri ng a kick, and they supply muscles involved in generating the kick. Th eir spikes occur in a distinctive and repeatable pattern that is close ly linked to the pattern of spikes in the flexor and extensor tibiae m otor neurons. When the extensor and flexor muscles cocontract, these t hree DUM neurons produce a burst of spikes at frequencies that can ris e to 25 Hz, and with the number of spikes (3-15) related to the durati on of this phase of the motor pattern. The spikes stop when the flexor muscle is inhibited and therefore before the tibia is extended rapidl y. The other DUM neurons which supply muscles that are not directly in volved in kicking are either inhibited or spike only sporadically.2. T he activation of a specific subset of DUM neurons during kicking may t hus be timed to influence the action of the muscles that participate i n this movement and appear to be controlled by the same circuits that determine the actions of the participating motor neurons. These modula tory neurons thus have specific individual actions in the control of m ovement.