THE ANTENNAL MOTOR SYSTEM OF CRICKETS - MODULATION OF MUSCLE CONTRACTIONS BY A COMMON INHIBITOR, DUM NEURONS, AND PROCTOLIN

In the crickets Gryllus bimaculatus and Gryllus campestris, the two in trinsic antennal muscles in the scape (first antennal segment) control antennal movements in the horizontal plane. Of the 17 excitatory ante nnal motoneurons, three motoneurons, two fast and one slow, can be sti mulated selectively and their effect on muscle contraction, i.e. anten nal movement, measured. Simultaneously, either a common inhibitor (CI) neuron or two DUM neurons can be stimulated and the effect on the slo w and/or fast muscle contraction measured. The activity of the common inhibitor affected only slow muscle contractions. It decreased contrac tion rate, increased relaxation rate and suppressed prolonged muscle t ension. This effect was blocked by picrotoxin. DUM neuron stimulation affected both slow and fast contractions. It reduced slow and enhanced fast contractions but in only 10% of the experiments could this effec t be detected. DUM neuron activity could be mimicked by octopamine app lication. Proctolin application enhanced both slow and fast contractio ns but did not increase muscle tension in the absence of motoneuron ac tivity. The results are discussed in relation to the large variability of possible antennal movements during behaviors.