Motoneuronal control of antennal muscles in Locusta migratoria

the antennal muscles and antennal motoneurones of Locusta migratoria were investigated with neuroanatomical and electrophysiological techniques. The motor nerves of each scape muscle contain five motoneurones. The antennal levator and depressor muscles within the head capsule are innervated by four and six motoneurones respectively including one common motoneurone. The central projections of the antennal motoneurones are described and possible connections to identified neurones are discussed. Intracellular recordings from the scape muscles revealed that the fibres are innervated by various combinations of “slow”, “intermediate”, “fast” and “inhibitory” motoneurones, defined by the occurrence and amplitude of the recorded junction potentials. “Slow”-innervation predominated in the fibres of the median scape muscle, whereas most fibres of the lateral scape muscle were found to be innervated by a “fast”-motoneurone. Scape muscle motoneurones were electrically stimulated and the resulting muscle tension was measured. In agreement with the histological findings, electrical stimulation with varying stimulus amplitude resulted in a maximum of five steps of contraction strength. Stronger tetanic contractures of both scape muscles were regularly followed by post-contractions which slowly decreased in amplitude with repeated stimulation. Similar post-contractions have recently been described in nerve-muscle systems of arthropods in connection with peptidergic cotransmitters, in particular the pentapeptide proctolin.