Function of Peripheral Inhibitory Axons in Insects

There are now many examples in insects of axons which elicithyperpolarizing junctional potentials in the muscle fibers theyinnervate. With the muscles bathed in haemolymph, electricalstimulation of these axons causes a decrease in the magnitudeof slow contractions. This property allows them to be definedas inhibitory. Although inhibitory axons have the ability toregulate the magnitude of maintained slow contractions, thereis little evidence that this is their normal function. The inhibitoryaxons supplying at least three insect muscles function to increasethe rate of relaxations following each contraction of a rhythmicsequence. Moreover, when the haemolymph potassium concentrationis high, some inhibitory axons probably ensure complete relaxationbetween rhythmic contractions by preventing potassium contractionsin tonic muscle fibers. There is no convincing evidence thatinhibitory axons can facilitate muscular contractions by becomingactive immediately before the excitatory input.