Invertebrate motor control and inhibitory motoneurons

The miniaturisation of animal body cells is limited, and as a consequence, muscle and nerve cells are of similar diameters in large and small animals, to within an order of magnitude. Small animals thus have to control their behaviour with much fewer muscle fibres and nerve cells. Insect muscles may consist of a few tens of fibres, and they are often controlled by just 3 motoneurons; a typical mammal has thousands of fibres per muscle and hundreds of motoneurons for comparable behavioural performances. In the typically small arthropods – insects, crustaceans, spiders – inhibitory (rather than the normal excitatory) motoneurons allow a motor control strategy that can deal with such small cell numbers, and that is radically different from that of the larger vertebrates. This motor control strategy shall be analysed on the basis of the available literature and described in a comprehensive form in a review.