Contrasting tactics in motor control by vertebrates and arthropods.

Vertebrates and arthropods share the common problem of controlling a rigid, articulated skeleton using neurally-controlled, striated muscle. Since this condition has arisen independently in the two groups, there is no reason to assume, a priori, that the control mechanisms used by the two groups will be the same. Indeed, there appear to be fundamental differences in the tactics used by the two groups. Insects and crustaceans use small numbers of heterogeneous motoneurons, while vertebrates (mammals especially) use many, more homogeneous, motor axons. In particular, arthropods make extensive use of peripheral neuromodulation to alter the properties of both neuromuscular junctions and muscle fibers. There has been little consideration of the functional consequences of these differences. I suggest that, faced with a size constraint on the number of motor units available, arthropods use peripheral modulation of muscle properties to achieve the flexibility and dynamic range that vertebrates achieve through recruitment of motor units.