For animals making their way through complex environments, walking is not only a question of moving their legs in rhythmic patterns, but they also may need to avoid, obstacles or to reach across gaps. Many insects are exceptionally good at maneuvering through complicated environments such as the forest canopy, where maintaining coordination is essential since the penalties for poor performance can be high: failing to find mates, food or, worse still,becoming someone else's food! Although much is known about how insects control the movements of a single limb, how they maintain coordination between limbs whilst negotiating obstacles or gaps is less clear. To do this, insects must take sensory information obtained by rhythmic movements of the antennae or forelimbs as well as by the visual system and use this information to plan leg movements during walking, which is difficult to investigate experimentally.
Angela Ridgel and colleagues at Case Western Reserve University in Cleveland used brain lesions in cockroaches, Blaberus discoidalis, to investigate the role of one enigmatic brain region that has been implicated in controlling limb coordination during obstacle avoidance and turning: the central body complex (CBC). Following 24 h recovery after surgery, the team assessed the cockroaches' behavior in two setups designed to make them turn. Using a U-shaped arena, they could assess more general turning and obstacle-avoidance behavior, while using a tethered setup in which a wall was moved to simulate an obstacle allowed them to examine the detailed movements of the legs.
Lesions made with a razor blade, which severed links between the left and right sides of the CBC or connections to and from one side of the brain and the CBC, had severe effects on the turning behavior of the cockroaches in the U-shaped arena. Over half the turns made by these animals were abnormal,including turning in circles, turning towards stimulation of one antenna but away from stimulation of the other or simply running into the wall. The team also made foil lesions at several regions within or closely associated with the CBC and likely to be involved in walking. In many cases, cockroaches with these lesions failed to turn and ran straight into the wall. This showed that lesions made with a razor blade mainly disrupt left-right coordination - the cockroaches still turn but do so incorrectly - whereas foil lesions prevent the cockroaches turning at all. More detailed analysis in tethered cockroaches showed that many of the lesions made with foil or razor blades prevented cockroaches from producing the appropriate leg movements to turn and avoid an obstacle.
As the authors point out, this study represents only the beginning of studies aimed at finding out how the CBC and its closely associated brain regions contribute to limb coordination during complex behaviors. Their results already emphasize the importance of communication between the two sides of the CBC for turning in the right direction and avoiding obstacles and of communication between the CBC and other parts of the brain in allowing the cockroaches to turn at all. The next step for Ridgel and colleagues may be to delve further into the CBC's circuits in the brain, where they may find answers to many of the questions the present study raises such as how the CBC integrates inputs from the left and right antennae to adjust the motor programs that control walking and initiate turning. These experiments are likely to prove challenging but have great potential for providing new insights into insect motor control.
