Interaction between descending input and thoracic reflexes for joint coordination in cockroach. II comparative studies on tethered turning and searching.

Tethered cockroaches turn from unilateral antennal contact using asymmetrical movements of mesothoracic (T2) legs (Mu and Ritzmannin J Comp Physiol A 191:1037-1054, 2005). During the turn, the leg on the inside of the turn (the inside T2 leg) has distinctly different motor patterns from those in straight walking. The transformation from walking to inside leg turning could be triggered by descending commands that alter a few critical reflexes that start a cascade of physical changes in leg movement or posture, leading to further alterations. This hypothesis has two implications: First, the descending activities must be able to influence thoracic reflexes. Second, one should be able to initiate the turning motor pattern in the absence of descending signals by mimicking a point farther down in the reflex cascade. We addressed the first implication in the companion paper. To examine the second implication, we compared kinematics and motor activities of the T2 leg during searching with that of inside leg turning. The reaching movements made during searching were found to be similar to the movements made by the inside leg during turning. Moreover, even after disconnecting the brain from the thoracic ganglia the reaching movements were similar. This observation is consistent with the second implication from the hypothesis.