Descending influences on escape behavior and motor pattern in the cockroach

The escape behavior of the cockroach is a ballistic behavior with well char acterized kinematics. The circuitry known to control the behavior lies in t he thoracic ganglia, abdominal ganglia, and abdominal nerve cord. Some evid ence suggests inputs may occur from the brain or suboesophageal ganglion. W e tested this notion by decapitating cockroaches, removing all descending i nputs, and evoking escape responses. The decapitated cockroaches exhibited directionally appropriate escape turns. However, there was a front-to-back gradient of change: the front legs moved little if at all, the middle legs moved in the proper direction but with reduced excursion, and the rear legs moved normally. The same pattern was seen when only inputs from the brain were removed, the suboesophageal ganglion remaining intact and connected to the thoracic ganglia. Electromyogram (EMG) analysis showed that the loss o f or reduction in excursion was accompanied by a loss of or reduction in fa st motor neuron activity. The loss of fast motor neuron activity was also o bserved in a reduced preparation in which descending neural signals were re versibly blocked via an isotonic sucrose solution superfusing the neck conn ectives, indicating that the changes seen were not due to trauma. Our data demonstrate that while the thoracic circuitry is sufficient to produce dire ctional escape, lesion or blockage of the connective affects the excitabili ty of components of the escape circuitry. Because of the rapidity of the es cape response, such effects are likely due to the elimination of tonic desc ending inputs. (C) 2001 John Wiley & Sons, Inc.