Meeting Abstract

42.6  Thursday, Jan. 5  Suppression of inappropriate reflexes during centrally commanded movements TAHIR, U*; EDWARDS, D.H; Georgia State University; Georgia State University

During centrally orchestrated movements, the nervous system must distinguish between appropriate and inappropriate reflex responses. For example, a contact-evoked startle response may be appropriate when the contact is unexpected but inappropriate when the contact is expected. In crayfish, a tail flip escape evoked by a phasic abdominal stimulus is suppressed when the animal is walking backward or escaping. In either of these contexts, abdominal contact is expected as the animal moves backward and the escape response would be inappropriate. In the present study we generalize this finding to postural reflexes of the crayfish abdomen that can also be evoked by unexpected touch. An isolated abdomen preparation was used that permitted recording and stimulating tailfan primary afferents, ventral nerve cord interneurons, and abdominal postural motor neurons. A brief (0.3 ms) electrical shock of the sensory tailfan nerve evoked an abdominal postural flexion response. The postural flexor motor nerve recording was analyzed to identify each of the six different evoked spikes with one of the six motor neurons of the motor nerve. An abdominal flexion motor program was then excited by repetitively stimulating bundles of descending interneurons. Sensory nerve stimulation evoked a smaller motor nerve response when delivered during the motor program than either before or afterwards, even though the motor neuron activity was higher during the motor program. These results indicate that postural reflex responses are inhibited during the motor program at a site presynaptic to the motor neurons. The most likely targets of inhibition are the presynaptic terminals of the primary afferents, which are known to receive primary afferent depolarization (PAD) during either an escape response or backward walking.