CRAWLING MOTOR PATTERNS INDUCED BY PILOCARPINE IN ISOLATED LARVAL NERVE CORDS OF MANDUCA-SEXTA

1. Larval crawling is a bilaterally symmetrical behavior that involves an anterior moving wave of motor activity in the body wall muscles in conjunction with sequential movements of the abdominal prolegs and th oracic legs. The purpose of this study was to determine whether the la rval CNS by itself and without phasic sensory feedback was capable of producing patterned activity associated with crawling. To establish th e extent of similarity between the output of the isolated nerve cord a nd crawling, the motor activity produced in isolated larval nerve cord s was compared with the motor activity from freely crawling larvae. 2. When exposed to the muscarinic receptor agonist pilocarpine (1.0 mM), isolated larval nerve cords produced long-lasting rhythmic activity i n the motor neurons that supply the thoracic leg, abdominal body wall, and abdominal proleg muscles. The rhythmic activity evoked by pilocar pine was abolished reversibly and completely by bath application of th e muscarinic-receptor antagonist atropine (0.01 mM) in conjunction wit h pilocarpine (1.0 mM), suggesting that the response was mediated by m uscarinic-like acetylcholine receptors. 3. Similar to crawling in inta ct animals, the evoked activity in isolated nerve cords involved bilat erally symmetrical motor activity that progressed from the most poster ior abdominal segment to the most anterior thoracic segment. The rhyth mic activity in thoracic leg, abdominal proleg, and abdominal body wal l motor neurons showed intrasegmental and intersegmental cycle-to-cycl e coupling. The average cycle period for rhythmic activity in the isol ated nerve cord was similar to 2.5 times slower than the cycle period for crawling in intact larvae, but not more variable. 4. Like crawling in intact animals, in isolated nerve cords, bursting activity in the dorsal body wall motor neurons occurred before activity in ventral/lat eral body wall motor neurons within an abdominal segment. The evoked b ursting activity recorded from the proleg nerve was superimposed on a high level of tonic activity. 5. In isolated nerve cords, bursts of ac tivity in the thoracic leg levator/extensor motor neurons alternated w ith bursts of activity in the depressor/flexor motor neurons. The burs t duration of the levator/extensor activity was brief and remained rel atively steady as cycle period increased. The burst duration of the de pressor/flexor activity occupied the majority of an average cycle and increased as cycle period increased. The phase of both levator/extenso r motor nerve activity and depressor/flexor motor nerve activity remai ned relatively stable over the entire range of cycle periods. The timi ng and patterning of thoracic leg motor neuron activity in isolated ne rve cords quantitatively resembled thoracic leg motor activity in free ly crawling larvae. 6. The rhythmic motor activity generated by an iso lated larval nerve cord resembled a slower version of normal crawling in intact larvae. Because of the many similarities between activity in duced in the isolated nerve cord and the muscle activity and movements of thoracic and abdominal segments during crawling, we concluded that central mechanisms can establish the timing and patterning of the cra wling motor pattern and that crawling may reflect the output of a cent ral pattern generating network.