CRAWLING MOTOR PATTERNS INDUCED BY PILOCARPINE IN ISOLATED LARVAL NERVE
CORDS OF MANDUCA SEXTA
Rebecca M. Johnston and Richard B. Levine.
ARL-Division of Neurobiology, Box 21077, University of Arizona, Tucson AZ
APStracts 3:0145N, 1996.
SUMMARY AND CONCLUSIONS
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 thoracic legs. The
purpose of this study was to determine whether the larval central nervous
system by itself and without phasic sensory feedback was capable of producing
patterned activity associated with crawling. To establish the extent of
similarity between the output of the isolated nerve cord and crawling, the
motor activity produced in isolated larval nerve cords was compared with the
motor activity from freely crawling larvae. 2. When exposed to the muscarinic
receptor agonist pilocarpine (1.0mM), isolated larval nerve cords produced
long-lasting rhythmic activity in the motor neurons that supply the thoracic
leg, abdominal body wall, and abdominal proleg muscles. The rhythmic activity
evoked by pilocarpine was reversibly and completely abolished by bath
application of the muscarinic-receptor antagonist atropine (0.01mM) in
conjunction with pilocarpine (1.0mM), suggesting that the response was
mediated by muscarinic-like acetylcholine receptors. 3. Similar to crawling in
intact animals, the evoked activity in isolated nerve cords involved
bilaterally symmetric motor activity that progressed from the most posterior
abdominal segment to the most anterior thoracic segment. The rhythmic activity
in thoracic leg, abdominal proleg and abdominal body wall motor neurons showed
intrasegmental and intersegmental cycle-to-cycle coupling. The average cycle
period for rhythmic activity in the isolated nerve cord was approximately 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/lateral body wall motor neurons within an abdominal segment. The
evoked bursting activity recorded from the proleg nerve was superimposed on a
high level of tonic activity. 5. In isolated nerve cords, bursts of activity
in the thoracic leg levator/extensor motor neurons alternated with bursts of
activity in the depressor/flexor motor neurons. The burst duration of the
levator/extensor activity was brief and remained relatively steady as cycle
period increased. The burst duration of the depressor/flexor activity occupied
the majority of an average cycle and increased as cycle period increased. The
phase of both levator/extensor motor nerve activity and depressor/flexor motor
nerve activity remained relatively stable over the entire range of cycle
periods. The timing and patterning of thoracic leg motor neuron activity in
isolated nerve cords quantitatively resembled thoracic leg motor activity in
freely crawling larvae. 6. The rhythmic motor activity generated by an
isolated larval nerve cord resembled a slower version of normal crawling in
intact larvae. Because of the many similarities between activity induced 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 crawling motor pattern and that
crawling may reflect the output of a central pattern generating network.
Received 9 May 1996; accepted in final form 25 June 1996.
APS Manuscript Number J376-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 25 July 1996