The Startle Phase of Escape Swimming is Controlled by Pedal Motoneurons in
the Pteropod Mollusc, Clione limacina .
Satterlie, Richard A., Tigran P. Norekian and Kirk J. Robertson.
Department of Zoology, Arizona State University, Tempe, AZ 85287-1501, USA
and Friday Harbor Laboratories, Friday Harbor, WA 98250, USA.
APStracts 3:0225N, 1996.
Escape swimming in the pteropod mollusc Clione limacina includes an initial
startle response, in which one or two powerful wing beats propel the animal up
to 18 body lengths per second, followed by a variable period of fast swimming,
with a maximal speed of 6 body lengths per second. The initial startle
response is the focus of this report. A pair of large pedal neurons (50-
60[mu]m) initiates wing contractions that are several times stronger than
those produced during slow or fast swimming. These "Startle" neurons are
silent with very low resting potentials and high activation thresholds. Each
Startle neuron has widespread innervation fields in the ipsilateral wing, with
one neuron innervating the dorsal musculature and producing dorsal flexion of
the wing (d-phase) and the other innervating the ventral musculature and
producing a ventral flexion of the wing (v-phase). Startle neurons are
motoneurons as they produce junctional potentials or spike-like responses in
both slow-twitch and fast-twitch muscle cells in 1 spike:1 EPSP ratio. Muscle
activation persists in high divalent saline, suggesting monosynaptic
connections. The musculature innervated by Startle neurons is the same used
during normal slow and fast swimming. However, Startle neuron activity is
independent of normal swimming activity: Startle neurons do not influence the
activity of swim pattern generator interneurons or motoneurons, nor do swim
neurons alter the activity of Startle neurons. The startle response shows
significant response depression with repetitive mechanical stimulation of the
tail or wings. A major focus for this depression is at the neuromuscular
junction. In reduced preparations, repetitive direct stimulation of a Startle
neuron does not result in a significant decrease in spike number or frequency,
but does produce a decrease in force generation (decrease to 20% of original
value after 5 stimuli delivered at 3 second intervals). Inputs that activate
the wing retraction reflex as well as swim inhibition, inhibit Startle
neurons. The inhibition appears to originate in the retraction interneurons as
direct connections from retraction sensory cells or retraction motoneurons are
not found. Mechanical stimulation of a wing or the tail, which usually
initiates startle response in intact animals, produces spikes or large EPSPs
in Startle neurons. The Startle neurons appear to be likely candidates for
direct control of the swim musculature during the startle phase of escape
swimming in Clione.
Received 22 April 1996; accepted in final form 18 September 1996.
APS Manuscript Number J333-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 5 November 1996