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.
ABSTRACT
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