Startle phase of escape swimming is controlled by pedal motoneurons in the pteropod mollusk Clione limacina.

Abstract

Escape swimming in the pteropod mollusk 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. Two pairs of large pedal neurons (50-60 microns) initiate 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 pair of neurons 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, because they produce junctional potentials or spike-like responses in both slow-twitch and fast-twitch muscle cells with 1:1 ratios of spikes to excitatory postsynaptic potentials. 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-s 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, because 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.