Motor Neuronal Control of Tail-Directed and Head-Directed Siphon Responses in Aplysia californica. Hickie, Christopher, and Edgar T. Walters. Department of Physiology and Cell Biology, University of Texas at Houston Medical School, P.O. Box 20708, Houston, TX 77225.
APStracts 2:0025N, 1995.
SUMMARY AND CONCLUSIONS
1. Cutaneous stimulation of opposite ends of the body causes qualitatively different siphon responses (Walters and Erickson 1986): tail stimulation causes flaring and backwards bending (the siphon T response) while head stimulation causes constriction and slight anterior bending. This paper characterizes the motor neuronal control of siphon T and siphon H responses. 2. The siphon response to tail nerve (p9) shock in a semi-intact preparation was indistinguishable from the siphon T response in intact or parapodectomized animals. Similarly, the siphon response to head nerve (c2) shock in this preparation was indistinguishable from the siphon H response in intact or parapodectomized animals. 3. Central siphon motor neurons (SMNs) were found to cause a wider variety of movements than previously reported. The movements produced by the LFSB cells strongly resemble the flaring response of the siphon to tail or tail nerve stimulation. The movements produced by RDS and LDS1 resemble components of the constricting response of the siphon to head or head nerve stimulation. 4. Among central SMNs, the LFSB cells show the strongest activation by posterior stimulation, while RDS and LDS1 show the strongest activation by anterior stimulation. The LFSA cells, which produce much weaker siphon constriction, are only activated slightly by posterior stimulation and are inhibited by anterior stimulation. Peripheral siphon motor neurons are inhibited by stimulation of head and tail nerves and thus their activity does not directly contribute to siphon T and H responses. 5. Artificially activating central SMNs with the pattern of activity previously exhibited after tail or head nerve stimulation indicated the sufficiency of the LFSB cells for the siphon T response, and of RDS and LDS1 for the siphon H response. 6. Dramatic behavioral deficits produced by hyperpolarizing the LFSB cells during tail nerve stimulation, or by hyperpolarizing RDS and LDS1 during head nerve stimulation, indicated the necessity of these cells for the expression of directed siphon responses to tail or head stimulation, respectively. 7. Because of their apparent necessity and sufficiency for directional siphon responses to anterior and posterior stimulation, these few cells provide well-defined vantage points for studying neural mechanisms underlying the motor control and transformation of siphon responses. The 4 LFSB cells offer a special advantage for cellular analysis because they form a homogeneous functional unit in which any sampled LFSB cell can be used as a precise monitor of the total motor output underlying the siphon T response. 8. Delayed, long-lasting excitation of motor neurons with actions antagonistic to the primary reflex response indicates that distributed activation of motor neurons at levels subthreshold for muscle contraction contributes to general defensive arousal following threatening stimulation. Thus, a function for the weak activation of numerous neurons by tactile stimulation (observed in the abdominal ganglion with optical recording methods) may be to ready many defensive systems for possible attack. 

Received 27 December 1994; accepted in final form 17 March 1995.
APS Manuscript Number J819-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on  3 April 1995.