The neural circuit mediating tentacle withdrawal in Helix aspersa, with
specific reference to the competence of the motoneuron C3.
Steven A. Prescott, Nishi Gill, and Ronald Chase.
Department of Biology, McGill University, 1205 Ave. Docteur Penfield,
Montr‚al, Qu‚bec, H3A 1B1, Canada.
APStracts 4:160N, 1997.
ABSTRACT
The tentacle withdrawal reflex in the terrestrial snail Helix aspersa involves
bending and retraction of the tentacles. When elicited by mechanical
stimulation of the tentacle, the reflex is mediated by the conjoint action of
the central and peripheral nervous systems. The neural circuit underlying the
stimulus-response pathways was studied in vitro using a combination of
morphological and physiological techniques. Sensory input caused by
stimulation of the nose (situated at the superior tentacle's tip) first passes
into the tentacle ganglion. Motor fibres are likely excited in the tentacle
ganglion to form a peripheral stimulus-response pathway. While still in the
tentacle ganglion, the excitation caused by a brief stimulus is transformed
into a prolonged neuronal discharge. This modified signal travels, via the
olfactory nerve, to the cerebral ganglion where it excites the giant
motoneuron C3 along with numerous smaller motoneurons. Afferent input to C3
also arrives from several other sources. The afferent convergence is followed
by a marked divergence of C3's output. C3 innervates the muscles mediating
both tentacle retraction and tentacle bending through multiple cerebral
nerves. Thus, C3's pattern of effector innervation allows this single cell to
elicit and coordinate both components of the tentacle withdrawal reflex.
Lesion experiments indicate that C3 is responsible for 85% of the central
contribution to tentacle retraction, though C3 is actually sufficient to
mediate maximal muscle contraction as evidenced by intracellular stimulation.
In addition to C3, three groups of putative central motoneurons were
identified through nerve backfills and nerve recordings. _The additional
motoneurons mediating tentacle retraction are important for maximizing the
rate of muscle contraction, while those mediating tentacle bending are likely
more important for non-defensive behaviours. These neurons are arranged in
parallel with C3, but unlike C3, each of these neurons innervates only a
single effector or portion thereof. Given C3's direct innervation of multiple
effectors and its sufficiency to evoke strong responses in those effectors, we
conclude that C3 is paramount in eliciting and coordinating tentacle
withdrawal.
Received 13 March 1997; accepted in final form 21 July 1997.
APS Manuscript Number J218-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 28 August 1997