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INTRODUCTION |
Specific coordination between
activities of motor neurons controlling different aspects of the same
complex behavior is vital for producing a functionally meaningful
behavioral act and achieving the required goal. Such coordination
ensures the orderly production of complex behaviors and represents an
important and universal principle of the CNS functioning in both
vertebrate and invertebrate animals. In the current study, we
investigated how rhythmic activities of the two major feeding
structures of the carnivorous pteropod mollusk Clione
limacina are coordinated during complex feeding behavior and
studied the neuronal mechanisms of this coordination.
Clione is a highly specialized carnivore that feeds on only
two species of shelled pteropod mollusks of the genus
Limacina (Lalli 1970
; Lalli and Gilmer
1989
; Wagner 1885
). To extract the soft body of
Limacina from its shell, Clione uses two
specialized feeding structures, chitinous hooks and the radula, which
pull the prey out of the shell to be swallowed whole (Lalli
1970
; Lalli and Gilmer 1989
; Wagner
1885
). Chitinous hooks, the toothed radula, and muscles
controlling their movements comprise the muscular buccal mass (Fig.
1). The radula is a feeding structure
found in all gastropod mollusks. The functional role of its rhythmic movements, which consist of the protraction and retraction phases, is
to grab the food and bring it to the opening of the esophagus. Chitinous hooks, which normally are retracted inside two symmetrical muscular hook sacs, are unique to Clione and other mollusks
from the order Gymnosomata and reflect a high food specialization
(Lalli 1970
; Lalli and Gilmer 1989
). The
functional role of the hooks is to grab the soft tissue of
Limacina and pull it out of the shell into the buccal
cavity. Hook activity is also rhythmic and consists of protraction and
retraction phases.

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Fig. 1.
Schematic drawing of the buccal mass and attached buccal ganglia (bg).
The buccal mass includes the toothed radula (rd) and paired hook sacs
(hs). Right hooks (hk) are protracted, left hooks are retracted inside
a hook sac. The salivary gland (sg) is attached to the buccal mass.
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We report here that the rhythmic movements of the radula and hooks are
highly coordinated in the phase-dependent manner. This phase-dependent
coordination was observed on the behavioral level and was also always
present on the motor neuronal level during both spontaneous and induced
rhythmic activity. Both hooks and radula movements are controlled by
neurons located in the small buccal ganglia attached to the buccal mass
(Fig. 1). The first and only observation of the rhythmically active
neurons in the buccal ganglia, although without a clear distinction
between radula and hook rhythmic neurons, was made by Arshavsky
et al. (1989)
. We have identified electrophysiologically and
morphologically specific motor neurons from four major functional
groups controlling radula and hooks protraction and retraction
movements. Neurons from different groups demonstrated coordinated
phase-locked rhythmic activity. Electrical and reciprocal inhibitory
connections between motor neurons from different groups are suggested
to underlie this coordination. The possible existence of a single
multifunctional central pattern generator for both radula and hook
motor centers is discussed.
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METHODS |
Adult specimens of Clione limacina were collected
from the breakwater at Friday Harbor Laboratories, University of
Washington (Friday Harbor, WA) in the spring-summer season and at the
White Sea Marine Laboratory of the Zoological Institute (White Sea, Russia) in the summer-autumn season. The animals were held in 1-gallon
jars in a refrigerator at 5
7°C. Prior to dissection, animals were
anesthetized in a 1:1 mixture of seawater and isotonic MgCl2 and then tightly pinned to a silicone
elastomer (Sylgard)-coated Petri dish. Reduced preparations consisted
of the wings, dissected head with the isolated buccal mass, and the
attached CNS. All central nerves innervating the buccal mass were
intact. Prior to electrophysiological recording, the sheaths of the
central ganglia were softened by bathing the preparation in a 1-mg/ml solution of protease (Sigma, type XIV) for 5 min, followed by 30-min wash.
A Sony video camera was mounted on the dissecting microscope and used
for recording hook and radula movements during experiments. Video
records from a standard video tape recorder were then digitized on an
IBM-PC compatible computer using a frame grabber from VideoVision Company and Personal AVI Editor program (FlickerFree
Multimedia Products). The digitized images were analyzed using video
analysis software PhysVis. The anterior tips of the radula and hooks
were monitored, and their movements along the y-axis were
measured. The y-axis was established as the plane of the
maximum amplitudes of the protraction-retraction movements. The
Y coordinates were calculated at each video frame (25 frames
per second) and plotted as a function of time. Two curves representing
radula and hook movements from the same video episode were
cross-correlated using correlation function built in MS Excel.
Cross-correlation coefficients from several episodes were presented as
means ± SE. In the experiments with intracellular recordings,
signals from the intracellular amplifier were captured on the sound
channel of the tape recorder to precisely connect them to video events.
Intracellular recordings from individual neurons were made with glass
microelectrodes (resistance 10-30 M
) filled with 2 M potassium
acetate. Electrophysiological signals were amplified, displayed, and
recorded using conventional electrophysiological techniques.
Intracellular stimulation was achieved via an amplifier bridge circuit.
Electrotonic coupling was demonstrated by applying depolarizing or
hyperpolarizing square current pulses to one cell and recording similar
but attenuated responses in other recorded neurons at the same time. To
test for monosynaptic connections, a high divalent cation solution was
used (in mM: 110 MgCl2, 25 CaCl2, 400 NaCl, 10 KCl, and 3 NaHCO3, pH 7.4). An extracellular suction
electrode filled with seawater was used to stimulate the cerebro-buccal
connective in reduced preparations. Each stimulus had duration of
0.5-1 s and intensity of 2-3 V. For morphological investigation of
recorded neurons, a 5% solution of 5(6)-carboxyfluorescein (Sigma)
prepared in 2 M potassium acetate was iontophoresed via the recording
electrodes (resistances, 20-40 M
) with 0.3- to 2-nA negative
current pulses for 5-30 min. Injected cells were observed live in the
recording dish either with a Nikon (Tokyo, Japan) epifluorescence
microscope equipped with filters for viewing fluorescein
epifluorescence or a BioRad (Hercules, CA) MRC 600 laser scanning
confocal microscope.
We used cross-correlation analysis to quantify the phase
relationship between motor neurons from different functional groups. Each trace of intracellular records with rhythmic spike activity was
converted into spike density function (SDF) by counting the number of
spikes in each 200-ms interval. Then cross-correlation coefficient was
calculated between SDF of two analyzed neurons using correlation
function built in MS Excel. When one of the neurons was not spiking but
still received prominent rhythmic inputs, we averaged membrane
potential during each 200-ms interval and calculated cross-correlation
coefficient between SDF in spiking neuron and averaged membrane
potential function (AMPF) in nonspiking neuron. Cross-correlation
coefficients from several pairs of neurons were presented as means ± SE.
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RESULTS |
Coordination between radula and hooks rhythmic movements
Spontaneous rhythmic radula protractions and retractions were
frequently observed in the reduced preparations. Hooks were less
active, although episodes of spontaneous rhythmic activity consisted of
hooks partial protraction and retraction were sometimes seen together
with radula movements. In quiescent preparations, rhythmic radula and
hooks protraction and retraction was easily induced by electrical
stimulation of the cerebro-buccal connective. This stimulation was very
effective and induced in all preparations a train of rhythmic movements
of both radula and hooks (n = 12). In all preparations,
during episodes of spontaneous and induced rhythmic activities, a
strict phase-dependent coordination was observed between rhythmic
movements of the hooks and radula. Hook protraction always coincided
with radula retraction, and hook retraction coincided with radula
protraction (Fig. 2). The
cross-correlation coefficient between the radula and hooks movements
was
0.72 ± 0.17 (mean ± SE, n = 5). Thus
hooks and radula always moved in functionally opposite directions with
their rhythmic activity locked in anti-phase.

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Fig. 2.
Rhythmic movements of the hooks and radula induced by stimulation of
the cerebro-buccal connective. Note that when the hooks are protracted,
the radula is retracted, and when the hooks are retracted, the radula
is protracted.
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Identification of hook and radula controlling motor neurons
We identified a group of neurons in the caudal part of the buccal
ganglia, on both dorsal and ventral sides, whose firing produced
protraction of the ipsilateral hooks (Fig.
3). These cells were designated Buccal
Hook Protractor (Bc-HP) neurons. A total of 83 Bc-HP neurons were
recorded in 36 preparations. Each buccal ganglion contained between 7 and 10 bilaterally symmetrical Bc-HP neurons, which had cell bodies
30-50 µm in diameter. Carboxyfluorescein staining revealed that each
Bc-HP neuron had one axon, which exited the buccal ganglia into the
ipsilateral hook nerve and innervated the ipsilateral muscular hook sac
(n = 11; Fig.
4A). The Bc-HP neurons were
normally silent when hooks were quiescent. During induced rhythmic
activity of the hooks, the Bc-HP neurons fired rhythmically in the hook
protraction phase. Intracellular stimulation of a Bc-HP neuron produced
protraction of the ipsilateral hooks, which persisted in high divalent
solution (Fig. 5A). Even a
single spike in a Bc-HP neuron induced a noticeable protraction
response in the ipsilateral hooks. In addition to the prominent
protraction of the ipsilateral hooks, strong induced bursts of spikes
in a Bc-HP neuron also induced radula retraction and protraction of the
contralateral hooks (n = 24). All recorded Bc-HP
neurons were electrically coupled with coupling coefficients ranging
between 0.1 and 0.2 (0.16 ± 0.03; n = 12).

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Fig. 3.
A map showing identified motor neurons on the dorsal and ventral
surfaces of the buccal ganglia. Identified neurons include the radula
protractors (Bc-RP) and retractors (Bc-RR), and the hook protractors
(Bc-HP) and retractors (Bc-HR). Also indicated are the cerebro-buccal
connectives (cer-buc), the bilaterally symmetrical hook nerves that
innervate the ipsilateral hook sac (hn), the bilaterally symmetrical
radula nerves 2 (rn2) that innervate radula muscles, the medial radula
nerve 1 (rn1), the nerves that innervate the proboscis (pn), and the
nerves that innervate the salivary glands (sgn).
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Fig. 4.
Morphological structure of identified motor neurons in the buccal
ganglia (bg). A: hook protractor motor neuron that
innervates the ipsilateral hook sac (hs). B: the hook
retractor motor neuron projects 2 axons into both ipsilateral and
contralateral hook nerves. C: the radula retractor motor
neuron innervates the radula via the lateral radula nerve.
D: the radula protractor motor neuron innervates the
radula via the medial radula nerve. Scale bar is 100 µm. White arrow
indicates the anterior side of a preparation. A,
C, and D, confocal microscope
reconstruction; B, microphotograph obtained using the
epifluorescence microscope.
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Fig. 5.
Intracellular stimulation of the hook-controlling motor neurons
produced prominent hook movements in high divalent solution.
A: each spike in a hook protractor Bc-HP motor neuron
produced prominent protraction response in the ipsilateral hooks
(top trace). B: stimulation of the Bc-HP
motor neurons triggered hook protraction, and subsequent stimulation of
a hook retractor Bc-HR motor neuron produced hook retraction. Arrows
indicate the beginning of the induced hook retraction.
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Only one Buccal Hook Retractor (Bc-HR) neuron has been identified in
the right buccal ganglion (n = 32 preparations). The Bc-HR neuron had a cell body 30 µm in diameter, which was located on
the medial side of the ganglion under the commissure (Fig. 3).
Carboxyfluorescein staining revealed that the Bc-HR neuron projected
two main axons into the ipsilateral and contralateral hook nerves and
produced extensive branching in the caudal part of both hook sacs
(n = 9; Fig. 4B). The Bc-HR neuron rhythmic bursts coincided with hook retraction episodes before hook protraction. The Bc-HR neurons were also active when hooks were constantly retracted
without demonstrating any rhythmic movements. When hooks were partially
protracted, for example, after stimulation of the Bc-HP neurons,
stimulation of a Bc-HR neuron always elicited complete retraction of
ipsilateral and contralateral hooks, which persisted in high divalent
solution (Fig. 5B).
A group of five or six Buccal Radula Protractor (Bc-RP) neurons was
identified in the ventrolateral part of each buccal ganglion (Fig. 3).
The neurons were small with soma diameters of 15-20 µm.
Intracellular carboxyfluorescein staining revealed that each Bc-RP
neuron projected a single axon into the unpaired medial radula nerve
rn1, which innervates the radula and muscles controlling its
movements (n = 6; Fig. 4D). When the radula
was quiescent, the Bc-RP neurons were silent. During spontaneous or
induced radula movements, the Bc-RP neurons fired rhythmically in the
radula protraction phase. Intracellular stimulation of a single Bc-RP neuron induced radula protraction, which persisted in high divalent solution (n = 22). All Bc-RP neurons were electrically
coupled with coupling coefficients ranging between 0.1 and 0.2 (0.17 ± 0.05; n = 9).
Three bilaterally symmetrical clusters of Buccal Radula Retractor
(Bc-RR) neurons were identified on both dorsal and ventral surfaces of
the buccal ganglia (Fig. 3). Two small clusters were located in the
ventromedial area above and below the commissure, and one cluster was
found in the dorsal-medial area. A total of 52 neurons were recorded in
28 preparations. All Bc-RR neurons sent their axons to the radula via
the ipsilateral radula nerve rn2 (n = 7;
Fig. 4C). Bursting rhythmic activity of the Bc-RR neurons
always correlated with the radula retraction phase of the feeding
rhythm. Intracellular stimulation of the Bc-RR neurons produced radula
retraction movement, which persisted in high divalent solution
(n = 21). Electrical coupling was found between all
Bc-RR neurons, within each cluster and between different clusters, with coupling coefficients ranging between 0.15 and 0.25 (0.21 ± 0.05; n = 9).
Rhythmic activity of the radula and hook motor neurons
During the episodes of spontaneous or induced "feeding"
activity in the buccal mass, which included rhythmic
protraction-retraction movements of both radula and hooks, the
following phase-dependent coordination between the rhythmic activities
of identified motor neurons was always observed. The Bc-RP neurons and
Bc-RR neurons burst, as expected, in opposite phases. Bursts of spikes
in Bc-RP neurons always coincided with the inhibitory episodes in the
Bc-RR neurons, and inhibition in Bc-RP neurons coincided with firing in
Bc-RR neurons (Figs. 6 and 7). At the
same time, Bc-HR neurons always burst in phase with the Bc-RP neurons
(Fig. 6), and Bc-HP neurons fired in phase with Bc-RR neurons (Fig.
7). Synchronization between the rhythmic
activities of the Bc-HP neurons and Bc-RR neurons was very high, with
their bursts occurring simultaneously and having similar duration
(n = 32; Fig.
8A). Cross-correlation coefficient between Bc-HP and Bc-RR neurons was 0.81 ± 0.12 (n = 7). Cross-correlation coefficient between Bc-HR
and Bc-RP neurons was 0.54 ± 0.17 (n = 7). Bursts
of spikes in the Bc-HR neurons and Bc-RP neurons occurred in the same
phase; however, a slight phase-shift was always observed between
neurons during high-speed recording (Fig.
9A). The Bc-HR neuron bursts
often ended 0.1-0.5 s after the bursts in the Bc-RR, and Bc-HP neurons
were initiated, thus creating a brief period of coactivation of the
Bc-HR and Bc-HP neurons (n = 24; Fig. 9B).
Duration of the Bc-HR and Bc-HP neurons coactivation was 0.42 ± 0.17 s (n = 8). This coactivation may be important
for producing a fast and powerful protraction movement of the hooks.

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Fig. 6.
Rhythmic movements of hooks and radula induced by a stimulation of the
cerebro-buccal connective and corresponding rhythmic activity of a
radula protractor Bc-RP motor neuron, radula retractor Bc-RR motor
neuron, and hook retractor Bc-HR motor neuron. Note that Bc-RP and
Bc-HR neurons were active in the same phase, while the Bc-RR neuron
burst in the opposite phase. Active retraction of the hooks (indicated
by asterisks) occurred immediately before hook protraction and
following passive, post protraction decay from the previous cycle,
which was very prominent in the absence of a prey.
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Fig. 7.
Rhythmic movements of hooks and radula induced by a stimulation of the
cerebro-buccal connective (arrow) and corresponding rhythmic activity
of a radula retractor Bc-RR motor neuron, radula protractor Bc-RP motor
neuron and hook protractor Bc-HP motor neuron. Note that Bc-RR and
Bc-HP neurons were active in the same phase, while the Bc-RP neuron
burst in the opposite phase.
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Fig. 8.
Hook protractor Bc-HP motor neurons and radula retractor Bc-RR motor
neurons are active strictly in the same phase with similar burst
duration, intervals, and even intensity (A). The
mechanism underlying this synchronization is electrical coupling
between these 2 types of motor neurons (B).
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Fig. 9.
A: rhythmic activity of the Bc-RP, Bc-RR, and Bc-HR
motor neurons shown at high-speed resolution. Although the Bc-RP and
Bc-HR motor neurons were active at the same phase, the Bc-HR neuron
burst occurred a little later than the Bc-RP neuron burst.
B: this slight phase shift of the Bc-HR neuron bursting
allows for the brief coactivation of hook protractor Bc-HP and hook
retractor Bc-HR motor neurons.
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Thus the four rhythmically active motor neuron groups that
control radula and hook protraction and retraction burst in two phases.
One phase included simultaneous activation of the Bc-RR and Bc-HP
neurons, while Bc-RP and Bc-HR neurons were inhibited (Fig.
10). During the second phase, Bc-RP and
Bc-HR neurons were activated, while Bc-RR and Bc-HP neurons were
inhibited (Fig. 10). This phase-dependent coordination between rhythmic
activities of the hook and radula controlling motor neurons was always
observed during spontaneous or induced "feeding" activity that
included rhythmic movements of both the hooks and radula. However, the radula was usually more active than the hooks, and there were frequent
episodes of rhythmic radula movements with quiescent hooks.
Intracellular recording revealed that during such episodes, Bc-HP
neurons did not spike, although they still received prominent rhythmic
inputs in the same phase-dependent manner (Fig.
11). These rhythmic subthreshold
depolarizing inputs in the Bc-HP neurons coincided with bursts of
spikes in the Bc-RR neurons (n = 12). The
cross-correlation coefficient between Bc-HP neurons (AMPF) and Bc-RR
neurons (SDF) was 0.65 ± 0.21 (n = 5). The Bc-HR
neurons received phase-dependent rhythmic inputs and spiked during
these episodes of active radula movements and quiescent hooks
(n = 7).

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Fig. 10.
The phase relationships of the buccal motor neurons from 4 major
groups. The beginning and end of each box represent the means ± SE onset and offset times of the impulse burst in the indicated neuron,
expressed as a fraction of the cycle period. Cycle period is
arbitrarily designated as beginning with burst onset in the RP neuron,
and ending with the onset of the next RP burst. Results are pooled from
8 preparations.
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Fig. 11.
A preparation in which the radula is rhythmically active, while the
hooks are quiescent. The Bc-RP and Bc-RR motor neurons, as expected,
generated rhythmic bursts of spikes in opposite phases. While the hook
protractor Bc-HP motor neuron did not fire, it still received prominent
rhythmic inputs in phase with the Bc-RR neuron.
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Interactions between radula and hook motor neurons
Strong electrical coupling was always observed between Bc-HP and
Bc-RR neurons, which rhythmically fired in the same phase (n = 21; Fig. 8B). Coupling coefficients
ranged between 0.15 and 0.25 (0.19 ± 0.05; n = 7). The Bc-HR and Bc-RP neurons, which also fired in the same phase,
but had a slight phase-shift that varied from one episode to another,
were not electrically coupled (n = 18).
In addition to electrical connections, buccal neurons from different
functional groups produced multiple chemical inhibitory connections
between each other. Induced bursts of spikes in the Bc-RP neurons
produced inhibitory inputs to the Bc-RR neurons (n = 4;
Fig. 12A). In turn,
stimulation of the Bc-RR neurons induced inhibition of the Bc-RP
neurons (n = 3; Fig. 12A). The Bc-HP neurons induced inhibition of the Bc-HR neurons (n = 4; Fig.
12B) and Bc-RP neurons (n = 7; Fig.
12C). The Bc-RP neurons, in turn, inhibited Bc-HP neurons
(n = 4). All these inhibitory connections were
polysynaptic since they were easily blocked by high divalent solution.

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Fig. 12.
Inhibitory connections between different radula and hooks controlling
motor neurons. A: intracellular stimulation of the Bc-RP
motor neuron induced inhibition of the Bc-RR neuron, while stimulation
of the Bc-RR produced inhibitory inputs in the Bc-RP neuron.
B: induced bursts of spikes in the Bc-HP motor neuron
produced inhibitory inputs to the Bc-HR neurons. C:
stimulation of the Bc-HP motor neuron induced inhibition of the Bc-RP
motor neuron.
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DISCUSSION |
Functional role of a phase-locked coordination between hooks and
radula movements
In Clione limacina, feeding behavior following prey
capture includes grasping movements of specialized feeding structures, chitinous hooks, and toothed radula, whose coordinated rhythmic activities extract the prey from its shell and bring it to the gut
(Lalli 1970
; Lalli and Gilmer 1989
;
Wagner 1885
). Clione does not bite small
pieces from the prey during feeding as many other animals do. It pulls
the entire prey from the shell, which takes 20-40 min of constant
efforts to accomplish (Lalli 1970
; Lalli and
Gilmer 1989
; Wagner 1885
). We show here that
coordination between rhythmic radula and hooks movements occurs in a
strict phase-dependent manner with hooks and radula moving in opposite phases, as is functionally appropriate. While the hooks are retracted, the radula is protracted attempting to seize the soft tissue of the
prey inside the shell. After seizing it, the radula retracts pulling
the prey from its shell, while the hooks protract and in turn grab the
tissue. Then the hooks retract pulling the prey out of the shell, while
the radula releases the tissue and protracts again to grasp it deeper
inside. In other words, the radula and hooks take turns in pulling the
prey out of the shell thus keeping a constant extracting pressure, the
same way as left and right hands take turns in gripping and pulling a rope.
Mechanisms of coordination between radula and hooks rhythmic
movements
During rhythmic fictive feeding, hook protraction always coincided
with radula retraction and hook retraction coincided with radula
protraction. On the neuronal level, Bc-HP and Bc-RR neurons always
fired in phase with each other. The Bc-HR and Bc-RP neurons also fired
in phase, although there was a slight phase shift in Bc-HR neuron
bursting, which allowed brief coactivation of Bc-HR and Bc-HP neurons.
Investigation of the interactions between motor neurons from different
functional groups revealed that Bc-HP and Bc-RR neurons, which fire in
the same phase, were electrically coupled to each other (Fig.
13). This electrical coupling is
apparently one mechanism of coordination between hook protractor and
radula retractor neurons. It also explains why a strong burst of spikes in the Bc-HP neurons produced, in addition to hook protraction, a
noticeable radula retraction. Electrical coupling between neurons has
been identified as a mechanism of behavioral coordination in several
animals (Collins 1983
; Syed and Winlow
1991
). The Bc-HR and Bc-RP neurons were not electrically
coupled, which would allow the observed Bc-HR phase shift and
coactivation of the hook retractor and protractor neurons. Brief
initial coactivation of functionally opposite motor neurons followed by
the inhibition of one neuron type is a well-known mechanism to produce
high movement speed and power in different animals (Heitler and
Burrows 1977
; Norekian and Satterlie 1993
).

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Fig. 13.
Schematic representation of the synaptic connections between the 4 different groups of radula and hooks controlling motor neurons. Hook
Protractor and Radula Retractor neurons, which fire in the same phase,
are electrically coupled. Identified inhibitory polysynaptic
connections between neurons are shown by filled circles.
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Reciprocal inhibitory connections found between different motor neurons
are a second mechanism for coordination between hook and radula
controlling neurons. Bc-HP neuron activity induced inhibitory inputs to
the Bc-RP neurons, while action potentials in the Bc-RP neurons, in
turn, induced inhibition of the Bc-HP neurons (Fig. 12). These
interactions ensure that radula protractor and hook protractor motor
neurons are rhythmically active in the opposite phases. The inhibitory
connections between motor neurons were polysynaptic. It is possible
that motor neurons activated interneurons of their central pattern
generator (CPG) via electrical coupling, as it happens in the
Clione swimming system in the pedal ganglia, where all motor
neurons are electrically coupled to the CPG interneurons of the same
phase (Arshavsky et al. 1993
). Interneurons active in
one phase would then produce excitatory inputs to the Bc-RP motor
neurons, inhibitory inputs to the Bc-RR motor neurons, and inhibitory
inputs to the Bc-HP motor neurons. Interneurons active in opposite
phase would produce excitatory inputs to the Bc-HP motor neurons,
inhibitory inputs to the Bc-HR motor neurons, and inhibitory inputs to
the Bc-RP motor neurons. This brings us to a suggestion that the radula
and hook motor neurons are driven not by two separate dedicated CPGs,
but rather one multifunctional pattern-generating network controlling
movements of both feeding structures. The existence of
multifunctional interneurons, or "distributed" networks was
demonstrated in many neural systems (Dickinson 1995
;
Getting and Dekin 1985
; Kristan et al.
1988
; Lockery and Sejnowski 1992
; Oku et
al. 1994
; Shaw and Kristan 1997
; Wu et
al. 1994
; Xin et al. 1996
). The concept of
multifunctional pattern generators was actively pursued in a
well-studied crustacean stomatogastric nervous system. The gastric and
pyloric networks were found to be not separate groups of neurons that
independently generate two different rhythmic behaviors, but rather
provide a synaptically connected pool of neurons from which many
different pattern-generating circuits can be assembled (Meyrand
et al. 1994
; Weimann et al. 1991
).
The idea that a single CPG drives both the radula and hooks
movements in Clione is also supported by the observation
that the hook-controlling motor neurons continue to receive
subthreshold rhythmic inputs coordinated with radula activity even when
the hooks are quiescent. The functional uncoupling of hooks and radula movement thus appears to occur simply because rhythmic depolarizing inputs to the hook protractor neurons do not reach spike threshold and
do not produce firing in the Bc-HP motor neurons. Thus the mechanism of
functional uncoupling of the radula and hooks systems need not be
inhibition of a separate hook CPG, but could result simply from a
decrease of Bc-HP motor neuron responsiveness to the incoming rhythmic
inputs, or a decrease in the strength of these inputs because of a
modulation. Such modulatory input from sensory afferent induces
restructuring of the pyloric neural network of the lobster
stomatogastric system, which results in cessation of rhythmic activity
of some neurons and therefore a reduced pyloric pattern (Hooper
et al. 1990
).
Comparative view on the feeding system
The buccal mass in many gastropod mollusks consists of the
muscles controlling rhythmic radula and jaw movements. It is important to note that jaw rhythmic activity is also coordinated with radula movements in a phase-dependent manner (Morton and Chiel
1993
; Nagahama et al. 1999
; Nagahama and
Takata 1989
; Willows 1980
). In most mollusks
from the order Gymnosomata, the buccal mass also includes hooks
(Lalli and Gilmer 1989
). In Clione, the jaws
disappeared, and the buccal mass consists of only the radula and
muscular hook sacs (Lalli and Gilmer 1989
). These
muscular hook sacs in gymnosomes presumably evolved from the ancestral
buccal mass that consisted of the radula and jaws. Thus hooks and
radula may represent two evolutionary very close feeding structures.
This consideration again raises the question of whether separate buccal
neural networks control hooks and radula movements in
Clione, or whether a single multifunctional CPG drives
the movements of both organs. Identification of the CPG(s)
underlying hooks and radula movements will be specifically targeted by
our following investigation of the coordinated rhythmic activity of
these two feeding structures of Clione.
This work was supported by National Science Foundation Grant
IBN-9630805, National Institute of Neurological Disorders and Stroke
Grant NS-34662, and Russian Foundation for Basic Research Grant
01-04-48259-a.
Address for reprint requests: T. P. Norekian (E-mail:
Tigran.Norekian{at}asu.edu).
Received 26 October 2001; accepted in final form 29 January 2002.