The Journal of Neuroscience, May 1, 1999, 19(9):3620–3628
Dynamic Restructuring of a Rhythmic Motor Program by a Single
Mechanoreceptor Neuron in Lobster
Denis Combes, Pierre Meyrand, and John Simmers
Laboratoire de Neurobiologie des Réseaux, Université Bordeaux I and Centre National de la Recherche Scientifique, Unité
Mixte de Recherche 5816, 33405 Talence, France
We have explored the synaptic and cellular mechanisms by
which a single primary mechanosensory neuron, the anterior
gastric receptor (AGR), reconfigures motor output of the gastric
mill central pattern generator (CPG) in the stomatogastric nervous system (STNS) of the lobster Homarus gammarus. AGR is
activated in vivo by contraction of the medial tooth protractor
muscle gm1 and accesses the gastric CPG via excitation of two
in-parallel interneurons, the excitatory commissural gastric
(CG) and the inhibitory gastric inhibitor (GI). In the spontaneously active STNS in vitro, weak firing of AGR in time with
gastric mill motoneurons (GM) reinforces an ongoing type I
gastric mill rhythm in which all gastric teeth power-stroke motoneurons are synchronously active. With strong AGR firing,
these phase relationships switch abruptly to a type II pattern in
which lateral and medial teeth power-stroke motoneurons fire in
antiphase. Our results suggest that these bimodal actions on
the gastric mill rhythm depend on the balance of firing of the CG
and GI interneurons and that selection of the pathway resides in
their different postsynaptic sensitivities to AGR. Whereas high
intrinsic firing rates of the CG neuron ensure that the excitatory
pathway predominates during low levels of sensory input,
strong synaptic facilitation in the GI neuron favors the inhibitory
pathway during high levels of receptor activity. Feedback from
a single mechanosensory neuron is thus able, in an activitydependent manner, to specify different motor programs from a
single central pattern-generating network.
Key words: Crustacea; Homarus gammarus; stomatogastric
system; gastric mill motor network; mechanosensory neuron;
sensorimotor integration; network reconfiguration; synaptic
facilitation
From a variety of studies on both vertebrates and invertebrates, it
is now clear that movement-related feedback from proprioceptors
plays a crucial role in adapting the intensity and timing of rhythmic motor programs to changing behavioral demands. For example, phasic sensory signals can entrain and reinforce locomotor
rhythms in lamprey (Grillner et al., 1981), crayfish (Sillar et al.,
1986), cat (Andersson and Grillner, 1983), and locust (Reye and
Pearson, 1988), thereby enabling central motor commands to
match perceived movements. Furthermore, proprioceptive signals may play an important role in reinforcing an ongoing phase
or initiating the transition between antagonistic phases of a movement cycle by positive and /or negative feedback reflexes (Pearson
and Duysens, 1976; Bässler, 1986; Rossignol et al., 1988). For
example, positive feedback from intraoral receptors allows us to
bite with increasing strength on soft foods, but, with excessive
closing forces, this input switches to negative feedback that inhibits jaw closing motoneurons (Sherrington, 1917).
In addition to transient adaptive adjustment of an ongoing
motor rhythm, proprioceptive input can be involved in switching
or selecting different patterns of motor output for different behavioral tasks. For example, in the mollusc Tritonia, light touch
activates a central circuit, resulting in a withdrawal response,
whereas stronger sensory stimuli induce the same circuit to produce escape swimming (Getting and Dekin, 1985). In Xenopus
embryos, the same central motor circuitry can produce swimming
or struggling behavior (Soffe, 1993), depending on the level of
input from a single population of cutaneous receptors (Soffe,
1997). In both cases, however, the precise cellular pathways and
mechanisms by which such sensory-induced switching occurs remain to be determined.
A suitable preparation for addressing this problem is the gastric
mill rhythm-generating network in the stomatogastric nervous
system (STNS) of the lobster Homarus gammarus. As a result of
intensive study over the last two decades, the crustacean gastric
mill motor network, including that of Homarus (Combes et al.,
1999), is now well understood in terms of underlying cellular and
synaptic mechanisms (Harris-Warrick et al., 1992). Moreover, a
mechanoreceptor that provides movement-related feedback to
the gastric mill central pattern generator (C PG) in Homarus has
been identified (Simmers and Moulins, 1988a,b; Combes et al.,
1995a). This sensor, the anterior gastric receptor (AGR), is
particularly attractive for the cellular study of proprioceptive
interactions with a cyclic motor program because it consists of a
single large neuron whose signaling properties are known in
detail. Moreover, as shown here, AGR accesses the lobster gastric
mill network via two interneuronal pathways that have been
identified previously (Combes et al., 1999).
E xploring mechanisms of sensorimotor integration is typically
bedeviled by a conflict between the needs of experimental accessibility for electrophysiological recording, persistence of motor
network activity, and conservation of proprioceptive feedback
pathways (Rossignol et al., 1988). In our in vitro study, we were
able to satisfy all three conditions by using rhythmically active,
deafferented (“open-loop”) STNS preparations in which the
AGR–gastric mill network loop was artificially closed with appropriately timed experimental activation of the receptor neuron.
Received Oct. 19, 1998; revised Feb. 9, 1999; accepted Feb. 12, 1999.
This work was supported in part by the Human Frontier Science Program.
Correspondence should be addressed to Denis Combes, Laboratoire de Neurobiologie des Réseaux, Université Bordeaux I and Centre National de la Recherche
Scientifique, Unité Mixte de Recherche 5816, Avenue des Facultés, 33405 Talence,
France.
Copyright © 1999 Society for Neuroscience 0270-6474/99/193620-09$05.00/0
Combes et al. • Sensory-Induced Motor Pattern Selection
J. Neurosci., May 1, 1999, 19(9):3620–3628 3621
Figure 1. C ellular pathway through which lobster primary sensory neuron AGR influences the gastric mill C PG network (schema). The receptor, which
arises from the tendon of the medial tooth protractor muscle gm1, projects to the STG via two descending CoG interneurons: the CG neuron, which
excites the lateral teeth opener ( O) LPG and the medial tooth protractor ( P) GM motoneurons, and the GI neuron, which inhibits the lateral teeth closer
( C) LG–MG and medial tooth retractor ( R) DG motoneurons. Numbers of motoneurons of each subtype are indicated in the circles. Note that the single
gastric mill network interneuron is not shown. A, Spontaneous and evoked AGR firing excites CG and GI interneurons. B, Each presynaptic AGR spike
is correlated 1:1 and at constant latency with an EPSP in both CG and GI.
We show that AGR input, by acting through dual interneuronal
pathways, has effects beyond simply reinforcing and adjusting
ongoing gastric mill rhythmicity or assisting in the transition
between cycle phases. Rather, according to the discharge of the
receptor, AGR can evoke fundamental and persistent restructuring of the gastric mill rhythm to produce different activity patterns, similar to those spontaneously expressed in the intact
animal.
A preliminary account of this work has been published previously (Combes et al., 1995b).
nerves, according to their different postsynaptic effects on gastric mill
motoneurons and on the basis of their synaptic responsiveness to AGR.
AGR itself was stimulated by either intrasomatic depolarizing current
injection or brief electrical shocks delivered via a platinum wire electrode placed on either of the two peripheral dendrites of the receptor.
In some experiments, the GI interneuron was selectively photoablated
by intrasomatically injecting L ucifer yellow (3% in distilled water; Sigma,
Quentin Fallavier, France) and then illuminating the commissural ganglion (CoG) for 30 min with intense blue light (450 – 490 nm). The
gradual membrane depolarization of the GI neuron to zero indicated a
successf ul photoinactivation (Miller and Selverston, 1979).
RESULTS
MATERIALS AND METHODS
All experiments were performed on in vitro preparations of the STNS of
the lobster Homarus gammarus using dissection, electrophysiological,
and data storage procedures as f ully described in our accompanying
paper (Combes et al., 1999).
Once isolated, the STNS was bathed in aerated saline maintained at
15–18°C and composed of (in mM) NaC l 479.12, KC l 12.74, C aC l2-2H2O
13.67, MgSO4 10, Na2SO4 3.91, and H EPES 5, buffered to pH 7.45.
Under these conditions, the STNS generates robust spontaneous gastric
mill rhythmicity, thereby allowing the study of AGR sensory input to an
already active C PG.
E xtracellular recordings were made with Vaseline-isolated platinum
wire electrodes placed against appropriate motor nerve branches. Intracellular recording– stimulation was made with glass microelectrodes (tip
resistance of 10 –30 MV) filled with 3 M KC l. The commissural gastric
(CG) and gastric inhibitor (GI) projection neurons were identified by
their axonal projections in the superior oesophageal and stomatogastric
The gastric mill CPG network in the lobster stomatogastric
ganglion (STG) consists of four motoneuron subsets, two of which
control protraction [10 gastric motoneurons (GMs)] and retraction [one dorsal gastric (DG) motoneuron] of the gastric medial
tooth, whereas the other two subsets drive opening [two lateral
posterior gastric (LPG) motoneurons] and closing [the single
lateral gastric (LG) and medial gastric (MG) motoneurons] of the
lateral teeth (Combes et al., 1999) (Fig. 1, schema). As we described in our accompanying article (Combes et al., 1999), the
gastric mill CPG receives two bilateral pairs of descending interneurons that originate in each CoG: the CG interneuron that
monosynaptically excites the L PG and GM motoneurons and the
GI interneuron that monosynaptically inhibits the LG–MG and
DG motoneurons. It was shown previously that the AGR mech-
3622 J. Neurosci., May 1, 1999, 19(9):3620–3628
Combes et al. • Sensory-Induced Motor Pattern Selection
Figure 2. Multiple effects of AGR on the gastric mill C PG. Simultaneous extracellular recordings from three gastric mill motoneurons (MG, LPG, and
GM) and intrasomatic recording of AGR. A, Spontaneous in vitro gastric mill rhythm in the absence of AGR input. (The small depolarizing events in
AGR are dendritic action potentials that do not generate axonal spikes.) In this pattern, the GM motoneurons fire in phase with the MG and out of phase
with the LPG motoneurons. B, C yclic depolarization of AGR by intracellular current injection ( i) causes it to fire axonal spikes weakly in time with GM
bursts and increases GM neuron firing, with little other effect on gastric mill activity. C, Stronger AGR activation causes a switch in the phase
relationships of the gastric mill pattern. Now, the GM motoneurons fire in phase with the LPG motoneurons. Dotted boxes indicate the pattern expressed
in B. C alibration: vertical bars, 10 mV, 2 nA; horizontal bar, 5 sec.
anoreceptor neuron arises from the tendon of the medial tooth
protractor muscle and directly excites the two CG interneurons
(Simmers and Moulins, 1988a,b) (Fig. 1 A, B). We find here that
AGR also excites the GI interneurons (Fig. 1 A); this excitation is
also probably direct, as indicated by the unitary fixed-latency
EPSPs elicited by AGR impulses (Fig. 1 B). Thus, with the STNS
in vitro, an open-loop preparation is available in which a single
primary mechanosensory neuron has access disynaptically to a
motor pattern-generating network via two antagonistic interneuronal pathways (Fig. 1, schema).
To examine the role of sensory integration through these two
projection pathways, we replaced the in vivo receptor activation
by muscle contraction with direct intrasomatic stimulation of
AGR in rhythmically active in vitro preparations. To reproduce
such “closed-loop” conditions, AGR was stimulated in time with
ongoing bursts in GM motoneurons, because it has been shown
previously (Combes et al., 1995a) that AGR is activated by
contraction of the muscle that these neurons innervate.
Figure 2 illustrates such an experiment in which simultaneous
extracellular recordings from three motoneuron types (MG,
L PG, and GM) of an already active gastric mill network before
( A) and during (B, C) rhythmic GM-timed spike trains evoked in
AGR. As was invariably seen in vitro during spontaneous gastric
cycling (Combes et al., 1999) (Fig. 2 A), the MG motoneuron
bursts in antiphase with the L PG motoneurons but in phase with
GM motoneurons. This coordination, which we refer to as the
type I gastric mill pattern, corresponds to motor activity in vivo in
which protraction of the medial tooth and closure of the lateral
teeth would occur simultaneously. [Note that the depolarizing
transients in the intrasomatic AGR recording in Fig. 2 A are
spontaneously generated dendritic potentials that fail to trigger
axonal action potentials (Combes et al., 1993).]
When AGR is cyclically depolarized to generate axonal spikes
at a mean frequency of ,20 Hz in time with spontaneous GM
motor bursts (Fig. 2 B), the phase relationships between the
different gastric mill motoneuron subsets remain unchanged, and
relatively little change is seen in gastric mill activity, although
GM neuron firing is enhanced. In contrast, rhythmic AGR stimulation that produces higher firing frequencies (in this experiment, .20 Hz) (Fig. 2C) causes a dramatic reorganization of the
gastric mill pattern. In this new pattern, medial tooth GM motoneurons now fire in phase with lateral teeth L PG motoneurons
rather than MG motoneurons (Fig. 2C, dotted boxes indicate type
I pattern expressed in B). These phase relationships comprise a
type II gastric mill pattern in which medial tooth protractor motor
bursting is now in time with lateral teeth opener bursts.
As is seen in Figure 3, the switch between the two patterns
occurs immediately after the onset of elevated AGR stimulation.
In this experiment, a tonically autoactive AGR was recorded,
along with the L PG, GM, and LG motoneurons. Note that the
LG and MG motoneurons are strongly electrically coupled and
therefore behave as a single functional entity (Selverston and
Moulins, 1987; Combes et al., 1999). During spontaneous tonic
AGR firing (Fig. 3, left), the typical type I gastric mill pattern
(ellipse, alternate bursting in the L PG and GM neurons) is expressed. When AGR is rhythmically depolarized to step its mean
firing rate from ;5 to 15–20 Hz, the gastric mill pattern almost
instantaneously switches to the type II pattern in which the L PG
and GM neurons are now coordinately active in antiphase with
the LG motoneuron.
Combes et al. • Sensory-Induced Motor Pattern Selection
J. Neurosci., May 1, 1999, 19(9):3620–3628 3623
Figure 3. The transition between types I and II gastric mill patterns occurs immediately at the onset of AGR bursting (compare ellipses). The gastric
mill C PG was monitored by extracellular recordings from the GM, LPG, and LG motoneurons. Note that the axon of the latter is in a nerve carrying
the ventricular dilator (VD) motoneuron axon of the pyloric network. Note also that AGR was spontaneously active in the absence of injected current.
C alibration: vertical bar, 10 mV; horizontal bar, 4 sec.
The first conclusion from these series of experiments (n 5 9)
therefore is that, when AGR is activated in time with GM
motoneuron bursts, depending on its firing frequency the receptor either reinforces the ongoing type I gastric mill pattern or
rapidly reconfigures the output of the network into a type II
gastric mill pattern.
Role of descending sensory interneurons
AGR has access to the gastric mill network via direct excitation of
two in-parallel interneurons, the excitatory CG and inhibitory GI
(Combes et al., 1999). Moreover, these two interneurons appear
to be the only pathway by which the receptor reaches the gastric
mill CPG. Simmers and Moulins (1988a) have already shown that
AGR has no direct access to gastric mill neurons in the STG. In
addition, the experiment shown in Figure 4 strongly suggests that
the receptor is unable to influence the gastric mill network
without the CG and GI neurons. Under control conditions in this
experiment (Fig. 4 A), extracellular AGR stimulation (shocks at
30 Hz for 3 sec) caused GM neuron excitation and MG neuron
inhibition, as already seen in Figures 2 and 3. In Figure 4 B, one
CoG had been removed by dissection, and the CG neuron in the
remaining CoG hyperpolarized. Moreover, the GI neuron in this
CoG had been photoinactivated by L ucifer yellow injection and
blue light illumination (see Materials and Methods). As a result of
these treatments, spontaneous gastric mill rhythmicity had
ceased. Under these conditions, AGR stimulation at even higher
frequencies than those used in Figure 4 A had no effect on the two
recorded gastric mill motoneurons.
Given that AGR appears to access the gastric mill network
uniquely via the CG and GI neurons, it should be possible to
mimic the effects of AGR input by direct manipulation of these
two interneurons. One of three such experiments is illustrated in
Figure 5 in which the CG and GI neurons were recorded intra-
Figure 4. AGR projects to the gastric mill C PG only via the two
descending CG and GI interneurons in each CoG. A, In control conditions, stimulation of AGR (30 Hz for 3 sec, horizontal bar) excites the GM
motoneuron and inhibits the MG neuron. B, When the right CoG was
removed and the two sensory interneurons in the left CoG were silenced
(by hyperpolarizing CG and photoablating GI), AGR stimulation no
longer affected the gastric mill motoneurons.
cellularly, along with extracellular recordings from motoneurons
of the three major gastric mill subgroups. Figure 5A shows spontaneous gastric mill cycling in the absence of phasic interneuronal
discharge. However, periodic stimulation of the interneurons,
either CG individually (Fig. 5B) or CG and GI simultaneously
3624 J. Neurosci., May 1, 1999, 19(9):3620–3628
Combes et al. • Sensory-Induced Motor Pattern Selection
Figure 5. Direct effects of the CG and GI interneurons on the gastric mill network. A, Spontaneous gastric mill rhythm monitored by extracellular
recordings of the MG, LPG, and GM motoneurons. The CG and GI interneurons were spontaneously silent (the depolarizing events in the CG neuron
are EPSPs; see expanded time base recording in inset). B, C yclic experimental activation of the CG neuron in GM neuron time reinforces the ongoing
gastric mill pattern (the smaller depolarizing events in the CG neuron are action potentials; see inset). C, Simultaneous cyclic depolarizations of both
interneurons reconfigure the gastric mill pattern. Dotted boxes represent the pattern expressed in B. C alibration: vertical bars, 20 mV; horizontal
bar, 4 sec.
(Fig. 5C) by current injection in time with GM neuron firing, had
effects on the rhythm identical to those produced by direct AGR
stimulation (Fig. 2). Thus, activation of the CG neuron alone
clearly enhanced (Fig. 5B) the ongoing type I pattern, whereas
conjoint activation of the CG and GI neurons induced type II
gastric mill activity (Fig. 5C) in which L PG and GM neurons fire
conjointly. In this experiment, the evoked discharge of both the
CG and GI neurons was similar (;50 Hz), but we have found that
phasic firing of the GI neuron as low as 20 Hz (in conjunction
with simultaneous CG neuron activity) can induce the type II
pattern.
Thus, like AGR stimulation at weak to moderate firing rates
(Fig. 2 B), activation of the CG neuron alone (Fig. 5C) is able to
promote the ongoing type I gastric mill rhythm. Similarly, coactivation of the CG and GI interneurons (Fig. 5B) produces a type
II gastric mill pattern that resembles the pattern AGR induces
when firing strongly (Fig. 2C). Can AGR itself under some
conditions preferentially activate the CG neuron and thereby
reinforce the gastric mill rhythm, and under others simultaneously drive the CG and GI neurons to reconfigure the gastric
mill pattern? This possibility is explored in the following
experiments.
Differential synaptic effects of AGR on the CG and
GI neurons
In a first step to assess the ability of the receptor to differentially
activate the two projection pathways, we measured the synaptic
responsiveness of the CG and GI interneurons to input from
AGR in three experiments. In the experiment shown in Figure
6 A, the GI and CG neurons were recorded simultaneously with
an AGR whose firing frequency was manipulated by intrasomatic
current injection (Fig. 6 A1). The mean 6 SEM evoked discharge
frequencies of postsynaptic CG and GI were then expressed as a
function of the firing rate of AGR (Fig. 6 A2). At lower receptor
discharge rates (,20 Hz), the response of the CG interneuron
was considerably higher (approximately three times) than that of
the GI interneuron. For example, when AGR fired at 10 Hz, the
CG neuron fired at an average of 12 Hz, whereas the GI neuron
fired at a mean rate of only 2.5 Hz. However, as AGR spike
frequency increased, the discharge rate of the GI interneuron
increased exponentially until its response curve approached and
eventually crossed that of the CG interneuron. Thus, when AGR
is firing at low frequencies, the CG interneuron is more active,
but at high receptor firing rates, the discharge of the GI neuron
increases relative to that of the CG interneuron so that eventually
both interneurons become equally active. The different responsiveness of the two interneurons to AGR input is confirmed in
Figure 6 B in which data from the three experiments analyzed
were pooled and the mean 6 SEM interneuronal discharge rates
(in each case expressed as a percentage of the firing rate when
AGR itself was firing at 15 Hz) are plotted as a function of
imposed AGR discharge at 15, 20, 25, and 30 Hz. In a strikingly
similar manner in all three experiments, whereas CG neuron
firing rose steadily by ;75% in response to a doubling of spike
frequency of AGR from 15 to 30 Hz, GI neuron firing increased
dramatically by .500%.
What mechanism could underlie these different sensitivities of
the two interneurons to AGR input? To address this, we compared the mean 6 SEM steady-state amplitude of EPSPs evoked
in the two interneurons at various AGR firing frequencies. (Note
that EPSPs in both interneurons attained steady-state levels
within ,1 sec of all depolarizing current-induced changes in
AGR firing rate.) As seen in the single experiment of Figure 7A
and the pooled data from the three experiments in Figure 7B,
whereas EPSPs recorded in the GI neuron increase smoothly with
stepwise increases in receptor firing rate, PSPs in the CG neuron
decrease in amplitude. In the example of Figure 7A, GI neuron
EPSPs increased from 10 to 30 mV, and CG neuron EPSPs
decreased from 10 to 4 mV, as firing frequency of AGR increased
from 8 to 30 Hz. In other words, the synaptic response of GI to
AGR is strongly facilitating, whereas that of CG appears to
Combes et al. • Sensory-Induced Motor Pattern Selection
Figure 6. Differential sensitivities of the CG and GI interneurons to
synaptic excitation by AGR. A, Single experiment. A1, Simultaneous intracellularly recorded responses of the two interneurons to an evoked increase
(by pulsed current injection) in AGR firing rate from a spontaneous mean
of 5–10 (left) and 21 (right) Hz. Calibration: vertical bars, 10 mV; horizontal
bar, 2 sec. A2, Plots of CG and GI neuron responses to a range of AGR
firing frequencies from 8 to 30 Hz. Each point is the mean 6 SEM firing
rate during at least three AGR stimulations. B, Pooled data from three
experiments. Each point is the mean 6 SEM firing rate (relative to control
rate when AGR fires at 15 Hz) of the three corresponding interneurons in
response to stepwise changes in AGR discharge as seen in A1.
defacilitate. However, note that the individual CG and GI neuron
EPSPs are superimposed on a depolarizing envelope because of
temporal summation that was itself proportional to receptor firing
rate (Fig. 6 A1). Thus, the apparent decrease in CG EPSP ampli-
J. Neurosci., May 1, 1999, 19(9):3620–3628 3625
Figure 7. The GI, but not the CG, interneuron displays strong synaptic
facilitation to AGR input. A, Single experiment (same as in Fig. 6 A1). A1,
Superimposed traces (n 5 4) show AGR-evoked EPSPs (measured at
steady-state levels) in the two interneurons at three different mean frequencies of receptor stimulation. Note strong increase in GI neuron EPSP
amplitude, whereas CG neuron EPSPs decrease. C alibration: vertical bars,
10 mV; horizontal bar, 5 msec. A2, Relationship between EPSP amplitude
in the two interneurons over a range (8 –30 Hz) of AGR firing frequencies. Each point is the mean 6 SEM amplitude of at least 10 synaptic
events. B, Pooled data from the same three experiments as in Figure 6 B.
Each point is the mean 6 SEM steady-state amplitude (relative to control
amplitude when AGR fires at 15 Hz) of each interneuron in response to
stepwise changes in receptor firing rate.
tude could be caused by the decrease in driving force resulting
from this underlying depolarization rather than true
defacilitation.
In conclusion, although a high spontaneous firing rate of the
CG neuron results in its activity predominating during low to
moderate levels of receptor input, strong synaptic facilitation in
3626 J. Neurosci., May 1, 1999, 19(9):3620–3628
the GI neuron ensures that at higher levels of AGR activity it
becomes relatively more effective and, in combination with the
CG neuron, reconfigures gastric mill output into pattern II.
DISCUSSION
We have explored a sensorimotor system that consists of a limited
number of neuronal elements, all of which have been identified
electrophysiologically and their synaptic relationships established. This system (Combes et al., 1999) is comprised of a single
primary mechanoreceptor neuron (AGR) that projects to the 16
neuron gastric mill CPG network via two intercalated interneurons, one excitatory (CG) and the other inhibitory (GI). We have
taken advantage of a further important feature of this preparation, namely the ability of the gastric mill CPG to remain spontaneously active in vitro in the absence of all sensory feedback.
This enabled us to examine the specific effects of AGR on an
already active gastric mill circuit in a manner that is generally
impossible in other preparations. In many other preparations, the
influence of sensory feedback on ongoing CPG activity has been
necessarily restricted to semi-intact or intact preparations in
which the central intervening pathways, themselves complex in
terms of both number of elements and distribution, remain inaccessible for electrophysiological investigation (Barnes and Gladden, 1985; Rossignol et al., 1988). Alternatively, the study of
reflexes in reduced in vitro preparations has almost invariably
been performed in the absence of activity in the target motor
circuitry (Burrows, 1992) and therefore under open-loop conditions that bear little functional relationship to normal rhythmic
behavior in vivo. To date, the only other examples in which
proprioceptive feedback to an active motor pattern generator has
been successfully studied at the cellular level is the influence of
stretch receptor input to the flight system of locust (Wolf and
Pearson, 1987, 1988) and the walking limb of crayfish (Sillar and
Skorupski, 1986; Sillar et al., 1986; Skorupski and Sillar, 1986).
Parallel sensorimotor processing can reinforce or
reorganize the gastric mill pattern
Of fundamental importance to the present study was the earlier
finding that active contraction of gastric mill power-stroke muscle
gm1 is the effective stimulus for AGR in vivo (Combes et al.,
1995a). Therefore, we were able to reproduce appropriately
timed input from AGR in our in vitro experimental conditions by
electrical stimulation of the receptor in time with rhythmic GM
neuron bursts. Our main finding is that feedback from this single
sensory neuron can have two distinctly different effects on spontaneous gastric mill network rhythmicity, inducing distinct motor
patterns that occur in the intact animal (Combes et al., 1999). We
have also shown that these effects are mediated by the CG and GI
interneurons and that selective direct activation of one (CG
alone) or both (CG and GI) interneurons closely mimics the
reinforcing and reconfiguring effects of the AGR neuron itself.
These results are summarized in Figure 8, which shows the
functional sensorimotor circuits and resultant gastric mill output
patterns in response to low and high frequencies of receptor
discharge. During moderate AGR firing (Fig. 8 A), the excitatory
sensorimotor pathway via the CG neuron predominates and
thereby evokes simultaneous excitation of GM and LG–MG motoneurons. Under these conditions, the spontaneous type I pattern typically observed in vitro in which medial (GM) and lateral
teeth (LG–MG) power-stroke motoneurons fire in phase (Fig. 2)
is promoted. During higher AGR discharge levels (Fig. 8 B), both
interneuronal pathways become active, and interneuron GI now
Combes et al. • Sensory-Induced Motor Pattern Selection
inhibits the LG–MG motoneurons, permitting the GM and L PG
neurons to be excited by interneuron CG. These combined effects
induce a new type II gastric mill pattern in which medial tooth
protractor (GM) and lateral teeth opener (LPG) motoneurons
fire in phase. In functional terms, this reorganization involves a
complete change in coordination (Fig. 8, ellipses) between motoneurons controlling the medial and lateral teeth subsystems and
in vivo is presumably responsible for adaptive changes in masticatory teeth movements, such as those reported previously in
spiny lobster (Heinzel, 1988).
Comparison with other sensorimotor systems
Studies in a variety of invertebrate and vertebrate preparations
have demonstrated repeatedly the crucial role of proprioceptive
feedback in adapting rhythmic motor programs to changing behavioral demands. Such influences include the regulation and
reinforcement of ongoing movement amplitude and timing, and
the control of phase transitions in a single movement cycle (Pearson, 1995). In this context, feedback from AGR can also participate in the entrainment and reinforcement of the gastric mill
rhythm (Elson et al., 1994) by promoting GM and (indirectly)
LG–MG motoneuron bursts via the excitatory CG interneuron
(Simmers and Moulins, 1988a,b). However, the recruitment of the
inhibitory GI interneuron and the consequent sustained reconfiguration of gastric mill motor coordination at higher rates of
receptor discharge constitute a completely different sensorimotor
effect that has not been demonstrated previously. Phenomenologically, this switch is analogous to the changes in interlimb coordination that, for example, permit a horse to alternate between
trotting or pacing, changes that are believed to arise from different longitudinal combinations of bilaterally alternating forelimb
and hindlimb locomotor programs (Pearson and Duysens, 1976).
Another reported example that resembles our results is found
in Xenopus embryos (Soffe, 1993, 1997) in which mild mechanical
stimulation of touch-sensitive skin sensory neurons activates the
central neuronal circuit for swimming, whereas stronger stimulation recruits additional neurons, resulting in struggling. Here,
however, neither the sensorimotor pathways nor the selection
mechanism are completely known, and, in contrast to the gastric
mill system, the different locomotor programs are triggered by
brief mechanosensory stimulation rather than being continuously
driven by phasic movement-related feedback.
There are also precedents for routing sensory information to a
target CPG via antagonistic interneurons. In mammals and arthropods alike, multiple proprioceptive pathways, both excitatory
and inhibitory, to the same functional group of leg motoneurons
have been well documented (Skorupski and Bush, 1992; De
Serres et al., 1995; Pearson, 1995; Leibrock et al., 1996). In these
cases, the selection between different sensory pathways depends
not on the activity level of the sensory neurons themselves but on
the animal’s behavioral state (static or locomotory) or on which
phase in an individual cycle the stimulus is delivered. In mammals, input from intraoral receptors also accesses the masticatory
CPG via two different interneuronal pathways, one excitatory
that provides positive feedback to jaw closure motoneurons and
one inhibitory that provides negative feedback and can prematurely terminate jaw closure (Rossignol et al., 1988; Appenteng,
1991). Although in this system a specific population of sensory
neurons can evoke different behavioral responses via a stimulusdependent selection of different cellular pathways, again unlike
our crustacean model, the selection process results in either
Combes et al. • Sensory-Induced Motor Pattern Selection
J. Neurosci., May 1, 1999, 19(9):3620–3628 3627
Figure 8. Schematic representation of the multiple effects of AGR on the gastric mill network and its motor pattern. During low receptor discharge ( A),
the higher spontaneous activity of the CG interneuron ensures that its excitatory pathway predominates, thereby reinforcing the type I gastric mill pattern
in which the GM and LPG neurons burst in antiphase. With intense receptor firing ( B), strong synaptic facilitation in the GI interneuron ensures that
its inhibitory pathway becomes effective, resulting in a reconfiguration of the gastric mill pattern in which the GM and LPG neurons are now in phase
with AGR firing. Open circles in gastric mill network wiring diagrams denote motoneuron subtypes that fire in phase with AGR. Hatched circles denote
motoneurons that fire in antiphase with AGR. Line thickness indicates strength of corresponding synaptic pathway.
reinforcement or protective termination of a single phase of
movement, not the induction of a new motor program.
Mechanisms subserving sensorimotor processing:
activity-dependent synaptic facilitation
Unlike the systems described above, the relative simplicity of our
preparation allowed access to the mechanisms responsible for the
selection of the sensorimotor pathways on which the differential
effects of AGR on the gastric mill network depend. Specifically,
our results indicate that the balance of firing in the two parallel
interneuronal pathways is determined by an interplay between
relative differences in both intrinsic excitability and synaptic
sensitivity to excitatory input from a single receptor. Whereas the
CG interneuron displays high spontaneous firing rates at or near
resting potential, the GI interneuron is less spontaneously active
but expresses strong activity-dependent synaptic facilitation in
response to AGR activity. Thus, with weak to moderate levels of
receptor discharge, the higher intrinsic activity of the CG neuron
ensures that this excitatory pathway is favored, whereas, at higher
levels of receptor firing, the strong facilitating capability of the GI
neuron ensures that the inhibitory pathway now also becomes
effective. As a consequence, feedback from AGR switches from a
reinforcing to a reconfiguring influence on gastric mill motor
output.
Our results thus reveal a crucial role for synaptic facilitation in
sensory information processing within the CNS. In the peripheral
nervous system of cricket (Davis and Murphey, 1993) and lobster
(Katz et al., 1993), mixed weakly and highly facilitating synapses
from a common sensory input generate different temporal sequences of muscle contraction. Such short-term activitydependent synaptic facilitation is generally considered to be a
presynaptic phenomenon involving calcium accumulation in input terminals (Katz and Miledi, 1968; Zucker, 1989). The differing influence of AGR on the CG and GI neurons suggests such
plasticity may be differentially expressed in different postsynaptic
targets of the same presynaptic neuron. Similar results have been
observed in several systems. In cat, for example, the same Ia
afferent makes central synapses that facilitate at some motoneurons and antifacilitate at others (Koerber and Mendel, 1991). In
cricket, a similar functional segregation of target responses to a
single cercal sensory neuron is believed to result from specific
3628 J. Neurosci., May 1, 1999, 19(9):3620–3628
retrograde influences on presynaptic terminals by the postsynaptic elements themselves (Davis and Murphey, 1993), whereas, at
the lobster neuromuscular junction, such divergent postsynaptic
effects appear to be associated with morphological particularities
of the presynaptic terminals (Katz et al., 1993). Whether similar
specializations are implicated in the different sensitivities of the
CG and GI neurons to AGR remains to be seen. An additional
possibility is that more than one neurotransmitter may be selectively released at the different synaptic sites (Whim and L loyd,
1989; Sossin et al., 1990; Blitz and Nusbaum, 1997) and /or which
may be sensed by different postsynaptic receptor complements.
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