Nonspiking local interneuron in the motor pattern generator for the crayfish swimmeret

1985 ◽  
Vol 54 (1) ◽  
pp. 28-39 ◽  
Author(s):  
D. H. Paul ◽  
B. Mulloney
Keyword(s):  
Nervous System ◽  
Membrane Potential ◽  
Motor Neuron ◽  
Motor Pattern ◽  
Pattern Generator ◽  
Local Interneuron ◽  
Postsynaptic Potentials ◽  
Essential Component ◽  
Local Interneurons

We describe a type of nonspiking premotor local interneuron (interneuron IA) in the abdominal nervous system of Pacifasticus leniusculus. All of its branches are restricted to one side of the midline. These interneurons are identifiable and occur as bilateral pairs, one neuron on each side of abdominal ganglia 3, 4, and 5. The membrane potential of interneuron IA oscillated in phase with the swimmeret rhythm, a motor pattern generated in each of these ganglia, because the neuron received postsynaptic potentials in phase with the rhythm. Sustained hyperpolarization of an individual interneuron IA initiated generation of the swimmeret rhythm in all the ganglia of a quiescent nervous system. Sustained depolarization stopped the swimmeret rhythm in all the active ganglia of a nervous system that was generating the rhythm. Currents injected into one interneuron reset the rhythm. Comparisons of the shapes of the IA interneurons in different ganglia showed that they are similar to each other and distinct from other local interneurons in these ganglia. Interneuron IA has a large integrative segment and relatively few branches that are largely restricted to the lateral neuropil, to which all other kinds of swimmeret neurons also project. We conclude that this interneuron occurs only once in each hemiganglion in abdominal segments 3, 4, and 5, and that it is identifiable. Furthermore, this interneuron is an essential component of the circuit in each hemiganglion that generates the swimmeret rhythm. The interneuron was dye coupled to a particular identifiable motor neuron and not to any other neurons. The motor neuron was not dye-coupled to any other local interneurons. The ability of this motor neuron to reset the rhythm is attributed to its being electrically coupled to interneuron IA in its ganglion.

Cycle Period of a Network Oscillator Is Independent of Membrane Potential and Spiking Activity in Individual Central Pattern Generator Neurons

2004 ◽  
Vol 92 (3) ◽  
pp. 1904-1917 ◽  
Author(s):  
Paul S. Katz ◽  
Akira Sakurai ◽  
Stefan Clemens ◽  
Deron Davis
Keyword(s):  
Nervous System ◽  
Membrane Potential ◽  
Central Pattern Generator ◽  
Temperature Sensitivity ◽  
Motor Pattern ◽  
Pattern Generator ◽  
Membrane Potentials ◽  
Cycle Period ◽  
Wide Range ◽  
Spiking Activity

Rhythmic motor patterns are thought to arise through the cellular properties and synaptic interactions of neurons in central pattern generator (CPG) circuits. Yet, when examining the CPG underlying the rhythmic escape response of the opisthobranch mollusc, Tritonia diomedea, we found that the cycle period of the fictive swim motor pattern recorded from the isolated nervous system was not altered by changing the resting membrane potential or the level of spiking activity of any of the 3 known CPG cell types: ventral swim interneuron-B (VSI-B), the dorsal swim interneurons (DSIs), and cerebral neuron 2 (C2). Furthermore, tonic firing in one or more DSIs or C2 evoked rhythmic bursting that did not differ from the cycle period of the motor pattern evoked by nerve stimulation, regardless of the firing frequency. In contrast, the CPG produced a large range of cycle periods as a function of temperature. The temperature sensitivity of the fictive motor pattern produced by the isolated nervous system was similar to the temperature sensitivity of the swimming behavior produced by the intact animal. Thus, although the CPG is capable of producing a wide range of cycle periods under the influence of temperature, the membrane potentials and spiking activity of the identified CPG neurons do not determine the periodicity of the motor pattern. This suggests that the timing of activity in this network oscillator may be determined by a mechanism that is independent of the membrane potentials and spike rate of its constituent neurons.

Fast Synaptic Connections From CBIs to Pattern-Generating Neurons in Aplysia: Initiation and Modification of Motor Programs

2003 ◽  
Vol 89 (4) ◽  
pp. 2120-2136 ◽  
Author(s):  
Itay Hurwitz ◽  
Irving Kupfermann ◽  
Klaudiusz R. Weiss
Keyword(s):  
Central Pattern Generator ◽  
Motor Pattern ◽  
Aplysia Californica ◽  
Pattern Generator ◽  
Synaptic Connections ◽  
Motor Patterns ◽  
Motor Programs ◽  
Postsynaptic Potentials ◽  
Buccal Ganglia ◽  
Very High

Consummatory feeding movements in Aplysia californica are organized by a central pattern generator (CPG) in the buccal ganglia. Buccal motor programs similar to those organized by the CPG are also initiated and controlled by the cerebro-buccal interneurons (CBIs), interneurons projecting from the cerebral to the buccal ganglia. To examine the mechanisms by which CBIs affect buccal motor programs, we have explored systematically the synaptic connections from three of the CBIs (CBI-1, CBI-2, CBI-3) to key buccal ganglia CPG neurons (B31/B32, B34, and B63). The CBIs were found to produce monosynaptic excitatory postsynaptic potentials (EPSPs) with both fast and slow components. In this report, we have characterized only the fast component. CBI-2 monosynaptically excites neurons B31/B32, B34, and B63, all of which can initiate motor programs when they are sufficiently stimulated. However, the ability of CBI-2 to initiate a program stems primarily from the excitation of B63. In B31/B32, the size of the EPSPs was relatively small and the threshold for excitation was very high. In addition, preventing firing in either B34 or B63 showed that only a block in B63 firing prevented CBI-2 from initiating programs in response to a brief stimulus. The connections from CBI-2 to the buccal ganglia neurons showed a prominent facilitation. The facilitation contributed to the ability of CBI-2 to initiate a BMP and also led to a change in the form of the BMP. The cholinergic blocker hexamethonium blocked the fast EPSPs induced by CBI-2 in buccal ganglia neurons and also blocked the EPSPs between a number of key CPG neurons within the buccal ganglia. CBI-2 and B63 were able to initiate motor patterns in hexamethonium, although the form of a motor pattern was changed, indicating that non-hexamethonium-sensitive receptors contribute to the ability of these cells to initiate bursts. By contrast to CBI-2, CBI-1 excited B63 but inhibited B34. CBI-3 excited B34 and not B63. The data indicate that CBI-1, -2, and -3 are components of a system that initiates and selects between buccal motor programs. Their behavioral function is likely to depend on which combination of CBIs and CPG elements are activated.

scholarly journals Feedback From Peripheral Musculature to Central Pattern Generator in the Neurogenic Heart of the Crab Callinectes sapidus: Role of Mechanosensitive Dendrites

2010 ◽  
Vol 103 (1) ◽  
pp. 83-96 ◽  
Author(s):  
Keyla García-Crescioni ◽  
Timothy J. Fort ◽  
Estee Stern ◽  
Vladimir Brezina ◽  
Mark W. Miller
Keyword(s):  
Motor Neuron ◽  
Central Pattern Generator ◽  
Motor Neurons ◽  
Callinectes Sapidus ◽  
Motor Pattern ◽  
Pattern Generator ◽  
Neuron Spike ◽  
Effector System ◽  
Spike Bursts

The neurogenic heart of decapod crustaceans is a very simple, self-contained, model central pattern generator (CPG)-effector system. The CPG, the nine-neuron cardiac ganglion (CG), is embedded in the myocardium itself; it generates bursts of spikes that are transmitted by the CG's five motor neurons to the periphery of the system, the myocardium, to produce its contractions. Considerable evidence suggests that a CPG-peripheral loop is completed by a return feedback pathway through which the contractions modify, in turn, the CG motor pattern. One likely pathway is provided by dendrites, presumably mechanosensitive, that the CG neurons project into the adjacent myocardial muscle. Here we have tested the role of this pathway in the heart of the blue crab, Callinectes sapidus . We performed “de-efferentation” experiments in which we cut the motor neuron axons to the myocardium and “de-afferentation” experiments in which we cut or ligated the dendrites. In the isolated CG, these manipulations had no effect on the CG motor pattern. When the CG remained embedded in the myocardium, however, these manipulations, interrupting either the efferent or afferent limb of the CPG-peripheral loop, decreased contraction amplitude, increased the frequency of the CG motor neuron spike bursts, and decreased the number of spikes per burst and burst duration. Finally, passive stretches of the myocardium likewise modulated the spike bursts, an effect that disappeared when the dendrites were cut. We conclude that feedback through the dendrites indeed operates in this system and suggest that it completes a loop through which the system self-regulates its activity.

Plasticity in the multifunctional buccal central pattern generator of Helisoma illuminated by the identification of phase 3 interneurons

1996 ◽  
Vol 75 (2) ◽  
pp. 561-574 ◽  
Author(s):  
E. M. Quinlan ◽  
A. D. Murphy
Keyword(s):  
Motor Activity ◽  
Motor Neuron ◽  
Central Pattern Generator ◽  
Motor Neurons ◽  
Motor Pattern ◽  
Pattern Generator ◽  
Neuron Activity ◽  
Phase 2 ◽  
Phase 3 ◽  
Standard Pattern

1. The mechanism for generating diverse patterns of buccal motor neuron activity was explored in the multifunctional central pattern generator (CPG) of Helisoma. The standard pattern of motor neuron activity, which results in typical feeding behavior, consists of three distinct phases of buccal motor neuron activity. We have previously identified CPG interneurons that control the motor neuron activity during phases 1 and 2 of the standard pattern. Here we identify a pair of interneurons responsible for buccal motor neuron activity during phase 3, and examine the variability in the interactions between this third subunit and other subunits of the CPG. 2. During the production of the standard pattern, phase 3 excitation in many buccal motor neurons follows a prominent phase 2 inhibitory postsynaptic potential. Therefore phase 3 excitation was previously attributed to postinhibitory rebound (PIR) in these motor neurons. Two classes of observations indicated that PIR was insufficient to account for phase 3 activity, necessitating phase 3 interneurons. 1) A subset of identified buccal neurons is inhibited during phase 3 by discrete synaptic input. 2) Other identified buccal neurons display discrete excitation during both phases 2 and 3. 3. A bilaterally symmetrical pair of CPG interneurons, named N3a, was identified and characterized as the source of phase 3 postsynaptic potentials in motor neurons. During phase 3 of the standard motor pattern, interneuron N3a generated bursts of action potentials. Stimulation of N3a, in quiescent preparations, evoked a depolarization in motor neurons that are excited during phase 3 and a hyperpolarization in motor neurons that are inhibited during phase 3. Hyperpolarization of N3a during patterned motor activity eliminated both phase 3 excitation and inhibition. Physiological and morphological characterization of interneuron N3a is provided to invite comparisons with possible homologues in other gastropod feeding CPGs. 4. These data support a model proposed for the organization of the tripartite buccal CPG. According to the model, each of the three phases of buccal motor neuron activity is controlled by discrete subsets of pattern-generating interneurons called subunit 1 (S1), subunit 2 (S2), and subunit 3 (S3). The standard pattern of buccal motor neuron activity underlying feeding is mediated by an S1-S2-S3 sequence of CPG subunit activity. However, a number of "nonstandard" patterns of buccal motor activity were observed. In particular, S2 and S3 activity can occur independently or be linked sequentially in rhythmic patterns other than the standard feeding pattern. Simultaneous recordings of S3 interneuron N3a with effector neurons indicated that N3a can account for phase-3-like postsynaptic potentials (PSPs) in nonstandard patterns. The variety of patterns of buccal motor neuron activity indicates that each CPG subunit can be active in the absence of, or in concert with, activity in any other subunit. 5. To explore how CPG activity may be regulated to generate a particular motor pattern from the CPG's full repertoire, we applied the neuromodulator serotonin. Serotonin initiated and sustained the production of an S2-S3 pattern of activity, in part by enhancing PIR in S3 interneuron N3a after the termination of phase 2 inhibition.

scholarly journals Maintenance of motor pattern phase relationships in the ventilatory system of the crab

1997 ◽  
Vol 200 (6) ◽  
pp. 963-974
Author(s):  
R Dicaprio ◽  
G Jordan ◽  
T Hampton
Keyword(s):  
Membrane Potential ◽  
Central Pattern Generator ◽  
Motor Pattern ◽  
Pattern Generator ◽  
Rate Of Change ◽  
Cycle Period ◽  
Potential Oscillation ◽  
Cycle Frequency ◽  
Membrane Potential Oscillation ◽  
Phase Relationships

The central pattern generator responsible for the gill ventilation rhythm in the shore crab Carcinus maenas can produce a functional motor pattern over a large (eightfold) range of cycle frequencies. One way to continue to generate a functional motor pattern over such a large frequency range would be to maintain the relative timing (phase) of the motor pattern as cycle frequency changes. This hypothesis was tested by measuring the phase of eight events in the motor pattern from extracellular recordings at different rhythm frequencies. The motor pattern was found to maintain relatively constant phase relationships among the various motor bursts in this rhythm over a large (sevenfold) range of cycle frequencies, although two phase-maintaining subgroups could be distinguished. Underlying this phase maintenance is a corresponding change in the time delay between events in the motor pattern ranging from 470 to 1800 ms over a sevenfold (300­2100 ms) change in cycle period. Intracellular recordings from ventilatory neurons indicate that there is very little change in the membrane potential oscillation in the motor neurons with changes in cycle frequency. However, recordings from nonspiking interneurons in the ventilatory central pattern generator reveal that the rate of change of the membrane potential oscillation of these neurons varies in proportion to changes in cycle frequency. The strict biomechanical requirements for efficient pumping by the gill bailer, and the fact that work is performed in all phases of the motor pattern, may require that this motor pattern maintain phase at all rhythm frequencies.

Spiking local interneurons as primary integrators of mechanosensory information in the locust

1983 ◽  
Vol 50 (6) ◽  
pp. 1281-1295 ◽  
Author(s):  
M. V. Siegler ◽  
M. Burrows
Keyword(s):  
Motor Neuron ◽  
Sensory Neuron ◽  
Motor Neurons ◽  
Synaptic Delay ◽  
Excitatory Postsynaptic Potential ◽  
Campaniform Sensilla ◽  
Local Interneuron ◽  
Sensory Stimuli ◽  
Local Interneurons ◽  
Afferent Fibers

A population of spiking local interneurons in the metathoracic ganglion of the locust is vigorously excited by particular sensory stimuli from the hindlegs and participates in local postural reflexes. We examined the inputs from singly innervated mechanoreceptors (hairs and campaniform sensilla) to these spiking local interneurons, to nonspiking local interneurons, and to motor neurons that are also elements of local reflex pathways. Recordings were made intracellularly from the interneurons and motor neurons and extracellularly from afferent fibers. The physiological evidence is consistent with the spiking local interneurons being excited by direct, chemically mediated synaptic inputs from the afferents. Each afferent spike is followed at a constant latency by an excitatory postsynaptic potential (EPSP) in a spiking local interneuron, even at instantaneous frequencies as high as 300 Hz. The estimated synaptic delay is 1.5 ms, similar to that measured at other presumed monosynaptic connections within the same ganglion. Cobalt stains of individual interneurons, and of the central projections of afferent fibers show that both branch within the same ventral region of neuropil. Afferents from several hairs and campaniform sensilla converge on an individual spiking local interneuron. One interneuron is shown to receive inputs from at least seven hairs and four campaniform sensilla, but these represent only a tiny fraction of the total number of such sensilla on a hindleg. Practical limitations to the number of sensilla that can be tested for each interneuron means that the degree of convergence is likely to be considerably underestimated. We found no evidence that nonspiking local interneurons or motor neurons receive direct inputs from the afferents tested. Neurons of both types are, however, affected by stimulation of individual hairs, and the resulting pattern of postsynaptic potentials (PSPs) is similar to the pattern of spikes evoked in the spiking local interneurons. We infer from the evidence presented here and elsewhere (10, 11) that the spiking local interneurons are involved in at least two types of pathways for local interactions: 1) sensory neuron-spiking local interneuron-motor neuron, and 2) sensory neuron-spiking local interneuron-nonspiking local interneuron-motor neuron. We conclude that the spiking local interneurons are major elements in the primary integration of inputs from external receptors on the hindlegs.

A Central Pattern Generator Producing Alternative Outputs: Phase Relations of Leech Heart Motor Neurons With Respect to Premotor Synaptic Input

2007 ◽  
Vol 98 (5) ◽  
pp. 2983-2991 ◽  
Author(s):  
Brian J. Norris ◽  
Adam L. Weaver ◽  
Angela Wenning ◽  
Paul S. García ◽  
Ronald L. Calabrese
Keyword(s):  
Motor Neuron ◽  
Central Pattern Generator ◽  
Motor Neurons ◽  
Coordination Mode ◽  
Motor Pattern ◽  
Pattern Generator ◽  
Neuron Activity ◽  
Intersegmental Coordination ◽  
Asymmetric Pattern ◽  
Neuron Ensemble

The central pattern generator (CPG) for heartbeat in leeches consists of seven identified pairs of segmental heart interneurons and one unidentified pair. Four of the identified pairs and the unidentified pair of interneurons make inhibitory synaptic connections with segmental heart motor neurons. The CPG produces a side-to-side asymmetric pattern of intersegmental coordination among ipsilateral premotor interneurons corresponding to a similarly asymmetric fictive motor pattern in heart motor neurons, and asymmetric constriction pattern of the two tubular hearts: synchronous and peristaltic. Using extracellular techniques, we recorded, in 61 isolated nerve cords, the activity of motor neurons in conjunction with the phase reference premotor heart interneuron, HN(4), and another premotor interneuron that allowed us to assess the coordination mode. These data were then coupled with a previous description of the temporal pattern of premotor interneuron activity in the two coordination modes to synthesize a global phase diagram for the known elements of the CPG and the entire motor neuron ensemble. These average data reveal the stereotypical side-to-side asymmetric patterns of intersegmental coordination among the motor neurons and show how this pattern meshes with the activity pattern of premotor interneurons. Analysis of animal-to-animal variability in this coordination indicates that the intersegmental phase progression of motor neuron activity in the midbody in the peristaltic coordination mode is the most stereotypical feature of the fictive motor pattern. Bilateral recordings from motor neurons corroborate the main features of the asymmetric motor pattern.

Pyloric motor pattern modification by a newly identified projection neuron in the crab stomatogastric nervous system

1996 ◽  
Vol 75 (1) ◽  
pp. 97-108 ◽  
Author(s):  
B. J. Norris ◽  
M. J. Coleman ◽  
M. P. Nusbaum
Keyword(s):  
Nervous System ◽  
Motor Pattern ◽  
Neuron Activity ◽  
Stomatogastric Nervous System ◽  
Pacemaker Neurons ◽  
Postsynaptic Potentials ◽  
Increased Activity ◽  
Phase Relationships ◽  
Stimulation Of ◽  
Pyloric Rhythm

1. We have used multiple, simultaneous intra- and extracellular recordings as well as Lucifer yellow dye-fills to identify modulatory commissural neuron 5 (MCN5) and characterize its effects in the stomatogastric nervous system (STNS) of the crab, Cancer borealis. MCN5 has a soma and neuropilar arborization in the commissural ganglion (CoG; Figs. 1 and 2), and it projects through the inferior esophageal nerve (ion) and stomatogastric nerve (stn) to the stomatogastric ganglion (STG; Figs. 1-3). 2. Within the CoGs, MCN5 receives esophageal rhythm-timed excitation and pyloric rhythm-timed inhibition (Fig. 4). Additionally, during the lateral teeth protractor phase of the gastric mill rhythm, the pyloric-timed inhibition of MCN5 is reduced or eliminated. 3. Intracellular stimulation of MCN5 excites the pyloric pacemaker ensemble, including the anterior burster (AB), pyloric dilator (PD), and lateral posterior gastric (LPG) neurons. This produces a faster pyloric rhythm. MCN5 stimulation also inhibits all nonpacemaker pyloric neurons, reducing or eliminating their activity (Figs. 5 and 6A; Tables 1 and 2). After MCN5 stimulation, bursting is enhanced for several cycles in some pyloric neurons when compared with their prestimulus activity (Figs. 5 and 6A; Tables 1 and 2). 4. MCN5 evokes distinct responses from each pyloric pacemaker neuron (Figs. 6-8). The AB and LPG neurons respond with increased activity. The AB response includes the presence of large amplitude excitatory postsynaptic potentials (EPSPs) that contribute to a depolarization of the trough of its rhythmic oscillations (Fig. 6). LPG responds by exhibiting increased activity that prolongs the duration of its burst beyond that of AB and PD (Fig. 7). In contrast, MCN5 stimulation initially produces decreased PD neuron activity, followed by a slight enhancement of each PD burst (Figs. 7 and 8). PD activity is further enhanced after MCN5 stimulation (Figs. 7 and 8). 5. MCN5-elicited action potentials evoke discrete, constant latency inhibitory postsynaptic potentials (IPSPs) in all nonpacemaker pyloric neurons, including the inferior cardiac (IC), lateral pyloric (LP), pyloric (PY), and ventricular dilator (VD) neurons (Fig. 9). MCN5 activity also inhibits these neurons indirectly, via its excitation of the pacemaker neurons. The pyloric pacemaker neurons synaptically inhibit all four nonpacemaker neurons. 6. The increased activity in the VD neuron, after MCN5 stimulation, is not mimicked by either direct hyperpolarization or by synaptically inhibiting VD via another pathway (Fig. 10). The poststimulation increase in IC neuron activity is stronger than that after hyperpolarizing current injection but is comparable with that resulting from stimulation of another inhibitory pathway (Fig. 10). The enhanced PY neuron activity is comparable with that resulting from either direct current injection or synaptic inhibition from another pathway (Fig. 10). 7. MCN5 activity increases the pyloric cycle frequency of both slow (< 1 Hz) and fast (1-2 Hz) rhythms (Fig. 11), and it significantly alters the phase relationships that define this motor pattern (Fig. 12). These phase relationships change again after MCN5 stimulation (Fig. 12). 8. MCN5 acts in concert with the pyloric pacemaker ensemble to elicit a pyloric rhythm that exhibits enhanced pacemaker neuron activity and reduced activity in all nonpacemaker neurons. Additionally, despite their electrical coupling, the three types of pacemaker neurons exhibit distinct responses to MCN5 stimulation. This partially uncouples their normally coactive bursts. The resulting motor pattern is distinct from all previously characterized pyloric rhythms.

Synaptic basis of swim initiation in the leech. III. Synaptic effects of serotonin-containing interneurones (cells 21 and 61) on swim CPG neurones (cells 18 and 208)

1986 ◽  
Vol 122 (1) ◽  
pp. 303-321
Author(s):  
M. P. Nusbaum
Keyword(s):  
Membrane Potential ◽  
Central Pattern Generator ◽  
Motor Pattern ◽  
Pattern Generator ◽  
Current Injection ◽  
Excitatory Effect ◽  
Direct Pathway ◽  
Macrobdella Decora ◽  
Synaptic Effects

Serotonin-containing cells 21 and 61 strongly excite a swim central pattern generator (CPG) neurone, cell 208, in nearby segmental ganglia in the leech Macrobdella decora. This excitatory effect is apparently independent of activity in the swim-initiating neurone cell 204, which monosynaptically excites cell 208 (Weeks, 1982b). Cell 208 excites cell 21, apparently directly. This is the first identified direct pathway for feedback from the swim central pattern generator to a swim initiator neurone. Focally applied serotonin has no effect on the soma of cell 208, but causes both excitatory and inhibitory responses in cell 208 when applied to different places within the neuropile. Cell 61 polysynaptically excites distant, posterior cells 208. This excitation is mediated at least in part by the activation of nearby cells 208, which polysynaptically excite posterior cells 208. Cell 208 is dye-coupled intraganglionically to a newly identified pair of neurones, designated cells 18. Cell 208 also excites posterior cells 18, apparently directly. This interaction may be the pathway whereby cell 61 polysynaptically excites posterior cells 208. During swimming, cell 18's membrane potential oscillates in phase with cell 208. Intracellular current injection into cell 18 during swimming perturbs the swim motor pattern. Therefore, cell 18 qualifies as a candidate swim CPG neurone.

An identified glutamatergic interneuron patterns feeding motor activity via both excitation and inhibition

1995 ◽  
Vol 73 (3) ◽  
pp. 945-956 ◽  
Author(s):  
E. M. Quinlan ◽  
K. Gregory ◽  
A. D. Murphy
Keyword(s):  
Motor Neuron ◽  
Motor Neurons ◽  
Activity Patterns ◽  
Motor Pattern ◽  
Neuron Activity ◽  
Inhibitory Effects ◽  
Standard Pattern ◽  
Glutamate Receptor Antagonist ◽  
Postsynaptic Potentials ◽  
Buccal Ganglia

1. Previously we demonstrated that glutamate is an important neurotransmitter in the CNS of Helisoma. Exogenous glutamate applied to the buccal ganglia mimicked both the excitatory and inhibitory effects of subunit 2 (S2) of the tripartite central pattern generator (CPG) on S2 postsynaptic motor neurons. Here we identify buccal interneuron B2 as an S2 interneuron by utilizing a combination of electrophysiology, pharmacology, and intracellular staining. In addition, neurons that were electrophysiologically and morphologically characterized as neuron B2 demonstrated antiglutamate immunoreactivity, suggesting that neuron B2 is a source of endogenous glutamate in the buccal ganglia. 2. Depolarization of neuron B2 evoked excitatory postsynaptic potentials in motor neurons excited by S2. The excitatory effects of B2 depolarization and S2 activation were reversibly antagonized by the ionotropic glutamate receptor antagonist 6-cyano-7-nitro-quinoxaline-2,3-dione, similar to the antagonism shown previously for application of exogenous glutamate. Depolarization of neuron B2 also evoked inhibitory postsynaptic potentials in motor neurons inhibited by S2. When such motor neurons were maintained in isolated cell culture, application of exogenous glutamate produced a direct hyperpolarization of the membrane potential. 3. The activity of neuron B2 is necessary for the production of the standard pattern of buccal motor neuron activity, which underlies functional feeding movements. The subunits of the tripartite buccal CPG must be active in the temporal sequence S1-S2-S3 to produce the standard feeding pattern. Rhythmic inhibition from neuron B2 terminated activity in S1 postsynaptic motor neurons and entrained the frequency of activity in S3 postsynaptic motor neurons. Hyperpolarization of neuron B2 disrupted the production of the standard motor pattern by eliminating S2 postsynaptic potentials in identified buccal motor neurons, thereby prolonging S1 activity and disrupting S3 bursting. 4. These data support the hypothesis that S2 neuron B2 is glutamatergic and demonstrate that glutamatergic transmission, and especially inhibition, is fundamental to the production of behaviorally critical motor neuron activity patterns in Helisoma.

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