The neuromuscular transform constrains the production of functional rhythmic behaviors

We continue our study of the properties and the functional role of the neuromuscular transform (NMT). The NMT is an input-output relation that formalizes the processes by which patterns of motor neuron firing are transformed to muscle contractions. Because the NMT acts as a dynamic, nonlinear, and modifiable filter, the transformation is complex. In the preceding paper we developed a framework for analysis of the NMT and identified with it principles by which the NMT transforms different firing patterns to contractions. The ultimate question is functional, however. In sending different firing patterns through the NMT, the nervous system is seeking to command different functional behaviors, with specific contraction requirements. To what extent do the contractions that emerge from the NMT actually satisfy those requirements? In this paper we extend our analysis to address this issue. We define representative behavioral tasks and corresponding measures of performance, for a single neuromuscular unit, for two antagonistic units, and, in a real illustration, for the accessory radula closer (ARC)-opener neuromuscular system of Aplysia. We focus on cyclical, rhythmic behaviors which reveal the underlying principles particularly clearly. We find that, although every pattern of motor neuron firing produces some state of muscle contraction, only a few patterns produce functional behavior, and even fewer produce efficient functional behavior. The functional requirements thus dictate certain patterns to the nervous system. But many desirable functional behaviors are not possible with any pattern. We examine, in particular, how rhythmic behaviors degrade and disintegrate as the nervous system attempts to speed up their cycle frequency. This happens because, with fixed properties, the NMT produces only a limited range of contraction shapes that are kinetically well matched to the firing pattern only on certain time scales. Thus the properties of the NMT constrain and restrict the production of functional behaviors. In the following paper, we see how the constraint may be alleviated and the range of functional behaviors expanded by appropriately tuning the properties of the NMT through neuromuscular plasticity and modulation.

The Aplysia mytilus inhibitory peptide-related peptides: identification, cloning, processing, distribution, and action

Neuropeptides are a ubiquitous class of signaling molecules. In our attempt to understand the generation of feeding behavior in Aplysia, we have sought to identify and fully characterize the neuropeptides operating in this system. Preliminary evidence indicated that Mytilus inhibitory peptide (MIP)-like peptides are present and operating in the circuitry that generates feeding in Aplysia. MIPs were originally isolated from the bivalve mollusc Mytilus edulis, and related peptides have been identified in other invertebrate species, but no precursor has been identified. In this study, we describe the isolation and characterization of novel Aplysia MIP-related peptides (AMRPs) and their precursor. Several AMRPs appear to have some structural and functional features similar to vertebrate opioid peptides. We use matrix-assisted laser desorption/ionization time-of-flight mass spectrometry to confirm that all 14 AMRPs predicted by the precursor are processed in isolated neurons. Northern analysis, whole-mount in situ hybridization, and immunohistochemistry are used to map the abundant expression of these peptides in the CNS and peripheral tissues such as the digestive tract, vasculature, and the reproductive organs. Physiological studies demonstrate that the rank order of the inhibitory actions of these peptides is different for three target muscles. These results underscore the importance of using a multidisciplinary approach to identifying and characterizing the actions of neuropeptides in an effort to gain understanding of their role in systems of interest. The widespread distribution of the AMRPs indicates that they may be operating in many different systems of Aplysia.

Insulin prohormone processing, distribution, and relation to metabolism in Aplysia californica

The first Aplysia californica insulin gene is characterized and its proteolytic processing from prohormone to final peptides elucidated using a combination of biochemical and mass spectrometric methods. Aplysia insulin (AI) is one of the largest insulins found, with a molecular weight of 9146 Da, and an extended A chain compared with other invertebrate and vertebrate insulins. The AI prohormone produces a series of C peptides and also a unique N-terminally acetylated D peptide. AI-producing cells are restricted to the central region of the cerebral ganglia mostly within the F and C clusters, and AI is transported to neurohemal release sites located on the upper labial and anterior tentacular nerves. The expression of AI mRNA decreases when the animal is deprived of food, and injections of AI reduce hemolymph glucose levels, suggesting that the function of insulin-regulating metabolism has been conserved.

Modulation of radula opener muscles in Aplysia

We observed fibers immunoreactive (IR) to serotonin (5-HT), the myomodulins (MMs), and FMRFamide on the I7-I10 complex in the marine mollusk Aplysia californica. The I7-I10 muscle complex, which produces radula opening, is innervated primarily by one motor neuron, B48. B48 is MM-IR and synthesizes authentic MM(A). When B48 is stimulated in a physiological manner, cAMP levels are increased in opener muscles. cAMP increases also are seen when the MMs are applied to opener muscles but are not seen with application of the B48 primary neurotransmitter acetylcholine (ACh). Possible physiological sources of 5-HT and FMRFamide are discussed. When modulators are applied to resting opener muscles, changes in membrane potential are observed. Specifically, 5-HT, MM(B), and low concentrations of MM(A) all depolarize muscle fibers. This depolarization is generally not sufficient to elicit myogenic activity in the absence of neural activity under "rest" conditions. However, if opener muscles are stretched beyond rest length, stretch- and modulator-induced depolarizations can summate and elicit contractions. This only occurs, however, if "depolarizing" modulators are applied alone. Thus other modulators (i.e., FMRFamide and high concentrations of MM(A)) hyperpolarize opener muscle fibers and can prevent depolarizing modulators from eliciting myogenic activity. All modulators tested affected parameters of motor neuron-elicited contractions of opener muscles. MM(B) and 5-HT increased contraction size over the range of concentrations tested, whereas MM(A) potentiated contractions when it was applied at lower concentrations but decreased contraction size at higher concentrations. FMRFamide decreased contraction size at all concentrations and did not affect relaxation rate. Additionally, the MMs and 5-HT increased muscle relaxation rate, decreased contraction latency, and decreased the rate at which tension was developed during motor neuron-elicited muscle contractions. Thus these modulators dramatically affect the ability of opener muscles to follow activity in the opener motor neuron B48. The possible physiological significance of these findings is discussed.

Decrement of the response of a serotonergic modulatory neuron (the metacerebral cell) in Aplysia, during repeated presentation of appetitive (food) stimuli

Application of food (seaweed, SW) stimuli to the lips evokes a burst of metacerebral cell (MCC) spikes, and it was found in free-moving animals that repeated presentation of the stimulus was associated with a rapid decrement of the evoked responses, even in the absence of ingestion of the food. To aid in discriminating between mechanisms that may be responsible for this decrement, SW was applied repeatedly to the lip ipsilateral or contralateral to one of the paired MCCs, and then generalization of the response decrement was tested by applying a SW stimulus to the opposite (non-stimulated) receptive field. There was statistically significant generalization of response decrement and the amount of generalization appeared to be a function of whether the decrementing stimuli were presented on the side ipsilateral vs. contralateral to the recorded MCC. The overall data suggest that MCC response decrement to repeated food stimuli results in a process analogous to behavioral habituation, and the data are consistent with a simple neural model.

Actions of a pair of identified cerebral-buccal interneurons (CBI-8/9) in Aplysia that contain the peptide myomodulin

A combination of biocytin back-fills of the cerebral-buccal connectives and immunocytochemistry of the cerebral ganglion demonstrated that of the 13 bilateral pairs of cerebral-buccal interneurons in the cerebral ganglion, a subpopulation of 3 are immunopositive for the peptide myomodulin. The present paper describes the properties of two of these cells, which we have termed CBI-8 and CBI-9. CBI-8 and CBI-9 were found to be dye coupled and electrically coupled. The cells have virtually identical properties, and consequently we consider them to be "twin" pairs and refer to them as CBI-8/9. CBI-8/9 were identified by electrophysiological criteria and then labeled with dye. Labeled cells were found to be immunopositive for myomodulin, and, using high pressure liquid chromatography, the cells were shown to contain authentic myomodulin. CBI-8/9 were found to receive synaptic input after mechanical stimulation of the tentacles. They also received excitatory input from C-PR, a neuron involved in neck lengthening, and received a slow inhibitory input from CC5, a cell involved in neck shortening, suggesting that CBI-8/9 may be active during forward movements of the head or buccal mass. Firing of CBI-8 or CBI-9 resulted in the activation of a relatively small number of buccal neurons as evidenced by extracellular recordings from buccal nerves. Firing also produced local movements of the buccal mass, in particular a strong contraction of the I7 muscle, which mediates radula opening. CBI-8/9 were found to produce a slow depolarization and rhythmic activity of B48, the motor neuron for the I7 muscle. The data provide continuing evidence that the small population of cerebral buccal interneurons is composed of neurons that are highly diverse in their functional roles. CBI-8/9 may function as a type of premotor neuron, or perhaps as a peptidergic modulatory neuron, the functions of which are dependent on the coactivity of other neurons.

C-PR neuron of Aplysia has differential effects on "Feeding" cerebral interneurons, including myomodulin-positive CBI-12

Head lifting and other aspects of the appetitive central motive state that precedes consummatory feeding movements in Aplysia is promoted by excitation of the C-PR neuron. Food stimuli activate C-PR as well as a small population of cerebral-buccal interneurons (CBIs). We wished to determine if firing of C-PR produced differential effects on the various CBIs or perhaps affected all the CBIs uniformly as might be expected for a neuron involved in producing a broad undifferentiated arousal state. We found that when C-PR was fired, it produced a wide variety of effects on various CBIs. Firing of C-PR evoked excitatory input to a newly identified CBI (CBI-12) the soma of which is located in the M cluster near the previously identified CBI-2. CBI-12 shares certain properties with CBI-2, including a similar morphology and a capacity to drive rhythmic activity of the buccal-ganglion. Unlike CBI-2, CBI-12 exhibits myomodulin immunoreactivity. Furthermore when C-PR is fired, CBI-12 receives a polysynaptic voltage-dependent slow excitation, whereas, CBI-2 receives relatively little input. C-PR also polysynaptically excites other CBIs including CBI-1 and CBI-8/9 but produces inhibition in CBI-3. In addition, firing of C-PR inhibits plateau potentials in CBI-5/6. The data suggest that activity of C-PR may promote the activity of one subset of cerebral-buccal interneurons, perhaps those involved in ingestive behaviors that occur during the head-up posture. C-PR also inhibits some cerebral-buccal interneurons that may be involved in behaviors in which C-PR activity is not required or may even interfere with other feeding behaviors such as rejection or grazing, that occur with the head down.

A pair of reciprocally inhibitory histaminergic sensory neurons are activated within the same phase of ingestive motor programs in Aplysia

Previous studies have shown that each buccal ganglion in Aplysia contains two B52 neurons, one in each hemiganglion. We now show that there are two B52 neurons in a single buccal hemiganglion and four cells in an animal. We also show that the B52 neurons are histamine-immunoreactive and use reverse phase HPLC to show that the histamine-immunoreactive substance is authentic histamine. Previous studies have shown that the B52 neurons make numerous inhibitory synaptic connections with neurons active during the radula closing/retraction phase of ingestive motor programs. A computational model of the Aplysia feeding central pattern generator has, therefore, suggested that the B52 neurons play a role in terminating closing/retraction. Consistent with this idea we show that both B52 neurons fire at the beginning of radula opening/protraction. We also show that both B52 neurons are sensory neurons. They are depolarized when a flap of connective tissue adjacent to the buccal commissural arch is stretched. During ingestive feeding this is likely to occur at the peak of closing/retraction as opening/protraction begins. In the course of this study we compare the two ipsilateral B52 neurons and show that these cells are virtually indistinguishable; e.g., they use a common neurotransmitter, make the same synaptic connections, and are both sensory as well as premotor neurons. Nevertheless we show that the B52 neurons are reciprocally inhibitory. Our results, therefore, strikingly confirm theoretical predictions made by others that neurons that inhibit each other will not necessarily participate in antagonistic phases of behavior.

Mass spectrometric survey of interganglionically transported peptides in Aplysia

The major ganglionic connectives in Aplysia are assayed to determine putative neuropeptides. Matrix-assisted laser desorption/ionization mass spectrometry allows direct measurement of peptides in a nerve. Many previously characterized peptides are observed, including APGWamide, buccalins, small cardioactive peptides, and egg-laying hormone. Several unreported peptides are detected in specific nerves, suggesting they may have important physiological roles. Furthermore, novel processing products of the L5-67 precursor peptide and the APGWamide/cerebral peptide 1 prohormone are strongly suggested, and their interganglionic transport demonstrated.

Analyzing the functional consequences of transmitter complexity

Neurons and other cells are regulated by a great multiplicity of neurotransmitters, modulators, hormones and other chemical messengers, which, through complex networks of extensively diverging and converging pathways, exert a multiplicity of effects. How do we analyze the functioning of such a complex network? If the effects of a transmitter depend on the presence of many other transmitters, how can we predict what they will be? If multiple transmitters act through the same network, how can their actions be specific? If they converge on the same effects, are some of the transmitters redundant? Why are there so many transmitters? Such questions can be addressed using an analytical approach that examines, qualitatively or quantitatively, how the operation of the network globally maps a multidimensional input space of transmitters to a multidimensional output space of effects.

Ion currents and mechanisms of modulation in the radula opener muscles of Aplysia

Ion currents and mechanisms of modulation in the radula opener muscles of Aplysia. J. Neurophysiol. 78: 2372-2387, 1997. Numerous studies of plasticity in the feeding behavior of Aplysia have shown that substantial plasticity is due to peripheral neuromodulation of the feeding musculature. Extensive previous work focusing on the accessory radula closer (ARC) muscle has led to the realization that a major function of the modulation in that muscle may be to ensure efficient coordination between its contractions and those of its antagonist muscles. For a more complete understanding, therefore, we must study these muscles also. Here we have studied the radula opener muscles I7-I10. Using single isolated muscle fibers under voltage clamp, we have characterized ion currents gated by voltage and by the physiological contraction-inducing neurotransmitter acetylcholine (ACh) and the effects of the physiological modulators serotonin, myomodulins A and B, and FMRFamide. Our results explain significant aspects of the electrophysiological behavior of the whole opener muscles, as well as why the opener and ARC muscles behave similarly in many ways yet differently in some key respects. Opener muscles express four types of K currents: inward rectifier, A-type [IK(A)], delayed rectifier [IK(V)], and Ca2+-activated [IK(Ca)]. They also express an L-type Ca current [ICa] and a leakage current. ACh activates a positive-reversing cationic current [IACh(cat)] and a negative-reversing Cl current [IACh(Cl)]. The opener muscles differ from the ARC in that, in the openers, activation of IK(A) occurs approximately 9 mV more positive and there is much less IACh(Cl). In both muscles, IACh(cat) most likely serves to depolarize the muscle until ICa activates to supply Ca2+ for contraction, but further depolarization and spiking is opposed by coactivation of IK(A), IK(V), IK(Ca), and IACh(Cl). Thus the differences in IK(A) and IACh(Cl) may well be key factors that prevent spikes in the ARC but often allow them in the opener muscles. As in the ARC, the modulators enhance ICa and so potentiate contractions. They also activate a modulator-specific K current, which causes hyperpolarization and depression of contractions. Finally, in the opener muscles but not in the ARC, the modulators activate a depolarizing cationic current that may help phase-advance the contractions. Each modulator exerts these effects to different degrees and thus has a distinct effect on voltage and contraction size and shape. The overall effect then will depend on the specific combinations of modulators released in different behaviors. By understanding the modulation in the opener muscles, as well as in the ARC, we are now in a position to understand how the behavior of the two muscles is coordinated under a variety of circumstances.

Control of time-dependent biological processes by temporally patterned input

Temporal patterning of biological variables, in the form of oscillations and rhythms on many time scales, is ubiquitous. Altering the temporal pattern of an input variable greatly affects the output of many biological processes. We develop here a conceptual framework for a quantitative understanding of such pattern dependence, focusing particularly on nonlinear, saturable, time-dependent processes that abound in biophysics, biochemistry, and physiology. We show theoretically that pattern dependence is governed by the nonlinearity of the input-output transformation as well as its time constant. As a result, only patterns on certain time scales permit the expression of pattern dependence, and processes with different time constants can respond preferentially to different patterns. This has implications for temporal coding and decoding, and allows differential control of processes through pattern. We show how pattern dependence can be quantitatively predicted using only information from steady, unpatterned input. To apply our ideas, we analyze, in an experimental example, how muscle contraction depends on the pattern of motorneuron firing.

Ganglionic distribution of inputs and outputs of C-PR, a neuron involved in the generation of a food-induced arousal state in Aplysia

Cerebral neuron C-PR is thought to play an important role in the appetitive phase of feeding behavior of Aplysia. Here, we describe the organization of input and output pathways of C-PR. Intracellular dye fills of C-PR revealed extensive arborization of processes within the cerebral and the pedal ganglia. Numerous varicosities of varying sizes may provide points of synaptic inputs and outputs. Blocking polysynaptic transmission in the cerebral ganglion eliminated the sensory inputs to C-PR from stimuli applied to the rhinophores or tentacles, indicating that this input is probably mediated by cerebral interneurons. Identified cerebral mechanoafferent sensory neurons polysynaptically excite C-PR. Stimulation of the eyes and rhinophores with light depresses C-PR spike activity, and this effect also appears to be mediated by cerebral interneurons. C-PR has bilateral synaptic actions on numerous pedal ganglion neurons, and also has effects on cerebral neurons, including the MCC, Bn cells, CBIs and the contralateral C-PR. Although the somata of these cerebral neurons are physically close to C-PR, experiments using high divalent cation-containing solutions and cutting of various connectives indicated that the effects of C-PR on other cerebral ganglion neurons (specifically Bn cells and the MCC) are mediated by interneurons that project back to the cerebral ganglion via the pedal and pleural connectives. The indirect pathways of C-PR to other cerebral neurons may help to ensure that consummatory motor programs are not activated until the appropriate appetitive motor programs, mediated by the pedal ganglia, have begun to be expressed.

Giant protein kinases: domain interactions and structural basis of autoregulation

The myosin-associated giant protein kinases twitchin and titin are composed predominantly of fibronectin- and immunoglobulin-like modules. We report the crystal structures of two autoinhibited twitchin kinase fragments, one from Aplysia and a larger fragment from Caenorhabditis elegans containing an additional C-terminal immunoglobulin-like domain. The structure of the longer fragment shows that the immunoglobulin domain contacts the protein kinase domain on the opposite side from the catalytic cleft, laterally exposing potential myosin binding residues. Together, the structures reveal the cooperative interactions between the autoregulatory region and the residues from the catalytic domain involved in protein substrate binding, ATP binding, catalysis and the activation loop, and explain the differences between the observed autoinhibitory mechanism and the one found in the structure of calmodulin-dependent kinase I.

Costorage and corelease of modulatory peptide cotransmitters with partially antagonistic actions on the accessory radula closer muscle of Aplysia californica

Many neurons that contain a classical neurotransmitter also contain modulatory peptides, but it has been difficult to establish unequivocally that these peptides are functional cotransmitters. Here, we provide evidence for functional cotransmission in a neuromuscular system of Aplysia. Using immunocytochemical techniques, we localize members of two peptide families, the small cardioactive peptides (SCPs) and the buccalins (BUCs), to a single subset of dense-core vesicles in the terminals of the cholinergic motorneuron B15. We describe a new preparation and method for the direct detection of released peptides and show that the SCPs and BUCs are released when neuron B15 is intracellularly stimulated. Consistent with their subcellular localization, the SCPs and BUCs are released in a stoichiometric ratio that is constant across conditions that change the absolute amount of peptides released. Peptide release is calcium-dependent but does not require muscle contractions. Thus, the release cannot be attributed to a displacement of peptides that may be present in the extracellular space. In previous studies, we characterized the physiological firing patterns of neuron B15. Here, we simulate these firing patterns and show that peptide release occurs. Additionally, we find that significant quantities of material are released under behaviorally relevant conditions. We find that concentrations of released peptides in the muscle are in the concentration range in which exogenously applied peptides exert characterized modulatory actions on muscle contractions. Together, our findings provide strong support for the hypothesis that peptides contained in neuron B15 are functional cotransmitters.

Release of peptide cotransmitters in Aplysia: regulation and functional implications

To gain insights into the physiological role of cotransmission, we measured peptide release from cell B15, a motorneuron that utilizes ACh as its primary transmitter but also contains putative peptide cotransmitters, the small cardioactive peptides (SCPs) and the buccalins (BUCs). All stimulation parameters used were in the range in which B15 fires in freely moving animals. We stimulated neuron B15 in bursts and systematically varied the interburst interval, the intraburst frequency, and burst duration. Both peptides were preferentially released when B15 was stimulated at higher intra- or interburst frequencies or with longer burst durations. Across stimulation patterns, the amount of peptide released depended on the mean frequency of stimulation and was independent of the specific pattern of stimulation. The parameters of stimulation that produce a larger release of peptides correspond to those that evoke larger contractions. Large and frequent contractions are likely to fuse or summate, thus disrupting the rhythmic behavior mediated by the muscle innervated by motorneuron B15. Because the combined effect of the SCPs and BUCs is to accelerate the relaxation and shorten the duration of muscle contractions, these peptides reduce the probability of the disruptive fusion or summation of muscle contractions. Because these cotransmitters regulate an aspect of muscle contractions that is not controlled by acetylcholine (ACh), the primary transmitter of B15, we suggest that peptides and ACh form parallel but functionally distinct lines of transmission at the neuromuscular junction. Both types of transmission may be necessary to ensure that behavior remains efficient over a wide range of conditions.

Substrate specificity and inhibitor sensitivity of Ca2+/S100-dependent twitchin kinases

Myosin-associated giant protein kinases of the titin/witchin-like superfamily have previously been implicated in the regulation of muscle function, based on genetic and physiological studies. We find that recombinant constitutively active Caenorhabditis elegans and Aplysia twitchin kinase fragments differ in their catalytic activities and peptide-substrate specificities, as well as in their sensitivities to the naphthalene sulfonamide inhibitors 1-(5-chloronaphthalenesulfonyl)-1H-hexahydro-1,4-diazepine (ML-7) and 1-(5-iodonaphthalenesulfonyl)-1H-hexahydro-1,4-diazepine (ML-9). The constitutively active Aplysia twitchin kinase fragment has a remarkably high activity (Vmax > 100 mumol.min-1.mg-1) towards some substrate peptides. The autoinhibited forms of these twitchin kinases can be activated in a Ca(2+)-dependent manner by the dimeric form of the S100A1 protein (S100A1(2)). The twitchin kinase S100A1(2)-binding site can also bind Ca2+/calmodulin but neither kinase is activated by calmodulin. The data provide a functional basis for the ongoing crystallographic study of twitchin kinase fragments.

Synaptic mechanisms in invertebrate pattern generation

Although individual neurons can be intrinsically oscillatory and can be network pacemakers, motor patterns are often generated in a more distributed manner. Synaptic connections with other neurons are important because they either modify the rhythm of the pacemaker cell or are essential for pattern generation in the first place. Computational studies of half-center oscillators have made much progress in describing how neurons make transitions between active and inactive phases in these simple networks. In addition to characterizing phase transitions, recent studies have described the synaptic mechanisms that are important for the initiation and maintenance of activity in half-center oscillators.

A cerebral central pattern generator in Aplysia and its connections with buccal feeding circuitry

Different feeding-related behaviors in Aplysia require substantial variations in the coordination of movements of two separate body parts, the lips and buccal mass. The central pattern generators (CPGs) and motoneurons that control buccal mass movements reside largely in the buccal ganglion. It was previously thought that control of the cerebral neuronal circuitry and motoneurons that generate lip movements was coordinated directly by feedback from buccal interneurons. Here, we describe cerebral lip motoneuron C15, which drives rhythmic activity in the isolated cerebral ganglion. Other lip motoneurons are active during this program, so we define it as a cerebral motor program (CMP). The C15 in each cerebral hemiganglion drives the CMP in ipsilateral neurons only, suggesting there are independent CPGs in each hemiganglion. The cerebral and buccal CPGs interact at several points. For example, cerebral-to-buccal interneurons (CBIs), which can drive the buccal CPG, receive excitatory input when the cerebral CPG is active. Likewise, C15, which can drive the cerebral CPG, is excited when the buccal CPG is active. This excitation is simultaneous in both C15s, coupling the activity in the two hemiganglionic cerebral CPGs. Therefore, there are independent cerebral and buccal CPGs, which can produce distinct rhythms, but which interact at several points. Furthermore, the connections between the cerebral and buccal CPGs alter during different forms of motor program. We suggest that such alterations in the interactions between these CPGs might contribute to the generation of the various forms of coordination of lip and buccal mass movements that are necessary during different feeding-related behaviors.

An identified interneuron contributes to aspects of six different behaviors in Aplysia

Previous results have indicated that the bilateral cerebral interneuron CC5 mediates the pedal artery shortening that is a component of defensive withdrawal responses involving the head. Current studies suggest that CC5 contributes to aspects of at least six different behaviors: locomotion, head turning, defensive head withdrawal, local tentacular withdrawal, rhythmic feeding, and head lifting. In addition to receiving input from mechanoreceptors in the head, CC5 receives synaptic input during fictive locomotor and feeding programs. Firing of CC5 produces widespread monosynaptic or polysynaptic actions in all ganglia in the animal. CC5 excites presumptive motor neurons for the neck, and its activity can contract neck muscles. The pedal artery shortener motor neuron (PAS), a key excitatory follower cell of CC5, fires during ipsilateral head turning, head withdrawal, tentacle withdrawal, feeding, and locomotion. For all behaviors, except locomotion and biting, responses of PAS were eliminated by cutting the ipsilateral-pleural connective, which interrupts the only direct connection of CC5 to the ipsilateral PAS. The data suggest that CC5 is a multifunctional interneuron that plays different roles during different behaviors. The neuron appears to be involved in producing coordinated movements of the head, involving both somatic and visceral muscles. For some behaviors, or for certain aspects of behaviors, CC5 appears to act as an individual command-like neuron; for other behaviors, CC5 appears to act more as an element of a distributed circuit and is neither necessary nor sufficient for any aspects of the behavior.

Functional uncoupling of linked neurotransmitter effects by combinatorial convergence

Physiological signaling pathways both diverge and converge-a single neurotransmitter can have multiple effects and multiple transmitters can have the same effects-in the same target cell. Divergence couples the effects of a transmitter together in a relatively fixed ratio. Different physiological circumstances may require a different ratio, however; the coupling must be made modifiable. This can be achieved through convergence. If two transmitters couple the effects in different ratios, then combinations of the transmitters can yield all intermediate ratios of the effects, thus functionally uncoupling them. This mechanism is analyzed in a well-understood, simple invertebrate neuromuscular circuit.