Biological Sciences 330/331, Smith College | Neurophysiology References to the Crayfish Swimmeret System Selected PubMed citations and abstracts through January 2005.
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Hooper, SL and R.A. DiCaprio (2004) Crustacean motor pattern generator networks. Neurosignals 13:50-69. • Crustacean motor pattern-generating networks have played central roles in understanding the cellular and network bases of rhythmic motor patterns for over half a century. We review here the four best investigated of these systems: the stomatogastric, ventilatory, cardiac, and swimmeret systems. Generally applicable observations arising from this work include (1) neurons with active, endogenous cell properties (endogenous bursting, postinhibitory rebound, plateau potentials), (2) nonhierarchical (distributed) network synaptic connectivity patterns characterized by high levels of inter-neuronal connections, (3) nonspiking neurons and graded transmitter release, (4) multiple modulatory inputs, (5) networks that produce multiple patterns and have flexible boundaries, and (6) peripheral properties (proprioceptive feedback loops, low-frequency muscle filtering) playing an important role in motor pattern generation or expression.   Mulloney, B. and W.M. Hall (2003) Local commissural interneurons integrate information from intersegmental coordinating interneurons.  J. Comp. Neurol. 466: 366-376. • The information that coordinates movements of swimmerets on different segments of the crayfish abdomen is conducted by interneurons that originate in each abdominal ganglion. These interneurons project axons to neighboring ganglia and beyond. To discover the anatomy of these axons in their target ganglia, we used Neurobiotin and dextran-Texas Red microelectrodes to fill them near their targets. Coordinating axons coursed through these target ganglia close to the midline and extended only a few short branches that did not approach the lateral neuropils. Two of the three types of coordinating axons made direct synaptic connections with a class of local commissural interneurons that relayed the information to targets in the swimmeret pattern-generating circuits. These commissural interneurons, named here ComInt 1 neurons, followed a particular route to cross the midline and reach their targets. ComInt 1 neurons were nonspiking; they received EPSPs from the coordinating axons near the midline and transmitted this information to their targets in the lateral neuropils using graded transmission. The output of each ComInt 1 was restricted to a single local circuit and had opposite effects on the power-stroke and return-stroke motor neurons driven by that circuit. ComInt 1 neurons were direct postsynaptic targets of both descending and ascending coordinating axons that originated in other anterior and posterior ganglia. Because of phase differences in the impulses in these different coordinating axons, their signals arrived simultaneously at each ComInt 1. We discuss these findings in the context of alternative models of the intersegmental coordinating circuit.   Mulloney, B. (2003)  During fictive locomotion, graded synaptic currents drive bursts of impulses in swimmeret motor neurons.  J. Neurosci. 23: 5953-5962. • During forward swimming, motor neurons that innervate each crayfish swimmeret fire periodic coordinated bursts of impulses. These bursts occur simultaneously in neurons that are functional synergists but alternate with bursts in their antagonists. These impulses ride on periodic oscillations of membrane potential that occur simultaneously in neurons of each type. A model of the local circuit that generates this motor pattern has been proposed. In this model, each motor neuron is driven alternately by excitatory and inhibitory synaptic currents from nonspiking local interneurons. I tested this model by perturbing individual interneurons and recording synaptic currents and changes in input resistance from each class of motor neuron. I also simulated the synaptic currents that would be observed in a cell subject to different patterns of presynaptic input. When the CNS was actively expressing the swimming motor pattern, changes in the membrane potential of individual local interneurons controlled firing of whole sets of motor neurons. Membrane currents in these motor neurons oscillated in phase with the motor output from their own local circuit. The phases of these oscillations differed in different functional classes of motor neurons. In neurons that could be clamped at the reversal potential of their outward currents, the model predicted that large periodic inward currents would be recorded. I observed no signs of periodic inward currents, even when the outward currents clearly had reversed. These results permit a simplification of the cellular model. They are discussed in the context of neural control of locomotion in crustacea and insects.   Jones, S.R., B. Mulloney, T.J. Kaper and N. Kopell (2003)  Coordination of cellular pattern-generating circuits that control limb movements: the sources of stable differences in intersegmental phases.  J.Neurosci.  23: 3457-3468 • Neuronal mechanisms in nervous systems that keep intersegmental phase lags the same at different frequencies are not well understood. We investigated biophysical mechanisms that permit local pattern-generating circuits in neighboring segments to maintain stable phase differences. We use a modified version of an existing model of the crayfish swimmeret system that is based on three known coordinating neurons and hypothesized intersegmental synaptic connections. Weakly coupled oscillator theory was used to derive coupling functions that predict phase differences between neurons in neighboring segments. We show how features controlling the size of the lag under simplified network configurations combine to create realistic lags in the full network. Using insights from the coupled oscillator theory analysis, we identify an alternative intersegmental connection pattern producing realistic stable phase differences. We show that the persistence of a stable phase lag to changes in frequency can arise from complementary effects on the network with ascending-only or descending-only intersegmental connections. To corroborate the numerical results, we experimentally constructed phase-response curves (PRCs) for two different coordinating interneurons in the swimmeret system by perturbing the firing of individual interneurons at different points in the cycle of swimmeret movement. These curves provide information about the contribution of individual intersegmental connections to the stable phase lag. We also numerically constructed PRCs for individual connections in the model. Similarities between the experimental and numerical PRCs confirm the plausibility of the network configuration that has been proposed and suggest that the same stabilizing balance present in the model underlies the normal phase-constant behavior of the swimmeret system.   Mulloney, B., T. Naranzogt and W.M. Hall (2003)   Architectonics of crayfish ganglia.  Microsc. Res. Tech. 60: 253-265 • The central nervous system of crayfish consists of a chain of segmental ganglia that are linked by cables of intersegmental axons. Each ganglion contains a highly-ordered core of longitudinal tracts, vertical tracts, commisures, and synaptic neuropils. We review from a techynical perspective the history of the description of these gagnlia, and recognize four episodes of progress. Each major innovation in anatomical methods has led to new insight into the structure and function of this nervous system, and new awareness of the structural patterns that are common to the CNS of all arthropods. Ganglia in different segments of the body differ in size, and appear to differ in anatomy. From a comparison of the structures of the cores of abdominal, thoracic, and subesophageal ganglia, we argue that this apparent difference is illusory. Rather, each of these ganglia is organized on the same plan, a plan also found in insect segmental ganglia. The apparent differences follow from longitudinal compression during development and from allometric growth of particular neuropils associated with innervation of the walking legs. Different authors have described the internal organization of ganglia in different segments, so we provide a cross-reference to the nomenclatures they have introduced. We compare the locations of cell bodies of motor neurons and accessory neurons that innervate different peripheral structures, and demonstrate double-labeling of certain GABAergic peripheral inhibiory neurons. Finally, we describe the construction of digital movies of serial sections of these ganglia, and discuss their utility.   Larimer JL, Moore D. (2003) Neural basis of a simple behavior: abdominal positioning in crayfish. Microsc Res Tech 60: 346 59 • Crustaceans have been used extensively as models for studying the nervous system. Members of the Order Decapoda, particularly the larger species such as lobsters and crayfish, have large segmented abdomens that are positioned by tonic flexor and extensor muscles. Importantly, the innervation of these tonic muscles is known in some detail. Each abdominal segment in crayfish is innervated bilaterally by three sets of nerves. The anterior pair of nerves in each ganglion controls the swimmeret appendages and sensory supply. The middle pair of nerves innervates the tonic extensor muscles and the regional sensory supply. The superficial branch of the most posterior pair of nerves in each ganglion is exclusively motor and supplies the tonic flexor muscles of that segment. The extension and flexion motor nerves contain six motor neurons, each of which is different in axonal diameter and thus produces impulses of different amplitude. Motor programs controlling each muscle can be characterized by the identifiable motor neurons that are activated. Early work in this field discovered that specific central interneurons control the abdominal positioning motor neurons. These interneurons were first referred to as "command neurons" and later as "command elements." Stimulation of an appropriate command element causes a complex, widespread output involving dozens of motor neurons. The output can be patterned even though the stimulus to the command element is of constant interval. The command elements are identifiable cells. When a stimulus is repeated in a command element, from either the same individual or from different individuals, the output is substantially the same. This outcome depends upon several factors. First, the command elements are not only identifiable, but they make many synapses with other neurons, and the synapses are substantially invariant. There are separate flexion producing and extension producing command elements. Abdominal flexion producing command elements excite other flexion elements and inhibit extensor command elements. The extension producing elements do the opposite. These interactions insure that interneurons of a particular class (flexion or extension producing) synaptically recruit perhaps twenty others of similar output, and that command elements promoting the opposing movements are inhibited. This strong reciprocity and the recruitment of similar command elements give a powerful motor program that appears to mimic behavior.   Okada J, Kuwasawa K. (2002) A push pull set of elastic strand stretch receptor neurons for the swimmeret in an isopod Bathynomus doederleini. J Comp Physiol A Neuroethol Sens Neural Behav Physiol 188(10):781 6 • There coexist two types of neuronal terminal processes attaching to elastic strands at the socket of the swimmeret in Bathynomus doederleini. One of the processes, stretch receptor I is derived from the 1st nerve root of the abdominal ganglion. The other, stretch receptor II is derived from the 2nd nerve root of the ganglion. Both axons of stretch receptors are very thick (30 60 micro m) at sites before the terminal arborization. Cell bodies of the stretch receptors are located in the ganglion of their own segments. The neuronal cell body of the stretch receptor I is located at the anterior half of the hemiganglion ipsilateral to the periphery, and the neuronal cell body of the stretch receptor II at the posterior half of the hemiganglion contralateral to the periphery. Their signaling modalities in response to swimmeret movements were analyzed from intracellular recordings from the cell bodies. Stretch receptor I produced a sustained hyperpolarizing potential in response to protraction of the swimmeret. Stretch receptor II produced a sustained depolarizing potential in response to the protraction, and moreover, generated spike potentials on the rising phase of the depolarizing potential according to its height and steepness. Both the stretch receptors are a push pull set of elastic strand stretch receptors for the angular position and velocity of swimmeret movements.   Copp NH, Hodes S. (2001) Asynchronous swimmeret beating during defense turns in the crayfish, Procambarus clarkii. J Comp Physiol [A] 187(9):737 45 • Swimmeret beating was monitored in freely moving specimens of the crayfish Procambarus clarkii as they exhibited defense turn responses to tactile stimuli. Analysis of videotape records revealed alterations in swimmeret beating during turning responses compared to straight, forward walking. During turns, swimmerets beat with shorter periods and smaller amplitude power strokes than during straight walking. Coordination between swimmerets also changed. Swimmerets on the side toward which the animal turned tended to lag behind their contralateral partners, rather than beat in synchrony as in straight walking, and ipsilateral coordination was loosened relative to straight walking. Asynchronous swimmeret beating accompanied asymmetric motions of the uropods in a manner similar to that observed during statocyst dependent equilibrium reactions in P. clarkii, but removal of the statoliths did not eliminate turn associated responses of the swimmerets. The coordinated action of the swimmerets and uropods may contribute to the torque that rotates the animal in the yaw plane. Implications of the observed changes in swimmeret coordination for understanding the underlying neuronal control system are discussed.   Naranzogt, T., W.M. Hall, and B. Mulloney (2001)  Limb movments during locomotion: Tests of a model of an intersegmental coordinating circuit. J. Neurosci. 21: 7859 7869. • During normal forward swimming, the swimmerets on neighboring segments of the crayfish abdomen make periodic power stroke movements that have a characteristic intersegmental difference in phase. Three types of intersegmental interneurons that originate in each abdominal ganglion are necessary and sufficient to maintain this phase relationship. A cellular model of the intersegmental coordinating circuit that also produces the same intersegmental phase has been proposed. In this model, coordinating axons synapse with local interneurons in their target ganglion and form a concatenated circuit that links neighboring segmental ganglia. This model assumed that coordinating axons projected to their nearest neighboring ganglion but not farther. We tested this assumption in two sets of experiments. If the assumption is correct, then blocking synaptic transmission in an intermediate ganglion should uncouple swimmeret activity on opposite sides of the block. We bathed individual ganglia in a low Ca2+ high Mg2+ saline that effectively silenced both motor output from the ganglion and the coordinating interneurons that originated in it. With this block in place, other ganglia on opposite sides of the block could nonetheless maintain their normal phase difference. Simultaneous recordings of spikes in coordinating axons on opposite sides of the blocked ganglion showed that these axons projected beyond the neighboring ganglion. Selective bilateral ablation of the tracts in which these axons ran showed that they were necessary and usually sufficient to maintain coordination across a blocked ganglion. We discuss revisions of the cellular model of the coordinating circuit that would incorporate these new results.   Mulloney, B. and W.M. Hall (2000) Functional organization of crayfish abdominal ganglia: III. Swimmeret motor neurons. J. Comp. Neurol. 419: 233 243. • Swimmerets are limbs on several segments of the crayfish abdomen that are used for forward swimming and other behaviors. We present evidence that the functional modules demonstrated previously in physiological experiments are reflected in the morphological disposition of swimmeret motor neurons. The single nerve that innervates each swimmeret divides into two branches that separately contain the axons of power stroke and return stroke motor neurons. We used Co++ or biocytin to backfill the entire pool of neurons that innervated a swimmeret, or functional subsets whose axons occurred in particular branches. Each filled cell body extended a single neurite that projected first to the Lateral Neuropil (LN), and there branched to form dendritic structures and its axon. All the motor neurons that innervated one swimmeret had cell bodies located in the ganglion from which their axons emerged, and the cell bodies of all but two of these neurons were located ipsilateral to their swimmeret. Counts of cell bodies filled from selected peripheral branches revealed about 35 power stroke motor neurons and 35 return stroke motor neurons. The cell bodies of these two types were segregated into different clusters within the ganglion, but both types sent their neurites into the ipsilateral LN and had their principle branches in this neuropil. We saw no significant differences in the numbers or distributions of these motor neurons in ganglia A2 through A5. These anatomical features are consistent with the physiological evidence that each swimmeret is controlled by its own neural module, which drives the alternating bursts of impulses in power stroke and return stroke motor neurons. We propose that the LN is the site of the synaptic circuit that generates this pattern.   Namba H., B. Mulloney (1999) Coordination of limb movements: Three types of intersegmental interneurons in the swimmeret system, and their responses to changes in excitation. J Neurophysiol. 81: 2437 2450. • During forward locomotion, the movements of swimmerets on different segments of the crayfish abdomen are coordinated so that more posterior swimmerets lead their anterior neighbors by ~25%. This coordination is accomplished by mechanisms within the abdominal nerve cord. Here we describe three different types of intersegmental swimmeret interneurons that are necessary and sufficient to accomplish this coordination. These interneurons could be identified both by their structures within their home ganglion and by their physiological properties. These interneurons occur as bilateral pairs in each ganglion that innervates swimmerets, and their axons traverse the minuscule tract (MnT) of their home ganglion before leaving to project to neighboring ganglia. Two types, ASCE and ASCL, projected an axon anteriorly; the third type, DSC, projected posteriorly. Each type fires a burst of impulses starting at a different phase of the swimmeret cycle in its home ganglion. In active preparations, excitation of individual ASCE or DSC interneurons at different phases in the cycle affected the timing of the next cycle in the interneuron's target ganglion. The axons of these interneurons that projected between two ganglia ran close together, and their firing often could be recorded by the same electrode. Experiments in which either this tract or the rest of the intersegmental connectives was cut bilaterally showed that these interneurons were both necessary and sufficient for coordination of neighboring swimmerets. When the level of excitation of the swimmeret system was increased by bath application of carbachol, the period of the system's cycle shortened, but the characteristic phase difference within and between ganglia was preserved. Each of these interneurons responded to this increase in excitation by increasing the frequency of impulses within each burst, but the phases and relative durations of their bursts did not change, and their activity remained coordinated with the cycle in their home ganglion. The decrease in duration of each burst was matched to the increase in impulse frequency within the burst so that the mean numbers of impulses per burst did not change significantly despite a threefold change in period. These three types of interneurons appear to form a concatenated intersegmental coordinating circuit that imposes a particular intersegmental phase on the local pattern generating modules innervating each swimmeret. This circuit is asymmetric, and forces posterior segments to lead each cycle of output.   Skinner, F.K. and B. Mulloney (1998) New advances in understanding intersegmental coordination in invertebrates and vertebrates. Curr. Opinion in Neurobiol. 8:725 732. • How does the CNS coordinate muscle contractions between different body segments during normal locomotion? Work on several preparations has shown that this coordination relies on excitability gradients and on differences between ascending and descending intersegmental coupling. Abstract models involving chains of coupled oscillators have defined properies of coordinating cirucuits that would permit them to establish a constant intersegmental phase in the face of changing periods. Analyses that combine computational and experimental strategies have led to new insights into the cellular organization of intersegmental coordinating circuits and the neural control of swinning in lamprey, tadpole, crayfish and leech.   Mulloney, B., H. Namba, F.K. Skinner and W.M. Hall (1998) Intersegmental coordination of swimmeret movements: mathematical models and interneurons. In: Neuronal mechanisms for generating locomotor activity (Kiehn, O et al., eds) Ann NY Acad Sci. 860:266-280.   Skinner, F.K. and B. Mulloney (1998) Intersegmental coordination of limb movements during locomotion: mathematical models predict circuits that drive swimmeret beating. J. Neurosci. 18: 38331 3842. • Normal locomotion in arthropods and vertebrates is a complex behavior, and the neural mechanisms that coordinate their limbs during locomotion at different speeds are unknown. The neural modules that drive cyclic movements of swimmerets respond to changes in excitation by changing the period of the motor pattern. As period changes, however, both intersegmental phase differences and the relative durations of bursts of impulses in different sets of motor neurons are preserved. To investigate these phenomena, we constructed a cellular model of the local pattern generating circuit that drives each swimmeret. We then constructed alternative intersegmental circuits that might coordinate these local circuits. The structures of both the model of the local circuit and the alternative models of the coordinating circuit were based on and constrained by previous experimental results on pattern generating neurons and coordinating interneurons. To evaluate the relative merits of these alternatives, we compared their dynamics with the performance of the real circuit when the level of excitation was changed. Many of the alternative coordinating circuits failed. One coordinating circuit, however, did effectively match the performance of the real system as period changed from 1 to 3.2 Hz. With this coordinating circuit, both the intersegmental phase differences and the relative durations of activity within each of the local modules fell within the ranges characteristic of the normal motor pattern and did not change significantly as period changed. These results predict a mechanism of coordination and a pattern of intersegmental connections in the CNS that is amenable to experimental test.   Mulloney B (1997) A test of the excitability gradient hypothesis in the swimmeret system of crayfish. Journal of Neuroscience. 17(5):1860 8. [See Laboratory 8.] • The motor pattern that drives coordinated movements of swimmerets in different segments during forward swimming characteristically begins with a power stroke by the most posterior limbs, followed progressively by power strokes of each of the more anterior limbs. To explain this caudal to rostral progression, the hypothesis was proposed that the neurons that drive the most posterior swimmerets are more excitable than their more anterior counterparts, and so reach threshold first. To test this excitability gradient hypothesis, I used carbachol to excite expression of the swimmeret motor pattern and used tetrodotoxin (TTX), sucrose solutions, and cutting to block the flow of information between anterior and posterior segments. I showed that the swimmeret activity elicited by carbachol is like that produced when the swimmeret system is spontaneously active and that blocking an intersegmental connective uncoupled swimmeret activity on opposite sides of the block. When anterior and posterior segments were isolated from each other, the frequencies of the motor patterns expressed by anterior segments were not slower than those expressed by posterior segments exposed to the same concentrations of carbachol. This result was independent of the concentration of carbachol applied and of the number of segmental ganglia that remained connected. When TTX was used to block information flow, the motor patterns produced in segments anterior to the block were significantly faster than those from segments posterior to the block. These observations contradict the predictions of the excitability gradient hypothesis and lead to the conclusion that the hypothesis is incorrect.   Skinner FK, Kopell N, Mulloney B. (1997) How does the crayfish swimmeret system work? Insights from nearest neighbor coupled oscillator models. J Comput Neurosci 4(2):151 60 • Rhythmic movements of crayfish swimmerets are coordinated by a neural circuit that links their four abdominal ganglia. Each swimmeret is driven by its own small local circuit, or pattern generating module. We modeled this network as a chain of four oscillators, bidirectionally coupled to their nearest neighbors, and tested the model's ability to reproduce experimentally observed changes in intersegmental phases and in period caused by differential excitation of selected abdominal ganglia. The choices needed to match the experimental data lead to the following predictions: coupling between ganglia is asymmetric; the ascending and descending coupling have approximately equal strengths; intersegmental coupling does not significantly affect the frequency of the system; and excitation affects the intrinsic frequencies of the oscillators and might also change properties of intersegmental coupling.   Mulloney B. Namba H. Agricola HJ. Hall WM (1997) Modulation of force during locomotion: differential action of crustacean cardioactive peptide on power stroke and return stroke motor neurons. Journal of Neuroscience. 17(18):6872 83. • Crustacean cardioactive peptide (CCAP) elicited expression of the motor pattern that drives coordinated swimmeret beating in crayfish and modulated this pattern in a dose dependent manner. In each ganglion that innervates swimmerets, neurons with CCAP like immunoreactivity sent processes to the lateral neuropils, which contain branches of swimmeret motor neurons and the local pattern generating circuits. CCAP affected each of the four functional groups of motor neurons, power stroke excitors (PSE), return stroke excitors (RSE), power stroke inhibitors (PSI), and return stroke inhibitors (RSI), that innervate each swimmeret. When CCAP was superfused, the membrane potentials of these neurons began to oscillate periodically about their mean potentials. The mean potentials of PSE and RSI neurons depolarized, and some of these neurons began to fire during each depolarization. Both intensity and durations of PSE bursts increased significantly. The mean potentials of RSE and PSI neurons hyperpolarized, and these neurons were less likely to fire during each depolarization. When CCAP was superfused in a low Ca2+ saline that blocked chemical transmission, these changes in mean potential persisted, but the periodic oscillations disappeared. These results are evidence that CCAP acts at two levels: activation of local premotor circuits and direct modulation of swimmeret motor neurons. The action on motor neurons is differential; PSEs and RSIs are excited, but RSEs and PSIs are inhibited. The consequences of this selectivity are to increase intensity of bursts of impulses that excite power stroke muscles.   Sherff, Carolyn M. and Brian Mulloney. (1997) Passive properties of swimmeret motor neurons. J. Neurophysiol. 78: 92 102. • Four different functional types of motor neurons innervate each swimmeret: return stroke excitors (RSEs), power stroke excitors (PSEs), return stroke inhibitors (RSIs), and power stroke inhibitors (PSIs). We studied the structures and passive electrical properties of these neurons, and tested the hypothesis that different types of motor neurons would have different passive properties that influenced generation of the swimmeret motor pattern. Cell bodies of neurons innervating one swimmeret were clustered in two anatomic groups in the same ganglion. The shapes of motor neurons in both groups were similar, despite the differences in locations of their cell bodies and in their functions. Diameters of their axons in the swimmeret nerve ranged from <2 to ~35 µm. Resting membrane potentials, input resistances, and membrane time constants were recorded with microelectrodes in the processes of swimmeret motor neurons in isolated abdominal nerve cord preparations. Membrane potentials had a median of 59 mV, with 25th and 75th percentiles of 66.0 and 53 mV. The median input resistance was 6.4 M , with 25th and 75th percentiles of 3.4 and 13.7 M. Membrane time constants had a median of 9.3 ms, with 25th and 75th percentiles of 5.7 and 15.0 ms. Excitatory and inhibitory motor neurons had similar passive properties. RSE motor neurons were typically more depolarized than the other types, but the passive properties of RSE, PSE, RSI, and PSI neurons were not significantly different. Membrane time constants measured from cell bodies were briefer than those measured from neuropil processes, but membrane potentials and input resistances were not significantly different. The relative sizes of different motor neurons were measured from the sizes of their impulses recorded extracellularly from the swimmeret nerve. Smaller motor neurons had lower membrane potentials and were more likely to be active in the motor pattern than were large motor neurons. Motor neurons of different sizes had similar input resistances and membrane time constants. Motor neurons that were either oscillating or oscillating and firing in phase with the swimmeret motor pattern had lower average membrane potentials and longer time constants than those that were not oscillating. When the state of the swimmeret system changed from quiescence to continuous production of the motor pattern, the resting potentials, input resistances, and membrane time constants of individual swimmeret motor neurons changed only slightly. On average, both input resistance and membrane time constant increased. These similarities are considered in light of the functional task each motor neuron performs, and a hypothesis is developed that links the brief time constants of these neurons and graded synaptic transmission by premotor interneurons to control of the swimmeret muscles and the performance of the swimmeret system.   CM Sherff and B Mulloney (1996) Tests of the motor neuron model of the local pattern generating circuits in the swimmeret system Journal of Neuroscience, Vol 16, 2839 2859. • The motor pattern that drives each crayfish swimmeret consists of alternating bursts of impulses in power stroke (PS) and return stroke (RS) motor neurons. A model of the neural circuit that generates this pattern focused on connections between motor neurons themselves (Heitler, 1978, 1981). The model predicts that synergist motor neurons are electrically coupled, whereas antagonists make mostly inhibitory synapses. We tested this model by observing the responses of motor neurons to pressure ejection of GABA and glutamate, transmitters that crayfish motor neurons release at neuromuscular junctions, and by measuring the strengths and delays of synapses between pairs of motor neurons. Both GABA and glutamate inhibited motor neurons. This inhibition persisted when synaptic transmitter release was blocked by high Mg2+. The effects of GABA were mimicked by muscimol, but not by baclofen or the GABAc receptor agonist cis 4 aminocrotonic acid, and they were not blocked by bicuculline. The effects of glutamate were mimicked by ibotenic acid. Picrotoxin partially blocked glutamate's inhibition of the motor pattern, but did not affect GABA responses. Most (87%) pairs of synergist motor neurons tested made weak, noninverting connections. Approximately half of these had synaptic delays of <2 msec, consistent with direct electrical or chemical synapses. Individual motor neurons were dye coupled to between one and three other motor neurons, and to interneurons. Less than half (44%) of the pairs of antagonist motor neurons tested made synaptic connections. These connections were weak, had long latencies (>4 msec), and therefore were probably polysynaptic. We conclude that direct synapses between swimmeret motor neurons cannot account for alternation of PS and RS bursts.   Braun G; Mulloney B (1995) Coordination in the crayfish swimmeret system: differential excitation causes changes in intersegmental phase. J Neurophysiol. 73(2): 880 5 • 1. Gradients of excitation in the swimmeret system were created by applying either pilocarpine or carbachol to selected ganglia in isolated abdominal nerve cords. The state of the system was monitored in each segment with extracellular electrodes on nerves that innervated swimmerets. In preparations that were quiescent before drugs were applied, these cholinergic agonists elicited well coordinated swimmeret motor patterns from the entire system, including ganglia that were not directly treated with pilocarpine or carbachol. 2. The periods of these patterns depended on the number of ganglia that were directly excited. As this number increased, period decreased. When the same numbers of ganglia were excited by direct application of a drug, the mean period of the swimmeret activity elicited by pilocarpine was greater than that elicited by carbachol. 3. Selective excitation of anterior or posterior ganglia caused significant changes in intersegmental phase at the boundary between excited and nonexcited regions of the nerve cord. When only anterior ganglia were excited directly, the phases of their power stroke activity relative to the most posterior ganglion were advanced. When only posterior ganglia were excited directly, the phases of power stroke activity in more anterior ganglia were retarded. Neither pilocarpine nor carbachol caused a complete reversal of the normal phase relations of the swimmeret motor patterns. 4. These results are consistent with an asymmetric coupling model of the intersegmental coordinating circuit of the swimmeret system but contradict an alternative excitability gradient model.   Chrachri A; Neil D; Mulloney B (1994) State dependent responses of two motor systems in the crayfish, Pacifastacus leniusculus. J. Comp Physiol A. 175(3): 371 80 • The expression of both swimmeret and postural motor patterns in crayfish (Pacifastacus leniusculus) were affected by stimulation of a second root of a thoracic ganglion. The response of the swimmeret system depended on the state of the postural system. In most cases, the response of the swimmeret system outlasted the stimulus. Stimulation of a thoracic second root also elicited coordinated responses from the postural system, that outlasted the stimulus. In different preparations, either the flexor excitor motor neurones or the extensor excitor motor neurones were excited by this stimulation. In every case, excitation of one set of motor neurones was accompanied by inhibition of that group's functional antagonists. This stimulation seemed to coordinate the activity of both systems; when stimulation inhibited the flexor motor neurones, then the extensor motor neurones and the swimmeret system were excited. When stimulation excited the flexor motor neurones, then the extensor motor neurones and the swimmeret system were inhibited. Two classes of interneurones that responded to stimulation of a thoracic second root were encountered in the first abdominal ganglion. These interneurones could be the pathway that coordinates the response of the postural and swimmeret systems to stimulation of a thoracic second root.   Acevedo LD, Hall WM, Mulloney B. (1994) Proctolin and excitation of the crayfish swimmeret system. J Comp Neurol 345(4):612 27 • The ventral nerve cord of crayfish contains axons of five pairs of excitatory interneurons, each of which can activate the swimmeret system. Perfusion of the ventral nerve cord with the neuropeptide proctolin also activates the swimmeret system. The experiments reported here were conducted to test the hypothesis that one or more of these excitatory interneurons uses proctolin as a transmitter. Each of the five excitatory axons was located and stimulated separately in an individual crayfish, and similar motor activity was elicited by stimulating each of them. Quantitative comparison of spontaneous swimmeret motor patterns with activity caused by stimulating one of these excitatory axons, EC, or by perfusing with proctolin solutions showed that the motor patterns produced under these three conditions were not significantly different (P > 0.05). By using a new, affinity purified proctolin antiserum, we labeled axons in the connective tissue between the last thoracic and first abdominal ganglion and compared the positions of labeled axons with the previously described positions of the excitatory axons. About 0.3% of the axons in these connective tissues showed proctolin like immunoreactivity, but heavily labeled pairs of axons did occur bilaterally in the regions of excitatory swimmeret axons. The projections of these labeled axons into the abdominal ganglia were traced in serial plastic sections. Labeled processes were abundant in the lateral neuropils, the loci of the swimmeret pattern generating circuitry. From this evidence, we propose that three of these excitatory swimmeret interneurons use proctolin as a transmitter, but that a fourth does not. The evidence for the fifth axon is ambiguous.   Murchison D, Chrachri A, Mulloney B. (1993) A separate local pattern-generating circuit controls the movements of each swimmeret in crayfish. J Neurophysiol. 70(6):2620-31 • 1. Within an abdominal segment, the motor output from the segmental ganglion to the swimmerets consists of coordinated bursts of impulses in the separate pools of motor neurons innervating the left and right limbs. This coordinated motor pattern features alternating (out-of-phase) bursts of impulses in the power-stroke (PS) and return-stroke (RS) motor axons that innervate each swimmeret. PS bursts on both sides of each segment occur simultaneously (in-phase), and so RS bursts on both sides are also in-phase. 2. With all intersegmental connections interrupted, isolated abdominal ganglia were able to sustain the normal swimmeret motor pattern of alternating PS/RS activity that was bilaterally in-phase. 3. After an isolated ganglion was surgically bisected down the midline, the isolated hemiganglia that resulted could produce stable, coordinated alternation of PS and RS bursts. 4. The neuropeptide proctolin could induce rhythmic oscillations of membrane potential in swimmeret neurons when spiking was blocked by tetrodotoxin (TTX). For neurons within the same hemiganglion, these oscillations retained the same phase relations they displayed in controls, but the oscillations of neurons in different hemiganglia became uncoordinated. 5. Synaptic transmission between swimmeret neurons in the same hemiganglion persisted in the presence of TTX. Swimmeret interneurons that could activate the pattern-generating circuitry under control conditions could induce membrane-potential oscillations in swimmeret neurons of the same hemiganglion when TTX was present. 6. We conclude that a separate hemisegmental pattern-generating circuit controls the rhythmic PS and RS movements of each swimmeret. Each circuit is located in the same hemiganglion as the population of motor neurons that innervates the local swimmeret. Graded transmission is sufficient to coordinate the timing of oscillatory activity within the hemisegmental circuitry. These hemisegmental circuits are coupled by intersegmental and bilateral coordinating pathways that are dependent on sodium action potentials for their operation.     Braun G; Mulloney B (1993) Cholinergic modulation of the swimmeret motor system in crayfish. J Neurophysiol. 70(6): 2391 8 [See Laboratory 8.] • 1. The muscarinic agonist pilocarpine induced the swimmeret motor pattern in resting isolated preparations of the crayfish abdominal nerve cord and modulated the burst frequency in a dose dependent manner. 2. Nicotine did not elicit rhythmic activity in resting isolated preparations but increased the burst frequency in active preparations. Nicotine produced higher burst frequencies than pilocarpine. 3. The acetylcholine (ACh) analogue carbachol combined the effects of pilocarpine and nicotine. It activated isolated resting preparations and increased the burst frequency as effectively as nicotine. The ACh esterase inhibitor eserine also increased the burst frequency in active preparations. 4. Neither muscarinic nor nicotinic antagonists disrupted the proctolin induced motor pattern, suggesting that proctolin and cholinergic agonists affect two different pathways for the activation of the swimmeret system. 5. We conclude that cholinergic interneurons participate in initiation of the swimmeret motor pattern and can modulate its burst frequency.   Barthe JY; Bevengut M; Clarac F (1993) In vitro, proctolin and serotonin induced modulations of the abdominal motor system activities in crayfish. Brain Res. 623(1): 101 9 • An in vitro thoraco abdominal preparation of the crayfish (Procambarus clarkii) ventral nerve cord was used to study the sites of action and the effects of proctolin and serotonin on the nervous activities of the two abdominal motor systems, namely the swimmeret and the abdominal positioning systems. In this preparation spontaneous motor activity was recorded corresponding to continuous rhythmic bursts in the swimmeret motor nerves and tonic discharge of motoneurons in both abdominal extensor and flexor motor nerves. Proctolin applied on the abdominal ganglia elicited bursts of spikes in the flexor motor nerve which were able to disturb and even stop the swimmeret activity. Increasing concentrations of serotonin applied on the thoracic ganglia were able, first, to increase the period durations of the swimmeret bursting activity and, second, to stop it. In this last condition, continuous swimmeret activity resumed by application of proctolin on the abdominal ganglia although period durations stayed slightly longer than in control. The actions of serotonin and proctolin on the two abdominal motor systems were discussed in terms of modulations and interactions between central neuronal networks and behaviors.   Killian KA, Page CH. (1992) Mechanosensory afferents innervating the swimmerets of the lobster. I. Afferents activated by cuticular deformation. J Comp Physiol [A] 170(4):491-500 • The mechanosensory innervation of the lobster (Homarus americanus) swimmeret was examined by electrophysiologically recording afferent spike responses initiated by localized mechanical stimulation of the caudal surface of the swimmeret. Two functional groups of subcuticular hypodermal mechanoreceptors innervate the swimmeret. Afferents of one group innervate the small discrete "ridges" of calcified cuticle lining the margins of both swimmeret rami. Putative ridge receptors are bipolar sensory neurons responding phasically to deformation of the ridge cuticle with the number and frequency of impulses produced dependent on stimulus strength and velocity. Afferents of the second group, which innervate substantial areas of hypodermis underlying the soft, flexible cuticular regions of the swimmeret, were designated "wide-field" hypodermal mechanoreceptors. These neurons have multiterminal receptive fields and respond phaso-tonically to cuticular distortion. The response properties of both types of hypodermal mechanoreceptors imply that they are activated during the characteristic beating movements of the swimmerets.   Killian KA, Page CH (1992) Mechanosensory afferents innervating the swimmerets of the lobster. II. Afferents activated by hair deflection. J Comp Physiol [A] 170(4):501-8 • Feathered hair sensilla fringe both rami of the lobster (Homarus americanus) swimmeret. The sensory response to hair displacement was characterized by recording afferent impulses extracellularly from the swimmeret sensory nerve while deflecting sensilla with a rigidly-coupled probe or controlled water movements. Two populations of hairs were observed: "distal" hairs localized to the distal 1/3 of each ramus and "proximal" hairs near its base. Distal hairs are not innervated by a mechanosensory neuron but instead act as levers producing strain within adjacent cuticle capable of activating a nearby hypodermal mechanoreceptor. Hair deflections of 25 degrees or more are required to evoke an afferent response and this response is dependent on hair deflection direction. The frequency and duration of the afferent discharge evoked are determined by the velocity of hair displacement. Each proximal hair is innervated by a single mechanosensory neuron responding phasically to hair deflections as small as 0.2 degrees in amplitude. Deflection at frequencies up to 5 Hz elicits a single action potential for each hair movement; at higher frequencies many deflections fail to evoke an afferent response. These sensilla, which are mechanically coupled, may be activated by the turbulent flow of water produced by the swimmerets during their characteristic beating movements.   Barthe JY; Bevengut M; Clarac F (1991) The swimmeret rhythm and its relationships with postural and locomotor activity in the isolated nervous system of the crayfish Procambarus clarkii. J Exp Biol. 157: 205 226 • An in vitro preparation was developed consisting of the five thoracic and six abdominal ganglia of the crayfish nerve cord, isolated from anterior nervous structures and from peripheral sensory inputs. The central activities of the thoracic leg, swimmeret and abdominal positioning motor systems and their relationships were studied. When motor outputs were tonic in the thoracic leg nerves (90% of the preparations), continuous rhythmic activity occurred and persisted for several hours in the swimmeret nerves. Interruptions of the swimmeret rhythm were associated with rhythmic motor outputs in the leg nerves (10% of the preparations). Motor activity in the abdominal positioning system was mainly tonic. Swimmeret rhythm reversibly disappeared during application of a sucrose block between the thoracic and abdominal parts of the nerve cord. Electrical stimulation of the connectives posterior to the block induced bouts of rhythmic swimmeret activity. Comparisons of the swimmeret rhythm (period) and the metachronal wave (duration, phase) showed that sectioning of the connectives between the thoracic and abdominal ganglia modified the period but did not affect the properties of the metachronal wave. We conclude that the presence of descending inputs from thoracic ganglia is necessary for persistent swimmeret activity.   Sherff CM, Mulloney B. (1991) Red pigment concentrating hormone is a modulator of the crayfish swimmeret system. J Exp Biol.155:21-35 • The crustacean red pigment concentrating hormone (RPCH) has been localized in neurons of the crayfish abdominal nerve cord and modulates the crayfish swimmeret rhythm. An antibody to RPCH labels a small set of cell bodies and axons in each abdominal ganglion. Physiological experiments in which RPCH was perfused into the ganglia of isolated nerve cords showed that RPCH modulated the swimmeret rhythm. In nerve cords that were spontaneously producing the swimmeret rhythm, RPCH lengthened both the period and the duration of bursts of action potentials, but did not alter the phase relationships between bursts in different segments. RPCH did not initiate the swimmeret rhythm in preparations that showed intermittent or no bursting activity. We believe that RPCH is released as a neurotransmitter in the lateral neuropil, where it exerts its effects on the local swimmeret circuits.   Murchison D; Larimer JL (1990) Dual motor output interneurons in the abdominal ganglia of the crayfish Procambarus clarkii: synaptic activation of motor outputs in both the swimmeret and abdominal positioning systems by single interneurons. J Exp Biol. 150: 269 93 • Many behavior patterns of the crayfish involve the positioning of the abdomen by the tonic motor system. Movements and positionings of the swimmerets are coordinated with these abdominal movements. Evidence from extracellular analyses suggested that single interneurons of the abdominal nerve cord could produce motor outputs in both the swimmeret and the abdominal positioning systems. Our intracellular investigation has revealed that many single cells can evoke outputs in both motor systems. Interneurons which produced fictive extension or flexion of the abdomen or inhibition of abdominal movement were also able to modulate a variety of swimmeret behavior including cyclic beating and excitation or inhibition of episodic outputs. Although interneurons were discovered that evoked each of the possible classes of dual output combinations, those that evoked combinations frequently observed in the freely behaving animal were more common than those that evoked infrequently observed combinations. Evidence also indicated that abdominal positioning inhibitors are present in greater numbers than previously suspected and that many are closely associated with the swimmeret circuitry. Interneurons with the ability to start and stop swimmeret cyclic outputs with current injections of opposite polarity are proposed to be higher order cells, and some are shown to have the properties of trigger neurons. It is proposed that most dual output cells are presynaptic to single output cells and that groups of related dual output cells may function together as command elements.   Mulloney B; Acevedo LD; Bradbury AG (1987) Modulation of the crayfish swimmeret rhythm by octopamine and the neuropeptide proctolin. J Neurophysiol. 58(3): 584 97 • 1. The swimmeret system can be excited by perfusing the neuropeptide proctolin through the isolated ventral nerve cord of the crayfish. Previously silent preparations begin to generate a characteristic motor pattern, the swimmeret rhythm, in the nerves that innervate the swimmerets. The response to proctolin is dose dependent and reversible. The threshold concentration of proctolin perfused through the ventral artery is approximately 10( 8) M. The EC50 is 1.6 X 10( 6) M. 2. Proctolin induced motor patterns have periods and phases similar to those of spontaneously generated motor patterns. The durations of the bursts of impulses in power stroke motor neurons generated in the presence of proctolin are, however, significantly longer than those that occur during spontaneous activity. 3. DL Octopamine inhibits the swimmeret system, both when the system is spontaneously active and when it has been excited by proctolin. The inhibition by octopamine is dose dependent and reversible. The threshold for inhibition is approximately 10( 6) M, and the EC50 is approximately 5 X 10( 5) M. 4. Octopamine's effect is mimicked by its agonists, synephrine and norepinephrine. Synephrine has a lower threshold concentration than does octopamine, but norepinephrine is much less effective than octopamine. 5. Octopamine's inhibition is partially blocked by an antagonist, phentolamine. 6. Phentolamine also blocks inhibition of the swimmeret system by inhibitory command interneurons. This block is dose dependent and can be partially overcome by stimulating the command interneurons at higher frequencies. 7. Perfusion with 11 other suspected crustacean neurotransmitters and transmitter analogues did not similarly excite or inhibit the swimmeret system, so we suggest that proctolin and octopamine are transmitters used by the neurons that normally control expression of the swimmeret rhythm.   Paul DH, Mulloney B. (1985) Nonspiking local interneuron in the motor pattern generator for the crayfish swimmeret. J Neurophysiol. 54(1):28-39 • 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.   Cattaert D, Clarac F. (1983) Influence of walking on swimmeret beating in the lobster Homarus gammarus. J Neurobiol 14(6):421-39 • Influence of walking on swimmeret beating in intact lobsters, Homarus gammarus, has been analyzed using a treadmill experimental device. Belt movement activates both leg stepping and swimmeret beating. The simultaneity of the onset of the two motor systems in this situation is demonstrated to be the result of a startle response initiated when the belt begins to move. This reaction consists of a non-specific motor activity involving several antagonist postural and dynamic muscles. Abdominal extension and vigorous swimmeret beating are the main features of this reaction. The main characteristics of the swimmeret beating as defined by Davis (1969) has been observed here in sequences without walking. However during long walking sequences a very different swimmeret beating pattern occurs. It is suggested that this slow swimmeret beating is completely subordinate to the walking rhythm during sequences of absolute coordination. In more rapid swimmeret beating a relative coordination with leg stepping is very common. The functional meaning of this linkage between legs and swimmerets is discussed.   West L, Jacobs G, Mulloney B. (1979) Intrasegmental proprioceptive influences on the period of the swimmeret rhythm in crayfish. J Exp Biol 82:281-8 • When the swimmerets of decapods beat, they do so because the muscles of each swimmeret are driven by a series of periodic bursts of impulses in its motor neurones. We investigated the effects of proprioceptive feedback on the period of this motor pattern by interfering with the movement of particular swimmerets. In different experiments, we observed three different kinds of results during interference with a swimmeret. Either the period decreased, or it did not change, or bursting was inhibited altogether. These different results are discussed in terms of the connectivity of different command fibres. Additional references Mulloney, B, D Murchison and A Chrachri (1993) Modular organization of pattern generating circuits in a segmental motor system: the swimmerets of crayfish. Seminars in the Neurosciences, vol 5, 1993: pp 49 57. [See excerpt in Laboratory 8.] An overview of the swimmeret pattern generator. Weise, Konrad (ed) (1990) Frontiers in crustacean neurobiology. QL435 F76 1990. See especially the chapters on swimmeret pattern generators by Mulloney et al, pp. 439 447; and on activation of the swimmeret pattern through stimulation of a thoracic root by Chrachri, pp. 279 287. Sandeman & Atwood (eds) Neural integration and behavior (Biology of Crustacea, v. 4). QP356 N47. See Chapter 2, Control of posture, by Charles Page (pp. 33 59). Cattaert, Daniel; Barthe, Jean Yves; Neil, Douglas M. (1992) Remote control of the swimmeret central pattern generator in crayfish (Procambarus clarkii and Pacifastacus leniusculus): effect of a walking leg proprioceptor. Journal of Experimental Biology 169: 181 206 Deller, Simon R. T.; Macmillan, David L. (1989) Entrainment of the swimmeret rhythm of the crayfish to controlled movements of some of the appendages. Journal of Experimental Biology 144: 257 78. Heitler, W. J. (1986) Aspects of sensory integration in the crayfish swimmeret system. Journal of Experimental Biology 120: 387 402. Heitler, W. J. (1985) Motor programme switching in the crayfish swimmeret system. Journal of Experimental Biology 114: 521 49.
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