Swimmeret Movements Research Articles

Five types of nonspiking interneurons in local pattern-generating circuits of the crayfish swimmeret system

We conducted a quantitative analysis of the different nonspiking interneurons in the local pattern-generating circuits of the crayfish swimmeret system. Within each local circuit, these interneurons control the firing of the power-stroke and return-stroke motor neurons that drive swimmeret movements. Fifty-four of these interneurons were identified during physiological experiments with sharp microelectrodes and filled with dextran Texas red, Neurobiotin, or both. Five types of neurons were identified on the basis of combinations of physiological and anatomical characteristics. Anatomical categories were based on 16 anatomical parameters measured from stacks of confocal images obtained from each neuron. The results support the recognition of two functional classes: inhibitors of power stroke (IPS) and inhibitors of return stroke (IRS). The IPS class of interneuron has three morphological types with similar physiological properties. The IRS class has two morphological types with physiological properties and anatomical features different from the IPS neurons but similar within the class. Three of these five types have not been previously identified. Reviewing the evidence for dye coupling within each type, we conclude that each type of IPS neuron and one type of IRS neuron occur as a single copy in each local pattern-generating circuit. The last IRS type includes neurons that might occur as a dye-coupled pair in each local circuit. Recognition of these different interneurons in the swimmeret pattern-generating circuits leads to a refined model of the local pattern-generating circuit that includes synaptic connections that encode and decode information required for intersegmental coordination of swimmeret movements.

Analysis of input-output relationships of CPG elements and their contributions to rhythmic output

We use the dynamic clamp technique [1] to explore how synaptic input patterns affect motor output (see Figure ​Figure1A).1A). We show that leech motor neuron intrinsic properties make a contribution to their output phasing. Then, we show that leech motor neurons receiving the same complement of synaptic inputs can still be organized into a coordinated motor pattern given that a gradient of synaptic strengths exists and that the premotor interneurons fire at different times [2]. In both of these cases, measuring motor neuron responses provides a direct assay for how premotor input patterns produce stereotyped motor output. We are currently extending this analysis to the crayfish swimmeret system, a system in which four segmental oscillators are interconnected by coordinating interneurons to produce a metachronal wave of swimmeret movements [3]. In this system, ascending (ASC) and descending (DSC) coordinating interneurons (see Figure ​Figure1B)1B) encode salient features about the activity pattern of their home segment and are exported (via spikes) to other segmental oscillators. Interestingly, when the system is driven across periods by different concentrations of neuromodulators, the number of spikes of a given coordinating interneuron remains constant although their duty cycles change. We are currently building a single-compartment, conductance-based model of the ASC and DSC coordinating neurons, with the goal of understanding how these coordinating neurons encode information in their home modules and how this encoding can be modulated when the swimmeret system is driven at different periods. Figure 1 Circuit diagrams and rhythmic motor output from the leech heartbeat central pattern generator (A) and the crayfish swimmeret system (B). A, left: Heart (HE) motor neurons in segments 8 and 12 both receive synaptic input from premotor hear (HN) interneurons ...

Understanding the mechanisms underlying phase-locking behavior in the crayfish swimmeret system

One of the fundamental challenges in neuroscience is to understand how the intrinsic properties of neurons and the properties of neural networks combine to produce behavior. Networks that produce rhythmic motor behaviors, such as locomotion, provide important model systems to address this problem. A particularly good model for this purpose is the neural circuit underlying the coordinated rhythmic limb movements in the crayfish swimmeret system. During forward swimming, rhythmic movements of swimmerets on different segments of the crayfish abdomen progress from back to front with the same period, but neighboring swimmerets are phase-lagged by 25% of the period. This coordination of limb movements is maintained over a wide range of frequency. The exact mechanisms underlying this robustly stable phase-locking are not known. Here, we use mathematical modeling and analysis in conjunction with recent experimental results to obtained insight into these mechanisms. The rhythmic behavior of each swimmeret is driven by a local pattern generating circuit consisting of a half-center oscillator (HCO). These local pattern generating circuits are connected through well-described intersegmental connections. We model the neural circuitry of the swimmeret system as a chain of HCOs. First, we examine the phase response properties of HCOs for two fundamentally different mechanisms that produce anti-phase activity in HCOs: the “escape” and “release” mechanisms. We demonstrate that the “escape” and “release” mechanisms give rise to very different phase response properties, and we use phase plane arguments to explain the different shapes of the phase response curves. We then examine a chain of four nearest-neighbor coupled HCOs. We use the coupled oscillator theory and symmetry arguments to show that the phase-locking 25% phase-locking observed in the crayfish swimmeret system arises naturally from the network connectivity, but this phase-locking is robustly stable for only some combinations of connectivity and the escape/release mechanisms.

Local and Intersegmental Interactions of Coordinating Neurons and Local Circuits in the Swimmeret System

During forward swimming, periodic movements of swimmerets on different segments of the crayfish abdomen progress from back to front with the same period. Information encoded as bursts of spikes by coordinating neurons in each segmental ganglion is necessary for this coherent organization. This information is conducted to targets in other ganglia. When an individual coordinating neuron is stimulated at different phases in the system's cycle of activity, the timing of motor output from other ganglia may be altered. In models of this coordinating circuit, we assumed that each coordinating neuron encodes information about the state of the local pattern-generating circuit in its home ganglion but is not part of that local circuit. We tested this assumption by stimulating individual coordinating neurons of two kinds -- ASC(E) and DSC -- at different phases under two conditions: with the target ganglion functional, and with the target ganglion silenced. Blocking a DSC neuron's target ganglion did not alter its negligible influence on the output from its home ganglion; the phase-response curves (PRC) remained flat. Blocking an ASC(E) neuron's target ganglion significantly affected its influence on the output from its home ganglion. We had predicted that ASC(E)'s modest phase-dependent influence would disappear with the target silenced, but instead the amplitude of the PRCs increased significantly. Thus we have two different results: DSC neurons conformed to prediction based on the models' assumptions, but ASC(E) neurons showed an unexpected property, one that is partially masked when the bidirectional flow of information between neighboring ganglia is operating normally.

Not by Spikes Alone: Responses of Coordinating Neurons and the Swimmeret System to Local Differences in Excitation

Swimmeret coordinating neurons in the crayfish CNS collectively encode a detailed cycle-by-cycle report on features of the motor output to each swimmeret. This information coordinates the motor output that drives swimmeret movements. To see how coordinating neurons responded to forced changes in intersegmental phase, we used a split-bath, repeated-measures experimental design to expose different regions of isolated abdominal nerve cords to different levels of excitation. We present a quantitative description of the firing of power-stroke (PS) motor units and two kinds of coordinating interneurons, ASC(E) and DSC, recorded simultaneously from each swimmeret ganglion under uniform and nonuniform excitation. When anterior and posterior ganglia were excited differently, several parameters of the swimmeret motor pattern were affected. Strengths of PS bursts in each ganglion were determined by local excitation. The phase of PS bursts in neighboring ganglia changed at the excitation boundary. Coordinating neurons from the two ganglia closest to the excitation boundary were most affected by nonuniform excitation. ASC(E) neurons tracked the timing and duration of each PS burst in their home ganglion, but did not follow changes in PS burst strength. DSC neurons changed the duration, phase, and number of spikes per burst. We propose two models to explain these results. First, the period expressed under nonuniform conditions is the sum of local intersegmental latencies and these latencies are determined by local excitation. Second, the phase change at the excitation boundary is determined by local modulation of the targets of the intersegmental coordinating neurons, not by modulation of the coordinating neurons themselves.

The Beat Goes On, and Up and Down.Focus on “Bursts of Information: Coordinating Interneurons Encode Multiple Parameters of a Periodic Motor Pattern”

The effective production of many motor behaviors requires the strict coordination of discrete motor pattern generating circuits ([Hill et al. 2003][1]). Locomotor behaviors usually require this coordination at several levels of the nervous system, within single limbs (interjoint coordination),

Local commissural interneurons integrate information from intersegmental coordinating interneurons

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.

A push-pull set of elastic strand stretch receptor neurons for the swimmeret in an isopod Bathynomus doederleini.

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.

Cardioacceleratory reflexes triggered by mechanoproprioceptors of the swimmerets in the stomatopod crustacean Squillaoratoria

The heart of Squilla oratoria is innervated by processes arising from the cardiac ganglion, which lies on the outer surface of the heart wall. The ganglion is regulated by one pair of cardioinhibitory nerves and two pairs of cardioacceleratory nerves. Cardiac acceleration accompanied activation of the five pairs of swimmerets in the first to the fifth abdominal segments. The cardioacceleratory nerves were activated when swimmerets beat strongly. Activation of the cardioacceleratory nerves was caused by electrical stimuli to a nerve branch extending to the swimmerets from the first nerve root of the abdominal ganglion. Bursts of afferent impulses were recorded from the nerve branch of the first nerve root corresponding to periods of protractive and retractive swimmeret movements. Afferent impulses were recorded from the nerve branch when the articular membrane was artificially boosted up. Cardiac acceleration during active swimmeret movements in Squilla is attributable to a reflexive response triggered by the movements. Putative mechanoproprioceptors on the articular membrane between the sterna and basipodite in the swimmerets may be responsible for the cardioacceleratory reflex.

Coordination of limb movements: three types of intersegmental interneurons in the swimmeret system and their responses to changes in excitation.

Coordination of limb movements: three types of intersegmental interneurons in the swimmeret system and their responses to changes in excitation. 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 approximately 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.

Intersegmental coordination of swimmeret movements: mathematical models and neural circuits.

Swimmerets move periodically through a cycle of power-strokes and return-strokes. Swimmerets on neighboring segments differ in phase by approximately 25%, and maintain this difference even when the period of the cycle changes from < 1 to > 4 Hz. We constructed a minimal cellular model of the segmental pattern-generating circuit which incorporated its essential components, and whose dynamics were like those of the local circuit. Three different intersegmental coordinating units were known to link neighboring ganglia, but their targets are unknown. We constructed different intersegmental circuits which these units might form between neighboring cellular models, and compared their dynamics with the real system. One intersegmental circuit could maintain an approximately 25% phase difference through a range of periods. In physiological experiments, we identified three types of intersegmental interneurons that originate in each ganglion and project to its neighbors. These neurons fire bursts at certain parts of the swimmeret cycle in their home ganglion. These three neurons are necessary and sufficient to maintain normal coordination between neighboring segments. Their properties conform to the predictions of the cellular model.

Intersegmental Coordination of Limb Movements during Locomotion: Mathematical Models Predict Circuits That Drive Swimmeret Beating

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.

A Test of the Excitability-Gradient Hypothesis in the Swimmeret System of Crayfish

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.

How does the crayfish swimmeret system work? Insights from nearest-neighbor coupled oscillator models.

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.

Neural mechanisms governing distribution of cardiac output in an isopod crustacean, Bathynomus doederleini: reflexes controlling the cardioarterial valves

In Bathynomus doederleini all of the cardioarterial valves located at the origin of the lateral arteries are dilated by impulses of lateral cardiac nerves. Tactile stimuli applied to sensillar setae depress impulse activities of the 1st and 5th lateral cardiac nerves. The 1st lateral cardiac nerve controls the valve of the lateral artery which runs to the walking-legs and viscera. The 5th lateral cardiac nerve controls the valve of the lateral artery which runs to the swimmeret muscles. The response indicates that tactile receptor reflexes bring about decreased haemolymph flow to the organs. Augmented swimmeret movements were always accompanied by an increased firing rate in the 5th lateral cardiac nerve. Artificial full protraction of swimmerets simultaneously induced excitation of the 5th lateral cardiac nerve and inhibition of the 1st lateral cardiac nerve. The excitation corresponds to an increase in haemolymph flow to the swimmerets, and the inhibition a decrease in haemolymph flow to walking-legs and viscera. Three kinds of mechanoproprioceptors which were activated by swimmeret movements were found. Two of the mechanoproprioceptors are located at the base of the basipodite. The other mechanoproprioceptor supplies processes to a nerve to the retractor muscles. Activation of three kinds of mechanoproprioceptors, induced by artificial swimmeret protraction, triggered lateral cardiac nerve reflex responses.

Synaptic interactions among neurons that coordinate swimmeret and abdominal movements in the crayfish.

1. Many interneurons in the crayfish (Procambarus clarkii) abdominal nervous system influence two behaviors, abdominal positioning and swimmeret movements. Such neurons are referred to as dual output cells. Other neurons which influence either one behavior or the other are single output cells. 2. Extensive synaptic interactions were observed between both dual and single output neurons involved in the control of abdominal positioning and swimmeret movements. Over 60% of all neuron pairs examined displayed interactions. Pairs of agonist neurons displayed excitatory interactions, while pairs of antagonists had inhibitory interactions. This pattern of interaction was observed in about 75% of interactive neuron pairs whether abdominal positioning or swimmeret outputs were considered. 3. Evidence for both serial and parallel connectivity, as well as, reciprocal or looping connections was observed. Looping connections can be found both between the abdominal positioning and swimmeret systems and within each system. 4. Most (28/34) single output neurons were not presynaptic to dual output neurons. No single output neurons were found to excite dual output neurons to spiking, although inhibitory interactions and weak excitations were observed. 5. Abdominal positioning inhibitors displayed properties consistent with a role in mediating some of the coordination between the swimmeret and abdominal positioning systems. 6. None of the dual output neurons examined influenced the swimmeret motoneurons directly.

Central outputs for extrinsic neural control of the heart in an isopod crustacean, Bathynomus doederleini: Neuroanatomy and electrophysiology

1. Free-moving and restrained animals ( Bathynomous doederleini) were used to correlate behavior with neural control of the heart. 2. The three paired cardio-regulatory axons were each traced to their ganglionic origins. The cardio-inhibitory axon and the 1st and 2nd cardio-acceleratory axons originate from, respectively, the 4th root of the 1st thoracic ganglion, the 3rd root of the 2nd thoracic ganglion and the 3rd root of the 3rd thoracic ganglion. 3. External stimuli such as turning lights on and off, and tactile or vibratory stimuli induced simultaneous cardiac inhibition and ventilatory inhibition of swimmerets. 4. Cardiac inhibition resulted from a reciprocal correlation between the activities of the cardio-inhibitory axon and the cardio-acceleratory axons, activation of the former and depression of the latter. 5. Tachycardia instantaneously accompanied three types of behavior, that are characterized by activation of swimmeret movements (paddling, walking and swimming). 6. Acute activation of swimmeret movements was not accompanied by tachycardia after the anterior cardiac nerves were severed. Activation of cardio-acceleratory axons and depression of cardie-inhibitory axons may be indispensable for tachycardia.

Short Communications: The Association of Uropod Steering with Postural Movement of the Abdomen in Crayfish

ABSTRACT Decapod crustaceans have elaborate statocysts to detect changes in the body position. They utilize these organs to initiate and control various equilibrium reactions so that they can always maintain an upright posture. In macrurans, these reactions consist of asymmetrical movements of walking legs, swimmerets and uropods, as well as compensatory movements of the eyestalks and antennae. They have been previously described in terms of a simple input (i.e. the tilt angle)-and-output (i.e. the magnitude of the resultant reaction) relationship (e.g. Schöne, 1954). But do these equilibrium reactions occur whenever the animal tilts? We report here that one of them, the uropod steering movement, does not occur by itself and always occurs together with the abdominal movement which is unrelated to the equilibrium reaction. It is shown that this association of two independent movements is not mediated by any peripheral sense organ. Quantitative aspects of the uropod steering movement were described elsewhere (Yoshino, Takahata &amp; Hisada, 1980).

Coupled motoneurones are part of the crayfish swimmeret central oscillator.

SWIMMERETS in the crayfish are the paired ventral abdominal appendages which beat in a metachronal rhythm during behaviours such as swimming and burrow ventilation. Each swimmeret is driven by alternating bursts of impulses in antagonistic power- and return-stroke motoneurones. Although proprioceptive input affects the detailed structure of the rhythm, the basic motor programme can be elicited from the deafferented abdominal central nervous system (CNS) by tonic stimulation of single ‘command fibres’ in the ventral nerve cord1–3. Each swimmeret is controlled by a hemiganglion capable of generating the rhythm in isolation, although normally the most posterior swimmeret ganglion (the 5th) acts as a pacemaker, regulating the activity of the other ganglia through a series of coordinating fibres located in the medial region of the ventral nerve cord4. I report here the preliminary results of an investigation into the structure of the central neural oscillator controlling swimmeret movements in crayfish. The results suggest that motoneurones in this system do not act merely as output elements which passively relay the activity of an oscillator formed from pre-motor neurones within the CNS, but that the motoneurones themselves are an integral part of the pattern generating circuits. There is both excitatory and inhibitory central coupling between motoneurones mediated either by electrical or graded chemical synapses. Furthermore, the activity of some motoneurones can directly influence the intensity, period, and phase of the oscillation.

Neural Control of Interappendage Phase During Locomotion

Interappendage phasing of crayfish swimmeret movements depends upon a central nervous system network of command, oscillator, and coordinating neurons. The command neurons serve to set the general excitation level in each of the segmental oscillators. The oscillator neurons in each hemi-ganglion generate the rhythmic alternations of powerstroke and returnstroke motor neuron activity. The coordinating neurons transmit the precise timing information about the state of one oscillator to other oscillators. This information can serve to advance or to delay the motor bursts driven by the other oscillators. Which effect is observed depends upon the arrival time of the coordinating neuron discharge within the cycle period of the modulated oscillator. This type of modulation leads to the prediction that a stable interappendage phase can result from situations where there is not a fixed excitability gradient among the segmental oscillators. This prediction has been verified using a cut command neuron preparation.