Pairs Of Motoneurons Research Articles

Two motor neuron synergies, invariant across ankle joint angles, activate the triceps surae during plantarflexion.

Recent studies have suggested that the nervous system generates movements by controlling groups of motor neurons (synergies) that do not always align with muscle anatomy. In this study, we determined whether these synergies are robust across tasks with different mechanical constraints. We identified motor neuron synergies using principal component analysis (PCA) and cross-correlations between smoothed discharge rates of motor neurons. In part 1, we used simulations to validate these methods. The results suggested that PCA can accurately identify the number of common inputs and their distribution across active motor neurons. Moreover, the results confirmed that cross-correlation can separate pairs of motor neurons that receive common inputs from those that do not receive common inputs. In part 2, 16 individuals performed plantarflexion at three ankle angles while we recorded EMG signals from the gastrocnemius lateralis (GL) and medialis (GM) and the soleus (SOL) with grids of surface electrodes. The PCA revealed two motor neuron synergies. These motor neuron synergies were relatively stable, with no significant differences in the distribution of motor neuron weights across ankle angles (P=0.62). When the cross-correlation was calculated for pairs of motor units tracked across ankle angles, we observed that only 13.0% of pairs of motor units from GL and GM exhibited significant correlations of their smoothed discharge rates across angles, confirming the low level of common inputs between these muscles. Overall, these results highlight the modularity of movement control at the motor neuron level, suggesting a sensible reduction of computational resources for movement control. KEY POINTS: The CNS might generate movements by activating groups of motor neurons (synergies) with common inputs. We show here that two main sources of common inputs drive the motor neurons innervating the triceps surae muscles during isometric ankle plantarflexions. We report that the distribution of these common inputs is globally invariant despite changing the mechanical constraints of the tasks, i.e. the ankle angle. These results suggest the functional relevance of the modular organization of the CNS to control movements.

Flexible neural control of transition points within the egg-laying behavioral sequence in Drosophila

Innate behaviors are frequently comprised of ordered sequences of component actions that progress to satisfy essential drives. Progression is governed by specialized sensory cues that induce transitions between components within the appropriate context. Here we have characterized the structure of the egg-laying behavioral sequence in Drosophila and found significant variability in the transitions between component actions that affords the organism an adaptive flexibility. We identified distinct classes of interoceptive and exteroceptive sensory neurons that control the timing and direction of transitions between the terminal components of the sequence. We also identified a pair of motor neurons that enact the final transition to egg expulsion. These results provide a logic for the organization of innate behavior in which sensory information processed at critical junctures allows for flexible adjustments in component actions to satisfy drives across varied internal and external environments.

Transitions of motor neuron activities during Ciona development.

Motor neurons (MNs) are one of the most important components of Central Pattern Generators (CPG) in vertebrates (Brown, Proceedings of The Royal Society B: Biological Sciences (The Royal Society), 1911, 84(572), 308-319). However, it is unclear how the neural activities of these components develop during their embryogenesis. Our previous study revealed that in Ciona robusta (Ciona intestinalis type A), a model organism with a simple neural circuit, a single pair of MNs (MN2L/MN2R) was determining the rhythm of its spontaneous early motor behavior (developmental stage St.22-24). MN2s are known to be one of the main components of Ciona CPG, though the neural activities of MN2s in the later larval period (St.25-) were not yet investigated. In this study, we investigated the neural activities of MN2s during their later stages and how they are related to Ciona's swimming CPG. Long-term simultaneous Ca2+ imaging of both MN2s with GCaMP6s/f (St.22-34) revealed that MN2s continued to determine the rhythm of motor behavior even in their later larval stages. Their activities were classified into seven phases (I-VII) depending on the interval and the synchronicity of MN2L and MN2R Ca2+ transients. Initially, each MN2 oscillates sporadically (I). As they develop into swimming larvae, they gradually oscillate at a constant interval (II-III), then start to synchronize (IV) and fully synchronize (V). Intervals become longer (VI) and sporadic again during the tail aggression period (VII). Interestingly, 76% of the embryos started to oscillate from MN2R. In addition, independent photostimulations on left and right MN2s were conducted. This is the first report of the live imaging of neural activities in Ciona's developing swimming CPG. These findings will help to understand the development of motor neuron circuits in chordate animals.

MiniSOG2-mediated Specific Photoablation of Motor Neurons in Ascidian Embryos.

When understanding the neuronal function of a specific neural circuit, single-cell level photoablation of a targeted cell is one of the useful experimental approaches. This protocol describes a method to photoablate specific motor neurons via the mini singlet oxygen generator (miniSOG2), a light-oxygen-voltage (LOV)-based optogenetic tool used for ablating targeted cells in arbitrary areas. MiniSOG2 could induce the cell death pathway by generating reactive oxygen species (ROS) upon blue light illumination. Photoablation of a specific cell using the miniSOG2 was performed to show that, in Ciona intestinalis type A ( Ciona robusta) , a single pair of motor neurons, MN2/A10.64, is necessary to drive their tail muscle contraction. The membrane targeted miniSOG2 combined with neuron-specific promoter (pSP-Neurog::miniSOG2-CAAX) was electroplated into the Ciona egg and transiently expressed at specific neurons of the embryo. MN2 labeled with pSP-Neurog:mCherry-CAAX was irradiated using a 440-nm laser from the lateral side for 10 min to ablate its neural function. The behavior of the embryo before and after the irradiation was recorded with a high-speed camera. Graphical abstract.

Presynaptic coupling by electrical synapses coordinates a rhythmic behavior by synchronizing the activities of a neuron pair

Electrical synapses are specialized structures that mediate the flow of electrical currents between neurons and have well known roles in synchronizing the activities of neuronal populations, both by mediating the current transfer from more active to less active neurons and by shunting currents from active neurons to their less active neighbors. However, how these positive and negative functions of electrical synapses are coordinated to shape rhythmic synaptic outputs and behavior is not well understood. Here, using a combination of genetics, behavioral analysis, and live calcium imaging in Caenorhabditis elegans, we show that electrical synapses formed by the gap junction protein INX-1/innexin couple the presynaptic terminals of a pair of motor neurons (AVL and DVB) to synchronize their activation in response to a pacemaker signal. Live calcium imaging reveals that inx-1/innexin mutations lead to asynchronous activation of AVL and DVB, due, in part, to loss of AVL-mediated activation of DVB by the pacemaker. In addition, loss of inx-1 leads to the ectopic activation of DVB at inappropriate times during the cycle through the activation of the L-type voltage-gated calcium channel EGL-19. We propose that electrical synapses between AVL and DVB presynaptic terminals function to ensure the precise and robust execution of a specific step in a rhythmic behavior by both synchronizing the activities of presynaptic terminals in response to pacemaker signaling and by inhibiting their activation in between cycles when pacemaker signaling is low.

Structural and Functional Synaptic Plasticity Induced by Convergent Synapse Loss in the Drosophila Neuromuscular Circuit.

Throughout the nervous system, the convergence of two or more presynaptic inputs on a target cell is commonly observed. The question we ask here is to what extent converging inputs influence each other's structural and functional synaptic plasticity. In complex circuits, isolating individual inputs is difficult because postsynaptic cells can receive thousands of inputs. An ideal model to address this question is the Drosophila larval neuromuscular junction (NMJ) where each postsynaptic muscle cell receives inputs from two glutamatergic types of motor neurons (MNs), known as 1b and 1s MNs. Notably, each muscle is unique and receives input from a different combination of 1b and 1s MNs; we surveyed multiple muscles for this reason. Here, we identified a cell-specific promoter that allows ablation of 1s MNs postinnervation and measured structural and functional responses of convergent 1b NMJs using microscopy and electrophysiology. For all muscles examined in both sexes, ablation of 1s MNs resulted in NMJ expansion and increased spontaneous neurotransmitter release at corresponding 1b NMJs. This demonstrates that 1b NMJs can compensate for the loss of convergent 1s MNs. However, only a subset of 1b NMJs showed compensatory evoked neurotransmission, suggesting target-specific plasticity. Silencing 1s MNs led to similar plasticity at 1b NMJs, suggesting that evoked neurotransmission from 1s MNs contributes to 1b synaptic plasticity. Finally, we genetically blocked 1s innervation in male larvae and robust 1b synaptic plasticity was eliminated, raising the possibility that 1s NMJ formation is required to set up a reference for subsequent synaptic perturbations.SIGNIFICANCE STATEMENT In complex neural circuits, multiple convergent inputs contribute to the activity of the target cell, but whether synaptic plasticity exists among these inputs has not been thoroughly explored. In this study, we examined synaptic plasticity in the structurally and functionally tractable Drosophila larval neuromuscular system. In this convergent circuit, each muscle is innervated by a unique pair of motor neurons. Removal of one neuron after innervation causes the adjacent neuron to increase neuromuscular junction outgrowth and functional output. However, this is not a general feature as each motor neuron differentially compensates. Further, robust compensation requires initial coinnervation by both neurons. Understanding how neurons respond to perturbations in adjacent neurons will provide insight into nervous system plasticity in both healthy and disease states.

The peripheral nervous system of the ascidian tadpole larva: Types of neurons and their synaptic networks.

Physical and chemical cues from the environment are used to direct animal behavior through a complex network of connections originating in exteroceptors. In chordates, mechanosensory and chemosensory neurons of the peripheral nervous system (PNS) must signal to the motor circuits of the central nervous system (CNS) through a series of pathways that integrate and regulate the output to motor neurons (MN); ultimately these drive contraction of the tail and limb muscles. We used serial-section electron microscopy to reconstruct PNS neurons and their hitherto unknown synaptic networks in the tadpole larva of a sibling chordate, the ascidian, Ciona intestinalis. The larva has groups of neurons in its apical papillae, epidermal neurons in the rostral and apical trunk, caudal neurons in the dorsal and ventral epidermis, and a single tail tip neuron. The connectome reveals that the PNS input arises from scattered groups of these epidermal neurons, 54 in total, and has three main centers of integration in the CNS: in the anterior brain vesicle (which additionally receives input from photoreceptors of the ocellus), the motor ganglion (which contains five pairs of MN), and the tail, all of which in turn are themselves interconnected through important functional relay neurons. Some neurons have long collaterals that form autapses. Our study reveals interconnections with other sensory systems, and the exact inputs to the motor system required to regulate contractions in the tail that underlie larval swimming, or to the CNS to regulate substrate preference prior to the induction of larval settlement and metamorphosis.

Correlation between discharge timings of pairs of motor units reveals the presence but not the proportion of common synaptic input to motor neurons.

We investigated whether correlation measures derived from pairs of motor unit (MU) spike trains are reliable indicators of the degree of common synaptic input to motor neurons. Several 50-s isometric contractions of the biceps brachii muscle were performed at different target forces ranging from 10 to 30% of the maximal voluntary contraction relying on force feedback. Forty-eight pairs of MUs were examined at various force levels. Motor unit synchrony was assessed by cross-correlation analysis using three indexes: the output correlation as the peak of the cross-histogram (ρ) and the number of synchronous spikes per second (CIS) and per trigger (E). Individual analysis of MU pairs revealed that ρ, CIS, and E were most often positively associated with discharge rate (87, 85, and 76% of the MU pairs, respectively) and negatively with interspike interval variability (69, 65, and 62% of the MU pairs, respectively). Moreover, the behavior of synchronization indexes with discharge rate (and interspike interval variability) varied greatly among the MU pairs. These results were consistent with theoretical predictions, which showed that the output correlation between pairs of spike trains depends on the statistics of the input current and motor neuron intrinsic properties that differ for different motor neuron pairs. In conclusion, the synchronization between MU firing trains is necessarily caused by the (functional) common input to motor neurons, but it is not possible to infer the degree of shared common input to a pair of motor neurons on the basis of correlation measures of their output spike trains.NEW & NOTEWORTHY The strength of correlation between output spike trains is only poorly associated with the degree of common input to the population of motor neurons. The synchronization between motor unit firing trains is necessarily caused by the (functional) common input to motor neurons, but it is not possible to infer the degree of shared common input to a pair of motor neurons on the basis of correlation measures of their output spike trains.

Using Computational Neuroscience to Define Common Input to Spinal Motor Neurons.

Common input is a widely used concept in motor neurophysiology. It embodies the notion that inputs to individual spinal motor neurons (MNs) are not unique, but partly shared across MNs, and is considered the main explanation for synchronized activity of MNs (Bremner et al., 1991; Farmer et al., 1993; Boonstra and Breakspear, 2012; Farina and Negro, 2015). Motor-unit synchronization was first observed in the time domain using cross-correlation histograms from pairs of MNs (Sears and Stagg, 1976). This was later extended to the frequency domain by estimating coherence between spike trains to reveal the frequency content of common input (Farmer et al., 1993). In addition to measurements of individual MNs, coherence can also be estimated between the surface EMG of different muscles—referred to as intermuscular coherence—to assess common input shared across motor-unit pools (Boonstra and Breakspear, 2012). Common input is considered relevant for motor control as it may provide a mechanism to reduce the dimensionality of the control signal, thereby simplifying motor control (Farmer, 1998).

An Interview with Professor Gian Garriga on Asymmetric Cell Divisions: Distinct Fates of Daughter Cells

An Interview with Professor Gian Garriga on Asymmetric Cell Divisions: Distinct Fates of Daughter Cells

The effects of interactions between intrinsic properties and network parameters on bilateral phasing in a reduced leech heartbeat system

The leech heartbeat central pattern generator (CPG) consists of a network of heart interneurons (HN) that coordinate heart excitor (HE) motor neuron activity via inhibitory chemical synapses. Each segmental pair of HE’s is connected to one another via electrical coupling. Depending on the segment, the pair of motor neurons in the living system is active across a wide range of phase differences from nearly in-phase to anti-phase [1]. Prior efforts to model this complete network have not quantitatively matched all contralateral phase values observed [2]. We have created a reduced network model in Simulink to explore parameters that contribute to these phase differences. This parameter search has been analyzed in MATLAB using the PANDORA toolbox [3]. In our network model, we implemented known neuronal properties and synaptic connections from a single segmental ganglion. The HN’s were modeled as endogenous bursters as previously described with a network cycle period of 9.45 ± 0.11 sec [4]; in previous studies [5], the HE’s were modeled as tonic firers. We varied three network parameters shown in Figure ​Figure11 in this study: phased delay of the right HN synaptic input [ΦSyn], the maximum conductances of the inhibitory synapse [gSyn], and electrical coupling strength [gcoup]. We varied ΦSyn to emulate the variable timing of inhibition onto HE’s in the full network of the living system. In addition, we varied the set of maximum conductances for the HE neurons in a linear fashion from classical HE (tonic firing) to HN (endogenous bursting) values to ascertain the role of intrinsic properties on phase-shifts. Figure 1 Leech heartbeat circuit diagram showing identified CPG heart interneurons (HN) and heart excitor motor neurons (HE). This linear variation of HE properties from tonic firing towards endogenous bursting led to a step-wise phase advance up to ~0.10 for both neurons. In-phase (ΦSyn: 0.0) and anti-phase (0.5) values of ΦSyn produced expected HE phase differences. However, intermediate synaptic delays (0.25, 0.75) caused a reduction in phase differences as the two motor neurons pull towards one another due to their electrical coupling. The amount of phase difference reduction was directly impacted in a nonlinear manner by the intrinsic properties of these HE neurons. Duty cycle (burst duration / cycle period) was not significantly impacted by intrinsic properties. In summary, increases in ΦSyn led to non-monotonic changes in HE motor neuron contralateral phase differences. Our search of parameter space continues to provide a foundation for understanding the mechanisms underlying variable phase differences in neuronal networks and reinforces the importance of interactions between endogenous properties and synaptic connections for producing functional motor patterns. The project described was supported by Vermont Genetics Network through an NIGMS (NIH) Institutional Development Award P20 {type:entrez-nucleotide,attrs:{text:GM103449,term_id:221402118,term_text:GM103449}}GM103449.

Identification of leech embryonic neurons that express a Hox gene required for the differentiation of a paired, segment-specific motor neuron

This study investigated the embryonic expression and function of the Hox gene Lox1 in the simple, well-characterized central nervous system (CNS) of the medicinal leech Hirudo medicinalis. Lox1 was expressed in an anterior–posterior domain, extending from the posterior aspect of the fourth segment (rostral neuromere 4, R4) to the seventeenth segment (midbody ganglion 13, M13). Lox1 expression was also found in both sex organ primordia (male and female). Lox1 expression was not detected in every cell of the ganglia included in its domain. It was detected in a specific subset that included several segmentally iterated neurons and segment-specific neurons. Several central neurons (neurons located in the central nervous system – CNS) that coexpressed both Lox1 and FMRFamide-like peptides were identified using antibody staining of leech embryos and epifluorescence and confocal microscopy. RNA interference was used to block the expression of Lox1. The expression pattern and the effect of RNA interference indicate that Lox1 is required for the differentiation of a segment-specific pair of motor neurons, the RPE (rostral penile evertor) neurons, which appear only in midbody ganglion 6 (M6) and innervate the male sex organ.

Factors Influencing the Estimates of Correlation between Motor Unit Activities in Humans

BackgroundAlpha motoneurons receive common synaptic inputs from spinal and supraspinal pathways. As a result, a certain degree of correlation can be observed between motoneuron spike trains during voluntary contractions. This has been studied by using correlation measures in the time and frequency domains. These measures are interpreted as reflecting different types of connectivity in the spinal networks, although the relation between the degree of correlation of the output motoneuron spike trains and of their synaptic inputs is unclear.Methodology/Principal FindingsIn this study, we analyze theoretically this relation and we complete this analysis by simulations and experimental data on the abductor digiti minimi muscle. The results demonstrate that correlation measures between motoneuron output spike trains are inherently influenced by the discharge rate and that this influence cannot be compensated by normalization. Because of the influence of discharge rate, frequency domain measures of correlation (coherence) do not identify the full frequency content of the common input signal when computed from pairs of motoneurons. Rather, an increase in sampling rate is needed by using cumulative spike trains of several motoneurons. Moreover, the application of averaging filters to the spike trains influences the magnitude of the estimated correlation levels calculated in the time, but not in the frequency domain (coherence).ConclusionsIt is concluded that the analysis of coherence in different frequency bands between cumulative spike trains of a sufficient number of motoneurons provides information on the spectrum of the common synaptic input. Nonetheless, the absolute values of coherent peaks cannot be compared across conditions with different cumulative discharge rates.

Compromise revisited: inhibitory synapse and electrical coupling effects on bilateral phasing in the leech heartbeat system

The leech heartbeat central pattern generator (CPG) consists of a network of heart interneurons (HN) that coordinate heart excitor (HE) motor neuron activity via inhibitory chemical synapses. Each segmental pair of HE’s is connected to one another via electrical coupling. Depending on the segment, the pair of motor neurons in the living system is active across a wide range of phase differences from nearly in-phase to anti-phase [1]. Prior efforts to model this complete network have not quantitatively matched the intersegmental phase differences observed [2]. We have created a reduced network model in Simulink to explore parameters that contribute to these phase differences. In our network model, we implemented known neuronal properties and synaptic connections from a single segmental ganglion as shown in Figure ​Figure1.1. In our initial model run, the HN’s were modeled as endogenous bursters as previously described [3]; the HE’s were modeled as tonic firers. We varied three parameters in this study: phased delay of the right HN synaptic input (ΦSyn) and the maximum conductances of the inhibitory synapse (gSyn) and electrical coupling (gcoup). Figure 1 Leech heartbeat circuit diagram showing identified heart interneurons (HN) of the CPG and their pattern of inhibitory synaptic connections (lines with circles) onto each other and the heart excitor motor neurons (HE). The resistor symbol indicates rectifying ... We found that in this network gSyn must be at least 150 times greater than gcoup in order to obtain 1:1 entrainment of the HE’s with the HN’s. Under conditions with zero ΦSyn, increased gcoup led to increased instantaneous spike frequencies (ISF) and reduced duty cycle, primarily due to a delayed burst beginning. Increasing gSyn alone with zero ΦSyn led to little change in HE phase or duty cycle, but saw increased ISF. With relatively high levels of gSyn (300-600 nS), increasing ΦSyn (0.2-0.5) caused one HE to decrease its duty cycle while the other increased. Higher levels of ΦSyn (0.5-0.8) caused the HE’s to switch their relative duty cycle patterns. With weak gcoup (0.25-0.50 nS) and moderate ΦSyn (0.4-0.6), increasing gSyn led to a reduced HE duty cycle and side-to-side phase difference. The largest phase differences were found when both gSyn and gcoup were relatively strong. In summary, increases in gcoup tended to lead to increased phase differences, while increases in gSyn led to decreased phase differences. Our search of parameter space has provided a foundation for understanding the mechanisms underlying variable phase differences in neuronal networks and reinforced the importance of compromise between synaptic and neuronal properties for producing functional motor patterns.

Individual differences in leech heart motor neuron models

Motor neurons are frequently overlooked as critical contributors to the programming of motor output except where they directly play a role in central pattern generators (CPGs). Leech heart motor neurons have been shown to contribute significant phase shifts to the rhythmic motor patterns they produce [1-3], although the most important factor in pattern formation is the interaction of CPG output and synaptic weights from the CPG onto the motor neurons [3]. We seek to address the question of which neural parameters, in particular active conductances, are important for functional pattern formation, and how they influence it. We are well positioned to address this question, as we have a unique dataset comprising the complete leech heart motor neuron input pattern, output pattern, and synaptic weights for multiple ganglia in many individual animals. We have begun to exploit these data to develop more realistic Hodgkin-Huxley style biophysical models of the leech heart motor neurons by constraining models optimized by a multi-objective evolutionary algorithm to fit this animal-specific data. In particular, we extended the single compartment Garcia model [1] to a multi-compartmental model and added a slow calcium and a calcium sensitive potassium channel, currents known to be present in the living motor neurons. Our model’s dimensions are derived from reconstructions of fluorophore filled leech heart motor neurons in segments 8-12. Membrane currents were distributed according to experimental data and the result of hand tuning; e.g., the fast sodium current is only present in the axonal compartment. This base model was then parameterized such that the maximal conductances of the active currents present in each compartment, as well as the electrical coupling conductance between each pair of motor neurons, were allowed to vary as free parameters in our evolutionary algorithm. We then utilized our input-output data by delivering a particular input pattern (specific animal and motor neuron pair) to a model motor neuron and then comparing the resulting output with that recorded in the same animal. In this manner, we constrained our definition of an ‘acceptable’ model by the input/output pattern recorded in individual animals – such a model must transform the input into the correct output within some reasonable error bound. To generate acceptable models we use a Multi Objective Evolutionary Algorithm (MOEA) to create a population of models from which a subset achieves ‘acceptable’ status. In this framework, multiple independent fitness functions are evaluated separately – the phase, duty cycle, spike height, slow-wave height, and within-burst inter-spike interval (ISI) for the two phases produced. The targets for these parameters are drawn from the same individual animal as the input/output data, and error bounds are based on the within-animal variability, although slightly widened for parameters such as ISI, slow-wave and spike height, as they vary greatly between animals (derived primarily from within-animal experimental variability). Acceptable model simulation traces were all but indistinguishable from intracellular recordings from the living system, with the exception of measurement noise, even though they are constrained by a small number of functional parameters. In order to identify key parameters and relationships between synaptic weights and membrane conductances which are constrained by the functional requirements of the system, we will conduct a sensitivity analysis of the acceptable models and inspect the dimensional stacking [4] of acceptable and failed models. The identified conductances can then be perturbed in the living system either via dynamic clamp or pharmacological manipulation to validate the modeling results.

Conservation of aTritoniaPedal peptides network in gastropods

Adults of the nudibranch mollusc Tritonia diomedea crawl using mucociliary locomotion. Crawling is controlled in part by the large Pedal 5 (Pd5) and Pedal 6 (Pd6) neurons that produce Tritonia Pedal peptides (TPeps). TPeps elicit an increase in ciliary beat frequency, thereby increasing crawling speed. In adults of T. diomedea, an extensive network of TPep-containing neurites adjacent to the basement membrane of the pedal epithelium delivers TPeps to the ciliated cells. In this study, we show that diverse nudibranchs all have a pattern of TPep-like immunoreactivity similar to that of T. diomedea, with thin tracts of TPep-like immunoreactive (TPep-LIR) neurites projecting to the epithelial layer. We also show that members of two non-nudibranch gastropod species have a pattern of TPep-innervation similar to that of the nudibranchs. In addition, we characterized two pairs of motor neurons in adults of the nudibranch Armina californica that are possible homologues of the Pd5 and Pd6 cells in T. diomedea. Activity in one of these pairs, the Pedal Peptidergic Dorsal 1 (PPD1) cells, was correlated with mucociliary locomotion. The second pair, the Pedal Peptidergic Ventral 1 cells, shared synchronous synaptic input with the PPD1 cells, a pattern consistent with the shared synaptic input of the T. diomedea Pd5 and Pd6 cells. These findings suggest that the roles of the Pd5 and Pd6 cells as mucociliary motor neurons in nudibranchs are conserved evolutionarily. Additionally, the extensive network of TPep-LIR neurites seen in the foot of T. diomedea appears likely to be a common feature among gastropods.

Flexibility in the Patterning and Control of Axial Locomotor Networks in Lamprey

In lower vertebrates, locomotor burst generators for axial muscles generally produce unitary bursts that alternate between the two sides of the body. In lamprey, a lower vertebrate, locomotor activity in the axial ventral roots of the isolated spinal cord can exhibit flexibility in the timings of bursts to dorsally-located myotomal muscle fibers versus ventrally-located myotomal muscle fibers. These episodes of decreased synchrony can occur spontaneously, especially in the rostral spinal cord where the propagating body waves of swimming originate. Application of serotonin, an endogenous spinal neurotransmitter known to presynaptically inhibit excitatory synapses in lamprey, can promote decreased synchrony of dorsal-ventral bursting. These observations suggest the possible existence of dorsal and ventral locomotor networks with modifiable coupling strength between them. Intracellular recordings of motoneurons during locomotor activity provide some support for this model. Pairs of motoneurons innervating myotomal muscle fibers of similar ipsilateral dorsoventral location tend to have higher correlations of fast synaptic activity during fictive locomotion than do pairs of motoneurons innervating myotomes of different ipsilateral dorsoventral locations, suggesting their control by different populations of premotor interneurons. Further, these different motoneuron pools receive different patterns of excitatory and inhibitory inputs from individual reticulospinal neurons, conveyed in part by different sets of premotor interneurons. Perhaps, then, the locomotor network of the lamprey is not simply a unitary burst generator on each side of the spinal cord that activates all ipsilateral body muscles simultaneously. Instead, the burst generator on each side may comprise at least two coupled burst generators, one controlling motoneurons innervating dorsal body muscles and one controlling motoneurons innervating ventral body muscles. The coupling strength between these two ipsilateral burst generators may be modifiable and weakening when greater swimming maneuverability is required. Variable coupling of intrasegmental burst generators in the lamprey may be a precursor to the variable coupling of burst generators observed in the control of locomotion in the joints of limbed vertebrates.

Notch-dependent induction of left/right asymmetry in C. elegans interneurons and motoneurons.

Although nervous systems are largely bilaterally symmetric on a structural level, they display striking degrees of functional left/right (L/R) asymmetry. In Caenorhabditis elegans, two structurally symmetric pairs of sensory neurons, ASE and AWC, display two distinctly controlled types of functional L/R asymmetries (stereotyped versus stochastic asymmetry). Beyond these two cases, the extent of neuronal asymmetry in the C. elegans nervous system was unclear. Here, we report that the Beta3/Olig-type bHLH transcription factor hlh-16 is L/R asymmetrically expressed in several distinct, otherwise bilaterally symmetric interneuron and motoneuron pairs that are part of a known navigation circuit. We find that hlh-16 asymmetry is generated during gastrulation by an asymmetric LAG-2/Delta signal originating from the mesoderm that promotes hlh-16 expression in neurons on the left side through direct binding of the Notch effector LAG-1/Su(H)/CBF to a cis-regulatory element in the hlh-16 locus. Removal of hlh-16 reveals an unanticipated asymmetry in the ability of the axons of the AIY interneurons to extend into the nerve ring, with the left AIY axon requiring elevated hlh-16 expression for correct extension. Our study suggests that the extent of molecular L/R asymmetry in the C. elegans nervous system is broader than previously anticipated, establishes a novel signaling mechanism that crosses germ layers to diversify bilaterally symmetric neuronal lineages, and reveals L/R asymmetric control of axonal outgrowth of bilaterally symmetric neurons.

Neural Networks: More about Flexibility Than Synaptic Strength

The leech heartbeat neural network is famous for its constancy in both architecture and functional output across animals. A recent study, however, has found that the synaptic strengths underlying this constancy are quite variable across animals.

Divergent mechanisms specify chordate motoneurons: evidence from ascidians

Ascidians are members of the vertebrate sister group Urochordata. Their larvae exhibit a chordate body plan, which forms by a highly accelerated embryonic strategy involving a fixed cell lineage and small cell numbers. We report a detailed analysis of the specification of three of the five pairs of motoneurons in the ascidian Ciona intestinalis and show that despite well-conserved gene expression patterns and embryological outcomes compared with vertebrates, key signalling molecules have adopted different roles. We employed a combination of cell ablation and gene manipulation to analyse the function of two signalling molecules with key roles in vertebrate motoneuron specification that are known to be expressed equivalently in ascidians: the inducer Sonic hedgehog, produced ventrally by the notochord and floorplate; and the inhibitory BMP2/4, produced on the lateral/dorsal side of the neural plate. Our surprising conclusion is that neither BMP2/4 signalling nor the ventral cell lineages expressing hedgehog play crucial roles in motoneuron formation in Ciona. Furthermore, BMP2/4 overexpression induced ectopic motoneurons, the opposite of its vertebrate role. We suggest that the specification of motoneurons has been modified during ascidian evolution, such that BMP2/4 has adopted a redundant inductive role rather than a repressive role and Nodal, expressed upstream of BMP2/4 in the dorsal neural tube precursors, acts as a motoneuron inducer during normal development. Thus, our results uncover significant differences in the mechanisms used for motoneuron specification within chordates and also highlight the dangers of interpreting equivalent expression patterns as indicative of conserved function in evo-devo studies.