Transmitter phenotypes of commissural interneurons in the lamprey spinal cord

Excitatory and Inhibitory Descending Commissural Interneurons Differentially Integrate Supraspinal and Segmental Sensory Signals

The limited information about how descending inputs from the brain and sensory inputs from the periphery use spinal cord interneurons (INs) is a major barrier to understanding how these inputs may contribute to motor functions under normal and pathologic conditions. Commissural interneurons (CINs) are a heterogeneous population of spinal INs that has been implicated in crossed motor responses and bilateral motor coordination (ability to use the right and left side of the body in a coordinated manner) and, therefore, are likely involved in many types of movement (e.g., dynamic posture stabilization, jumping, kicking, walking). In this study, we incorporate mouse genetics, anatomy, electrophysiology, and single-cell calcium imaging to investigate how a subset of CINs, those with descending axons called dCINs, are recruited by descending reticulospinal and segmental sensory signals independently and in combination. We focus on two groups of dCINs set apart by their principal neurotransmitter (glutamate and GABA) and identified as VGluT2+ dCINs and GAD2+ dCINs. We show that VGluT2+ and GAD2+ dCINs are both extensively recruited by reticulospinal and sensory input alone but that VGluT2+ and GAD2+ dCINs integrate these inputs differently. Critically, we find that when recruitment depends on the combined action of reticulospinal and sensory inputs (subthreshold inputs), VGluT2+ dCINs, but not GAD2+ dCINs, are recruited. This difference in the integrative capacity of VGluT2+ and GAD2+ dCINs represents a circuit mechanism that the reticulospinal and segmental sensory systems may avail themselves of to regulate motor behaviors both normally and after injury.SIGNIFICANCE STATEMENT The way supraspinal and peripheral sensory inputs use spinal cord interneurons is fundamental to defining how motor functions are supported both in health and disease. This study, which focuses on dCINs, a heterogeneous population of spinal interneurons critical for crossed motor responses and bilateral motor coordination, shows that both glutamatergic (excitatory) and GABAergic (inhibitory) dCINs can be recruited by supraspinal (reticulospinal) or peripheral sensory inputs. Additionally, the study demonstrates that in conditions where the recruitment of dCINs depends on the combined action of reticulospinal and sensory inputs, only excitatory dCINs are recruited. The study uncovers a circuit mechanism that the reticulospinal and segmental sensory systems may avail themselves of to regulate motor behaviors both normally and after injury.

Key role of 5-HT1A receptors in the modulation of the neuronal network underlying the respiratory rhythm generation in lampreys

In mammals, 5-HTexcitatory respiratory effects imply 5-HT_1A receptor-mediated disinhibition of pre-Bötzinger complex neurons. In the lamprey, 5-HT_1A receptors are involved in the neural control of locomotion, but their role in the respiratory regulation, particularly at the level of the putative respiratory rhythm generator, the paratrigeminal respiratory group (pTRG), is not known. We here investigate the respiratory function of inhibitory 5-HT_1A receptors within the pTRG of the isolated brainstem of the adult lamprey. The 5-HT_1A receptor agonists either bath applied or microinjected into the pTRG did not cause significant effects. However, the selective 5-HT_1A receptor antagonist (S)-WAY 100135 bath applied or microinjected into the pTRG induced depressing respiratory effects or even apnoea, thus revealing that 5-HT exerts a 5-HT_1A receptor-mediated potent tonic influence on respiration and contributes to maintain baseline levels of respiratory activity. Microinjections of strychnine or bicuculline, either alone or in combination, into the pTRG prevented (S)-WAY 100135-induced apnoea. In addition, immunohistochemical studies corroborate the present findings suggesting that 5-HT_1A receptors are widely expressed in close apposition to the soma of glycine-immunoreactive cells located within the pTRG region. The results show that in the lamprey respiratory network, 5-HT exerts a tonic influence on respiration by a potent inhibitory control on both GABAergic and glycinergic mechanisms. The observed disinhibitory effects resemble the excitatory respiratory modulation exerted by 5-HT_1A receptor-mediated inhibition of glycinergic and/or GABAergic neurons present in mammals, supporting the notion that some features of the neuronal network subserving respiratory rhythm generation are highly conserved throughout phylogeny.

Large-Scale Analysis of the Diversity and Complexity of the Adult Spinal Cord Neurotransmitter Typology

The development of nervous system atlases is a fundamental pursuit in neuroscience, since they constitute a fundamental tool to improve our understanding of the nervous system and behavior. As such, neurotransmitter maps are valuable resources to decipher the nervous system organization and functionality. We present here the first comprehensive quantitative map of neurons found in the adult zebrafish spinal cord. Our study overlays detailed information regarding the anatomical positions, sizes, neurotransmitter phenotypes, and the projection patterns of the spinal neurons. We also show that neurotransmitter co-expression is much more extensive than previously assumed, suggesting that spinal networks are more complex than first recognized. As a first direct application, we investigated the neurotransmitter diversity in the putative glutamatergic spinal V2a-interneuron assembly. These studies shed new light on the diverse and complex functions of this important interneuron class in the neuronal interplay governing the precise operation of the central pattern generators.

Complementary expression of calcium binding proteins delineates the functional organization of the locomotor network

Neuronal networks in the spinal cord generate and execute all locomotor-related movements by transforming descending signals from supraspinal areas into appropriate rhythmic activity patterns. In these spinal networks, neurons that arise from the same progenitor domain share similar distribution patterns, neurotransmitter phenotypes, morphological and electrophysiological features. However, subgroups of them participate in different functionally distinct microcircuits to produce locomotion at different speeds and of different modalities. To better understand the nature of this network complexity, here we characterized the distribution of parvalbumin (PV), calbindin D-28 k (CB) and calretinin (CR) which are regulators of intracellular calcium levels and can serve as anatomical markers for morphologically and potential functionally distinct neuronal subpopulations. We observed wide expression of CBPs in the adult zebrafish, in several spinal and reticulospinal neuronal populations with a diverse neurotransmitter phenotype. We also found that several spinal motoneurons express CR and PV. However, only the motoneuron pools that are responsible for generation of fast locomotion were CR-positive. CR can thus be used as a marker for fast motoneurons and might potentially label the fast locomotor module. Moreover, CB was mainly observed in the neuronal progenitor cells that are distributed around the central canal. Thus, our results suggest that during development the spinal neurons utilize CB and as the neurons mature and establish a neurotransmitter phenotype they use CR or/and PV. The detailed characterization of CBPs expression, in the spinal cord and brainstem neurons, is a crucial step toward a better understanding of the development and functionality of neuronal locomotor networks.

Entrainment Ranges for Chains of Forced Neural and Phase Oscillators

Sensory input to the lamprey central pattern generator (CPG) for locomotion is known to have a significant role in modulating lamprey swimming. Lamprey CPGs are known to have the ability to entrain to a bending stimulus, that is, in the presence of a rhythmic signal, the CPG will change its frequency to match the stimulus frequency. Bending experiments in which the lamprey spinal cord has been removed and mechanically bent back and forth at a single point have been used to determine the range of frequencies that can entrain the CPG rhythm. First, we model the lamprey locomotor CPG as a chain of neural oscillators with three classes of neurons and sinusoidal forcing representing edge cell input. We derive a phase model using the connections described in the neural model. This results in a simpler model yet maintains some properties of the neural model. For both the neural model and the derived phase model, entrainment ranges are computed for forcing at different points along the chain while varying both intersegmental coupling strength and the coupling strength between the forcer and chain. Entrainment ranges for chains with nonuniform intersegmental coupling asymmetry are larger when forcing is applied to the middle of the chain than when it is applied to either end, a result that is qualitatively similar to the experimental results. In the limit of weak coupling in the chain, the entrainment results of the neural model approach the entrainment results for the derived phase model. Both biological experiments and the robustness of non-monotonic entrainment ranges as a function of the forcing position across different classes of CPG models with nonuniform asymmetric coupling suggest that a specific property of the intersegmental coupling of the CPG is key to entrainment.

Inhibitory control of ascending glutamatergic projections to the lamprey respiratory rhythm generator

Neurons within the vagal motoneuron region of the lamprey have been shown to modulate respiratory activity via ascending excitatory projections to the paratrigeminal respiratory group (pTRG), the proposed respiratory rhythm generator. The present study was performed on in vitro brainstem preparations of the lamprey to provide a characterization of ascending projections within the whole respiratory motoneuron column with regard to the distribution of neurons projecting to the pTRG and related neurochemical markers. Injections of Neurobiotin were performed into the pTRG and the presence of glutamate, GABA and glycine immunoreactivity was investigated by double-labeling experiments. Interestingly, retrogradely labeled neurons were found not only in the vagal region, but also in the facial and glossopharyngeal motoneuron regions. They were also present within the sensory octavolateral area (OLA). The results show for the first time that neurons projecting to the pTRG are immunoreactive for glutamate, surrounded by GABA-immunoreactive structures and associated with the presence of glycinergic cells. Consistently, GABAA or glycine receptor blockade within the investigated regions increased the respiratory frequency. Furthermore, microinjections of agonists and antagonists of ionotropic glutamate receptors and of the GABAA receptor agonist muscimol showed that OLA neurons do not contribute to respiratory rhythm generation. The results provide evidence that glutamatergic ascending pathways to the pTRG are subject to a potent inhibitory control and suggest that disinhibition is one important mechanism subserving their function. The general characteristics of inhibitory control involved in rhythmic activities, such as respiration, appear to be highly conserved throughout vertebrate evolution.

Decoding the organization of spinal circuits that control locomotion

Unravelling the functional operation of neuronal networks and linking cellular activity to specific behavioural outcomes are among the biggest challenges in neuroscience. In this broad field of research, substantial progress has been made in studies of the spinal networks that control locomotion. Through united efforts using electrophysiological and molecular genetic network approaches and behavioural studies in phylogenetically diverse experimental models, the organization of locomotor networks has begun to be decoded. The emergent themes from this research are that the locomotor networks have a modular organization with distinct transmitter and molecular codes and that their organization is reconfigured with changes to the speed of locomotion or changes in gait.

Characterization of dendritic morphology and neurotransmitter phenotype of thoracic descending propriospinal neurons after complete spinal cord transection and GDNF treatment

After spinal cord injury (SCI), poor regeneration of damaged axons of the central nervous system (CNS) causes limited functional recovery. This limited spontaneous functional recovery has been attributed, to a large extent, to the plasticity of propriospinal neurons, especially the descending propriospinal neurons (dPSNs). Compared with the supraspinal counterparts, dPSNs have displayed significantly greater regenerative capacity, which can be further enhanced by glial cell line-derived neurotrophic factor (GDNF). In the present study, we applied a G-mutated rabies virus (G-Rabies) co-expressing green fluorescence protein (GFP) to reveal Golgi-like dendritic morphology of dPSNs. We also investigated the neurotransmitters expressed by dPSNs after labeling with a retrograde tracer Fluoro-Gold (FG). dPSNs were examined in animals with sham injuries or complete spinal transections with or without GDNF treatment. Bilateral injections of G-Rabies and FG were made into the 2nd lumbar (L2) spinal cord at 3 days prior to a spinal cord transection performed at the 11th thoracic level (T11). The lesion gap was filled with Gelfoam containing either saline or GDNF in the injury groups. Four days post-injury, the rats were sacrificed for analysis. For those animals receiving G-rabies injection, the GFP signal in the T7-9 spinal cord was visualized via 2-photon microscopy. Dendritic morphology from stack images was traced and analyzed using a Neurolucida software. We found that dPSNs in sham injured animals had a predominantly dorsal-ventral distribution of dendrites. Transection injury resulted in alterations in the dendritic distribution with dorsal-ventral retraction and lateral-medial extension. Treatment with GDNF significantly increased the terminal dendritic length of dPSNs. The density of spine-like structures was increased after injury, and treatment with GDNF enhanced this effect. For the group receiving FG injections, immunohistochemistry for glutamate, choline acetyltransferase (ChAT), glycine, and GABA was performed in the T7-9 spinal cord. We show that the majority of FG retrogradely-labeled dPSNs were located in the Rexed Lamina VII. Over 90% of FG-labeled neurons were glutamatergic, with the other three neurotransmitters contributing less than 10% of the total. To our knowledge this is the first report describing the morphologic characteristics of dPSNs and their neurotransmitter expressions, as well as the dendritic response of dPSNs after transection injury and GDNF treatment.

The intrinsic operation of the networks that make us locomote

The spinal cord of all vertebrates contains the networks that coordinate the locomotor movements. In lamprey, zebrafish and amphibian tadpoles these networks generate the swimming movements and depend primarily on ipsilateral excitatory premotor interneurons of the V2a type (zebrafish) generate the segmental burst pattern. In zebrafish they can be further subdivided into three subclasses activating slow, intermediate and fast muscle fibers. Inhibitory commissural neurons are responsible for the alternating pattern between the two sides of the body. Stretch receptor neurons sense the movements and provide sensory feedback. In mammals the locomotor pattern in each limb comprises four different phases including flexor-extensor alternation. Also in this case local ipsilateral excitatory V2 interneurons can drive rhythmic burst activity in individual muscle groups. The coordination between the two hind limbs appears to be controlled by separate sets of commissural interneurons (V0) most likely engaged in walk, trot and gallop respectively.

Dynamics and plasticity of spinal locomotor circuits

Spinal circuits generate coordinated locomotor movements. These hardwired circuits are supplemented with neuromodulation that provide the necessary flexibility for animals to move smoothly through their environment. This review will highlight some recent insights gained in understanding the functional dynamics and plasticity of the locomotor circuits. First the mechanisms governing the modulation of the speed of locomotion will be discussed. Second, advantages of the modular organization of the locomotor networks with multiple circuits engaged in a task-dependent manner will be examined. Finally, the neuromodulation and the resulting plasticity of the locomotor circuits will be summarized with an emphasis on endocannabinoids and nitric oxide. The intention is to extract general principles of organization and discuss some onto-genetic and phylogenetic divergences.

Neuronal release and successful astrocyte uptake of aminoacidergic neurotransmitters after spinal cord injury in lampreys

In contrast to mammals, the spinal cord of lampreys spontaneously recovers from a complete spinal cord injury (SCI). Understanding the differences between lampreys and mammals in their response to SCI could provide valuable information to propose new therapies. Unique properties of the astrocytes of lampreys probably contribute to the success of spinal cord regeneration. The main aim of our study was to investigate, in the sea lamprey, the release of aminoacidergic neurotransmitters and the subsequent astrocyte uptake of these neurotransmitters during the first week following a complete SCI by detecting glutamate, GABA, glycine, Hu and cytokeratin immunoreactivities. This is the first time that aminoacidergic neurotransmitter release from neurons and the subsequent astrocytic response after SCI are analysed by immunocytochemistry in any vertebrate. Spinal injury caused the immediate loss of glutamate, GABA and glycine immunoreactivities in neurons close to the lesion site (except for the cerebrospinal fluid-contacting GABA cells). Only after SCI, astrocytes showed glutamate, GABA and glycine immunoreactivity. Treatment with an inhibitor of glutamate transporters (DL-TBOA) showed that neuronal glutamate was actively transported into astrocytes after SCI. Moreover, after SCI, a massive accumulation of inhibitory neurotransmitters around some reticulospinal axons was observed. Presence of GABA accumulation significantly correlated with a higher survival ability of these neurons. Our data show that, in contrast to mammals, astrocytes of lampreys have a high capacity to actively uptake glutamate after SCI. GABA may play a protective role that could explain the higher regenerative and survival ability of specific descending neurons of lampreys.

From lamprey to salamander: an exploratory modeling study on the architecture of the spinal locomotor networks in the salamander

The evolutionary transition from water to land required new locomotor modes and corresponding adjustments of the spinal "central pattern generators" for locomotion. Salamanders resemble the first terrestrial tetrapods and represent a key animal for the study of these changes. Based on recent physiological data from salamanders, and previous work on the swimming, limbless lamprey, we present a model of the basic oscillatory network in the salamander spinal cord, the spinal segment. Model neurons are of the Hodgkin-Huxley type. Spinal hemisegments contain sparsely connected excitatory and inhibitory neuron populations, and are coupled to a contralateral hemisegment. The model yields a large range of experimental findings, especially the NMDA-induced oscillations observed in isolated axial hemisegments and segments of the salamander Pleurodeles waltlii. The model reproduces most of the effects of the blockade of AMPA synapses, glycinergic synapses, calcium-activated potassium current, persistent sodium current, and [Formula: see text]-current. Driving segments with a population of brainstem neurons yields fast oscillations in the in vivo swimming frequency range. A minimal modification to the conductances involved in burst-termination yields the slower stepping frequency range. Slow oscillators can impose their frequency on fast oscillators, as is likely the case during gait transitions from swimming to stepping. Our study shows that a lamprey-like network can potentially serve as a building block of axial and limb oscillators for swimming and stepping in salamanders.

Modulation of calcium currents and membrane properties by substance P in the lamprey spinal cord

Substance P is endogenously released within the locomotor network of the adult lamprey, accelerates the burst frequency of fictive locomotion, and reduces the reciprocal inhibition. Previous studies have shown that dopamine, serotonin, and GABA regulate calcium channels, which control neurotransmitter release, action potential duration, and slow afterhyperpolarization (sAHP). Here we examine the effect of substance P on calcium channels in motoneurons and commissural interneurons using whole cell patch clamp in the lamprey spinal cord. This study analyzed the effects of substance P on calcium currents activated in voltage clamp. We examined the calcium-dependent sAHP in current clamp, to determine the involvement of three calcium channel subtypes modulated by substance P. The effects of substance P on membrane potential and during N-methyl-d-aspartic acid (NMDA) induced oscillations were also analyzed. Depolarizing voltage steps induced inward calcium currents. Substance P reduced the currents carried by calcium by 61% in commissural interneurons and by 31% in motoneurons. Using specific calcium channel antagonists, we show that substance P reduces the sAHP primarily by inhibiting N-type (CaV2.2) channels. Substance P depolarized both motoneurons and commissural interneurons, and we present evidence that this occurs due to an increased input resistance. We also explored the effects of substance P on NMDA-induced oscillations in tetrodotoxin and found it caused a frequency increase. Thus the reduction of calcium entry by substance P and the accompanying decrease of the sAHP amplitude, combined with substance P potentiation of currents activated by NMDA, may both contribute to the increase in fictive locomotion frequency.

Glutamatergic neuronal populations in the brainstem of the sea lamprey, Petromyzon marinus: an in situ hybridization and immunocytochemical study

Glutamate is the major excitatory neurotransmitter in vertebrates, and glutamatergic cells probably represent a majority of neurons in the brain. Physiological studies have demonstrated a wide presence of excitatory (glutamatergic) neurons in lampreys. The present in situ hybridization study with probes for the lamprey vesicular glutamate transporter (VGLUT) provides an anatomical basis for the general distribution and precise localization of glutamatergic neurons in the sea lamprey brainstem. Most glutamatergic neurons were found within the periventricular gray layer throughout the brainstem, with the following regions being of particular interest: the optic tectum, torus semicircularis, isthmus, dorsal and medial nuclei of the octavolateral area, dorsal column nucleus, solitary tract nucleus, motoneurons, and reticular formation. The reticular population revealed a high degree of cellular heterogeneity including small, medium-sized, large, and giant glutamatergic neurons. We also combined glutamate immunohistochemistry with neuronal tract-tracing methods or γ-aminobutyric acid (GABA) immunohistochemistry to better characterize the glutamatergic populations. Injection of Neurobiotin into the spinal cord revealed that retrogradely labeled small and medium-sized cells of some reticulospinal-projecting groups were often glutamate-immunoreactive, mostly in the hindbrain. In contrast, the large and giant glutamatergic reticulospinal perikarya mostly lacked glutamate immunoreactivity. These results indicate that glutamate immunoreactivity did not reveal the entire set of glutamatergic populations. Some spinal-projecting octaval populations lacked both VGLUT and glutamate. As regards GABA and glutamate, their distribution was largely complementary, but colocalization of glutamate and GABA was observed in some small neurons, suggesting that glutamate immunohistochemistry might also detect non-glutamatergic cells or neurons that co-release both GABA and glutamate.

The glutamatergic neurons in the spinal cord of the sea lamprey: an in situ hybridization and immunohistochemical study

Glutamate is the main excitatory neurotransmitter involved in spinal cord circuits in vertebrates, but in most groups the distribution of glutamatergic spinal neurons is still unknown. Lampreys have been extensively used as a model to investigate the neuronal circuits underlying locomotion. Glutamatergic circuits have been characterized on the basis of the excitatory responses elicited in postsynaptic neurons. However, the presence of glutamatergic neurochemical markers in spinal neurons has not been investigated. In this study, we report for the first time the expression of a vesicular glutamate transporter (VGLUT) in the spinal cord of the sea lamprey. We also study the distribution of glutamate in perikarya and fibers. The largest glutamatergic neurons found were the dorsal cells and caudal giant cells. Two additional VGLUT-positive gray matter populations, one dorsomedial consisting of small cells and another one lateral consisting of small and large cells were observed. Some cerebrospinal fluid-contacting cells also expressed VGLUT. In the white matter, some edge cells and some cells associated with giant axons (Müller and Mauthner axons) and the dorsolateral funiculus expressed VGLUT. Large lateral cells and the cells associated with reticulospinal axons are in a key position to receive descending inputs involved in the control of locomotion. We also compared the distribution of glutamate immunoreactivity with that of γ-aminobutyric acid (GABA) and glycine. Colocalization of glutamate and GABA or glycine was observed in some small spinal cells. These results confirm the glutamatergic nature of various neuronal populations, and reveal new small-celled glutamatergic populations, predicting that some glutamatergic neurons would exert complex actions on postsynaptic neurons.

Glutamatergic neuronal populations in the forebrain of the sea lamprey, Petromyzon marinus: an in situ hybridization and immunocytochemical study

Despite the importance of glutamate as a major excitatory neurotransmitter in the brain, the distribution of glutamatergic populations in the brain of most vertebrates is still unknown. Here, we studied for the first time the distribution of glutamatergic neurons in the forebrain of the sea lamprey (Petromyzon marinus), belonging to the most ancient group of vertebrates (agnathans). For this, we used in situ hybridization with probes for a lamprey vesicular glutamate transporter (VGLUT) in larvae and immunofluorescence with antiglutamate antibodies in both larvae and adults. We also compared glutamate and γ-aminobutyric acid (GABA) immunoreactivities in sections using double-immunofluorescence methods. VGLUT-expressing neurons were observed in the olfactory bulb, pallium, septum, subhippocampal lobe, preoptic region, thalamic eminence, prethalamus, thalamus, epithalamus, pretectum, hypothalamus, posterior tubercle, and nucleus of the medial longitudinal fascicle. Comparison of VGLUT signal and glutamate immunoreactivity in larval forebrain revealed a consistent distribution of positive cells, which were numerous in most regions. Glutamate-immunoreactive cell populations were also found in similar regions of the adult forebrain. These include mitral-like cells of the olfactory bulbs and abundant cells in the lateral pallium, septum, and various diencephalic regions, mainly in the prethalamus, thalamus, habenula, pineal complex, and pretectum. Only a small portion of the glutamate-immunoreactive cells showed colocalization with GABA, which was observed mainly in the olfactory bulb, telencephalon, hypothalamus, ventral thalamus, and pretectum. Comparison with glutamatergic cells observed in rodent forebrains suggests that the regional distribution of glutamatergic cells does not differ greatly in lampreys and mammals.

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.

Development and functional organization of spinal locomotor circuits

The coordination and timing of muscle activities during rhythmic movements, like walking and swimming, are generated by intrinsic spinal motor circuits. Such locomotor networks are operational early in development and are found in all vertebrates. This review outlines and compares recent advances that have revealed the developmental and functional organization of these fundamental spinal motor networks in limbed and non-limbed animals. The comparison will highlight common principles and divergence in the organization of the spinal locomotor network structure in these different species as well as point to unresolved issues regarding the assembly and functioning of these networks.

Beyond connectivity of locomotor circuitry-ionic and modulatory mechanisms

Discrete neural networks in the central nervous system generate the repertoire of motor behavior necessary for animal survival. The final motor output of these networks is the result of the anatomical connectivity between the individual neurons and also their biophysical properties as well as the dynamics of their synaptic transmission. To illustrate how this processing takes place to produce coordinated motor activity, we have summarized some of the results available from the lamprey spinal locomotor network. The detailed knowledge available in this model system on the organization of the network together with the properties of the constituent neurons and the modulatory systems allows us to determine how the impact of specific ion channels and receptors is translated to the global activity of the locomotor circuitry. Understanding the logic of the neuronal and synaptic processing within the locomotor network will provide information about not only their normal operation but also how they react to disruption such as injuries or trauma.

Neuronal control of swimming behavior: comparison of vertebrate and invertebrate model systems

Swimming movements in the leech and lamprey are highly analogous, and lack homology. Thus, similarities in mechanisms must arise from convergent evolution rather than from common ancestry. Despite over 40 years of parallel investigations into this annelid and primitive vertebrate, a close comparison of the approaches and results of this research is lacking. The present review evaluates the neural mechanisms underlying swimming in these two animals and describes the many similarities that provide intriguing examples of convergent evolution. Specifically, we discuss swim initiation, maintenance and termination, isolated nervous system preparations, neural-circuitry, central oscillators, intersegmental coupling, phase lags, cycle periods and sensory feedback. Comparative studies between species highlight mechanisms that optimize behavior and allow us a broader understanding of nervous system function.

Rhythmogenesis in axial locomotor networks: an interspecies comparison

During locomotion, specialized neural networks referred to as "central pattern generators" ensure precise temporal relations between the axial segments, both in limbed and limbless vertebrates. These neural networks are intrinsically capable of generating coordinated patterns of rhythmic activity in the absence of sensory feedback or descending command from higher brain centers. Rhythmogenesis in these neural circuits lies on several mechanisms, both at the cellular and the network levels. In this chapter, we compare the anatomical organization of the axial networks, the role of identified spinal neurons, and their interactions in rhythmogenesis in four species: lamprey, zebrafish, Xenopus tadpole, and salamander. The comparison suggests that several principles in axial network design are phylogenetically conserved among vertebrates.