Locomotor-related presynaptic modulation of primary afferents in the lamprey

A New Perspective on Predictive Motor Signaling

Adaptive behavior relies on complex neural processing in multiple interacting networks of both motor and sensory systems. One such interaction employs intrinsic neuronal signals, so-called 'corollary discharge' or 'efference copy', that may be used to predict the sensory consequences of a specific behavioral action, thereby enabling self-generated (reafferent) sensory information and extrinsic (exafferent) sensory inflow to be dissociated. Here, by using well-established examples, we seek to identify the distinguishing features of corollary discharge and efference copy within the framework of predictive motor-to-sensory system coordination. We then extend the more general concept of predictive signaling by showing how neural replicas of a particular motor command not only inform sensory pathways in order to gate reafferent stimulation, but can also be used to directly coordinate distinct and otherwise independent behaviors to the original motor task. Moreover, this motor-to-motor pairing may additionally extend to a gating of sensory input to either or both of the coupled systems. The employment of predictive internal signaling in such motor systems coupling and remote sensory input control thus adds to our understanding of how an organism's central nervous system is able to coordinate the activity of multiple and generally disparate motor and sensory circuits in the production of effective behavior.

State-dependent sensorimotor gating in a rhythmic motor system

Sensory feedback influences motor circuits and/or their projection neuron inputs to adjust ongoing motor activity, but its efficacy varies. Currently, less is known about regulation of sensory feedback onto projection neurons that control downstream motor circuits than about sensory regulation of the motor circuit neurons themselves. In this study, we tested whether sensory feedback onto projection neurons is sensitive only to activation of a motor system, or also to the modulatory state underlying that activation, using the crab Cancer borealis stomatogastric nervous system. We examined how proprioceptor neurons (gastropyloric receptors, GPRs) influence the gastric mill (chewing) circuit neurons and the projection neurons (MCN1, CPN2) that drive the gastric mill rhythm. During gastric mill rhythms triggered by the mechanosensory ventral cardiac neurons (VCNs), GPR was shown previously to influence gastric mill circuit neurons, but its excitation of MCN1/CPN2 was absent. In this study, we tested whether GPR effects on MCN1/CPN2 are also absent during gastric mill rhythms triggered by the peptidergic postoesophageal commissure (POC) neurons. The VCN and POC pathways both trigger lasting MCN1/CPN2 activation, but their distinct influence on circuit feedback to these neurons produces different gastric mill motor patterns. We show that GPR excites MCN1 and CPN2 during the POC-gastric mill rhythm, altering their firing rates and activity patterns. This action changes both phases of the POC-gastric mill rhythm, whereas GPR only alters one phase of the VCN-gastric mill rhythm. Thus sensory feedback to projection neurons can be gated as a function of the modulatory state of an active motor system, not simply switched on/off with the onset of motor activity.NEW & NOTEWORTHY Sensory feedback influences motor systems (i.e., motor circuits and their projection neuron inputs). However, whether regulation of sensory feedback to these projection neurons is consistent across different versions of the same motor pattern driven by the same motor system was not known. We found that gating of sensory feedback to projection neurons is determined by the modulatory state of the motor system, and not simply by whether the system is active or inactive.

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.

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.

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.

Measured motion: searching for simplicity in spinal locomotor networks

Spinal interneurons are organized into networks that control the activity and output of the motor system. This review outlines recent progress in defining the rules that govern the assembly and function of spinal motor networks, focusing on three main areas. We first examine how subtle variations in the wiring diagrams and organization of locomotor networks in different vertebrates permits animals to adapt their motor programs to the demands of their physical environment. We discuss how the membrane properties of spinal interneurons, and their synaptic interactions, underlie the modulation of motor circuits and encoded motor behaviors. We also describe recent molecular genetic approaches to map and manipulate the connectivity and interactions of spinal interneurons and to assess the impact of such perturbations on network function and motor behavior.

Nitric oxide potentiation of locomotor activity in the spinal cord of the lamprey

To understand the intrinsic operation of spinal networks generating locomotion, we need to not only characterize the constituent neurons and their connectivity, but also determine the role of intrinsic modulation in shaping the final motor output. We have focused on the effects of nitric oxide (NO) on the locomotor frequency and the underlying synaptic mechanisms in the lamprey spinal cord. To identify the source of NO, we used NADPH-diaphorase histochemistry and nNOS immunocytochemistry. Gray matter and sensory neurons were positively labeled using both methods. Preparations preincubated with NO synthase inhibitors displayed slower locomotor frequency that increased upon washout of the inhibitors, suggesting that NO is an endogenous neuromodulator in the spinal cord. Application of NO donors increased the locomotor frequency that was blocked by an NO scavenger and partially reduced by an inhibitor of sGC. To analyze the synaptic modulation underlying the NO-induced increase of the locomotor frequency we performed intracellular recordings from motoneurons and interneurons. The NO-induced increase in locomotor frequency was associated with a decrease in the midcycle inhibition and an increase in on-cycle excitation. To determine the site of action of NO, we examined the effect of NO donors on miniature PSCs. NO increased both the frequency and amplitude of mEPSCs while it only decreased the frequency of mIPSCs, suggesting the increased excitation is mediated by both presynaptic and postsynaptic mechanisms, while the decrease in inhibition involves only presynaptic mechanisms. Our results demonstrate a significant role of NO in adult vertebrate motor control which, via modulation of both excitatory and inhibitory transmission, increases the locomotor burst frequency.

Transmitter phenotypes of commissural interneurons in the lamprey spinal cord

The fundamental network for locomotion in all vertebrates contains a central pattern generator or CPG that produces the required motor output in the spinal cord. In the lamprey spinal cord different classes of interneuron's forming the core CPG circuitry have been characterized based on their morphological and electrophysiological features. The commissural interneuron's (C-INs) represent one essential component of CPG that have been implicated in controlling left-right alternation of the motor activity during swimming. However, it is still unclear if the C-INs displays a homogenous neurotransmitter phenotype and how they are distributed. In this paper we investigated the segmental distribution of glycine, glutamate and GABA-immunoreactive (ir) C-INs by combining retrograde Neurobiotin tracing with specific antibodies for these transmitters. The C-INs were more abundant in caudal and rostral segments adjacent to the injection site and their number gradually decreased in more distal segments, suggesting that these interneurons project over a short distance. The glycine-ir neurons represented around 50% of the total C-INs, while glutamate-ir neurons represented only 29%. Both types of C-INs were homogenously distributed over different segments along the spinal cord. Finally, no Neurobiotin labeled C-INs displayed GABA-ir, although many interneurons were ir to GABA, suggesting that GABAergic interneurons are not directly responsible for controlling left-right alternation of activity during locomotion in lamprey. Overall, these results show that the C-INs display a gradual rostrocaudal distribution and consist of both glycine- and glutamate-ir neurons. The difference in the proportion of inhibitory and excitatory C-INs represents an anatomical substrate that can ensure the predominance of alternating activity during locomotion.

Multiple mechanisms for integrating proprioceptive inputs that converge on the same motor pattern-generating network

Movement-derived sensory feedback adapts centrally generated motor programs to changing behavioral demands. Motor circuit output may also be shaped by distinct proprioceptive systems with different central actions, although little is known about the integrative processes by which such convergent sensorimotor regulation occurs. Here, we explore the combined actions of two previously identified proprioceptors on the gastric mill motor network in the lobster stomatogastric nervous system. Both mechanoreceptors [anterior gastric receptor (AGR) and posterior stomach receptor (PSR)] access the gastric circuit via the same pair of identified projection interneurons that either excite [commissural gastric (CG)] or inhibit [gastric inhibitor (GI)] different subsets of gastric network neurons. Mechanosensory information from the two receptors is integrated upstream to the gastric circuit at two levels: (1) postsynaptically, where both receptors excite the GI neuron while exerting opposing effects on the CG neuron, and (2) presynaptically, where PSR reduces AGR's excitation of the CG projection neuron. Concomitantly PSR selectively enhances AGR's activation of the GI neuron, possibly also via a presynaptic action. PSR's influences also far outlast its transient synaptic effects, indicating the additional involvement of modulatory processes. Consequently, PSR activation causes parallel input from AGR to be conveyed preferentially via the GI interneuron, resulting in a prolonged switch in the pattern of gastric circuit output. Therefore, via a combination of short- and long-lasting, presynaptic and postsynaptic actions, one proprioceptive system is able to promote its impact on a target motor network by biasing the access of a different sensory system to the same circuit.

Mechanosensory gating of proprioceptor input to modulatory projection neurons

Sensorimotor gating commonly occurs at sensory neuron synapses onto motor circuit neurons and motor neurons. Here, using the crab stomatogastric nervous system, we show that sensorimotor gating also occurs at the level of the projection neurons that activate motor circuits. We compared the influence of the gastro-pyloric receptor (GPR) muscle stretch-sensitive neuron on two projection neurons, modulatory commissural neuron 1 (MCN1) and commissural projection neuron 2 (CPN2), with and without a preceding activation of the mechanosensory ventral cardiac neurons (VCNs). MCN1 and CPN2 project from the paired commissural ganglia (CoGs) to the stomatogastric ganglion (STG), where they activate the gastric mill (chewing) motor circuit. When stimulated separately, the GPR and VCN neurons each elicit the gastric mill rhythm by coactivating MCN1 and CPN2. When GPR is instead stimulated during the VCN-gastric mill rhythm, it slows this rhythm. This effect results from a second GPR synapse onto MCN1 that presynaptically inhibits its STG terminals. Here, we show that, during the VCN-triggered rhythm, the GPR excitation of MCN1 and CPN2 in the CoGs is gated out, leaving only its influence in the STG. This gating effect appears to occur within the CoG and does not result from a ceiling effect on projection neuron firing frequency. Additionally, this gating action enables GPR to either activate rhythmic motor activity or act as a phasic sensorimotor feedback system. These results also indicate that the site of sensorimotor gating can occur at the level of the projection neurons that activate a motor circuit.

GABA distribution in lamprey is phylogenetically conserved

The localization of gamma-aminobutyric acid (GABA) has been well described in most classes of vertebrates but not in adult lampreys. The question if the GABA distribution is similar throughout the vertebrate subphylum is therefore still to be addressed. We here investigate two lamprey species, the sea lamprey, Petromyzon marinus, and the river lamprey, Lampetra fluviatilis, and compare the GABA pattern with that of other vertebrates. The present immunohistochemical study provides an anatomical basis for the general distribution and precise localization of GABAergic neurons in the adult lamprey forebrain and brainstem. GABA-immunoreactive cells were organized in a virtually identical manner in the two species. They were found throughout the brain, with the following regions being of particular interest: the granular cell layer of the olfactory bulb, the nucleus of the anterior commissure, the septum, the lateral and medial pallia, the striatum, the nucleus of the postoptic commissure, the thalamus, the hypothalamus, and pretectal areas, the optic tectum, the torus semicircularis, the mesencephalic tegmentum, restricted regions of the rhombencephalic tegmentum, the octavolateral area, and the dorsal column nucleus. The GABA distribution found in cyclostomes is very similar to that of other classes of vertebrates, including mammals. Since the lamprey diverged from the main vertebrate line around 450 million years ago, this implies that already at that time the basic vertebrate plan for the GABA innervation in different parts of the brain had been developed.

Intersegmental coordination: influence of a single walking leg on the neighboring segments in the stick insect walking system

A key element of walking is the coordinated interplay of multiple limbs to achieve a stable locomotor pattern that is adapted to the environment. We investigated intersegmental coordination of walking in the stick insect, Carausius morosus by examining the influence a single stepping leg has on the motoneural activity of the other hemiganglia, and whether this influence changes with the walking direction. We used a reduced single leg walking preparation with only one intact front, middle, or hind leg. The intact leg performed stepping movements on a treadmill, thus providing intersegmental signals about its stepping to the other hemiganglia. The activity of coxal motoneurons was simultaneously recorded extracellularly in all other segments. Stepping sequences of any given single leg in either walking direction were accompanied by an increase in coxal motoneuron (MN) activity of all other segments, which was mostly modulated and slightly in phase with stance of the walking leg. In addition, forward stepping of the front leg and, to a lesser extent, backward stepping of the hind leg elicited alternating activity in mesothoracic coxal MNs. Forward and backward stepping of the middle leg did not elicit alternating activity in coxal MNs in any other hemiganglia, indicating that the influence of middle leg stepping is qualitatively different from that of forward front and backward hind leg stepping. Our results indicate that in an insect walking system individual segments differ with respect to their intersegmental influences and thus cannot be treated as similar within the chain of segmental walking pattern generators. Consequences for the current concepts on intersegmental coordination are discussed.

Biological pattern generation: the cellular and computational logic of networks in motion

In 1900, Ramón y Cajal advanced the neuron doctrine, defining the neuron as the fundamental signaling unit of the nervous system. Over a century later, neurobiologists address the circuit doctrine: the logic of the core units of neuronal circuitry that control animal behavior. These are circuits that can be called into action for perceptual, conceptual, and motor tasks, and we now need to understand whether there are coherent and overriding principles that govern the design and function of these modules. The discovery of central motor programs has provided crucial insight into the logic of one prototypic set of neural circuits: those that generate motor patterns. In this review, I discuss the mode of operation of these pattern generator networks and consider the neural mechanisms through which they are selected and activated. In addition, I will outline the utility of computational models in analysis of the dynamic actions of these motor networks.

Dynamic sensorimotor interactions in locomotion

Locomotion results from intricate dynamic interactions between a central program and feedback mechanisms. The central program relies fundamentally on a genetically determined spinal circuitry (central pattern generator) capable of generating the basic locomotor pattern and on various descending pathways that can trigger, stop, and steer locomotion. The feedback originates from muscles and skin afferents as well as from special senses (vision, audition, vestibular) and dynamically adapts the locomotor pattern to the requirements of the environment. The dynamic interactions are ensured by modulating transmission in locomotor pathways in a state- and phase-dependent manner. For instance, proprioceptive inputs from extensors can, during stance, adjust the timing and amplitude of muscle activities of the limbs to the speed of locomotion but be silenced during the opposite phase of the cycle. Similarly, skin afferents participate predominantly in the correction of limb and foot placement during stance on uneven terrain, but skin stimuli can evoke different types of responses depending on when they occur within the step cycle. Similarly, stimulation of descending pathways may affect the locomotor pattern in only certain phases of the step cycle. Section ii reviews dynamic sensorimotor interactions mainly through spinal pathways. Section iii describes how similar sensory inputs from the spinal or supraspinal levels can modify locomotion through descending pathways. The sensorimotor interactions occur obviously at several levels of the nervous system. Section iv summarizes presynaptic, interneuronal, and motoneuronal mechanisms that are common at these various levels. Together these mechanisms contribute to the continuous dynamic adjustment of sensorimotor interactions, ensuring that the central program and feedback mechanisms are congruous during locomotion.

Tuning the network: modulation of neuronal microcircuits in the spinal cord and hippocampus

Adaptation of an organism to its changing environment ultimately depends on the modification of neuronal activity. The dynamic interaction between cellular components within neuronal networks relies on fast synaptic interaction via ionotropic receptors. However, neuronal networks are also subject to modulation mediated by various metabotropic G-protein-coupled receptors that modify synaptic and neuronal function. Modulation increases the functional complexity of a network, because the same cellular components can produce different outputs depending on the behavioural state of the animal. This review, which is part of the TINS Microcircuits Special Feature, provides an overview of neuromodulation in two neuronal circuits that both produce oscillatory activity but differ fundamentally in function. Hippocampal circuits are compared with the spinal networks generating locomotion, with a view to exploring common principles of neuromodulatory activity.

Corollary discharge inhibition and audition in the stridulating cricket

The romantic notion of crickets singing on a warm summer's evening is quickly dispelled when one comes ear to ear with a stridulating male. Remarkably, stridulating male crickets are able to hear sounds from the environment despite generating a 100 db song (Heiligenberg 1969; Jones and Dambach 1973). This review summarises recent work examining how they achieve this feat of sensory processing. While the responsiveness of the crickets' peripheral auditory system (tympanic membrane, tympanic nerve, state of the acoustic spiracle) is maintained during sound production, central auditory neurons are inhibited by a feedforward corollary discharge signal precisely timed to coincide with the auditory neurons' maximum response to self-generated sound. In this way, the corollary discharge inhibition prevents desensitisation of the crickets' auditory pathway during sound production.

Travelling wave patterns in a model of the spinal pattern generator using spiking neurons

The aim of this study is to produce travelling waves in a planar net of artificial spiking neurons. Provided that the parameters of the waves--frequency, wavelength and orientation--can be sufficiently controlled, such a network can serve as a model of the spinal pattern generator for swimming and terrestrial quadruped locomotion. A previous implementation using non-spiking, sigmoid neurons lacked the physiological plausibility that can only be attained using more realistic spiking neurons. Simulations were conducted using three types of spiking neuronal models. First, leaky integrate-and-fire neurons were used. Second, we introduced a phenomenological bursting neuron. And third, a canonical model neuron was implemented which could reproduce the full dynamics of the Hodgkin-Huxley neuron. The conditions necessary to produce appropriate travelling waves corresponded largely to the known anatomy and physiology of the spinal cord. Especially important features for the generation of travelling waves were the topology of the local connections--so-called off-centre connectivity--the availability of dynamic synapses and, to some extent, the availability of bursting cell types. The latter were necessary to produce stable waves at the low frequencies observed in quadruped locomotion. In general, the phenomenon of travelling waves was very robust and largely independent of the network parameters and emulated cell types.

The spinal GABAergic system is a strong modulator of burst frequency in the lamprey locomotor network

The spinal network coordinating locomotion is comprised of a core of glutamate and glycine interneurons. This network is modulated by several transmitter systems including spinal GABA interneurons. The purpose of this study is to explore the contribution of GABAergic neurons to the regulation of locomotor burst frequency in the lamprey model. Using gabazine, a competitive GABAA antagonist more specific than bicuculline, the goal was to provide a detailed analysis of the influence of an endogenous activation of GABAA receptors on fictive locomotion, as well as their possible interaction with GABAB and involvement of GABAC receptors. During N-methyl-D-aspartate (NMDA)-induced fictive locomotion (ventral root recordings in the isolated spinal cord), gabazine (0.1-100 microM) significantly increased the burst rate up to twofold, without changes in regularity or "burst quality." Gabazine had a proportionately greater effect at higher initial burst rates. Picrotoxin (1-7.5 microM), a less selective GABAA antagonist, also produced a pronounced increase in frequency, but at higher concentrations, the rhythm deteriorated, likely due to the unspecific effects on glycine receptors. The selective GABAB antagonist CGP55845 also increased the frequency, and this effect was markedly enhanced when combined with the GABAA antagonist gabazine. The GABAC antagonist (1,2,5,6-tetrahydropyridine-4-yl)methylphosphinic acid (TPMPA) had no effect on locomotor bursting. Thus the spinal GABA system does play a prominent role in burst frequency regulation in that it reduces the burst frequency by < or =50%, presumably due to presynaptic and soma-dendritic effects documented previously. It is not required for burst generation, but acts as a powerful modulator.

Muscarinic modulation of the trigemino-reticular pathway in lampreys

In lampreys, reticulospinal neurons integrate sensory inputs to adapt their control onto the spinal locomotor networks. Whether and how sensory inputs to reticulospinal neurons are modulated remains to be determined. We showed recently that cholinergic inputs onto reticulospinal neurons play a key role in the initiation of locomotion elicited by stimulation of the mesencephalic locomotor region in semi intact lampreys. Here, we examined the possible role of muscarinic acetylcholine receptors in modulating trigeminal inputs to reticulospinal neurons. A local application of muscarinic agonists onto an intracellularly recorded reticulospinal cell depressed the disynaptic responses to trigeminal stimulation. A depression was also observed when muscarinic agonists were pressure ejected over the brain stem region containing second-order neurons relaying trigeminal inputs to reticulospinal neurons. Conversely, muscarinic antagonists increased the trigeminal-evoked responses, suggesting that a muscarinic depression of sensory inputs to RS neurons is exerted tonically. The muscarinic modulation affected predominantly the N-methyl-d-aspartate (NMDA) component of the trigeminal-evoked responses. Moreover, atropine perfusion facilitated the occurrence of sustained depolarizations induced by stimulation of the trigeminal nerve, and it revealed NMDA-induced intrinsic oscillations in reticulospinal neurons. The functional significance of a muscarinic modulation of a sensory transmission to reticulospinal neurons is discussed.

Corollary discharge inhibition of ascending auditory neurons in the stridulating cricket

Acoustically communicating animals are able to process external acoustic stimuli despite generating intense sounds during vocalization. We have examined how the crickets' ascending auditory pathway copes with self-generated, intense auditory signals (chirps) during singing (stridulation). We made intracellular recordings from two identified ascending auditory interneurons, ascending neuron 1 (AN1) and ascending neuron 2 (AN2), during pharmacologically elicited sonorous (two-winged), silent (one-winged), and fictive (isolated CNS) stridulation. During sonorous chirps, AN1 responded with bursts of spikes, whereas AN2 was inhibited and rarely spiked. Low-amplitude hyperpolarizing potentials were recorded in AN1 and AN2 during silent chirps. The potentials were also present during fictive chirps. Therefore, they were the result of a centrally generated corollary discharge from the stridulatory motor network. The spiking response of AN1 and AN2 to acoustic stimuli was inhibited during silent and fictive chirps. The maximum period of inhibition occurred in phase with the maximum spiking response to self-generated sound in a sonorously stridulating cricket. In some experiments (30%) depolarizing potentials were recorded during silent chirps. Reafferent feedback elicited by wing movement was probably responsible for the depolarizing potentials. In addition, two other sources of inhibition were present in AN1: (1) IPSPs were elicited by stimulation with 12.5 kHz stimuli and (2) a long-lasting hyperpolarization followed spiking responses to 4.5 kHz stimuli. The hyperpolarization desensitized the response of AN1 to subsequent quieter stimuli. Therefore, the corollary discharge will reduce desensitization by suppressing the response of AN1 to self-generated sounds.

A corollary discharge mechanism modulates central auditory processing in singing crickets

Crickets communicate using loud (100 dB SPL) sound signals that could adversely affect their own auditory system. To examine how they cope with this self-generated acoustic stimulation, intracellular recordings were made from auditory afferent neurons and an identified auditory interneuron-the Omega 1 neuron (ON1)-during pharmacologically elicited singing (stridulation). During sonorous stridulation, the auditory afferents and ON1 responded with bursts of spikes to the crickets' own song. When the crickets were stridulating silently, after one wing had been removed, only a few spikes were recorded in the afferents and ON1. Primary afferent depolarizations (PADs) occurred in the terminals of the auditory afferents, and inhibitory postsynaptic potentials (IPSPs) were apparent in ON1. The PADs and IPSPs were composed of many summed, small-amplitude potentials that occurred at a rate of about 230 Hz. The PADs and the IPSPs started during the closing wing movement and peaked in amplitude during the subsequent opening wing movement. As a consequence, during silent stridulation, ON1's response to acoustic stimuli was maximally inhibited during wing opening. Inhibition coincides with the time when ON1 would otherwise be most strongly excited by self-generated sounds in a sonorously stridulating cricket. The PADs and the IPSPs persisted in fictively stridulating crickets whose ventral nerve cord had been isolated from muscles and sense organs. This strongly suggests that the inhibition of the auditory pathway is the result of a corollary discharge from the stridulation motor network. The central inhibition was mimicked by hyperpolarizing current injection into ON1 while it was responding to a 100 dB SPL sound pulse. This suppressed its spiking response to the acoustic stimulus and maintained its response to subsequent, quieter stimuli. The corollary discharge therefore prevents auditory desensitization in stridulating crickets and allows the animals to respond to external acoustic signals during the production of calling song.

Spinal inhibitory neurons that modulate cutaneous sensory pathways during locomotion in a simple vertebrate

During locomotion, reflex responses to sensory stimulation are usually modulated and may even be reversed. This is thought to be the result of phased inhibition, but the neurons responsible are usually not known. When the hatchling Xenopus tadpole swims, responses to cutaneous stimulation are modulated. This occurs because sensory pathway interneurons receive rhythmic glycinergic inhibition broadly in phase with the motor discharge on the same side of the trunk. We now describe a new whole-cell recording preparation of the Xenopus tadpole CNS. This has been used with neurobiotin injection to define the passive and firing properties of spinal ascending interneurons and their detailed anatomy. Paired recordings show that they make direct, glycinergic synapses onto spinal sensory pathway interneurons, and the site of contact can be seen anatomically. During swimming, ascending interneurons fire rhythmically. Analysis shows that their firing is more variable and not as reliable as other interneurons, but the temporal pattern of their impulse activity is suitable to produce the main peak of gating inhibition in sensory pathway interneurons. Ascending interneurons are not excited at short latency after skin stimulation but are strongly active after repetitive skin stimulation, which evokes vigorous and slower struggling movements. We conclude that ascending interneurons are a major class of modulatory neurons producing inhibitory gating of cutaneous sensory pathways during swimming and struggling.

Contributions of identifiable neurons and neuron classes to lamprey vertebrate neurobiology

Among the advantages offered by the lamprey brainstem and spinal cord for studies of the structure and function of the nervous system is the unique identifiability of several pairs of reticulospinal neurons in the brainstem. These neurons have been exploited in investigations of the patterns of sensory input to these cells and the patterns of their outputs to spinal neurons, but no doubt these cells could be used much more effectively in exploring their roles in descending control of the spinal cord. The variability of cell positions of neurons in the spinal cord has precluded the recognition of unique spinal neurons. However, classes of nerve cells can be readily defined and characterized within the lamprey spinal cord and this has led to progress in understanding the cellular and synaptic mechanisms of locomotor activity. In addition, both the identifiable reticulospinal cells and the various spinal nerve cell classes and their known synaptic interactions have been used to demonstrate the degree and specificity of regeneration within the lamprey nervous system. The lack of uniquely identifiable cells within the lamprey spinal cord has hampered progress in these areas, especially in gaining a full understanding of the locomotor network and how neuromodulation of the network is accomplished.

The intrinsic function of a motor system--from ion channels to networks and behavior

The forebrain, brainstem and spinal cord contribution to the control of locomotion is reviewed in this article. The lamprey is used as an experimental model since it allows a detailed cellular analysis of the neuronal network underlying locomotion. The focus is on cellular mechanisms that are important for the pattern generation, as well as different types of pre- and postsynaptic modulation. This experimental model is bridging the gap between the molecular and cellular level to the network and behavioral level.

Central control components of a 'simple' stretch reflex

The monosynaptic stretch reflex is a fundamental feature of sensory-motor organization in most animal groups. In isolation, it serves largely as a negative feedback devoted to postural controls; however, when it is involved in diverse movements, it can be modified by central command circuits. In order to understand the implications of such modifications, a model system has been chosen that has been studied at many different levels: the crayfish walking system. Recent studies have revealed several levels of control and modulation (for example, at the levels of the sensory afferent and the output synapse from the sensory afferent, and via changes in the membrane properties of the postsynaptic neuron) that operate complex and highly adaptive sensory-motor processing. During a given motor task, such mechanisms reshape the sensory message completely, such that the stretch reflex becomes a part of the central motor command.

Bridging the gap - from ion channels to networks and behaviour

A major challenge for current research in neuroscience is to understand the intrinsic operation of the functional modules of the central nervous system, such as those formed by cortical columns and the neuronal networks controlling motor behaviour. Most vertebrate experimental models used in network analyses involve developing nervous systems, which are in rapid transition with regard to their cellular properties and the expression of different ion channels. Recent advances in our understanding of the cellular and circuit properties of motor networks are making it possible to decipher the mechanisms involved in vertebrate motor pattern generation.

Presynaptic inhibition and antidromic discharges in crayfish primary afferents

The mechanisms of presynaptic inhibition have been studied in sensory afferents of a stretch receptor in an in vitro preparation of the crayfish. Axon terminals of these sensory afferents display primary afferent depolarisations (PADs) mediated by the activation of GABA receptors that open chloride channels. Intracellular labeling of sensory axons by Lucifer yellow combined with GABA immunohistochemistry revealed the presence of close appositions between GABA-immunoreactive boutons and sensory axons close to their first branching point within the ganglion. Electrophysiological studies showed that GABA inputs mediating PADs appear to occur around the first axonal branching point, which corresponds to the area of transition between active and passive propagation of spikes. Moreover, this study demonstrated that whilst shunting appeared to be the sole mechanism involved during small amplitude PADs, sodium channel inactivation occurred with larger amplitude PADs. However, when the largest PADs (>25 mV) are produced, the threshold for spike generation is reached and antidromic action potentials are elicited. The mechanisms involved in the initiation of antidromic discharges were analyzed by combining electrophysiological and simulation studies. Three mechanisms act together to ensure that PAD-mediated spikes are not conveyed distally: 1) the lack of active propagation in distal regions of the sensory axons; 2) the inactivation of the sodium channels around the site where PADs are produced; and 3) a massive shunting through the opening of chloride channels associated with the activation of GABA receptors. The centrally generated spikes are, however, conveyed antidromically in the sensory nerve up to the proprioceptive organ, where they inhibit the activity of the sensory neurons for several hundreds of milliseconds.

The effects of antidromic discharges on orthodromic firing of primary afferents in the cat

This study investigated the effects of antidromically conducted nerve impulses on the transmission of orthodromic volleys in primary afferents of the hindlimb in decerebrated paralyzed cats. Two protocols were used: (A) Single skin and muscle afferents (N=20) isolated from the distal part of cut dorsal rootlets (L7-S1) were recorded while stimulation was applied more caudally. The results showed that during the trains of three to 20 stimuli, the orthodromic firing frequency decreased or ceased, depending on the frequency of stimulation. Remarkably, subsequent to these trains, the occurrence of orthodromic spikes could be delayed for hundreds of ms (15/20 afferents) and sometimes stopped for several seconds (10/20 afferents). Longer stimulation trains, simulating antidromic bursts reported during locomotion, caused a progressive decrease, and a slow recovery of, orthodromic firing frequency (7/20 afferents), indicating a cumulative long-lasting depressing effect from successive bursts. (B) Identified stretch-sensitive muscle afferents were recorded intra-axonally and antidromic spikes were evoked by the injection of square pulses of current through the micropipette. In this case, one to three antidromic spikes were sufficient to delay the occurrence of the next orthodromic spike by more than one control inter-spike interval. If the control inter-spike interval was decreased by stretching the muscle, the delay evoked by antidromic spikes decreased proportionally. Overall, these findings suggest that antidromic activity could alter the mechanisms underlying spike generation in peripheral sensory receptors and modify the orthodromic discharges of afferents during locomotion.

The modulation of presynaptic inhibition in single muscle primary afferents during fictive locomotion in the cat

The aim of this study is to understand the functional organization of presynaptic inhibition in muscle primary afferents during locomotion. Primary afferent depolarization (PAD) associated with presynaptic inhibition was recorded intra-axonally in identified afferents from various hindlimb muscles in L6-L7 spinal segments during fictive locomotion in the decerebrate cat. PADs were evoked by the stimulation of peripheral muscle nerves and were averaged in the different epochs of the fictive step cycle. Fifty-three trials recorded from 39 muscle axons (37 from group I and two from group II) were retained for analysis. The results showed that there was a significant phase-dependent modulation of PAD amplitude (p < 0.05) in a majority of muscle afferents (30 of 39, 77%). However, not all stimulated nerves led to significantly modulated PADs in a given axon (36 of 53 trials, 68%). We also observed that the pattern of modulation (phase for maximum and minimum PAD amplitude and the depth of modulation) varied with each recorded afferent, as well as with each stimulated nerve. We further evaluated the effect of PAD modulation on the phasic transmission of the monosynaptic reflex (MSR) and found that PADs decreased the MSR amplitude in all phases of the fictive step cycle, independent of the PAD pattern in individual group I fibers. We conclude that (1) PAD modulation patterns of all group I fibers contacting motoneurons led to an overall reduction in monosynaptic transmission, and (2) individual PAD patterns could participate in the control of transmission in specific reflex pathways during locomotion.

Sensory pathways and their modulation in the control of locomotion

Recent experiments have extended our understanding of how sensory information in premotor networks controlling motor output is processed during locomotion, and at what level the efficacy of specific sensory-motor pathways is determined. Phasic presynaptic inhibition of sensory transmission combined with postsynaptic alterations of excitatory and inhibitory synaptic transmission from interneurons of the premotor networks contribute to the modulation of reflex pathways and to the generation of reflex reversal. These mechanisms play an important role in adapting the operation of central networks to external demands and thus help optimize sensory-motor integration.

Vertebrate locomotion--a lamprey perspective

The forebrain, brain stem, and spinal cord contribution to the control of locomotion is reviewed in this chapter. The lamprey is used as an experimental model because it allows a detailed cellular analysis of the neuronal network underlying locomotion. The focus is on cellular mechanisms that are important for the pattern generation, as well as different types of pre- and postsynaptic modulation. Neuropeptides target different cellular and synaptic mechanisms and cause long-lasting changes (> 24 h) in network function.