Generation of a locomotory rhythm by a neural network with recurrent cyclic inhibition

An oscillatory intersegmental neuronal network drives the swimming rhythm of the leech. This network consists of interneurons joined via inhibitory connections to form a series of segmentally iterated, concatenated rings. Recurrent cyclic inhibition in these rings produces a multiphasic activity rhythm. By theoretical analysis of such concatenated interneuronal rings and construction of their electronic analogs it is shown that the interneural network identified in the central nervous system of the leech has properties appropriate for generating the observed motor output.     

Embryonic electrical connections appear to prefigure a behavioral circuit in the leech CNS

During development, many embryos show electrical coupling among neurons that is spatially and temporally regulated. For example, in vertebrate embryos extensive dye coupling is seen during the period of circuit formation, suggesting that electrical connections could prefigure circuits, but it has been difficult to identify which neuronal types are coupled. We have used the leech Hirudo medicinalis to follow the development of electrical connections within the circuit that produces local bending. This circuit consists of three layers of neurons: four mechanosensory neurons (P cells), 17 identified interneurons, and approximately 24 excitatory and inhibitory motor neurons. These neurons can be identified in embryos, and we followed the spatial and temporal dynamics as specific connections developed. Injecting Neurobiotin into identified cells of the circuit revealed that electrical connections were established within this circuit in a precise manner from the beginning. Connections first appeared between motor neurons; mechanosensory neurons and interneurons started to connect at least a day later. This timing correlates with the development of behaviors, so the pattern of emerging connectivity could explain the appearance first of spontaneous behaviors (driven by a electrically coupled motor network) and then of evoked behaviors (when sensory neurons and interneurons are added to the circuit).     

A Neural Network Model of Sensorimotor Transformations in the Local Bending Reflex of the Medicinal Leech

SYNOPSIS. In response to a moderate mechanical stimulus, theleech withdraws by forming a U-shaped local bend beneath thestimulus. Interneurons in the local bending reflex receive multiplesensory inputs, and have outputs to many motor neurons, suggestingthat stimulus location on the body is represented in a distributedfashion. However, it is possible that as yet unidentified interneuronsdedicated to the detection of specific spatial locations arenecessary for the full range of the local bending motor output.Using the backpropagation algorithm to optimize connectionsin a model of the reflex, we showed that the distributed processingmodel of sensorimotor integration was consistent with our knowledgeof the connection strengths between identified neurons in thereflex. The model further showed that the spatial and temporalconstraints on the linkage between sensory representations andmotor output can be satisfied by the appropriate connectivitybetween layers of interneurons and motor neurons in the leech,as well as in other systems with similar circuitry.     

The whole-body shortening reflex of the medicinal leech: motor pattern,sensory basis,and interneuronal pathways

The leech whole-body shortening reflex consists of a rapid contraction of the body elicited by a mechanical stimulus to the anterior of the animal. We used a variety of reduced preparations — semi-intact, body wall, and isolated nerve cord — to begin to elucidate the neural basis of this reflex in the medicinal leech Hirudo medicinalis. The motor pattern of the reflex involved an activation of excitatory motor neurons innervating dorsal and ventral longitudinal muscles (dorsal excitors and ventral excitors respectively), as well as the L cell, a motor neuron innervating both dorsal and ventral longitudinal muscles. The sensory input for the reflex was provided primarily by the T (touch) and P (pressure) types of identified mechanosensory neuron. The S cell network, a set of electrically-coupled interneurons which makes up a fast conducting pathway in the leech nerve cord, was active during shortening and accounted for the shortest-latency excitation of the L cells. Other, parallel, interneuronal pathways contributed to shortening as well. The whole-body shortening reflex was shown to be distinct from the previously described local shortening behavior of the leech in its sensory threshold, motor pattern, and (at least partially) in its interneuronal basis.Abbreviations conn connective - DE dorsal excitor motor neuron - DI dorsal inhibitor motor neuron - DP dorsal posterior nerve - DP:B1 dorsal posterior nerve branch 1 - DP:B2 dorsal posterior nerve branch 2 - MG midbody ganglion - VE ventral excitor motor neuron - VI ventral inhibitor motor neuron     

Motor pattern selection by combinatorial code of interneuronal pathways

We use a modeling approach to examine ideas derived from physiological network analyses, pertaining to the switch of a motor control network between two opposite control modes. We studied the femur–tibia joint control system of the insect leg, and its switch between resistance reflex in posture control and “active reaction” in walking, both elicited by the same sensory input. The femur–tibia network was modeled by fitting the responses of model neurons to those obtained in animals. The strengths of 16 interneuronal pathways that integrate sensory input were then assigned three different values and varied independently, generating a database of more than 43 million network variants. We demonstrate that the same neural network can produce the two different behaviors, depending on the combinatorial code of interneuronal pathways. That is, a switch between behaviors, such as standing to walking, can be brought about by altering the strengths of selected sensory integration pathways. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Spatial-specific action of serotonin within the leech midbody ganglion

Serotonin is a conspicuous neuromodulator in the nervous system of many vertebrates and invertebrates. In previous experiments performed in the leech nervous system, we compared the effect of the amine released from endogenous sources [using selective serotonin reuptake inhibitors (SSRIs), e.g. fluoxetine] with that of bath-applied serotonin. The results suggested that the amine does not reach all its targets in a uniform way, but produces the activation of an interneuronal pathway that generated specific synaptic responses on different neurons. Taking into account that the release of the amine is often regulated at the presynaptic level, we have investigated whether autoreceptor antagonists mimic the SSRIs effect. We found that methiothepin (100 microM) produced similar effects than fluoxetine. To further test the hypothesis that endogenous serotonin produce its effect by acting locally at specific sites, we analyzed the effect of iontophoretic applications of serotonin. We found a site in the neuropil of the leech ganglia where serotonin application mimicked the effect of the SSRIs and the 5-HT antagonist. The results further support the view that the effect of serotonin exhibits a spatial specificity that can be relevant to understand its modulatory actions.     

Nuclear microsatellite and mitochondrial DNA analyses reveal the regional genetic structure and phylogeographical history of a sanguivorous land leech,Haemadipsa japonica,in Japan

Recent molecular studies have indicated that phylogeographical history of Japanese biota is likely shaped by geohistory along with biological events, such as distribution shifts, isolation, and divergence of populations. However, the genetic structure and phylogeographical history of terrestrial Annelida species, including leech species, are poorly understood. Therefore, we aimed to understand the genetic structure and phylogeographical history across the natural range of Haemadipsa japonica, a sanguivorous land leech species endemic to Japan, by using nine polymorphic nuclear microsatellites (nSSR) and cytochrome oxidase subunit one (COI) sequences of mitochondrial DNA (mtDNA). Analyses using nSSR revealed that H. japonica exhibited a stronger regional genetic differentiation among populations (G'_ST = 0.77) than other animal species, probably because of the low mobility of land leech. Analyses using mtDNA indicated that H. japonica exhibited two distinct lineages (A and B), which were estimated to have diverged in the middle Pleistocene and probably because of range fragmentation resulting from climatic change and glacial and interglacial cycles. Lineage A was widely distributed across Japan, and lineage B was found in southwestern Japan. Analyses using nSSR revealed that lineage A was roughly divided into two population groups (i.e., northeastern and southwestern Japan); these analyses also revealed a gradual decrease in genetic diversity with increasing latitude in lineage A and a strong genetic drift in populations of northeastern Japan. Combined with the largely unresolved shallow polytomies from the mtDNA phylogeny, these results implied that lineage A may have undergone a rapid northward migration, probably during the Holocene. Then, the regional genetic structure with local unique gene pools may have been formed within each lineage because of the low mobility of this leech species.     

Two forms of sensitization of the local bending reflex of the medicinal leech

Sensitization of the local bending reflex of the medicinal leech Hirudo medicinalis was studied in a semi-intact preparation in which behavioral and electrophysiological recordings were made simultaneously. 1. Sensitization of local bending could be produced in two ways: by repeated stimulation of the mechanoreceptor sensitive to pressure (the P cell), and by stimulation of the mechanoreceptor sensitive to noxious stimuli (the N cell). 2. Both forms of sensitization produced a central neuronal change, measured as an increase in the number of stimulus-evoked action potentials in cell 3 (an excitor of dorsal longitudinal muscles). 3. Intracellular stimulation of serotonin-containing neurons 21 and 61 mimicked the sensitizing stimuli, but stimulation of the Retzius cell, which also contains serotonin, did not. 4. Stimulation of the Leydig cell, which releases octopamine, decreased the strength of local bending.     

Sensory processing and the network mechanisms for reading neuronal population codes

In many neuronal systems, information appears to be represented in the activity of populations of neurons. Such neuronal population codes must also be read out, or interpreted, by downstream networks. Recent studies in both vertebrate and invertebrate systems have begun to elucidate some of the general mechanisms underlying these processes. Directed behaviors, that involve a directional response to a directional sensory input, have been a particularly useful context for these studies because, among other things, their input-output relationship is easily defined and experimentally controlled. We have recently shown that the neuronal network underlying a directed behavior in the medicinal leech utilizes a specific population coding scheme based on a neuronal population vector. A population vector of mechanosensory neuron activity correlates well with behavioral output and the connectivity of the downstream network is well suited for accurately reading out this population code. Accepted: 17 April 1999     

Generation of motor patterns for walking and flight in motoneurons supplying bifunctional muscles in the locust

In the flight system of Locusta migratoria certain muscles move a wing and a leg (bifunctional muscles) and are active during the performance of walking and flight. A preparation that allowed intracellular recordings during these behaviors was developed to analyze the activity of motoneurons supplying these and other muscles. Motoneurons innervating bifunctional muscles were active during walking and flight, whereas motoneurons innervating unifunctional flight muscles were active only during flight. Both motor patterns, walking and flight, were sometimes generated simultaneously in our preparation. In bifunctional motoneurons the two patterns were superimposed, whereas in unifunctional motoneurons only the flight motor pattern was observed. All flight interneurons we examined were either inactive or tonically inhibited during walking. All interneurons that were strongly modulated during walking were either inactive, inhibited, or only weakly modulated during flight. Anatomical investigations showed that unifunctional flight motoneurons have their main processes in the extreme dorsal region of neuropil. With the exception of the second basalar motoneurons, all bifunctional motoneurons have their processes extending more ventrally in the neuropil. Flight interneurons have their processes restricted to the dorsal neuropil. Interneurons that were rhythmically active during walking had their processes distributed more ventrally. We conclude that motoneurons innervating bifunctional muscles are active during both motor patterns, walking and flight, and that these patterns are produced by two distinct interneuronal networks. The pattern-generating network for flight appears to be located in the extreme dorsal regions of the thoracic ganglia, and the network for walking is located more ventrally.     

Functionally heterogeneous segmental oscillators generate swimming in the medicinal leech

Swimming behavior in the leech Hirudo medicinalis arises from neuronal circuits within the ventral nerve cord. Although the ventral nerve cord comprises a series of homologous segmental ganglia, it remains unresolved whether the swim oscillator circuits within individual ganglia are functionally equivalent. We have extended previous studies on pairs of ganglia to test whether individual ganglia throughout the nerve cord are capable of generating swim oscillations and to measure the cycle periods of local oscillations. We found that the swim-generating function of individual ganglia is broadly distributed, but not uniform. The swim-like oscillations in isolated ganglia from the anterior ganglia nerve cord were less robust than those from mid-cord. Swimming activity in posterior cord ganglia is even weaker we were unable to obtain swim-like oscillations from individual ganglia of the nerve cord posterior to segment 12. Swim-cycle periods exhibited a U-shaped function: those recorded in the most anterior individual ganglia (2.3 s for ganglion M2) and short chains of posterior ganglia (up to 4.0 s) were two to four times longer than those obtained from mid-cord ganglia (near 1.0 s). We conclude that the leech swim system comprises a functionally heterogeneous set of local oscillator units.     

RFamide Peptides in the Leech, Hirudo medicinalis

RFamide peptides have been localized to a number of neuronsof the CNS of the leech, Hirudo medicinalis, using immunocytochemicaltechniques. The majority of this immunoreactivity appears tobe due to the peptide FMRFamide. Most of the identified RFamideimmunoreactive cells are cholinergic motor neurons, though someare interneurons. Superfused FMRFamide is active on the targetsof these identified neurons; in a few well studied cases, ithas been possible to show that FMRFamide mimics a specific physiologicalaction of an identified neuron on its target. In the leech as in other phyla where they occur, RFamide peptidesare widely distributed in neurons, and are neuromodulators withdiverse physiological effects.     

The interrelations between neurons of the amygdala and hypothalamus during conditioning with selection of food reinforcement quality in cats

Three cats were subjected to appetitive instrumental conditioning to light by the method of the "active choice" of the reinforcement quality. The short-delayed conditioned bar-pressings were reinforced by bread-meat mixture and the delayed response by meat. The animals differed in behavior strategy: two animals preferred bar-pressing with long delay (the so-called "self-control" group) and one animal preferred bar-pressing with short delay (the so-called "impulsive" group). The multiunit activity of the basolateral amygdala and nucleus lateralis of the hypothalamus was recorded through chronically implanted nichrome wire semimicroelecrodes. The interactions between the neighboring neurons in the lateral hypothalamus and basolateral amygdala (within the local neuronal network) and between the neurons of the basolateral amygdala and lateral hypothalamus (distributed neuronal networks in the direction amygdala--hypothalamus and vice versa) were evaluated by means of statistical crosscorrelation analysis of spike trains. The crosscorrelational interneuronal connections in the delay range of 0-100 ms were examined. It was shown that the number of crosscorrelations between the discharges on neurons both in the local networks of basolateral amygdala and distributed networks was significantly higher in "impulsive" cats. In both groups of animals, the percentage of crosscorrelations between neighbouring neurons in the local networks of the lateral hypothalamus was similar. We suggest that the local networks of the basolateral amygdala and amygdalar-hypothalamic distributed neuronal networks are involved in the system of brain structures which determine the individual features of animal behavior.     

Generation of variants of a motor act in a modular and hierarchical motor network

BACKGROUND: Most motor systems can generate a variety of behaviors, including categorically different behaviors and variants of a single motor act within the same behavioral category. Previous work indicated that many pattern-generating interneuronal networks may have a modular organization and that distinct categories of behaviors can be generated through flexible combinations of a small number of modules or building blocks. However, it is unclear whether and how a small number of modules could possibly generate a large number of variants of one behavior. RESULTS: We show that the modular feeding motor network of Aplysia mediates variations in protraction duration in biting-like programs. Two descending commands are active during biting behavior and trigger biting-like responses in a semiintact preparation. In the isolated CNS, when activated alone, the two commands produce biting-like programs of either long or short protraction duration by acting specifically on two modules that have opposite effects on protraction duration. More importantly, when coactivated at different frequencies, the two commands produce biting programs with an intermediate protraction duration. CONCLUSIONS: It was previously hypothesized that behavioral variants may be produced by combining different activity levels of multiple descending commands. Our data provide direct evidence for such a scheme and show how it is implemented in a modularly organized network. Thus, within a modular and hierarchical architecture, in addition to generating different categories of behavior, a small number of modules also efficiently implements variants of a single behavior.     

From synapses to behavior: development of a sensory-motor circuit in the leech

The development of neuronal circuits has been advanced greatly by the use of imaging techniques that reveal the activity of neurons during the period when they are constructing synapses and forming circuits. This review focuses on experiments performed in leech embryos to characterize the development of a neuronal circuit that produces a simple segmental behavior called "local bending." The experiments combined electrophysiology, anatomy, and FRET-based voltage-sensitive dyes (VSDs). The VSDs offered two major advantages in these experiments: they allowed us to record simultaneously the activity of many neurons, and unlike other imaging techniques, they revealed inhibition as well as excitation. The results indicated that connections within the circuit are formed in a predictable sequence: initially neurons in the circuit are connected by electrical synapses, forming a network that itself generates an embryonic behavior and prefigures the adult circuit; later chemical synapses, including inhibitory connections, appear, "sculpting" the circuit to generate a different, mature behavior. In this developmental process, some of the electrical connections are completely replaced by chemical synapses, others are maintained into adulthood, and still others persist and share their targets with chemical synaptic connections.     

A hemicord locomotor network of excitatory interneurons: a simulation study

Locomotor burst generation is simulated using a full-scale network model of the unilateral excitatory interneuronal population. Earlier small-scale models predicted that a population of excitatory neurons would be sufficient to produce burst activity, and this has recently been experimentally confirmed. Here we simulate the hemicord activity induced under various experimental conditions, including pharmacological activation by NMDA and AMPA as well as electrical stimulation. The model network comprises a realistic number of cells and synaptic connectivity patterns. Using similar distributions of cellular and synaptic parameters, as have been estimated experimentally, a large variation in dynamic characteristics like firing rates, burst, and cycle durations were seen in single cells. On the network level an overall rhythm was generated because the synaptic interactions cause partial synchronization within the population. This network rhythm not only emerged despite the distributed cellular parameters but relied on this variability, in particular, in reproducing variations of the activity during the cycle and showing recruitment in interneuronal populations. A slow rhythm (0.4–2 Hz) can be induced by tonic activation of NMDA-sensitive channels, which are voltage dependent and generate depolarizing plateaus. The rhythm emerges through a synchronization of bursts of the individual neurons. A fast rhythm (4–12 Hz), induced by AMPA, relies on spike synchronization within the population, and each burst is composed of single spikes produced by different neurons. The dynamic range of the fast rhythm is limited by the ability of the network to synchronize oscillations and depends on the strength of synaptic connections and the duration of the slow after hyperpolarization. The model network also produces prolonged bouts of rhythmic activity in response to brief electrical activations, as seen experimentally. The mutual excitation can sustain long-lasting activity for a realistic set of synaptic parameters. The bout duration depends on the strength of excitatory synaptic connections, the level of persistent depolarization, and the influx of Ca²⁺ ions and activation of Ca²⁺-dependent K⁺ current.     

A kinematic study of crawling behavior in the leech,Hirudo medicinalis

The medicinal leech crawls along a solid substrate by repeated alternating extensions and shortenings of the body. Extension occurs with the posterior sucker attached and the head sucker free. The head sucker then attaches, followed by shortening and release of the tail sucker. The tail sucker is then pulled toward the head, where it reattaches to the substrate. The head sucker then releases, and another crawling cycle begins (Figs. 1, 5). There are two crawling variants: inchworm crawling, in which the head and tail suckers are closely apposed at the end of a cycle and the body forms a loop above the substrate, and vermiform crawling, in which the suckers are placed farther apart and the body remains fairly close to the substrate (Fig. 1). The cycle period and the distance traveled during a cycle are greater in inchworm than in vermiform crawling; however, the velocity of travel is the same for both (Fig. 2). For both variants, the interval between head sucker attachment and tail sucker release is similar at all cycle periods and has a value consistent with direct interneuronal conduction of a signal from head sucker sensory neurons to tail sucker motor neurons. The interval between tail sucker attachment and head sucker release, however, is longer and varies with the cycle period, suggesting a more complex interneuronal circuit in the pathway from tail sucker sensory neurons to head sucker motor neurons (Fig. 4). The onsets of the components of the crawling cycle (extension, post-extension pause, shortening, and post-shortening pause) show an anteroposterior lag (Figs. 5, 7). For both variants, the travel time between segments varies directly with the period (Fig. 8). For both crawl types, the durations of the cycle components vary directly with the period, with several exceptions (Figs. 9, 10). A model is presented that summarizes the coordination of the various motor events in a cycle of leech crawling (Figs. 11 and 12).     

Synchronization of neuron population subject to steady DC electric field induced by magnetic stimulation

Electric fields, which are ubiquitous in the context of neurons, are induced either by external electromagnetic fields or by endogenous electric activities. Clinical evidences point out that magnetic stimulation can induce an electric field that modulates rhythmic activity of special brain tissue, which are associated with most brain functions, including normal and pathological physiological mechanisms. Recently, the studies about the relationship between clinical treatment for psychiatric disorders and magnetic stimulation have been investigated extensively. However, further development of these techniques is limited due to the lack of understanding of the underlying mechanisms supporting the interaction between the electric field induced by magnetic stimulus and brain tissue. In this paper, the effects of steady DC electric field induced by magnetic stimulation on the coherence of an interneuronal network are investigated. Different behaviors have been observed in the network with different topologies (i.e., random and small-world network, modular network). It is found that the coherence displays a peak or a plateau when the induced electric field varies between the parameter range we defined. The coherence of the neuronal systems depends extensively on the network structure and parameters. All these parameters play a key role in determining the range for the induced electric field to synchronize network activities. The presented results could have important implications for the scientific theoretical studies regarding the effects of magnetic stimulation on human brain.     

The organization of the network characteristics of the cells in local and distributed neuronal networks of the cat brain]

In cats with elaborated alimentary instrumental reflexes to light net characteristics of neurones of visual, motor cortex and the hypothalamus lateral nucleus were studied on the basis of revealed interneuronal interactions by means of cross-correlation method of analysis. Different organization of net properties of the cortical neurones in organization of local and distributed neuronal networks was shown, namely: predominance of the divergent characteristics over the convergent ones for cells in local networks and levelling of these relations in distributed nets. Neurones of the lateral hypothalamus nucleus had equal presentation of divergent and convergent properties in organization of local and distributed networks. Net characteristics of neurones of the cortical and subcortical structures were manifested in the background after the elaboration and the extinction of conditioned reflexes. Only small cells of the visual cortex were functionally dependent and changed correlation of net characteristics in local networks at CR extinction.