Neural network simulations of coupled locomotor oscillators in the lamprey spinal cord

The segmental locomotor network in the lamprey spinal cord was simulated on a computer using a connectionist-type neural network. The cells of the network were identical except for their excitatory levels and their synaptic connections. The synaptic connections used were based on previous experimental work. It was demonstrated that the connectivity of the circuit is capable of generating oscillatory activity with the appropriate phase relations among the cells. Intersegmental coordination was explored by coupling two identical segmental networks using only the cells of the network. Each of the possible couplings of a bilateral pair of cells in one oscillator with a bilateral pair of cells in the other oscillator produced stable phase locking of the two oscillators. The degree of phase difference was dependent upon synaptic weight, and the operating range of synaptic weights varied among the pairs of connections. The coupling was tested using several criteria from experimental work on the lamprey spinal cord. Coupling schemes involving several pairs of connecting cells were found which 1) achieved steadystate phase locking within a single cycle, 2) exhibited constant phase differences over a wide range of cycle periods, and 3) maintained stable phase locking in spite of large differences in the intrinsic frequencies of the two oscillators. It is concluded that the synaptic connectivity plays a large role in producing oscillations in this network and that it is not necessary to postulate a separate set of coordinating neurons between oscillators in order to achieve appropriate phase coupling.     

The nature of the coupling between segmental oscillators of the lamprey spinal generator for locomotion: A mathematical model

We present a theoretical model which is used to explain the intersegmental coordination of the neural networks responsible for generating locomotion in the isolated spinal cord of lamprey.A simplified mathematical model of a limit cycle oscillator is presented which consists of only a single dependent variable, the phase (t). By coupling N such oscillators together we are able to generate stable phase locked motions which correspond to traveling waves in the spinal cord, thus simulating fictive swimming. We are also able to generate irregular drifting motions which are compared to the experimental data obtained from cords with selective surgical lesions.     

Presynaptic mechanisms of changes in segmental responses accompanying the formation of a spinal locomotor generator in the cat

Using unanesthetized and decorticated (or decerebrated at level A 13) cats, it was found that spinalization leads to depolarization of the central terminals of primary afferents and an increase in the N₁ component of dorsal surface potential and dorsal root potential (DRP) produced by stimulating the low-threshold cutaneous and muscle afferents. Other effects include an increase in early polysynaptic responses and DRP produced by stimulation of high-threshold muscle afferents, a reduction in the intensity of interneuron activation in the nucleus interpositus mono- and polysynaptically connected with primary afferents, and a rise in the activity of n. interpositus interneurons di- and oligo-synaptically connected with afferent terminals. Changes in the opposite direction were produced by injecting DOPA into spinal animals. The connection between changes in the state of the segmental neuronal apparatus of the lumbosacral spinal cord and the level of spinal locomotor generator activity is discussed in the light of the findings obtained.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 18, No. 5, pp. 669–678, September–October, 1986.     

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 $h$ -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.     

Inhibition in the lamprey spinal cord]

Glycine and GABA play the role of inhibitory transmitters in the lamprey spinal cord. The mechanisms of action of both amino acids to the membrane receptors producing the postsynaptic inhibition as well as role and mechanism of GABA action producing the presynaptic inhibition are considered in this paper. The data concerned with morphological substrates of both type inhibitions are discussed.     

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.     

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.     

The development of the lamprey pattern generator for locomotion

The life cycle of the lamprey includes a larval stage that can last for several years. The motor behavior of the larval lamprey, the ammocoete, has been only minimally studied and little is known of the neural correlates of that behavior. Comparison of known larval behavior to that of adults leaves unclear whether there are large or small changes in the spinal nervous system during transformation. The motor output of isolated larval and transforming spinal cords when stimulated to "swim" with D-glutamate has some differences from that of comparable adult preparations, but shares many important features with adults. Primarily, the fictive swimming is less well regulated and less stable than adults of the same species. We propose that a major difference in the structure and organization of the central pattern generator for locomotion between adults and ammocoetes is a relative lack or immaturity of some cell types that participate in the coordination of the segments and the generation of the rhythm of the periodic bursting.     

DCC mediated axon guidance of spinal interneurons is essential for normal locomotor central pattern generator function

Coordinated limb rhythmic movements take place through organized signaling in local spinal cord neuronal networks. The establishment of these circuitries during development is dependent on the correct guidance of axons to their targets. It has previously been shown that the well-known axon guidance molecule netrin-1 is required for configuring the circuitry that provides left-right alternating coordination in fictive locomotion. The attraction of commissural axons to the midline in response to netrin-1 has been shown to involve the netrin-1 receptor DCC (deleted in Colorectal Cancer). However, the role of DCC for the establishment of CPG coordination has not yet been resolved. We show that mice carrying a null mutation of DCC displayed an uncoordinated left-right activity during fictive locomotion accompanied by a loss of interneuronal subpopulations originating from commissural progenitors. Thus, DCC plays a crucial role in the formation of spinal neuronal circuitry coordinating left-right activities. Together with the previously published results from netrin-1 deficient mice, the data presented in this study suggest a role for the most ventral originating V3 interneurons in synchronous activities over the midline. Further, it provides evidence that axon crossing in the spinal cord is more intricately controlled than in previously suggested models of DCC-netrin-1 interaction.     

Lobster locomotor activity as a measure of GABAAreceptor modulation

Gamma‐aminobutyric acid (GABA) is a well‐known neurotransmitter. A crustacean behavioural assay was developed for the examination of the effects of a benzodiazepine agonist and an antagonist of GABA_A‐type receptors. Both adult and juvenile male and female lobsters, Homarus americanus, were treated with single (or follow‐up) empirically determined doses of the agonist, diazepam, one of its metabolites (desmethyldiazepam), the antagonist, N‐methyl‐ß‐carboline‐3‐carboxamide (MBC), or vehicle alone. The effects of the compounds were monitored in a submerged circular open‐field for differences in locomotor activity, which was significantly inhibited by diazepam. Treatment with MBC following injection of diazepam reversed the latter's effects on locomotion. These results suggest that benzodiazepines may affect crustacean GABA receptors in a fashion similar to the GABA_A type found in the vertebrates, and that they may be involved in the regulation of locomotor behaviour.     

Effect of spinal level and loading conditions on the production of vertebral burst fractures in a porcine model

Vertebral burst fractures are commonly studied with experimental animal models. There is however a lack of consensus as to what parameters are important to create an unstable burst fracture with a significant canal encroachment on such model. This study aims to assess the effect of the loading rate, flexion angle, spinal level, and their interactions on the production of a vertebral thoracolumbar burst fracture on a porcine model. Sixteen functional spinal units composed of three vertebrae were harvested from mature Yucatan minipigs. Two loading rates (0.01 and 500 mm/s), two flexion angles (0° and 15°), and two spinal levels (T11-T13 and T14-L2) were studied, following a full factorial experimental plan with one repetition. Compression was applied to each functional unit to create a vertebral fracture. The load-to-failure, loss of compressive stiffness, final canal encroachment, and fracture type were used as criteria to evaluate the resulting fracture. All specimens compressed without flexion resulted in burst fractures. Half of the specimens compressed with the 15° flexion angle resulted in compression fractures. Specimens positioned without flexion lost more of their compressive stiffness and had more significant canal encroachment. Fractured units compressed with a higher loading rate resulted in a greater loss of compressive stiffness. The spinal level had no significant effect on the resulting fractures. The main parameters which affect the resulting fracture are the loading rate and the flexion angle. A higher loading rate and the absence of flexion favors the production of burst fractures with a greater canal encroachment.     

Traveling-wave pattern generator controls movement and organization of sensory feedback in a spinal cord model

 A traveling wave in a two-dimensional spinal cord model constitutes a stable pattern generator for quadruped gaits. In the context of the somatotopic organization of the spinal cord, this pattern generator is sufficient to generate stable locomotive limb trajectories. The elastic properties of muscles alone, providing linear negative feedback, are sufficient to stabilize stance and locomotion in the presence of perturbative forces. We further show that such a pattern generator is capable of organizing sensory processing in the spinal cord. A single-layer perceptron was trained to associate the sensory feedback from the limb (coding force, length, and change of length for each muscle) with the two-dimensional activity profile of the traveling wave. This resulted in a well-defined spatial organization of the connections within the spinal network along a rostrocaudal axis. The spinal network driven by peripheral afferents alone supported autonomous locomotion in the positive feedback mode, whereas in the negative feedback mode stance was stabilized in response to perturbations. Systematic variation of a parameter representing the effect of gamma-motor neurons on muscle spindle activity in our model led to a corresponding shift of limb position during stance and locomotion, resulting in a systematic displacement alteration of foot positions. Received: 30 July 2001 / Accepted in revised form: 17 April 2002 Correspondence to: A. Kaske (e-mails: alexander.kaske@mtc.ki.se, alexander.kaske@vglab.com)     

Intersegmental coordinating system of the lamprey central pattern generator for locomotion

Summary In the lamprey,Ichthyomyzon unicuspis, the wave of activity required for normal swimming movements can be generated by a central pattern generator (CPG) residing in the spinal cord. A constant phase coupling between spinal segments can be organized by intersegmental coordinating neurons intrinsic to the cord. The rostral and caudal segmental oscillators of the CPG have different preferred frequencies when separated from each other. Therefore the system must maintain the segmental oscillators of the locomotor CPG at a single common frequency and with the proper relative timing. Using selective lesions and a split-bath, it is demonstrated that the coordinating system is comprised of at least 3 subsystems, short-axon systems in the lateral and medial tracts and a long axon system in the lateral tracts. Each alone can sustain relatively stable coordinated activity.Abbreviations CPG central pattern generator - NMDA N-methyl-D-aspartate - VR ventral root     

Functional regeneration and recovery of locomotor activity in spinally transected lamprey.

Spinally transected lamprey recovery locomotor function within 3-6 weeks, and recovery is due, in part, to functional regeneration of neural pathways in the central nervous system (CNS). Our data demonstrate for the first time in the lamprey that descending axons arising from brainstem command neurons can functionally regenerate and restore locomotor initiation below a healed spinal transection site. Immediately after behavioral recovery (3-6 weeks) the locomotor pattern was incomplete but returned to normal during the remainder of the recovery period (6-40 weeks). Initially, the extent of regeneration of descending axons was limited but increased to at least 30-50 mm at recovery times of 24-40 weeks. Regenerated giant Muller axons do not contribute significantly to recovery of locomotor function; rather, regenerated axons of smaller reticulospinal neurons appear to restore locomotor initiation. The restoration of locomotor coordination across a spinal lesion is dependent on two mechanisms: regeneration of spinal coordinating neurons and mechanosensory inputs. Comparisons are made to spinal cord regeneration in other lower vertebrates and to the relative lack of CNS regeneration and behavioral recovery in higher vertebrates.     

Homogeneity among mitochondria revealed by a constant proportion of their enzymes

The homogeneity or heterogeneity at the enzyme level of mitochondria has not been directly demonstrated and is important for many studies. To clarify this point, carbamoyl phosphate synthase (ammonia), glutamate dehydrogenase and mitochondrial adenosine triphosphatase (F1) were located in rat liver by immunolabeling using protein A-gold. Measurements of the number of gold particles per square micron of cross sectional images of mitochondria permit to assess the relative molecular concentration of the three enzymes and, most interestingly, it presents the first evidence that different mitochondria in rat liver cells have the same relative proportion of the three enzymes. Since they have vastly different half-lives, bulk or unregulated autophagy as the main mechanism regulating the turnover of these enzymes seems unlikely.     

Manipulations of spinal cord excitability evoke developmentally-dependent compensatory changes in the lamprey spinal cord

We have examined homeostatic or compensatory plasticity evoked by tonic changes in spinal cord excitability in the lamprey, a model system for investigating spinal cord function. In larval animals, reducing excitability by incubating in tetrodotoxin or the glutamate receptor antagonists CNQX or CNQX/AP5 for 20–48 h resulted in a diverse set of cellular and synaptic changes that together were consistent with an increase in spinal cord excitability. Similar changes occurred to a tonic increase in excitation evoked by incubating in high potassium physiological solution (i.e. responses were unidirectional). We also examined developmental influences on these effects. In animals developing from the larval to adult form effects were reduced or absent, suggesting that at this stage the spinal cord was more tolerant of changes in activity levels. Responses had returned in adult animals, but they were now bi-directional (i.e. opposite effects were evoked by an increase or decrease in excitability). The spinal cord can thus monitor and adapt cellular and synaptic properties to tonic changes in excitability levels. This should be considered in analyses of spinal cord plasticity and injury.     

Homogeneity among mitochondria revealed by a constant proportion of their enzymes

Summary The homogeneity or heterogeneity at the enzyme level of mitochondria has not been directly demonstrated and is important for many studies. To clarify this point, carbamoyl phosphate synthase (ammonia), glutamate dehydrogenase and mitochondrial adenosine triphosphatase (F₁) were located in rat liver by immunolabeling using protein A-gold. Measurements of the number of gold particles per square micron of cross sectional images of mitochondria permit to assess the relative molecular concentration of the three enzymes and, most interestingly, it presents the first evidence that different mitochondria in rat liver cells have the same relative proportion of the three enzymes. Since they have vastly different half-lives, bulk or unregulated autophagy as the main mechanism regulating the turnover of these enzymes seems unlikely.     

Seeing the body produces limb-specific modulation of skin temperature

Vision of the body, even when non-informative about stimulation, affects somatosensory processing. We investigated whether seeing the body also modulates autonomic control in the periphery by measuring skin temperature while manipulating vision. Using a mirror box, the skin temperature was measured from left hand dorsum while participants: (i) had the illusion of seeing their left hand, (ii) had the illusion of seeing an object at the same location or (iii) looked directly at their contralateral right hand. Skin temperature of the left hand increased when participants had the illusion of directly seeing that hand but not in the other two view conditions. In experiment 2, participants viewed directly their left or right hand, or the box while we recorded both hand dorsum temperatures. Temperature increased in the viewed hand but not the contralateral hand. These results show that seeing the body produces limb-specific modulation of thermal regulation.