Messages conveyed by descending tracts during scratching in the cat. I. Activity of vestibulospinal neurons

Model of a bilateral Brown-type central pattern generator for symmetric and asymmetric locomotion

The coordinated activity of muscles is produced in part by spinal rhythmogenic neural circuits, termed central pattern generators (CPGs). A classical CPG model is a system of coupled oscillators that transform locomotor drive into coordinated and gait-specific patterns of muscle recruitment. The network properties of this conceptual model can be simulated by a system of ordinary differential equations with a physiologically inspired coupling locus of interactions capturing the timing relationship for bilateral coordination of limbs in locomotion. Whereas most similar models are solved numerically, it is intriguing to have a full analytical description of this plausible CPG architecture to illuminate the functionality within this structure and to expand it to include steering control. Here, we provided a closed-form analytical solution contrasted against the previous numerical method. The evaluation time of the analytical solution was decreased by an order of magnitude when compared with the numerical approach (relative errors, <0.01%). The analytical solution tested and supported the previous finding that the input to the model can be expressed in units of the desired limb locomotor speed. Furthermore, we performed parametric sensitivity analysis in the context of controlling steering and documented two possible mechanisms associated with either an external drive or intrinsic CPG parameters. The results identify specific propriospinal pathways that may be associated with adaptations within the CPG structure. The model offered several network configurations that may generate the same behavioral outcomes. NEW & NOTEWORTHY Using a simple process of leaky integration, we developed an analytical solution to a robust model of spinal pattern generation. We analyzed the ability of this neural element to exert locomotor control of the signal associated with limb speeds and tested the ability of this simple structure to embed steering control using the velocity signal in the model's inputs or within the internal connectivity of its elements.

Innate and Cultural Spatial Time: A Developmental Perspective

We reviewed literature to understand when a spatial map for time is available in the brain. We carefully defined the concepts of metrical map of time and of conceptual representation of time as the mental time line (MTL) in order to formulate our position. It is that both metrical map and conceptual representation of time are spatial in nature. The former should be innate, related to motor/implicit timing, it should represent all magnitudes with an analogic and bi-dimensional structure. The latter MTL should be learned, available at about 8-10 years-old and related to cognitive/explicit time. It should have uni-dimensional, linear and directional structure (left-to-right in Western culture). We bear the centrality of the development of number cognition, of time semantic concepts and of reading/writing habits for the development of ordinality and linearity of the MTL.

Younger is not always better: development of locomotor adaptation from childhood to adulthood

New walking patterns can be learned over short timescales (i.e., adapted in minutes) using a split-belt treadmill that controls the speed of each leg independently. This leads to storage of a modified spatial and temporal motor pattern that is expressed as an aftereffect in regular walking conditions. Because split-belt walking is a novel task for adults and children alike, we used it to investigate how motor adaptation matures during human development. We also asked whether the immature pattern resembles that of people with cerebellar dysfunction, because we know that this adaptation depends on cerebellar integrity. Healthy children (3-18 years old) and adults, and individuals with cerebellar damage were adapted while walking on split belts (1:2 speed ratio). Adaptation and de-adaptation rates were quantified separately for temporal and spatial parameters. All healthy children and adults tested could learn the new timing at the same rate and showed significant aftereffects. However, children younger than 6 years old were unable to learn the new spatial coordination. Furthermore, children as old as age 11 years old showed slower rates of adaptation and de-adaptation of spatial parameters of walking. Young children showed patterns similar to cerebellar patients, with greater deficits in spatial versus temporal adaptation. Thus, although walking is a well-practiced, refined motor skill by late childhood (i.e., 11 years of age), the processes underlying learning new spatial relationships between the legs are still developing. The maturation of locomotor adaptation follows at least two time courses, which we propose is determined by the developmental state of the cerebellum.

Do premotor interneurons act in parallel on spinal motoneurons and on dorsal horn spinocerebellar and spinocervical tract neurons in the cat?

It has previously been established that ventral spinocerebellar tract (VSCT) neurons and dorsal spinocerebellar tract neurons located in Clarke's column (CC DSCT neurons) forward information on actions of premotor interneurons in reflex pathways from muscle afferents on α-motoneurons. Whether DSCT neurons located in the dorsal horn (dh DSCT neurons) and spinocervical tract (SCT) neurons are involved in forwarding similar feedback information has not yet been investigated. The aim of the present study was therefore to examine the input from premotor interneurons to these neurons. Electrical stimuli were applied within major hindlimb motor nuclei to activate axon-collaterals of interneurons projecting to these nuclei, and intracellular records were obtained from dh DSCT and SCT neurons. Direct actions of the stimulated interneurons were differentiated from indirect actions by latencies of postsynaptic potentials evoked by intraspinal stimuli and by the absence or presence of temporal facilitation. Direct actions of premotor interneurons were found in a smaller proportion of dh DSCT than of CC DSCT neurons. However, they were evoked by both excitatory and inhibitory interneurons, whereas only inhibitory premotor interneurons were previously found to affect CC DSCT neurons [as indicated by monosynaptic excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) in dh DSCT and only IPSPs in CC DSCT neurons]. No effects of premotor interneurons were found in SCT neurons, since monosynaptic EPSPs or IPSPs were only evoked in them by stimuli applied outside motor nuclei. The study thus reveals a considerable differentiation of feedback information provided by different populations of ascending tract neurons.

A trans-spinal loop between neurones in the reticular formation and in the cerebellum

Voluntary limb movements are initiated in the brain but the neurones responsible for activating the muscles (motoneurones and interneurones) are located in the spinal cord. The spinal cord also contains neurones that provide the brain, and especially the cerebellum, with continuous information on effects of the descending commands. We show that one population of such neurones provide the cerebellum with information on how likely the brain's commands (mediated by descending reticulospinal neurones) are to be executed as planned, depending on the degree of inhibition of motoneurones. They may therefore play an important role in preventing errors in activation of motoneurones and thereby help the brain to correct its signals to the spinal cord before such errors have been committed.

Reciprocal Ia inhibition contributes to motoneuronal hyperpolarisation during the inactive phase of locomotion and scratching in the cat

Despite decades of research, the classical idea that 'reciprocal inhibition' is involved in the hyperpolarisation of motoneurones in their inactive phase during rhythmic activity is still under debate. Here, we investigated the contribution of reciprocal Ia inhibition to the hyperpolarisation of motoneurones during fictive locomotion (evoked either by electrical stimulation of the brainstem or by l-DOPA administration following a spinal transection at the cervical level) and fictive scratching (evoked by stimulation of the pinna) in decerebrate cats. Simultaneous extracellular recordings of Ia inhibitory interneurones and intracellular recordings of lumbar motoneurones revealed the interneurones to be most active when their target motoneurones were hyperpolarised (i.e. in the inactive phase of the target motoneurones). To date, these results are the most direct evidence that Ia inhibitory interneurones contribute to the hyperpolarisation of motoneurones during rhythmic behaviours. We also estimated the amount of Ia inhibition as the amplitude of Ia IPSC in voltage-clamp mode. In both flexor and extensor motoneurones, Ia IPSCs were always larger in the inactive phase than in the active phase during locomotion (n = 14) and during scratch (n = 11). Results obtained from spinalised animals demonstrate that the spinal rhythm-generating network simultaneously drives the motoneurones of one muscle group and the Ia interneurones projecting to motoneurones of the antagonist muscles in parallel. Our results thus support the classical view of reciprocal inhibition as a basis for relaxation of antagonist muscles during flexion-extension movements.

Thinking about walking: effects of conscious correction versus distraction on locomotor adaptation

Control of the human walking pattern normally requires little thought, with conscious control used only in the face of a challenging environment or a perturbation. We have previously shown that people can adapt spatial and temporal aspects of walking to a sustained perturbation generated by a split-belt treadmill. Here we tested whether conscious correction of walking, versus distraction from it, modifies adaptation. Conscious correction of stepping may expedite the adaptive process and help to form a new walking pattern. However, because walking is normally an automatic process, it is possible that conscious effort could interfere with adaptation, whereas distraction might improve it by removing competing voluntary control. Three groups of subjects were studied: a control group was given no specific instructions, a conscious correction group was instructed how to step and given intermittent visual feedback of stepping during adaptation, and a distraction group performed a dual-task during adaptation. After adaptation, retention of aftereffects was assessed in all groups during normal treadmill walking without conscious effort, feedback, or distraction. We found that conscious correction speeds adaptation, whereas distraction slows it. Subjects trained with distraction retained aftereffects longest, suggesting that the training used during adaptation predicts the time course of deadaptation. An unexpected finding was that these manipulations affected the adaptation rate of spatial but not temporal elements of walking. Thus conscious processes can preferentially access the spatial walking pattern. It may be that spatial and temporal controls of locomotion are accessible through distinct neural circuits, with the former being most sensitive to conscious effort or distraction.

The spinobulbar system in lamprey

Locomotor networks in the spinal cord are controlled by descending systems which in turn receive feedback signals from ascending systems about the state of the locomotor networks. In lamprey, the ascending system consists of spinobulbar neurons which convey spinal network signals to the two descending systems, the reticulospinal and vestibulospinal neurons. Previous studies showed that spinobulbar neurons consist of both ipsilaterally and contralaterally projecting cells distributed at all rostrocaudal levels of the spinal cord, though most numerous near the obex. The axons of spinobulbar neurons ascend in the ventrolateral spinal cord and brainstem to the caudal mesencephalon and within the dendritic arbors of reticulospinal and vestibulospinal neurons. Compared to mammals, the ascending system in lampreys is more direct, consisting of excitatory and inhibitory monosynaptic inputs from spinobulbar neurons to reticulospinal neurons. The spinobulbar neurons are rhythmically active during fictive locomotion, representing a wide range of timing relationships with nearby ventral root bursts including those in phase, out of phase, and active during burst transitions between opposite ventral roots. The spinobulbar neurons are not simply relay cells because they can have mutual synaptic interactions with their reticulospinal neuron targets and they can have synaptic outputs to other spinal neurons. Spinobulbar neurons not only receive locomotor inputs but also receive direct inputs from primary mechanosensory neurons. Due to the relative simplicity of the lamprey nervous system and motor control system, the spinobulbar neurons and their interactions with reticulospinal neurons may be advantageous for investigating the general organization of ascending systems in the vertebrate.

Activity of the Motor Cortex During Scratching

In awake cats sitting with the head restrained, scratching was evoked using stimulation of the ear. Cats scratched the shoulder area, consistently failing to reach the ear. Kinematics of the hind limb movements and the activity of ankle muscles, however, were similar to those reported earlier in unrestrained cats. The activity of single neurons in the hind limb representation of the motor cortex, including pyramidal tract neurons (PTNs), was examined. During the protraction stage of the scratch response, the activity in 35% of the neurons increased and in 50% decreased compared with rest. During the rhythmic stage, the motor cortex population activity was approximately two times higher compared with rest, because the activity of 53% of neurons increased and that of 33% decreased in this stage. The activity of 61% of neurons was modulated in the scratching rhythm. The average depth of frequency modulation was 12.1 ± 5.3%, similar to that reported earlier for locomotion. The phases of activity of different neurons were approximately evenly distributed over the scratch cycle. There was no simple correlation between resting receptive field properties and the activity of neurons during the scratch response. We conclude that the motor cortex participates in both the protraction and the rhythmic stages of the scratch response.

Reticulospinal neurons receive direct spinobulbar inputs during locomotor activity in lamprey

Reticulospinal neurons of the lamprey brain stem receive rhythmic input from the spinal cord during locomotor activity. The goal of the present study was to determine whether such spinal input has a direct component to reticulospinal neurons or depends on brain stem interneurons. To answer this question, an in vitro lamprey brain stem-spinal cord preparation was used with a diffusion barrier placed caudal to the obex, separating the experimental chamber into two baths. Locomotor activity was induced in the spinal cord by perfusion of d-glutamate or N-methyl-dl-aspartate into the spinal cord bath. The brain stem bath was first perfused with normal Ringer solution followed by a high-Ca(2+), -Mg(2+) solution, which reduced polysynaptic transmission. The amplitudes of membrane potential oscillations of reticulospinal neurons in the posterior and middle rhombencephalic reticular nuclei (PRRN and MRRN, respectively) recorded with sharp intracellular microelectrodes did not significantly change from normal to high-divalent solution. This finding suggests a large part of the spinal input creating the oscillations is direct to the reticulospinal neurons. Application of strychnine to the high-Ca(2+), -Mg(2+) solution decreased membrane potential oscillation amplitude, and injection of Cl(-) reversed presumed inhibitory postsynaptic potentials, indicating a role for direct spinal inhibitory inputs. Although reduced, the persistence of oscillations in strychnine suggests that spinal excitatory inputs also contribute to the oscillations. Thus it was concluded that both excitatory and inhibitory spinal neurons provide direct rhythmic inputs to reticulospinal cells of the PRRN and MRRN during locomotor activity. These inputs provide reticulospinal cells with information regarding the activity of the spinal locomotor networks.

Neurophysiology: cerebral carbon copies

Predicting the consequences of our actions is essential for sensorimotor control. A candidate neural pathway underlying the prediction of eye position during saccades has been reported.

Vestibulospinal and reticulospinal neuronal activity during locomotion in the intact cat. II. Walking on an inclined plane

The experiments described in this report were designed to determine the contribution of vestibulospinal neurons (VSNs) in Deiters' nucleus and of reticulospinal neurons (RSNs) in the medullary reticular formation to the modifications of the walking pattern that are associated with locomotion on an inclined plane. Neuronal discharge patterns were recorded from 44 VSNs and 63 RSNs in cats trained to walk on a treadmill whose orientation was varied from +20 degrees (uphill) to -10 degrees (downhill), referred to as pitch tilt, and from 20 degrees roll tilt left to 20 degrees roll tilt right. During uphill locomotion, a majority of VSNs (25/44) and rhythmically active RSNs (24/39) showed an increase in peak discharge frequency, above that observed during locomotion on a level surface. VSNs, unlike some of the RSNs, exhibited no major deviations from the overall pattern of the activity recorded during level walking. The relative increase in discharge frequency of the RSNs (on average, 31.8%) was slightly more than twice that observed in the VSNs (on average, 14.4%), although the average absolute change in discharge frequency was similar (18.2 Hz in VSNs and 21.6 Hz in RSNs). Changes in discharge frequency during roll tilt were generally more modest and were more variable, than those observed during uphill locomotion as were the relative changes in the different limb muscle electromyograms that we recorded. In general, discharge frequency in VSNs was more frequently increased when the treadmill was rolled to the right (ear down contralateral to the recording site) than when it was rolled to the left. Most VSNs that showed significant linear relationships with treadmill orientation in the roll plane increased their activity during right roll and decreased activity during left roll. Discharge activity in phasically modulated RSNs was also modified by roll tilt of the treadmill. Modulation of activity in RSNs that discharged twice in each step cycle was frequently reciprocal in that one burst of activity would increase during left roll and the other during right roll. The overall results indicate that each system contributes to the changes in postural tone that are required to adapt the gait for modification on an inclined surface. The characteristics of the discharge activity of the VSNs suggest a role primarily in the overall control of the level of electromyographic activity, while the characteristics of the RSNs suggest an additional role in determining the relative level of different muscles, particularly when the pattern is asymmetric.

Mechanisms of supraspinal correction of scratching generator

The influences of signals in descending systems on the parameters of scratching generator activity were studied on decerebrate immobilized cats. It was shown that phasic electric stimulation of descending systems evoked certain phase-dependent reorganization of the parameters of scratching generator efferent activity. Maximum increase in scratching cycle duration during electric stimulation of Deiters' nucleus, red nucleus and pyramidal tract is observed during stimulation in the first half of aiming phase. Stimulation in the second half of aiming phase and at the beginning of scratching jerk phase virtually does not change the scratching cycle duration. Maximum increase in scratching cycle duration during electric stimulation of the nucleus reticularis gigantocellularis is observed in the second half of aiming phase. Electric activation of descending pathways during aiming phase increases its intensity and decreases the intensity of scratching jerk phase. Activation of descending pathways during scratching jerk phase increases its intensity and virtually does not change the aiming phase intensity. Influences of electric activation of descending systems on scratching generator work reveal dependence on limb position. They are increased when the limb is deflected to the rear and are decreased during over-aimed position. Decerebellation leads to a decrease of scratching generator activity parameters rearrangement under influence of electric stimulation of the red nucleus and nucleus reticularis gigantocellularis, and to its increase during Deiters' nucleus stimulation. On the basis of these results the principles of supraspinal correction of scratching generator work are discussed.

Transneuronal transport of wheat germ agglutinin conjugated horseradish peroxidase into last order spinal interneurones projecting to acromio- and spinodeltoideus motoneurones in the cat. 2. Differential labelling of interneurones depending on movement type

Transneuronal transport of wheat germ agglutinin conjugated horseradish peroxidase was used to define the location of last order spinal interneurones projecting to deltoideus motoneurones during voluntary target-reaching and/or in unrestricted walking on the ground. Labelled interneurones were found bilaterally from C2 to Th1 in target-reaching cats and almost exclusively in the C5-Th1 segments in walking cats, although the total number of labelled interneurones in these cats was considerably higher than in the target-reaching cats. These results confirm the previous finding that propriospinal neurones in the C3-C4 segments can mediate the descending command for target-reaching movements with the forelimb. In both groups of cats labelled interneurones were found ipsilaterally in laminae V-IX, while contralaterally they were mainly restricted to lamina VIII. In the forelimb segments there was a larger number of labelled interneurones in the walking cats in the lateral part of laminae V-VII and in laminae VIII and IX. There was a positive, almost linear correlation between the total number of labelled interneurones and motoneurones in all cats. The results suggest that both excitatory and inhibitory last order interneurones can be transneuronally labelled. It is concluded that this method can be used for functional identification of last order interneurones active during the preparation and/or execution of different movements.

Red nucleus: past and future

The red nucleus has greatly interested scientists for almost a century. This can be explained by the fact that problems of general interest are encountered when studying this nucleus. Some of them are outlined in this paper, such as the phylogenetic evolution of the rubrospinal tract, the respective roles of the rubrospinal and pyramidal tracts in the execution of various types of movements, and the respective roles of these two tracts in movement automatization.

Effects of selective spinal cord lesions on hind limb locomotion in birds

In birds, a variety of subtotal thoracic spinal cord lesions were made to determine the spinal cord pathways essential for avian hind limb locomotion (bipedal walking). The various surgical disruptions included: section of the dorsal half of the cord, a hemisection, section of the dorsal cord and ventromedial funiculi, section of the dorsal cord ventrolateral funiculi, section of the entire thoracic cord except for one ventrolateral quadrant, section of the ventral half of the cord, and complete transection of the thoracic cord. The study compared the locomotion following these lesions in both chronic surviving and acutely decerebrated, brain stem-stimulated birds. Behavioral assessments and electromyographic recording techniques were used to evaluate locomotor activity. Our results showed that transectioning pathways within the dorsal cord did not hinder the activation and maintenance of self-supported walking in either preparation. However, sparing the spinal cord pathways within either the ventromedial or ventrolateral funiculi of the thoracic spinal cord was essential for the activation of self-supported walking in both preparations. When our findings are integrated with previous studies, medullary reticulospinal pathways (projecting through the ventral funiculi) are strongly implicated as a common descending projection for the activation of spinal cord locomotor networks and the initiation of locomotion. Similar findings have been found in quadrapedal mammals and, as a complement, birds may make an excellent model for the study of bipedal locomotion.

Activity of C3-C4 propriospinal neurons during fictitious forelimb locomotion in the cat

The activity of C3-C4 propriospinal neurons was recorded during fictitious forelimb locomotion in immobilized decerebrated cats with the spinal cord transected at the lower thoracic level. The discharge frequency of most neurons was rhythmically modulated in relation to the cycle of fictitious stepping in spite of the absence of any rhythmic signals from the limb receptors. Thus, the intraspinal mechanisms present a powerful input to the C3-C4 propriospinal neurons.

Activity of cerebellar Purkinje cells during fictitious scratch reflex in the cat

The activity of Purkinje cells (PCs) was recorded in the anterior lobe (the vermis and pars intermedia) and in the paramedian lobule of the cerebellum during the fictitious scratch reflex in thalamic cats immobilized with Flaxedil. In the anterior lobe, the activity of many PCs was rhythmically modulated in relation to the scratch cycle: they generated bursts of impulses separated by periods of silence. Different PCs were active in different phases of the scratch cycle. In many cases the discharge modulation was irregular: the burst duration and the discharge rate in the burst varied considerably in subsequent cycles. The rhythmical activity of PCs was determined by modulation of the frequency of 'simple spikes' reflecting the mossy fiber input. Generation of 'complex spikes' reflecting the climbing fiber input in most PCs was not related with the scratch rhythm. In the paramedian lobule, rhythmical modulation of PCs was practically absent. Rhythmical modulation of PCs in immobilized cats is determined by signals coming from the central spinal mechanism of scratching via the ventral spinocerebellar tract (VSCT) and the spinoreticulocerebellar pathway (SRCP). Results of separate transections of these pathways demonstrated that the VSCT plays the crucial role in modulating the PCs.

Activity of neurons of cerebellar nuclei during fictitious scratch reflex in the cat. II. Interpositus and lateral nuclei

The activity of neurons of the interpositus and lateral cerebellar nuclei was recorded during fictitious scratch reflex in thalamic cats immobilized with Flaxedil. Interpositus neurons were identified by antidromic response to stimulation of the contralateral red nucleus. The interpositus neurons responding to passive movements of the ipsilateral hindlimb manifested rhythmical modulation of the discharge in relation with the scratch cycle. The neurons generated bursts of impulses separated by periods of silence. Different neurons were active in different parts of the scratch cycle, but most of them were active in the second half of the flexor phase. When the scratch reflex was evoked on the contralateral side, rhythmical modulation was observed in about half of the neurons, and it was less pronounced than in the case of ipsilateral scratching. Rhythmical modulation of cerebellar neurons during fictitious scratching is determined by signals coming from the central spinal mechanism, generating rhythmical oscillations, via the ventral spinocerebellar tract (VSCT) and the spinoreticulocerebellar pathway (SRCP). Results of separate transections of these pathways showed that the VSCT plays the crucial role in modulating interpositus neurons. Neurons of the lateral nucleus exhibited no modulation during fictitious scratching.

Activity of neurons of cerebellar nuclei during fictitious scratch reflex in cat. I. Fastigial nucleus

The activity of neurons located in the rostral part of the fastigial nucleus was recorded during the fictitious scratch reflex in thalamic cats immobilized with Flaxedil. The activity of most of the neurons was rhythmically modulated in relation to the scratch cycle: they generated bursts of impulses separated by periods of silence or, in a few cases, by periods of reduced activity. The discharge pattern of neurons was usually the same during both ipsilateral and contralateral scratch reflex: in both cases their maximum activity could be observed in the extensor phase of the scratch cycle. Rhythmical modulation of fastigial neurons is determined by signals coming from the central spinal mechanism, generating rhythmical oscillations, via the ventral spinocerebellar tract (VSCT) and the spinoreticulo-cerebellar pathway (SRCP). Results of separate transections of these pathways suggest that the SRCP determines the periodical excitationm of fastigial neurons and the VSCT their periodical inhibition.

Messages conveyed by descending tracts during scratching in the cat. II. Activity of rubrospinal neurons

(1) The activity of rubrospinal (RS) neurons giving axons to the lumbosacral spinal cord was recorded during actual and fictitious8 scratching in thalamic cats. (2) During both actual and fictitious scratching, the discharge frequency of many RS neurons was rhythmically modulated. Different neurons were active in different parts of the scratch cycle, but most neurons were active in the flexor phase. (3) The discharge frequency within the bursts during fictitious scratching was, on the average, equal to that during actual scratching. Immobilization usually resulted only in a small displacement of the burst position in the scratch cycle. Therefore, rhythmical modulation of RS neurons is determined mainly by central mechanisms and not by a rhythmical sensory input. (4) In decerebellate cats, the overwhelming majority of RS neurons had no rhythmical modulation. Very weak modulation was found only in a few neurons. (5) Transection of the ventral spinocerebellar tract (VSCT) resulted in considerable reduction or complete cessation of rhythmical modulation in RS neurons during fictitious scratching. On the contrary, transection of the spino-reticulocerebellar pathway (SRCP) resulted in just a small decrease of modulation. Therefore, of the two pathways (the VSCT and SRCP) transmitting messages about intraspinal processes to the cerebellum during scratching2,3, the VSCT is of major importance for modulating RS neurons.