Supraspinal monosynaptic excitation and inhibition of thoracic back montoneurons

The Vestibulospinal Nucleus Is a Locus of Balance Development

Mature vertebrates maintain posture using vestibulospinal neurons that transform sensed instability into reflexive commands to spinal motor circuits. Postural stability improves across development. However, due to the complexity of terrestrial locomotion, vestibulospinal contributions to postural refinement in early life remain unexplored. Here we leveraged the relative simplicity of underwater locomotion to quantify the postural consequences of losing vestibulospinal neurons during development in larval zebrafish of undifferentiated sex. By comparing posture at two timepoints, we discovered that later lesions of vestibulospinal neurons led to greater instability. Analysis of thousands of individual swim bouts revealed that lesions disrupted movement timing and corrective reflexes without impacting swim kinematics, and that this effect was particularly strong in older larvae. Using a generative model of swimming, we showed how these disruptions could account for the increased postural variability at both timepoints. Finally, late lesions disrupted the fin/trunk coordination observed in older larvae, linking vestibulospinal neurons to postural control schemes used to navigate in depth. Since later lesions were considerably more disruptive to postural stability, we conclude that vestibulospinal contributions to balance increase as larvae mature. Vestibulospinal neurons are highly conserved across vertebrates; we therefore propose that they are a substrate for developmental improvements to postural control.

The vestibulospinal nucleus is a locus of balance development

Mature vertebrates maintain posture using vestibulospinal neurons that transform sensed instability into reflexive commands to spinal motor circuits. Postural stability improves across development. However, due to the complexity of terrestrial locomotion, vestibulospinal contributions to postural refinement in early life remain unexplored. Here we leveraged the relative simplicity of underwater locomotion to quantify the postural consequences of losing vestibulospinal neurons during development in larval zebrafish of undifferentiated sex. By comparing posture at two timepoints, we discovered that later lesions of vestibulospinal neurons led to greater instability. Analysis of thousands of individual swim bouts revealed that lesions disrupted movement timing and corrective reflexes without impacting swim kinematics, and that this effect was particularly strong in older larvae. Using a generative model of swimming, we showed how these disruptions could account for the increased postural variability at both timepoints. Finally, late lesions disrupted the fin/trunk coordination observed in older larvae, linking vestibulospinal neurons to postural control schemes used to navigate in depth. Since later lesions were considerably more disruptive to postural stability, we conclude that vestibulospinal contributions to balance increase as larvae mature. Vestibulospinal neurons are highly conserved across vertebrates; we therefore propose that they are a substrate for developmental improvements to postural control.

Vertebral level specific modulation of paraspinal muscle activity based on vestibular signals during walking

Evoking muscle responses by electrical vestibular stimulation (EVS) may help to understand the contribution of the vestibular system to postural control. Although paraspinal muscles play a role in postural stability, the vestibulo-muscular coupling of these muscles during walking has rarely been studied. This study aimed to investigate how vestibular signals affect paraspinal muscle activity at different vertebral levels during walking with preferred and narrow step width. Sixteen healthy participants were recruited. Participants walked on a treadmill for 8 min at 78 steps/min and 2.8 km/h, at two different step width, either with or without EVS. Bipolar electromyography was recorded bilaterally from the paraspinal muscles at eight vertebral levels from cervical to lumbar. Coherence, gain, and delay of EVS and EMG responses were determined. Significant EVS-EMG coupling (P < 0.01) was found at ipsilateral and/or contralateral heel strikes. This coupling was mirrored between left and right relative to the midline of the trunk and between the higher and lower vertebral levels, i.e. a peak occurred at ipsilateral heel strike at lower levels, whereas it occurred at contralateral heel strike at higher levels. EVS-EMG coupling only partially coincided with peak muscle activity. EVS-EMG coherence slightly, but not significantly, increased when walking with narrow steps. No significant differences were found in gain and phase between the vertebral levels or step width conditions. In summary, vertebral level specific modulation of paraspinal muscle activity based on vestibular signals might allow a fast, synchronized, and spatially co-ordinated response along the trunk during walking. KEY POINTS: Mediolateral stabilization of gait requires an estimate of the state of the body, which is affected by vestibular afference. During gait, the heavy trunk segment is controlled by phasic paraspinal muscle activity and in rodents the medial and lateral vestibulospinal tracts activate these muscles. To gain insight in vestibulospinal connections in humans and their role in gait, we recorded paraspinal surface EMG of cervical to lumbar paraspinal muscles, and characterized coherence, gain and delay between EMG and electrical vestibular stimulation, during slow walking. Vestibular stimulation caused phasic, vertebral level specific modulation of paraspinal muscle activity at delays of around 40 ms, which was mirrored between left, lower and right, upper vertebral levels. Our results indicate that vestibular afference causes fast, synchronized, and spatially co-ordinated responses of the paraspinal muscles along the trunk, that simultaneously contribute to stabilizing the centre of mass trajectory and to keeping the head upright.

Muscle synergy analysis yields an efficient and physiologically relevant method of assessing stroke

The Fugl-Meyer Assessment is widely used to test motor function in stroke survivors. In the Fugl-Meyer Assessment, stroke survivors perform several movement tasks and clinicians subjectively rate the performance of each task item. The individual task items in the Fugl-Meyer Assessment are selected on the basis of clinical experience, and their physiological relevance has not yet been evaluated. In the present study, we aimed to objectively rate the performance of task items by measuring the muscle activity of 41 muscles from the upper body while stroke survivors and healthy participants performed 37 Fugl-Meyer Assessment upper extremity task items. We used muscle synergy analysis to compare muscle activity between subjects and found that 13 muscle synergies in the healthy participants (which we defined as standard synergies) were able to reconstruct all of the muscle activity in the Fugl-Meyer Assessment. Among the standard synergies, synergies involving the upper arms, forearms and fingers were activated to varying degrees during different task items. In contrast, synergies involving posterior trunk muscles were activated during all tasks, which suggests the importance of posterior trunk muscle synergies throughout all sequences. Furthermore, we noted the inactivation of posterior trunk muscle synergies in stroke survivors with severe but not mild impairments, suggesting that lower trunk stability and the underlying activity of posterior trunk muscle synergies may have a strong influence on stroke severity and recovery. By comparing the synergies of stroke survivors with standard synergies, we also revealed that some synergies in stroke survivors corresponded to merged standard synergies; the merging rate increased with the impairment of stroke survivors. Moreover, the degrees of severity-dependent changes in the merging rate (the merging rate–severity relationship) were different among different task items. This relationship was significant for 26 task items only and not for the other 11 task items. Because muscle synergy analysis evaluates coordinated muscle activities, this different dependency suggests that these 26 task items are appropriate for evaluating muscle coordination and the extent of its impairment in stroke survivors. Overall, we conclude that the Fugl-Meyer Assessment reflects physiological function and muscle coordination impairment and suggest that it could be performed using a subset of the 37 task items.

A Neural Controller Model Considering the Vestibulospinal Tract in Human Postural Control

Humans are able to control their posture in their daily lives. It is important to understand how this is achieved in order to understand the mechanisms that lead to impaired postural control in various diseases. The descending tracts play an important role in controlling posture, particularly the reticulospinal and the vestibulospinal tracts (VST), and there is evidence that the latter is impaired in various diseases. However, the contribution of the VST to human postural control remains unclear, despite extensive research using neuroscientific methods. One reason for this is that the neuroscientific approach limits our understanding of the relationship between an array of sensory information and the muscle outputs. This limitation can be addressed by carrying out studies using computational models, where it is possible to make and validate hypotheses about postural control. However, previous computational models have not considered the VST. In this study, we present a neural controller model that mimics the VST, which was constructed on the basis of physiological data. The computational model is composed of a musculoskeletal model and a neural controller model. The musculoskeletal model had 18 degrees of freedom and 94 muscles, including those of the neck related to the function of the VST. We used an optimization method to adjust the control parameters for different conditions of muscle tone and with/without the VST. We examined the postural sway for each condition. The validity of the neural controller model was evaluated by comparing the modeled postural control with (1) experimental results in human subjects, and (2) the results of a previous study that used a computational model. It was found that the pattern of results was similar for both. This therefore validated the neural controller model, and we could present the neural controller model that mimics the VST.

Functional organization of vestibulospinal inputs on thoracic motoneurons responsible for trunk postural control in Xenopus

KEY POINTS

Vestibulospinal reflexes participate in postural control. How this is achieved has not been investigated fully. We combined electrophysiological, neuroanatomical and imaging techniques to decipher the vestibulospinal network controlling the activation of back and limb muscles responsible for postural adjustments. We describe two distinct pathways activating either thoracic postural motoneurons alone or thoracic and lumbar motoneurons together, with the latter co-ordinating specifically hindlimb extensors and postural back muscles.

ABSTRACT

In vertebrates, trunk postural stabilization is known to rely mainly on direct vestibulospinal inputs on spinal axial motoneurons. However, a substantial role of central spinal commands ascending from lumbar segments is not excluded during active locomotion. In the adult Xenopus, a lumbar drive dramatically overwhelms the descending inputs onto thoracic postural motoneurons during swimming. Given that vestibulospinal fibres also project onto the lumbar segments that shelter the locomotor generators, we investigated whether such a lumbo-thoracic pathway may relay vestibular information and consequently, also be involved in the control of posture at rest. We show that thoracic postural motoneurons exhibit particular dendritic spatial organization allowing them to gather information from both sides of the cord. In response to passive head motion, these motoneurons display both early and delayed discharges, with the latter occurring in phase with ipsilateral hindlimb extensor bursts. We demonstrate that both vestibulospinal and lumbar ascending fibres converge onto postural motoneurons, and that thoracic motoneurons monosynaptically respond to the electrical stimulation of either pathway. Finally, we show that vestibulospinal fibres project to and activate lumbar interneurons with thoracic projections. Taken together, our results complete the scheme of the vestibulospinal control of posture by illustrating the existence of a novel, indirect pathway, which implicates lumbar interneurons relaying vestibular inputs to thoracic motoneurons, and participating in global body postural stabilization in the absence of active locomotion.

Crossed activation of thoracic trunk motoneurons by medullary reticulospinal neurons

Activation of contralateral muscles by supraspinal neurons, or crossed activation, is critical for bilateral coordination. Studies in mammals have focused on the neural circuits that mediate cross activation of limb muscles, but the neural circuits involved in crossed activation of trunk muscles are still poorly understood. In this study, we characterized functional connections between reticulospinal (RS) neurons in the medial and lateral regions of the medullary reticular formation (medMRF and latMRF) and contralateral trunk motoneurons (MNs) in the thoracic cord (T7 and T10 segments). To do this, we combined electrical microstimulation of the medMRF and latMRF and calcium imaging from single cells in an ex vivo brain stem-spinal cord preparation of neonatal mice. Our findings substantiate two spatially distinct RS pathways to contralateral trunk MNs. Both pathways originate in the latMRF and are midline crossing, one at the level of the spinal cord via excitatory descending commissural interneurons (reticulo-commissural pathway) and the other at the level of the brain stem (crossed RS pathway). Activation of these RS pathways may enable different patterns of bilateral trunk coordination. Possible implications for recovery of trunk function after stroke or spinal cord injury are discussed.NEW & NOTEWORTHY We identify two spatially distinct reticulospinal pathways for crossed activation of trunk motoneurons. Both pathways cross the midline, one at the level of the brain stem and the other at the level of the spinal cord via excitatory commissural interneurons. Jointly, these pathways provide new opportunities for repair interventions aimed at recovering trunk functions after stroke or spinal cord injury.

Reticulospinal Systems for Tuning Motor Commands

The pontomedullary reticular formation (RF) is a key site responsible for integrating descending instructions to execute particular movements. The indiscrete nature of this region has led not only to some inconsistencies in nomenclature, but also to difficulties in understanding its role in the control of movement. In this review article, we first discuss nomenclature of the RF, and then examine the reticulospinal motor command system through evolution. These command neurons have direct monosynaptic connections with spinal interneurons and motoneurons. We next review their roles in postural adjustments, walking and sleep atonia, discussing their roles in movement activation or inhibition. We propose that knowledge of the internal organization of the RF is necessary to understand how the nervous system tunes motor commands, and that this knowledge will underlie strategies for motor functional recovery following neurological injuries or diseases.

Segmental organization of vestibulospinal inputs to spinal interneurons mediating crossed activation of thoracolumbar motoneurons in the neonatal mouse

Vestibulospinal pathways activate contralateral motoneurons (MNs) in the thoracolumbar spinal cord of the neonatal mouse exclusively via axons descending ipsilaterally from the vestibular nuclei via the lateral vestibulospinal tract (LVST; Kasumacic et al., 2010). Here we investigate how transmission from the LVST to contralateral MNs is mediated by descending commissural interneurons (dCINs) in different spinal segments. We test the polysynaptic nature of this crossed projection by assessing LVST-mediated ventral root (VR) response latencies, manipulating synaptic responses pharmacologically, and tracing the pathway transynaptically from hindlimb extensor muscles using rabies virus (RV). Longer response latencies in contralateral than ipsilateral VRs, near-complete abolition of LVST-mediated calcium responses in contralateral MNs by mephenesin, and the absence of transsynaptic RV labeling of contralateral LVST neurons within a monosynaptic time window all indicate an overwhelmingly polysynaptic pathway from the LVST to contralateral MNs. Optical recording of synaptically mediated calcium responses identifies LVST-responsive ipsilateral dCINs that exhibit segmental differences in proportion and dorsoventral distribution. In contrast to thoracic and lower lumbar segments, in which most dCINs are LVST responsive, upper lumbar segments stand out because they contain a much smaller and more ventrally restricted subpopulation of LVST-responsive dCINs. A large proportion of these upper lumbar LVST-responsive dCINs project to contralateral L5, which contains many of the hindlimb extensor MNs activated by the LVST. A selective channeling of LVST inputs through segmentally and dorsoventrally restricted subsets of dCINs provides a mechanism for targeting vestibulospinal signals differentially to contralateral trunk and hindlimb MNs in the mammalian spinal cord.

Task, muscle and frequency dependent vestibular control of posture

The vestibular system is crucial for postural control; however there are considerable differences in the task dependence and frequency response of vestibular reflexes in appendicular and axial muscles. For example, vestibular reflexes are only evoked in appendicular muscles when vestibular information is relevant to postural control, while in neck muscles they are maintained regardless of the requirement to maintain head on trunk balance. Recent investigations have also shown that the bandwidth of vestibular input on neck muscles is much broader than appendicular muscles (up to a factor of 3). This result challenges the notion that vestibular reflexes only contribute to postural control across the behavioral and physiological frequency range of the vestibular organ (i.e., 0-20 Hz). In this review, we explore and integrate these task-, muscle- and frequency-related differences in the vestibular system's contribution to posture, and propose that the human nervous system has adapted vestibular signals to match the mechanical properties of the system that each group of muscles controls.

A neuromuscular strategy to prevent spinal torsion: backward perturbation alters asymmetry of transversus abdominis muscle thickness into symmetry

Symmetric co-contraction of the transversus abdominis (TrA) muscle is beneficial in terms of increasing trunk stability. The aim of this study was to investigate the symmetry of lateral abdominal muscle thickness during static and dynamic conditions. Fifteen male subjects (27.13 ± 5.51 years old) were instructed to sit on a chair and maintain upright posture. Every individual subject wore a jacket harness that could be backwardly attached to a 9-kg weight through a pulley system. An unexpected drop of the weight induced the transition from static to dynamic condition. The thickness of external oblique, internal oblique, and TrA muscles was measured with ultrasonography. Our results revealed more symmetry of TrA thickness during the dynamic condition (21% vs. 13%, p = 0.019) compared with the static. The symmetric muscle thickness of TrA during the dynamic condition is considered a result of more contraction on the non-dominant side. This phenomenon could be a possible strategy of deep abdominal muscles to prevent spinal torsion during sudden trunk perturbation.

Projections from the lateral vestibular nucleus to the spinal cord in the mouse

The present study investigated the projections from the lateral vestibular nucleus (LVe) to the spinal cord using retrograde and anterograde tracers. Retrogradely labeled neurons were found after fluoro-gold injections into both the cervical and lumbar cord, with a smaller number of labeled neurons seen after lumbar cord injections. Labeled neurons in the LVe were found in clusters at caudal levels of the nucleus, and a small gap separated these clusters from labeled neurons in the spinal vestibular nucleus (SpVe). In the anterograde study, BDA-labeled fiber tracts were found in both the ventral and ventrolateral funiculi on the ipsilateral side. These fibers terminated in laminae 6-9. Some fibers were continuous with boutons in contact with motor neurons in both the medial and lateral motor neuron columns. In the lumbar and sacral segments, some collaterals from the ipsilateral vestibulospinal tracts were found on the contralateral side, and these fibers mainly terminated in laminae 6-8. The present study reveals for the first time the fiber terminations of the lateral vestibular nucleus in the mouse spinal cord and therefore enhances future functional studies of the vestibulospinal system.

Segmental patterns of vestibular-mediated synaptic inputs to axial and limb motoneurons in the neonatal mouse assessed by optical recording

Proper control of movement and posture occurs partly via descending projections from the vestibular nuclei to spinal motor circuits. Days before birth in rodents, vestibulospinal neurons develop axonal projections that extend to the spinal cord. How functional these projections are just after birth is unknown. Our goal was to assess the overall functional organization of vestibulospinal inputs to spinal motoneurons in a brainstem-spinal cord preparation of the neonatal mouse (postnatal day (P) 0-5). Using calcium imaging, we recorded responses evoked by electrical stimulation of the VIIIth nerve, in many motoneurons simultaneously throughout the spinal cord (C2, C6, T7, L2 and L5 segments), in the medial and lateral motor columns. Selective lesions in the brainstem and/or spinal cord distinguished which tracts contributed to the responses: those in the cervical cord originated primarily from the medial vestibulospinal tracts but with a substantial contribution from the lateral vestibulospinal tract; those in the thoracolumbar cord originated exclusively from the lateral vestibulospinal tract. In the thoracolumbar but not the cervical cord, excitatory commissural connections mediated vestibular responses in contralateral motoneurons. Pharmacological blockade of GABA(A) receptors showed that responses involved a convergence of excitatory and inhibitory inputs which in combination produced temporal response patterns specific for different segmental levels. Our results show that by birth vestibulospinal projections in rodents have already established functional synapses and are organized to differentially regulate activity in neck and limb motoneurons in a tract- and segment-specific pattern similar to that in adult mammals. Thus, this particular set of descending projections develops several key features of connectivity appropriately at prenatal stages. We also present novel information about vestibulospinal inputs to axial motoneurons in mammals, providing a more comprehensive platform for future studies into the overall organization of vestibulospinal inputs and their role in regulating postural stability.

Direct and indirect connections with upper limb motoneurons from the primate reticulospinal tract

Although the reticulospinal tract is a major descending motor pathway in mammals, its contribution to upper limb control in primates has received relatively little attention. Reticulospinal connections are widely assumed to be responsible for coordinated gross movements primarily of proximal muscles, whereas the corticospinal tract mediates fine movements, particularly of the hand. In this study, we used intracellular recording in anesthetized monkeys to examine the synaptic connections between the reticulospinal tract and antidromically identified cervical ventral horn motoneurons, focusing in particular on motoneurons projecting distally to wrist and digit muscles. We found that motoneurons receive monosynaptic and disynaptic reticulospinal inputs, including monosynaptic excitatory connections to motoneurons that innervate intrinsic hand muscles, a connection not previously known to exist. We show that excitatory reticulomotoneuronal connections are as common and as strong in hand motoneuron groups as in forearm or upper arm motoneurons. These data suggest that the primate reticulospinal system may form a parallel pathway to distal muscles, alongside the corticospinal tract. Reticulospinal neurons are therefore in a position to influence upper limb muscle activity after damage to the corticospinal system as may occur in stroke or spinal cord injury, and may be a target site for therapeutic interventions.

Responses of thoracic spinal interneurons to vestibular stimulation

Vestibular influences on outflow from the spinal cord are largely mediated via spinal interneurons, although few studies have recorded interneuronal activity during labyrinthine stimulation. The present study determined the responses of upper thoracic interneurons of decerebrate cats to electrical stimulation of the vestibular nerve or natural stimulation of otolith organs and the anterior and posterior semicircular canals using rotations in vertical planes. A majority of thoracic interneurons (74/102) responded to vestibular nerve stimulation at median latencies of 6.5 ms (minimum of approximately 3 ms), suggesting that labyrinthine inputs were relayed to these neurons through trisynaptic and longer pathways. Thoracic interneuronal responses to vertical rotations were similar to those of graviceptors such as otolith organs, and a wide array of tilt directions preferentially activated different cells. Such responses were distinct from those of cells in the cervical and lumbar enlargements, which are mainly elicited by ear-down tilts and are synchronous with stimulus position when low rotational frequencies are delivered, but tend to be in phase with stimulus velocity when high frequencies are employed. The dynamic properties of thoracic interneuronal responses to tilts were instead similar to those of thoracic motoneurons and sympathetic preganglionic neurons. However, the preferred tilt directions of the interneurons were more heterogeneous than thoracic spinal outputs, showing that the outputs do not simply reflect an addition of local interneuronal activity.

The anatomy of the central control of posture: consistency and plasticity

Posture is usually thought to be steered by brainstem and cortical structures that have access to the motoneurons and their premotor interneurons of the axial and neck muscles. The present paper describes these pathways and their relation with gaze control structures. All these systems belong to the medial component of the so-called voluntary motor system. On the other hand, in the cat there also exist several postures that are not steered by the somatic, but by the emotional motor system. Examples are arching of the back and mating postures. The pathways thought to be involved in these behaviours are described. They belong to the lateral component of the emotional motor system and, especially in case of mating postures, are extremely plastic. They have been demonstrated to be almost 10 times as strong in oestrus than in non-oestrus cats.

Vestibular influences on cat lumbar paravertebral muscles

Recordings were made from nerves innervating ventral lumbar paravertebral muscles (quadratus lumborum) during natural vestibular stimulation in vertical planes. The best direction of vestibular stimulation for producing an increase in nerve activity was near nose-up pitch in two-thirds of the animals, but was near nose-down pitch in the others. The response gain (re. position) was flat across the frequency range of 0.05-1 Hz, and the responses persisted throughout 11-s static tilts. These data suggest that otolith organs activated by pitch influence activity in the lumbar paravertebral muscles. These responses may serve to stabilize the vertebral column during movements involving body pitch, such as vertical climbing.

Primary motor cortex influences on the descending and ascending systems

The motor cortex plays a crucial role in the co-ordination of movement and posture. This is possible because the pyramidal tract fibres have access both directly and through collateral branches to structures governing eye, head, neck trunk and limb musculature. Pyramidal tract axons also directly reach the dorsal laminae of the spinal cord and the dorsal column nuclei, thus aiding in the selection of the sensory ascendant transmission. No other neurones in the brain besides pyramidal tract cells have such a wide access to different structures within the central nervous system. The majority of the pyramidal tract fibres that originate in the motor cortex and that send collateral branches to multiple supraspinal structures do not reach the spinal cord. Also, the great majority of the corticospinal neurones that emit multiple intracraneal collateral branches terminate at the cervical spinal cord level. The pyramidal tract fibres directed to the dorsal column nuclei that send collateral branches to supraspinal structures also show a clear tendency to terminate at supraspinal and cervical cord levels. These facts suggest that a substantial co-ordination between descending and ascending pathways might be produced by the same motor cortex axons at both supraspinal and cervical spinal cord sites. This may imply that the motor cortex co-ordination will be mostly directed to motor responses involving eye-neck-forelimb muscle synergies. The review makes special emphasis in the available evidence pointing to the role of the motor cortex in co-ordinating the activities of both descending and ascending pathways related to somatomotor integration and control. The motor cortex may function to co-operatively select a unique motor command by selectively filter sensory information and by co-ordinating the activities of the descending systems related to the control of distal and proximal muscles.

Pontine-induced generalized suppression of postural muscle tone in a reflexively standing acute decerebrate cat

In a reflexively standing acute decerebrate cat, the pontine tegmentum was electrically stimulated and the effects on postural muscle tone and locomotor movements evoked by stimulation of the mesencephalic locomotor region (MLR) were studied. A stimulating microelectrode was placed systematically at 1-mm increments throughout the pons (H - 2 to H - 10) at levels ranging from P0.0 to P6.5 dorsoventrally and mediolaterally from LR0 to L4 or R4. Another stimulating microelectrode was placed in the physiologically identified MLR. Stimuli delivered to the dorsomedial regions of the pontine tegmentum (P3 to P4, LR1.5 to 2.5, H - 4 to H - 6) resulted in simultaneous and bilateral suppression of tonic activities in the neck, lumbar back, forelimb and hindlimb muscles. The pontine inhibitory sites corresponded to the medial area of the central tegmental field (FTC) and the central area of the gigantocellular tegmental field (FTG), bilaterally. Effects of pontine induced suppression on those muscles were stimulus frequency and stimulus intensity-dependent and the effects persisted even after termination of the stimulation. With concomitant pontine stimulation, MLR-evoked locomotor movements were suppressed along with prolongation of the forelimb and hindlimb step cycles.

Physiological abnormalities in hereditary hyperekplexia

Five patients from a kindred with hereditary hyperekplexia had physiological testing. The surface-recorded electromyographic pattern of audiogenic muscle jerks was identical to that of the normal acoustic startle reflex. Testing at graded stimulus intensities indicated an increase in the gain of the acoustic startle reflex. Nose-tap stimuli resulted in short-latency generalized electromyographic bursts that were similar to the R1 component of the blink reflex. Electrical stimulation of peripheral nerves elicited a pattern of generalized muscle jerks that was similar to that of the acoustic startle reflex. Somatosensory evoked potentials, brainstem auditory evoked potentials, and cortical auditory evoked potentials were normal. The primary physiological abnormality in hereditary hyperekplexia is widespread elevated gain of vestigial withdrawal reflexes in the brainstem and possibly the spinal cord, most likely resulting from increased excitability of reticular neurons.

Evidence of a Monosynaptic Pathway Between Cells of the Ventromedial Medulla and the Motoneuron Pool of the Thoracic Spinal Cord in Rat: Electron Microscopic Analysis of Synaptic Contacts

Previous electrophysiological and anatomical data have suggested the existence of a descending pathway from the ventromedial medulla into the thoracic motoneuron pool. However, systematic light and electron microscopic analysis have not yet been done to reveal such a projection. In the present study, the anterograde tracer, Phaseolus vulgaris leucoagglutinin (PHA-L) was injected into several discrete regions of the medioventral medulla and descending PHA-L-labelled axons were investigated in the thoracic ventral horn using both light and electron microscopy. Light microscopic analysis of descending projections from 20 distinct areas of the medioventral medulla showed that neurons that project predominantly to the intermediate and ventral regions of the thoracic spinal grey matter are located caudal to the facial nucleus. Monosynaptic contacts were found between axons originating from five distinct regions of the medioventral medulla (containing raphé and/or gigantocellular reticular neurons) and cells in the thoracic motoneuron pool. PHA-L-labelled boutons formed synaptic contacts with large calibre dendrites and with somata. Seventy-two per cent of the investigated 32 boutons appeared to have symmetrical synaptic membrane specializations. The majority of the boutons contained only small, pleomorphic vesicles. Our findings show the existence of a direct monosynaptic pathway between the neurons of the ventromedial medulla and thoracic motor nuclei, providing anatomical support for previous physiological data.

Vestibular and cerebellar modulation of expiratory motor activities in the cat

1. The purpose of our investigation was to evaluate the hypothesis that components of the vestibular and cerebellar systems regulate efferent respiratory-modulated activities of cranial and spinal nerves. The hypothesis was based upon the observation that spinal neural activities during expiration are greatly altered subsequent to a change in posture. 2. In decerebrate and paralysed cats, efferent activities were recorded from the central cut ends of the phrenic nerve, intercostal nerve, branch of the intercostal nerve innervating the triangularis sterni, cranial iliohypogastric (abdominal) nerve and recurrent laryngeal nerve. 3. Animals were artificially ventilated. Those with intact vagi were ventilated by a servo-respirator which produced changes in lung volume in parallel with alterations in integrated activity of the phrenic nerve. Animals with bilateral vagotomy were ventilated with a standard respirator. 4. Aspiration of the entire cerebellar cortex did not produce alterations in levels of neural activities; the respiratory frequency was increased modestly. Following ablation of the ventrolateral portion of corpus medullare and cerebellar peduncles, expiratory activities of spinal nerves were completely eliminated whereas inspiratory activities were not greatly altered. Results were similar in animals having either intact or sectioned vagi. 5. Electrical stimulation or chemical stimulation by glutamate of regions of the ventrolateral cerebellum produced little change in respiratory neural activities except when these stimulations were within the infracerebellar nucleus. Stimulations in this nucleus caused pronounced increases in expiratory activities of spinal nerves. Neither inspiratory activities of spinal nerves nor inspiratory or expiratory activities of the recurrent laryngeal nerve were altered. Studies in animals having intact or sectioned vagi yielded similar results. 6. Bilateral lesions of neurons in the infracerebellar nucleus by injections of kainic acid in animals having intact or sectioned vagi caused an irreversible loss of expiratory activities of spinal nerves with neither inspiratory spinal activities nor inspiratory and expiratory laryngeal activities being altered. Similar findings were obtained following unilateral ablation of the infracerebellar nucleus in vagotomized cats. However, in cats with intact vagi, unilateral ablation of the infracerebellar nucleus produced only transient changes in either inspiratory or expiratory neural activities.(ABSTRACT TRUNCATED AT 400 WORDS)

Functional relationships between the muscles of mastication and the muscles of the leg. An electromyographic study

The authors have shown, with the help of electromyography, the functional relationship which exists between the muscles of mastication and certain muscles of the leg after artificially creating interdental occlusal dysfonction. While the activity of the tibialis anterior was not affected by the alteration in occlusion, that of the peroneus longus and gastrocnemius was markedly changed. This is due to the activation of long muscle chains as a consequence of the activation of complex nervous reflexes.

Pedunculopontine tegmental nucleus-induced inhibition of muscle activity in the rat

The connections of the pedunculopontine tegmental nucleus (PPN) have led us to propose that this structure mediates striatally induced inhibition of muscle activity by directing basal ganglia output to an inhibitory reticulospinal system (nucleus reticularis gigantocellularis and ventralis, nrGi-V). We conducted experiments in order to examine the effects of electrical stimulation of the PPN on the activity of selected neck and shoulder muscles. PPN stimulation at low rates (0.1 Hz) elicited bilateral muscle excitation. As the rate of stimulation was increased (e.g. to 10 Hz), less excitation was observed. Anodal DC current inactivation of the nrGi-V during concurrent 10 Hz PPN stimulation resulted in an augmentation of muscle activity above the levels observed during 10 Hz PPN stimulation alone. PPN stimulation (10 Hz) also profoundly inhibited cortically-induced muscle activity. Further support for our proposal stems from increased baseline activity (0.1 Hz PPN-induced excitation) in animals with ibotenic acid lesions of the PPN as compared to normal animals. Apparently, destruction of the PPN releases the musculature from tonic and/or phasic inhibition. A model is discussed which attempts to account for both the rate-dependent changes in excitation and the inhibition of cortically induced muscle activity.

Brainstem projections to cats' upper lumbar spinal cord: implications for abdominal muscle control

Unilateral L1-L2 gray matter HRP injections labeled neurons bilaterally in nucleus retroambiguus (expiratory neuron region of caudal ventral respiratory group) and ventromedial reticular formation. Minor labeling occurred in raphe, vestibular, and medial parabrachial nuclei, lateral reticular formation, and C1-C2. Midsagittal lesions between C1 and obex prevented nucleus retroambiguus labeling (except 1 contralateral cell adjacent an incomplete lesion), indicating that these neurons decussate between C1 and obex and have collaterals that recross the spinal cord.

Comparison of the branching patterns of lateral and medial vestibulospinal tract axons in the cervical spinal cord

The morphology of single physiologically-identified lateral and medial vestibulospinal tract (LVST and MVST) axons was analysed, using intracellular staining with horseradish peroxidase (HRP) and three-dimensional reconstruction of axonal trajectories in the cat. Axons were penetrated in the cervical cord at C1-C8 with a microelectrode filled with 7% HRP. These axons were identified as vestibulospinal axons by their monosynaptic responses to stimulation of the vestibular nerve and further classified as either LVST or MVST axons by their responses to stimulation of the LVST and MVST. The stained axons could be traced over distances of 3-16 mm rostrocaudally. Within these lengths, both LVST and MVST axons were found to have multiple axon collaterals at different segments in the cervical cord. Up to seven collaterals were given off from the stems of MVST axons and LVST axons. The LVST axons included both neurones terminating at the cervical cord and those projecting further caudally to the thoracic or lumbar cord. Each collateral of these LVST axons, after entering into the gray matter, ramified successively in a delta-like fashion and terminated mainly in lamina VIII and in the medial part of lamina VII. Many boutons of both terminal and en passant types seemed to make contact with the cell bodies and proximal dendrites of neurones in the ventromedial nucleus (VM). Each collateral had a narrow rostrocaudal extension (0.2-1.6 mm, average 0.8 mm) in the gray matter in contrast to a much wider intercollateral interval (average 1.5 mm), so that there were gaps free from terminal boutons between adjacent collateral arborizations. The morphology of axon collaterals of MVST axons was very similar to that of LVST axons. The rostrocaudal extent of single axon collaterals was very restricted (0.3-2.1 mm) in contrast to the wide spread in a mediolateral or a dorsoventral direction. MVST axons had intensive projections to the upper cervical cord with multiple axon collaterals. One to seven collaterals of single MVST axons were found at C1-C3. Terminals of MVST axons were distributed in laminae VII, VIII and IX, including the VM, the nucleus spinalis n. accessorii (SA), and the commissural nucleus. Many terminals seemed to make contact with retrogradely-labelled motoneurones of neck muscles. Both axosomatic and axodendritic contacts were observed on motoneurones in various sizes. Some collaterals gave rise to terminal arborizations in both the VM and the SA. These results suggest that single LVST and MVST axons may control excitability of multiple dorsal axial muscles concurrently with their multiple axon collaterals at multisegmental levels.

The influence of adequate vestibular stimulation on evoked locomotor muscle activity in the decerebrated guinea-pig

The influence of adequate vestibular stimulation on locomotor muscle activity has been investigated in the decerebrated guinea-pig. Locomotor activity was evoked by electrical stimulation of the mesencephalic locomotor region, whose location has been ascertained in this animal. Vestibular stimulation was performed by cyclic tiltings about the longitudinal and transverse axes and swinging along the vertical axis. The translation frequency was in the range of 0.02-0.8 Hz with an amplitude of +/-20% for tilting and 40 mm for swinging. Vestibular stimulation was accompanied by distinct changes in locomotor electromyographic activity of fore- and hindlimb antagonist muscles. During stimulation the intensity of discharges in extensor and flexor muscles corresponding to the stance and swing phases of the locomotor cycle was modulated; the alternation of antagonist muscle activity was not as a rule disturbed. The changes in muscle activity had the same pattern and similar phase-frequency properties to those observed under analogous vestibular stimulation during the maintenance of steady posture. It is suggested that the vestibular system is of considerable importance for the regulation of locomotor muscle activity. During locomotion the vestibular system influences mainly spinal motor output but does not act on the locomotor generator.

Anatomical distribution and response patterns of reticular neurons active in relation to acoustic startle

A population of reticulospinal neurons with short latency response to startle-inducing stimuli was identified in the nucleus reticularis pontis caudalis (NRPC) and nucleus gigantocellularis (NRGC) of the medial pontomedullary reticular formation. The threshold and magnitude of response to auditory stimuli was correlated in these cells and in the muscles mediating startle. Startle-related neurons were significantly more likely to have high conduction velocity spinal projections than adjacent cells not related to startle. Startle-related cells were not 'dedicated' to startle, but were active in relation to spontaneous movements. Both the unit response of the startle-related cells and the startle response recorded in muscles were suppressed by the prior presentation of a weak prepulse. Thus, prepulse inhibition of startle occurs at, or prior to, the medial pontomedullary reticular formation. We conclude that these reticulospinal cells convey the output of the brainstem system modulating and triggering startle.

Mesodiencephalic projections to the vestibular complex in the cat

The distribution of cells in the rostral medial mesencephalon and caudal diencephalon which project to the vestibular complex was mapped in the cat by using retrograde axonal transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Subsequent experiments using anterograde transport of WGA-HRP clarified the position of the terminations of the mesodiencephalic-derived afferents in the vestibular complex. After large injections which involved the entire vestibular complex, retrogradely labeled cells were seen in both the ipsilateral and contralateral interstitial nucleus of Cajal (INC) and were more numerous in its rostral pole. Labeled cells also occurred in the perifascicular region, both immediately adjacent to the fasciculus retroflexus and rostroventral to it. Fusiform midline cells of the Edinger-Westphal nucleus were also labeled, as well as a number of cells in the adjacent somatic portion of the oculomotor complex (OMC). Another group of labeled cells was observed within the contralateral medial terminal nucleus of the accessory optic tract (MTN) and in the posterior hypothalamic nucleus. Injections limited to subregions of the vestibular complex resulted in similar but slightly varying distributions and numbers of retrogradely labeled cells. After injections covering the caudal half of the medial vestibular nucleus (MVN) and descending vestibular nucleus (DVN), labeled cells in the INC and tegmentum dorsal to it were especially prominent, but none was seen in the MTN or OMC. Injections placed in the rostral MVN, lateral vestibular nucleus, y group, and superior vestibular nucleus resulted in a distribution of labeled cells similar to that seen following global vestibular injections, but these cells were fewer in number. After an injection confined to the y group, a small number of retrogradely labeled cells were seen in the rostral pole of the INC and immediately ventral to the fasciculus retroflexus. Projections from the rostral medial mesencephalon and caudal diencephalon to the MVN, DVN, and y group were confirmed by using anterograde transport of WGA-HRP. Direct projections from the INC-perifascicular regions and somatic neurons of the OMC to the caudal vestibular complex could play a role in eye-head coordination. Those projections from the rostral INC and MTN to the rostral vestibular complex may play a role in vertical eye movements and responses to visual stimuli which move in the vertical plane.

Vestibulospinal and reticulospinal interactions in the activation of back muscle EMG in the rat

The effects of electrical stimulation of the lateral vestibular nucleus (LVN) and medullary reticular formation (RF) on electromyographic activity in axial muscles medial longissimus (ML) and lateral longissimus (LL) in the rat were studied. Long trains (150-500 ms) at 200-330 Hz and 20-100 microA were sufficient to activate ML and LL at latencies of 20-100 ms from the beginning of the train. Results of stimulation at 200-330 Hz to RF or LVN showed that muscle units were activated at a fixed latency from any effective pulse in the stimulus train. Using high frequency (1 kHz) trains of 3-6 pulses to LVN, EMG activity was detected at minimum latencies of 3.5-6 ms. When conduction times from the medulla to the spinal cord, and the spinal cord to the muscle are subtracted, this latency range is consistent with monosynaptic activation. In many cases, muscle units were recruited in order of size, with both RF and LVN stimulation. Combined stimulation of LVN and RF sites in n. gigantocellularis led to EMG activity in ML and LL at currents which were insufficient to evoke activity when presented singly. When stimulation of one site (300-400 ms train) was just sufficient to evoke a response, a shorter, overlapping train (100-150 ms) to the other site led to a higher rate of muscle activity that continued through the end of the long train, even after the short train had ended. In all cases, the effect of RF facilitating LVN was similar to the effect of LVN facilitating RF. The evidence for convergence between these two systems in the medulla and the spinal cord is discussed.

Brainstem reticular stimulation facilitates back muscle motoneuronal responses to pudendal nerve input

In urethane or pentobarbital anesthetized female rats, stimulation with bipolar electrodes in the medullary reticular formation evoked activity in lateral longissimus muscle nerves, when trains of pulses were applied. Combined stimulation of the medullary reticular formation and the pudendal nerve revealed a marked mutual facilitation of their two separate effects on the excitability of the lateral longissimus muscle nerves. These results match previous conclusions based on the effects of spinal column transection on the pudendal nerve-evoked response, and are consistent with a role for this mutual facilitation in the control of lordosis behavior.

Effects of total and partial spinal transections on the pudendal nerve-evoked response in rat lumbar axial muscle

The pudendal nerve-evoked activity in lumbar axial muscle nerves 24-48 h following spinal transections was studied in order to examine the supraspinal control of the pudendal nerve-evoked response (PNER). After total transections of the spinal cord in the thoracic region the overall activity in the muscle nerves was much reduced; the evoked responses consisted only of a single peak in activity 19.8 ms (+/- 1.4 ms) following the last shock of a 3-shock pudendal nerve stimulus demonstrating that some of the earliest responses previously described for the PNER are segmental in origin. To help identify supraspinal inputs to the PNER a series of partial transections were made to cut selectively different fiber columns within the cord. Four different surgical procedures were performed: transections of the entire dorsal half of the cord; ablations of all of the medial columns; combined lesions of the dorsal half and medial columns, sparing the tracts within the ventrolateral columns and bilateral transections of the lateral columns. Following all partial transections except those of the lateral columns, the PNER was similar to that seen in intact animals. After lateral column transections however, the responses were like those seen in the totally transected animals. The combined results suggest that supraspinal influences upon the PNER are conveyed primarily via the ventrolateral columns of the spinal cord. Bilateral convergence of afferent information was evident in the totally transected animals, indicating that such convergence can occur at segmental levels. The lesions which substantially altered the PNER closely paralleled those which eliminated lordotic responsiveness in behaving female rats.

The morphology of single lateral vestibulospinal tract axons in the lower cervical spinal cord of the cat

The intraspinal morphology of single lateral vestibulospinal tract (LVST) axons was investigated with the method of intra-axonal staining with horseradish peroxidase (HRP) and three-dimensional reconstruction of the axonal trajectory. Axons penetrated in the ventral funiculus at C5-C8 were identified as LVST axons by their monosynaptic responses to stimulation of the ipsilateral vestibular nerve and by their direct responses to stimulation of the ipsilateral Deiters' nucleus and LVST. Reconstructions were made from 34 well-stained LVST axons. Of these, 23 terminated in the brachial segments (C5-Th1) and the other 11 projected below Th2. These axons were traced over distances of 2.9-16.3 mm rostrocaudally. Within these lengths, one to seven axon collaterals (mean +/- S.D., 3.2 +/- 2.0, N = 19) were given off at right angles from the stem axons of LVST axons terminating in the brachial segments. The mean diameters of stem axons and primary collaterals were 4.5 microns and 1.6 micron, respectively. In the gray matter, collaterals ramified successively, pursued a delta-like path, and terminated mainly in laminae VII and VIII or lamina IX. The rostrocaudal extension of a single collateral was very restricted (mean +/- S.D., 760 +/- 220 microns, N = 16), in contrast to the extensive dorsoventral and mediolateral extent of the terminal arborization. There were usually gaps between adjacent collateral arborizations from the same stem axons, since the intercollateral distances ranged from 400 to 4,300 microns (mean = 1,490 microns). LVST axons terminating in brachial segments were divided into two groups--a medial group and a lateral group--on the basis of their projection sites in the transverse plane of the gray matter. The axons of the medial type had their main projection to laminae VII and VIII of Rexed, while those of the lateral type terminated in lamina IX. The terminal arborizations of the medial type LVST axons were mainly distributed over lamina VIII, where synaptic boutons appeared to make contact with proximal dendrites or somata of medium-sized and large neurons in the ventromedial nucleus and also in the medial portion of lamina VII adjacent to the central canal and dorsal to lamina VIII. Five out of 15 medial type axons had a bilateral projection. One or two collaterals of each of these axons crossed the midline through the anterior commissure and terminated in lamina VII or VIII. It was concluded that the contralateral projection was sparse.(ABSTRACT TRUNCATED AT 400 WORDS)

Separation of the medullo-spinal descending pathway for somatic and autonomic outflow in the cat

The present study investigated differences between vestibulo-somatic and vestibulo-sympathetic reflexes, along with differences between somatic and autonomic spino-bulbo-spinal (SBS) reflexes in chloralose-urethane anesthetized cats. Electrical stimulation was applied to the vestibular nerve (V) for a duration of 0.3 ms. The potential responses in the sympathetic renal nerve (RN) and somatic lumbar nerve were recorded simultaneously. Responses were recorded for a variety of conditioning stimulus to testing stimulus intervals, and the results were plotted to form a recovery curve. The recovery curve for the test response from the somatic nerve was very different from that of the sympathetic nerve. Following transection of the lateral part of the thoracic cord, in the case of the sympathetic renal nerve, recorded responses were still present on vestibular and lumbar nerve stimulation, whereas in the case of the vestibulo-somatic and somatic SBS reflexes, the reflex response had disappeared after transection. These findings suggest that sympathetic and somatic reactions as a result of vestibular stimulation have different descending pathways in the spinal cord, and that their physiological characteristics are different.

Fastigial influences on postural tonus as studied by kainate lesions and by local infusion of GABAergic drugs in the rat

Localized lesions with kainic acid (KA) and local infusion of GABAergic drugs were used to study the role of the nucleus fastigii (NF) in postural tonus. Unilateral axon-sparing KA lesions of NF resulted in ipsilateral limb extensor atonia and contralateral limb extensor hypertonus and abduction. This effect lasted for 3-8 days depending on the dose of KA. Lesions of sites adjacent to the NF failed to produce postural asymmetries. Local infusion of the GABA agonist muscimol (10-50 ng) in the NF produced a reversible postural asymmetry fenomenologically similar to that produced by KA lesions of NF. Infusion of agents blocking GABAergic transmission in the NF (bicuculline, picrotoxin) resulted in a postural asymmetry of inverted laterality in respect to that produced by muscimol. Bilateral KA lesions of NF or bilateral infusion of muscimol resulted in bilateral hyperextension-abduction of the limbs. Infusion of GABAergic drugs in areas adjacent to the NF failed to produce postural changes. The results are interpreted to indicate that the NF exerts a crossed inhibitory and a direct excitatory influence on limb postural tonus.