Interdependence of spatial properties and projection patterns of medial vestibulospinal tract neurons in the cat

Vestibulospinal reflexes elicited with a tone burst method are dependent on spatial orientation

Balance involves several sensory modalities including vision, proprioception and the vestibular system. This study aims to investigate vestibulospinal activation elicited by tone burst stimulation in various muscles and how head position influences these responses. We recorded electromyogram (EMG) responses in different muscles (sternocleidomastoid-SCM, cervical erector spinae-ES-C, lumbar erector spinae-ES-L, gastrocnemius-G, and tibialis anterior-TA) of healthy participants using tone burst stimulation applied to the vestibular system. We also evaluated how head position affected the responses. Tone burst stimulation elicited reproducible vestibulospinal reflexes in the SCM and ES-C muscles, while responses in the distal muscles (ES-L, G, and TA) were less consistent among participants. The magnitude and polarity of the responses were influenced by the head position relative to the cervical spine. When the head was rotated or tilted, the polarity of the vestibulospinal responses changed, indicating the integration of vestibular and proprioceptive inputs in generating these reflexes. Overall, our study provides valuable insights into the complexity of vestibulospinal reflexes and their modulation by head position. However, the high variability in responses in some muscles limits their clinical application. These findings may have implications for future research in understanding vestibular function and its role in posture and movement control.

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.

Ancestral persistence of vestibulospinal reflexes in axial muscles in humans

Most studies addressing the role of vestibulospinal reflexes in balance maintenance have mainly focused on responses in the lower limbs, while limited attention has been paid to the output in trunk and back muscles. To address this issue, we tested whether electromyographic (EMG) responses to galvanic vestibular stimulations (GVS) were modulated similarly in back and leg muscles, in situations where the leg muscle responses to GVS are known to be attenuated. Body sway and surface EMG signals were recorded in the paraspinal and limb muscles of humans (n = 19) under three complementary conditions. During treadmill locomotion, EMG responses in the lower limbs were observed only during stance, whereas responses in trunk muscles were observed during all phases of the locomotor cycle. During upright standing, a slight head contact abolished the responses in the lower limbs, while the responses remained present in back muscles. Similarly, during parabolic flight-induced microgravity, EMG responses in lower limb muscles were suppressed but remained in axial muscles despite the abolished gravitational otolithic drive. Our results suggest a differentiated control of axial and appendicular muscles when a perturbation is detected by vestibular inputs. The persistence and low modulation of axial muscle responses suggests that a hard-wired reflex is functionally efficient to maintain posture. By contrast, the ankle responses to GVS occur only in balance tasks when proprioceptive feedback is congruent. This study using GVS in microgravity is the first to present an approach delineating feedforward vestibular control in unconstrained environment.NEW & NOTEWORTHY This study addresses the extent of conservation of trunk muscle control in humans. Results show that galvanic vestibular stimulation-evoked vestibular responses in trunk muscles remain strong in conditions where leg muscle responses are downmodulated (walking, standing, microgravity). This suggests a phylogenetically conserved blueprint of sensorimotor organization, with strongly hardwired vestibulospinal inputs to axial motoneurons and a higher degree of flexibility in the later emerging limb control system.

Descending Influences on Vestibulospinal and Vestibulosympathetic Reflexes

This review considers the integration of vestibular and other signals by the central nervous system pathways that participate in balance control and blood pressure regulation, with an emphasis on how this integration may modify posture-related responses in accordance with behavioral context. Two pathways convey vestibular signals to limb motoneurons: the lateral vestibulospinal tract and reticulospinal projections. Both pathways receive direct inputs from the cerebral cortex and cerebellum, and also integrate vestibular, spinal, and other inputs. Decerebration in animals or strokes that interrupt corticobulbar projections in humans alter the gain of vestibulospinal reflexes and the responses of vestibular nucleus neurons to particular stimuli. This evidence shows that supratentorial regions modify the activity of the vestibular system, but the functional importance of descending influences on vestibulospinal reflexes acting on the limbs is currently unknown. It is often overlooked that the vestibulospinal and reticulospinal systems mainly terminate on spinal interneurons, and not directly on motoneurons, yet little is known about the transformation of vestibular signals that occurs in the spinal cord. Unexpected changes in body position that elicit vestibulospinal reflexes can also produce vestibulosympathetic responses that serve to maintain stable blood pressure. Vestibulosympathetic reflexes are mediated, at least in part, through a specialized group of reticulospinal neurons in the rostral ventrolateral medulla that project to sympathetic preganglionic neurons in the spinal cord. However, other pathways may also contribute to these responses, including those that dually participate in motor control and regulation of sympathetic nervous system activity. Vestibulosympathetic reflexes differ in conscious and decerebrate animals, indicating that supratentorial regions alter these responses. However, as with vestibular reflexes acting on the limbs, little is known about the physiological significance of descending control of vestibulosympathetic pathways.

Reduced performance in balance, walking and turning tasks is associated with increased neck tone in Parkinson's disease

Rigidity or hypertonicity is a cardinal symptom of Parkinson's disease (PD). We hypothesized that hypertonicity of the body axis affects functional performance of tasks involving balance, walking and turning. The magnitude of axial postural tone in the neck, trunk and hip segments of 15 subjects with PD (both ON and OFF levodopa) and 15 control subjects was quantified during unsupported standing in an axial twisting device in our laboratory as resistance to torsional rotation. Subjects also performed six functional tests (walking in a figure of eight [Figure of Eight], Timed Up and Go, Berg Balance Scale, supine rolling task [rollover], Functional Reach, and standing 360-deg turn-in-place) in the ON and OFF state. Results showed that PD subjects had increased tone throughout the axis compared to control subjects (p=0.008) and that this increase was most prominent in the neck. In PD subjects, axial tone was related to functional performance, but most strongly for tone at the neck and accounted for an especially large portion of the variability in the performance of the Figure of Eight test (r(OFF)=0.68 and r(ON)=0.74, p<0.05) and the Rollover test (r(OFF)=0.67 and r(ON)=0.55, p<0.05). Our results suggest that neck tone plays a significant role in functional mobility and that abnormally high postural tone may be an important contributor to balance and mobility disorders in individuals with PD.

Stability of output effects from motor cortex to forelimb muscles in primates

Stimulus-triggered averaging (StTA) of electromyographic (EMG) activity is a form of intracortical microstimulation that enables documentation in awake animals of the sign, magnitude, latency, and distribution of output effects from cortical and brainstem areas to motoneurons of different muscles. In this study, we show that the properties of effects in StTAs are stable and mostly independent of task conditions. StTAs of EMG activity from 24 forelimb muscles were collected from two male rhesus monkeys while they performed three tasks: (1) an isometric step tracking wrist task, (2) an isometric whole-arm push-pull task, and (3) a reach-to-grasp task. Layer V sites in primary motor cortex were identified and microstimuli were applied (15 muA) at a low rate (15 Hz). Our results show that the sign of effects (facilitation or suppression) in StTAs of EMG activity are remarkably stable in the presence of joint angle position changes (96% stable), whole-arm posture changes (97% stable), and across fundamentally different types of tasks such as arm push-pull versus reach-to-grasp (81% stable). Furthermore, comparing effects across different phases of a task also yielded remarkable stability (range, 84-96%). At different shoulder, elbow, and wrist angles, the magnitudes of effects in individual muscles were highly correlated. Our results demonstrate that M1 output effects obtained with StTA of EMG activity are highly stable across widely varying joint angles and motor tasks. This study further validates the use of StTA for mapping and other studies of cortical motor output.

Modeling Gravity-Dependent Plasticity of the Angular Vestibuloocular Reflex With a Physiologically Based Neural Network

A neural network model was developed to explain the gravity-dependent properties of gain adaptation of the angular vestibuloocular reflex (aVOR). Gain changes are maximal at the head orientation where the gain is adapted and decrease as the head is tilted away from that position and can be described by the sum of gravity-independent and gravity-dependent components. The adaptation process was modeled by modifying the weights and bias values of a three-dimensional physiologically based neural network of canal–otolith-convergent neurons that drive the aVOR. Model parameters were trained using experimental vertical aVOR gain values. The learning rule aimed to reduce the error between eye velocities obtained from experimental gain values and model output in the position of adaptation. Although the model was trained only at specific head positions, the model predicted the experimental data at all head positions in three dimensions. Altering the relative learning rates of the weights and bias improved the model-data fits. Model predictions in three dimensions compared favorably with those of a double-sinusoid function, which is a fit that minimized the mean square error at every head position and served as the standard by which we compared the model predictions. The model supports the hypothesis that gravity-dependent adaptation of the aVOR is realized in three dimensions by a direct otolith input to canal–otolith neurons, whose canal sensitivities are adapted by the visual-vestibular mismatch. The adaptation is tuned by how the weights from otolith input to the canal–otolith-convergent neurons are adapted for a given head orientation.

Spatial properties of central vestibular neurons

We studied the spatial characteristics of 45 vestibular-only (VO) and 12 vestibular-plus-saccade (VPS) neurons in two cynomolgus monkeys using angular rotation and static tilt. The purpose was to determine the contribution of canal and otolith-related inputs to central vestibular neurons whose activity is associated with the central velocity storage integrator. Lateral canal-related neurons responded maximally during vertical axis rotation when the head was tilted 25 +/- 6 and 22 +/- 3 degrees forward relative to the axis of rotation in the two animals, and vertical canal-related neurons responded maximally with the head tilted back 63+/- 5 and 57 +/- 7 degrees . The origin of the vertical canal-related input was verified by rotation about a spatial horizontal axis. Thirty-one percent of cells received input in a single canal plane. Sixty-seven percent of canal-related cells received otolith input, 31% of vertical canal neurons had lateral canal input, and 43% of lateral canal neurons had vertical canal input. Twenty percent of neurons had convergent input from the lateral canals, the vertical canals, and the otolith organs. Some VO and VPS cells had spatial-temporal convergent (STC) properties; more of these cells had STC properties at lower frequencies of rotation. Thus VO and VPS neurons associated with velocity storage receive a broad range of convergent inputs from each portion of the vestibular labyrinth. This convergence could provide the basis for gravity-dependent eye velocity orientation induced through velocity storage.

Electromyographic activity of dorsal neck muscles in squirrel monkeys during rotations in an upright or upside down posture

Electromyographic (EMG) activity was recorded from occipitoscapularis, semispinalis, and splenius neck muscles in five alert squirrel monkeys during 0.25-Hz rotations about horizontal axes oriented at 22.5 degrees intervals, including pitch, roll, and intermediate axes. The animals were oriented in either upright or upside down posture. In the upright posture, all monkeys exhibited compensatory EMG activity with maximal activation during rotations about axes between pitch in the pitch forward direction and contralaterally directed roll. Response timing varied across animals with EMG peaks ranging from near pitch forward head velocity to near pitch forward head position. When the head was upside down, response dynamics and directionality were altered to varying degrees in different monkeys. The greatest change in response to head inversion was seen in the monkey that had response phases closest to head position, the least in the animal with phases closest to head velocity. The monkey with EMG response peaks closest to position phase showed nearly 180 degrees inversion of responses when the head was upside down, suggesting that in this monkey a righting reflex mediated by utricular signals was activated in the upside down posture. The monkey with EMG response peaks closest to velocity phase may have lacked a righting response and exhibited only a canal-mediated compensatory vestibulocervical reflex in both upright and upside down postures. The results suggest that reflex contraction of neck muscles in response to passive head rotation includes an interplay of compensatory and righting responses that varies from animal to animal.

An Integrative Neural Network for Detecting Inertial Motion and Head Orientation

The ability to navigate in the world and execute appropriate behavioral responses depends critically on the contribution of the vestibular system to the detection of motion and spatial orientation. A complicating factor is that otolith afferents equivalently encode inertial and gravitational accelerations. Recent studies have demonstrated that the brain can resolve this sensory ambiguity by combining signals from both the otoliths and semicircular canal sensors, although it remains unknown how the brain integrates these sensory contributions to perform the nonlinear vector computations required to accurately detect head movement in space. Here, we illustrate how a physiologically relevant, nonlinear integrative neural network could be used to perform the required computations for inertial motion detection along the interaural head axis. The proposed model not only can simulate recent behavioral observations, including a translational vestibuloocular reflex driven by the semicircular canals, but also accounts for several previously unexplained characteristics of central neural responses such as complex otolith–canal convergence patterns and the prevalence of dynamically processed otolith signals. A key model prediction, implied by the required computations for tilt–translation discrimination, is a coordinate transformation of canal signals from a head-fixed to a spatial reference frame. As a result, cell responses may reflect canal signal contributions that cannot be easily detected or distinguished from otolith signals. New experimental protocols are proposed to characterize these cells and identify their contributions to spatial motion estimation. The proposed theoretical framework makes an essential first link between the computations for inertial acceleration detection derived from the physical laws of motion and the neural response properties predicted in a physiologically realistic network implementation.

Current approaches and future directions to understanding control of head movement

This chapter reviews four key issues that must be addressed to advance our knowledge of control of head movement by the central nervous system (CNS). (1) Researchers must consider how the CNS utilizes the multiple muscle patterns that can produce the same head movement in carrying out tasks in an optimal way. (2) More attention must be paid to the dynamics of neck muscle activation that are required to implement head movements and show they are produced by CNS circuits. (3) Research is required to determine how the multiple pathways that impinge upon neck motor centers are utilized in a variety of tasks including eye-head gaze shifts, smooth head tracking, head stabilization and manipulating objects with the head. These pathways include corticospinal, vestibulospinal, reticulospinal (three subdivisions), fastigiospinal, tectospinal and interstitiospinal tracts. (4) Further analysis is needed to understand how vestibular signals are modulated during each of the above-mentioned tasks. This ambitious agenda is justified by the fact that the head-neck motor system is an ideal model for understanding issues of complex motor control.

Vestibular convergence patterns in vestibular nuclei neurons of alert primates

Sensory signal convergence is a fundamental and important aspect of brain function. Such convergence may often involve complex multidimensional interactions as those proposed for the processing of otolith and semicircular canal (SCC) information for the detection of translational head movements and the effective discrimination from physically congruent gravity signals. In the present study, we have examined the responses of primate rostral vestibular nuclei (VN) neurons that do not exhibit any eye movement-related activity using 0.5-Hz translational and three-dimensional (3D) rotational motion. Three distinct neural populations were identified. Approximately one-fourth of the cells exclusively encoded rotational movements (canal-only neurons) and were unresponsive to translation. The canal-only central neurons encoded head rotation in SCC coordinates, exhibited little orthogonal canal convergence, and were characterized with significantly higher sensitivities to rotation as compared to primary SCC afferents. Another fourth of the neurons modulated their firing rates during translation (otolith-only cells). During rotations, these neurons only responded when the axis of rotation was earth-horizontal and the head was changing orientation relative to gravity. The remaining one-half of VN neurons were sensitive to both rotations and translations (otolith + canal neurons). Unlike primary otolith afferents, however, central neurons often exhibited significant spatiotemporal (noncosine) tuning properties and a wide variety of response dynamics to translation. To characterize the pattern of SCC inputs to otolith + canal neurons, their rotational maximum sensitivity vectors were computed using exclusively responses during earth-vertical axis rotations (EVA). Maximum sensitivity vectors were distributed throughout the 3D space, suggesting strong convergence from multiple SCCs. These neurons were also tested with earth-horizontal axis rotations (EHA), which would activate both vertical canals and otolith organs. However, the recorded responses could not be predicted from a linear combination of EVA rotational and translational responses. In contrast, one-third of the neurons responded similarly during EVA and EHA rotations, although a significant response modulation was present during translation. Thus this subpopulation of otolith + canal cells, which included neurons with either high- or low-pass dynamics to translation, appear to selectively ignore the component of otolith-selective activation that is due to changes in the orientation of the head relative to gravity. Thus contrary to primary otolith afferents and otolith-only central neurons that respond equivalently to tilts relative to gravity and translational movements, approximately one-third of the otolith + canal cells seem to encode a true estimate of the translational component of the imposed passive head and body movement.

Dynamic and kinematic strategies for head movement control

This paper describes our analysis of the complex head-neck system using a combination of experimental and modeling approaches. Dynamical analysis of head movements and EMG activation elicited by perturbation of trunk position has examined functional contributions of biomechanically and neurally generated forces in lumped systems with greatly simplified kinematics. This has revealed that visual and voluntary control of neck muscles and the dynamic and static vestibulocollic and cervicocollic reflexes preferentially govern head-neck system state in different frequency domains. It also documents redundant control, which allows the system to compensate for lesions and creates a potential for substantial variability within and between subjects. Kinematic studies have indicated the existence of reciprocal and co-contraction strategies for voluntary force generation, of a vestibulocollic strategy for stabilizing the head during body perturbations and of at least two strategies for voluntary head tracking. Each strategy appears to be executed by a specific muscle synergy that is presumably optimized to efficiently meet the demands of the task.

Spatiotemporal processing of linear acceleration: primary afferent and central vestibular neuron responses

Spatiotemporal convergence and two-dimensional (2-D) neural tuning have been proposed as a major neural mechanism in the signal processing of linear acceleration. To examine this hypothesis, we studied the firing properties of primary otolith afferents and central otolith neurons that respond exclusively to horizontal linear accelerations of the head (0.16-10 Hz) in alert rhesus monkeys. Unlike primary afferents, the majority of central otolith neurons exhibited 2-D spatial tuning to linear acceleration. As a result, central otolith dynamics vary as a function of movement direction. During movement along the maximum sensitivity direction, the dynamics of all central otolith neurons differed significantly from those observed for the primary afferent population. Specifically at low frequencies (</=0.5 Hz), the firing rate of the majority of central otolith neurons peaked in phase with linear velocity, in contrast to primary afferents that peaked in phase with linear acceleration. At least three different groups of central response dynamics were described according to the properties observed for motion along the maximum sensitivity direction. "High-pass" neurons exhibited increasing gains and phase values as a function of frequency. "Flat" neurons were characterized by relatively flat gains and constant phase lags (approximately 20-55 degrees ). A few neurons ("low-pass") were characterized by decreasing gain and phase as a function of frequency. The response dynamics of central otolith neurons suggest that the approximately 90 degrees phase lags observed at low frequencies are not the result of a neural integration but rather the effect of nonminimum phase behavior, which could arise at least partly through spatiotemporal convergence. Neither afferent nor central otolith neurons discriminated between gravitational and inertial components of linear acceleration. Thus response sensitivity was indistinguishable during 0.5-Hz pitch oscillations and fore-aft movements. The fact that otolith-only central neurons with "high-pass" filter properties exhibit semicircular canal-like dynamics during head tilts might have important consequences for the conclusions of previous studies of sensory convergence and sensorimotor transformations in central vestibular neurons.

Spatial alignment of rotational and static tilt responses of vestibulospinal neurons in the cat

The responses of vestibulospinal neurons to 0.5-Hz, whole-body rotations in three-dimensional space and static tilts of whole-body position were studied in decerebrate and alert cats. The neurons' spatial properties for earth-vertical rotations were characterized by maximum and minimum sensitivity vectors (R(max) and R(min)) in the cat's horizontal plane. The orientation of a neuron's R(max) was not consistently related to the orientation of its maximum sensitivity vector for static tilts (T(max)). The angular difference between R(max) and T(max) was widely distributed between 0 degrees and 150 degrees, and R(max) and T(max) were aligned (i.e., within 45 degrees of each other) for only 44% (14/32) of the neurons. The alignment of R(max) and T(max) was not correlated with the neuron's sensitivity to earth-horizontal rotations, or to the orientation of R(max) in the horizontal plane. In addition, the extent to which a neuron exhibited spatiotemporal convergent (STC) behavior in response to vertical rotations was independent of the angular difference between R(max) and T(max). This suggests that the high incidence of STC responses in our sample (56%) reflects not only canal-otolith convergence, but also the presence of static and dynamic otolith inputs with misaligned directionality. The responses of vestibulospinal neurons reflect a complex combination of static and dynamic vestibular inputs that may be required by postural reflexes that vary depending on head, trunk, and limb orientation, or on the frequency of stimulation.

Relation between axon morphology in C1 spinal cord and spatial properties of medial vestibulospinal tract neurons in the cat

Twenty-one secondary medial vestibulospinal tract neurons were recorded intraaxonally in the ventromedial funiculi of the C1 spinal cord in decerebrate, paralyzed cats. Antidromic stimulation in C6 and the oculomotor nucleus identified the projection pattern of each neuron. Responses to sinusoidal, whole-body rotations in many planes in three-dimensional space were characterized before injection of horseradish peroxidase or Neurobiotin. The spatial response properties of 19 neurons were described by a maximum activation direction vector (MAD), which defines the axis and direction of rotation that maximally excites the neuron. The other two neurons had spatio-temporal convergent behavior and no MAD was calculated. Collateral morphologies were reconstructed from serial frontal sections to reveal terminal fields in the C1 gray matter. Axons gave off multiple collaterals that terminated ipsilaterally to the stem axon. Collaterals of individual axons rarely overlapped longitudinally but projected to similar regions in the ventral horn when viewed in transverse sections. The number of primary collaterals in C1 was different for vestibulo-collic, vestibulo-oculo-collic, and C6-projecting neurons: on average one every 1.34, 1.72, and 4.25 mm, respectively. The heaviest arborization and most terminal boutons were seen in the ventral horn, in laminae VIII and IX. Varicosities on terminal branches in lamina IX were observed adjacent to large cell bodies-putative neck motoneurons-in counterstained tissue. Some collaterals had branches that extended dorsally to lamina VII. Neurons with different spatial properties had terminal fields in different regions of the ventral horn. Axons with type I responses and MADs near those of a semicircular canal pair had widely distributed collateral branches and numerous terminations in the dorsomedial, ventromedial, and spinal accessory nuclei and in lamina VIII. Axons with type I responses that suggested convergent canal pair input, with type II responses, and with spatio-temporal convergent behavior had smaller terminal fields. Some neurons with these more complex spatial properties projected to the dorsomedial and spinal accessory but not to the ventromedial nuclei. Others had focused projections to dorsolateral regions of the ventral horn with few branches in the motor nuclei.