Neck muscle synergies during stimulation and inactivation of the interstitial nucleus of Cajal (INC)

The mesencephalic locomotor region recruits V2a reticulospinal neurons to drive forward locomotion in larval zebrafish

The mesencephalic locomotor region (MLR) is a brain stem area whose stimulation triggers graded forward locomotion. How MLR neurons recruit downstream vsx2+ (V2a) reticulospinal neurons (RSNs) is poorly understood. Here, to overcome this challenge, we uncovered the locus of MLR in transparent larval zebrafish and show that the MLR locus is distinct from the nucleus of the medial longitudinal fasciculus. MLR stimulations reliably elicit forward locomotion of controlled duration and frequency. MLR neurons recruit V2a RSNs via projections onto somata in pontine and retropontine areas, and onto dendrites in the medulla. High-speed volumetric imaging of neuronal activity reveals that strongly MLR-coupled RSNs are active for steering or forward swimming, whereas weakly MLR-coupled medullary RSNs encode the duration and frequency of the forward component. Our study demonstrates how MLR neurons recruit specific V2a RSNs to control the kinematics of forward locomotion and suggests conservation of the motor functions of V2a RSNs across vertebrates.

Head-nodding: a driving force for the circulation of cerebrospinal fluid

The myodural bridge (MDB) is a dense connective tissue bridge connecting the suboccipital muscles to the spinal dura mater, and it has been proven to be a normal common existing structure in humans and mammals. Some scholars believe that the suboccipital muscles can serve as a dynamic cerebrospinal fluid (CSF) pump via the MDB, and they found head rotations promote the CSF flow in human body, which provided evidence for this hypothesis. Head movement is a complex motion, but the effects of other forms of head movement on CSF circulation are less known. The present study explored the effects of head-nodding on CSF circulation. The CSF flow of 60 healthy volunteers was analyzed via cine phase-contrast magnetic resonance imaging at the level of the occipitocervical junction before and after one-minute-head-nodding period. Furthermore, the CSF pressures of 100 volunteers were measured via lumbar puncture before and after 5 times head-nodding during their anesthetizing for surgical preparation. As a result, it was found that the maximum and average CSF flow rates at the level of the upper border of atlas during ventricular diastole were significantly decreased from 1.965 ± 0.531 to 1.839 ± 0.460 ml/s and from 0.702 ± 0.253 to 0.606 ± 0.228 ml/s respectively. In the meantime, the changes in the ratio of cranial and caudal orientation of the net flow volume were found differed significantly after the one-minute-head-nodding period (p = 0.017). And on the other hand, the CSF pressures at the L3–L4 level were markedly increased 116.03 ± 26.13 to 124.64 ± 26.18 mmH₂O. In conclusion, the head-nodding has obvious effects on CSF circulation and head movement is one of the important drivers of cerebrospinal fluid circulation. We propose that the suboccipital muscles, participating in various head movements, might pull the dura sac via the myodural bridge, and thus, head movement provides power for the CSF circulation.

Implications of asymmetric neural activity patterns in the basal ganglia outflow in the integrative neural network model for cervical dystonia

Cervical dystonia (CD) is characterized by abnormal twisting and turning of the head with associated head oscillations. It is the most common form of dystonia, which is a third most common movement disorder. Despite frequent occurrence there is paucity in adequate therapy, much of which is attributed to its uncertain pathophysiology. Recently we proposed a unifying network model highlighting the role of head neural integrator (hNI) for the pathophysiology of CD. According to our hypothesis the CD is due to abnormal output of hNI; the latter itself is not affected but its dysfunction is secondary to abnormal feedback. We hypothesized that asymmetry in the feedback to hNI is associated with severity in CD; the feedback asymmetry is greater in CD with lateralized head postures, such as turning of head in yaw plane (torticollis) or roll plane (laterocollis). The hypothesis also specifies that feedback to hNI-cerebellum, proprioception, and basal ganglia outflow (pallidus) are connected in a network; thus asymmetry is distributed through the feedback network. In 15 CD patients undergoing deep brain stimulation (DBS) surgery, with their informed consent, we used the opportunity to collect single unit neural responses and local field potential from the globus pallidus to measure whether feedback to hNI is asymmetric. Using machine learning algorithms developed to analyze single unit data, we found: (1) globus pallidus interna (GPi) firing rate, discharge pattern and gamma oscillation were asymmetric in patients with robust torticollis; (2) there was no asymmetry in these parameters in retrocollis; and (3) in those patients with oppositely directed laterocollis and torticollis. Firing rate was higher in GPi cells ipsilateral to the direction of head rotation; the asymmetry was more pronounced in tonic cells compared to burst neurons. In addition to confirming that CD is associated with an asymmetric pallidal activity, our data showed that neuronal asymmetry correlated with the degree of involuntary head turning. We propose that asymmetric pallidal activity results in asymmetric feedback to hNI causing its dysfunction.

The role of pallidum in the neural integrator model of cervical dystonia

Dystonia is the third most common movement disorder affecting three million people worldwide. Cervical dystonia is the most common form of dystonia. Despite common prevalence the pathophysiology of cervical dystonia is unclear. Traditional view is that basal ganglia is involved in pathophysiology of cervical dystonia, while contemporary theories suggested the role of cerebellum and proprioception in the pathophysiology of cervical dystonia. It was recently proposed that the cervical dystonia is due to malfunctioning of the head neural integrator - the neuron network that normally converts head velocity to position. Most importantly the neural integrator model was inclusive of traditional proposal emphasizing the role of basal ganglia while also accommodating the contemporary view suggesting the involvement of cerebellum and proprioception. It was hypothesized that the head neural integrator malfunction is the result of impairment in cerebellar, basal ganglia, or proprioceptive feedback that converge onto the integrator. The concept of converging input from the basal ganglia, cerebellum, and proprioception to the network participating in head neural integrator explains that abnormality originating anywhere in the network can lead to the identical motor deficits - drifts followed by rapid corrective movements - a signature of neural integrator dysfunction. We tested this hypothesis in an experiment examining simultaneously recorded globus pallidal single-unit activity, synchronized neural activity (local field potential), and electromyography (EMG) measured from the neck muscles during the standard-of-care deep brain stimulation surgery in 12 cervical dystonia patients (24 hemispheres). Physiological data were collected spontaneously or during voluntary shoulder shrug activating the contralateral trapezius muscle. The activity of pallidal neurons during shoulder shrug exponentially decayed with time constants that were comparable to one measured from the pretectal neural integrator and the trapezius electromyography. These results show that evidence of abnormal neural integration is also seen in globus pallidum, and that latter is connected with the neural integrator. Pretectal single neuron responses consistently preceded the muscle activity; while the globus pallidum internus response always lagged behind the muscle activity. Globus pallidum externa had equal proportion of lag and lead neurons. These results suggest globus pallidum receive feedback from the muscles or the efference copy from the integrator or the other source of the feedback. There was bi-hemispheric asymmetry in the pallidal single-unit activity and local field potentials. The asymmetry correlated with degree of lateral head turning in cervical dystonia patients. These results suggest that bihemispheric asymmetry in the feedback leads to asymmetric dysfunction in the neural integrator causing head turning.

The Anatomical Basis for Dystonia: The Motor Network Model

Background

The dystonias include a clinically and etiologically very diverse group of disorders. There are both degenerative and non-degenerative subtypes resulting from genetic or acquired causes. Traditionally, all dystonias have been viewed as disorders of the basal ganglia. However, there has been increasing appreciation for involvement of other brain regions including the cerebellum, thalamus, midbrain, and cortex. Much of the early evidence for these other brain regions has come from studies of animals, but multiple recent studies have been done with humans, in an effort to confirm or refute involvement of these other regions. The purpose of this article is to review the new evidence from animals and humans regarding the motor network model, and to address the issues important to translational neuroscience.

Methods

The English literature was reviewed for articles relating to the neuroanatomical basis for various types of dystonia in both animals and humans.

Results

There is evidence from both animals and humans that multiple brain regions play an important role in various types of dystonia. The most direct evidence for specific brain regions comes from animal studies using pharmacological, lesion, or genetic methods. In these studies, experimental manipulations of specific brain regions provide direct evidence for involvement of the basal ganglia, cerebellum, thalamus and other regions. Additional evidence also comes from human studies using neuropathological, neuroimaging, non-invasive brain stimulation, and surgical interventions. In these studies, the evidence is less conclusive, because discriminating the regions that cause dystonia from those that reflect secondary responses to abnormal movements is more challenging.

Discussion

Overall, the evidence from both animals and humans suggests that different regions may play important roles in different subtypes of dystonia. The evidence so far provides strong support for the motor network model. There are obvious challenges, but also advantages, of attempting to translate knowledge gained from animals into a more complete understanding of human dystonia and novel therapeutic strategies.

Physiology of midbrain head movement neurons in cervical dystonia

BACKGROUND

Early theories for cervical dystonia, as promoted by Hassler, emphasized the role of the midbrain interstitial nucleus of Cajal. Focus then shifted to the basal ganglia, and it was further supported with the success of deep brain stimulation. Contemporary theories suggested the role of the cerebellum, but even more recent hypotheses renewed interest in the midbrain. Although the pretectum was visited on several occasions, we still do not know about the physiology of midbrain neurons in cervical dystonia.

METHODS

We analyzed the unique database of pretectal neurons collected in the 1970s and 1980s during historic stereotactic surgeries aimed to treat cervical dystonia. This database is valuable because such recordings could otherwise never be obtained from humans.

RESULTS

We found the following 3 types of eye or neck movement sensitivity: eye-only neurons responded to pure vertical eye movements, neck-only neurons were sensitive to pure neck movements, and the combined eye-neck neurons responded to eye and neck movements. There were the 2 neuronal subtypes: burst-tonic and tonic. The eye-neck or eye-only neurons sustained their activity during eccentric gaze holding. In contrast, the response of neck-only and eye-neck neurons exponentially decayed during neck movements.

CONCLUSIONS

Modern quantitative analysis of a historic database of midbrain single units from patients with cervical dystonia might support novel hypotheses for normal and abnormal head movements. This data, collected almost 4 decades ago, must be carefully viewed, especially because it was acquired using a less sophisticated technology available at that time and the aim was not to address specific hypothesis, but to make an accurate lesion providing optimal relief from dystonia. © 2017 International Parkinson and Movement Disorder Society.

Cervical dystonia: a neural integrator disorder

Ocular motor neural integrators ensure that eyes are held steady in straight-ahead and eccentric positions of gaze. Abnormal function of the ocular motor neural integrator leads to centripetal drifts of the eyes with consequent gaze-evoked nystagmus. In 2002 a neural integrator, analogous to that in the ocular motor system, was proposed for the control of head movements. Recently, a counterpart of gaze-evoked eye nystagmus was identified for head movements; in which the head could not be held steady in eccentric positions on the trunk. These findings lead to a novel pathophysiological explanation in cervical dystonia, which proposed that the abnormalities of head movements stem from a malfunctioning head neural integrator, either intrinsically or as a result of impaired cerebellar, basal ganglia, or peripheral feedback. Here we briefly recapitulate the history of the neural integrator for eye movements, then further develop the idea of a neural integrator for head movements, and finally discuss its putative role in cervical dystonia. We hypothesize that changing the activity in an impaired head neural integrator, by modulating feedback, could treat dystonia.

Short-duration stimulation of the supplementary eye fields perturbs anti-saccade performance while potentiating contralateral head orienting

Many forms of brain stimulation utilize the notion of state dependency, whereby greater influences are observed when a given area is more engaged at the time of stimulation. Here, by delivering intracortical microstimulation (ICMS) to the supplementary eye fields (SEF) of monkeys performing interleaved pro- and anti-saccades, we show a surprising diversity of state-dependent effects of ICMS-SEF. Short-duration ICMS-SEF passed around cue presentation selectively disrupted anti-saccades by increasing reaction times and error rates bilaterally, and also recruited neck muscles, favoring contralateral head turning to a greater degree on anti-saccade trials. These results are consistent with the functional relevance of the SEF for anti-saccades. The multiplicity of stimulation-evoked effects, with ICMS-SEF simultaneously disrupting anti-saccade performance and facilitating contralateral head orienting, probably reflects both the diversity of cortical and subcortical targets of SEF projections, and the response of this oculomotor network to stimulation. We speculate that the bilateral disruption of anti-saccades arises via feedback loops that may include the thalamus, whereas neck muscle recruitment arises via feedforward polysynaptic pathways to the motor periphery. Consideration of both sets of results reveals a more complete picture of the highly complex and multiphasic response to ICMS-SEF that can play out differently in different effector systems.

Keeping your head on target

The mechanisms by which the human brain controls eye movements are reasonably well understood, but those for the head less so. Here, we show that the mechanisms for keeping the head aimed at a stationary target follow strategies similar to those for holding the eyes steady on stationary targets. Specifically, we applied the neural integrator hypothesis that originally was developed for holding the eyes still in eccentric gaze positions to describe how the head is held still when turned toward an eccentric target. We found that normal humans make head movements consistent with the neural integrator hypothesis, except that additional sensory feedback is needed, from proprioceptors in the neck, to keep the head on target. We also show that the complicated patterns of head movements in patients with cervical dystonia can be predicted by deficits in a neural integrator for head motor control. These results support ideas originally developed from animal studies that suggest fundamental similarities between oculomotor and cephalomotor control, as well as a conceptual framework for cervical dystonia that departs considerably from current clinical views.

Relationships between neck muscle electromyography and three-dimensional head kinematics during centrally induced torsional head perturbations

The relationship between neck muscle electromyography (EMG) and torsional head rotation (about the nasooccipital axis) is difficult to assess during normal gaze behaviors with the head upright. Here, we induced acute head tilts similar to cervical dystonia (torticollis) in two monkeys by electrically stimulating 20 interstitial nucleus of Cajal (INC) sites or inactivating 19 INC sites by injection of muscimol. Animals engaged in a simple gaze fixation task while we recorded three-dimensional head kinematics and intramuscular EMG from six bilateral neck muscle pairs. We used a cross-validation-based stepwise regression to quantitatively examine the relationships between neck EMG and torsional head kinematics under three conditions: 1) unilateral INC stimulation (where the head rotated torsionally toward the side of stimulation); 2) corrective poststimulation movements (where the head returned toward upright); and 3) unilateral INC inactivation (where the head tilted toward the opposite side of inactivation). Our cross-validated results of corrective movements were slightly better than those obtained during unperturbed gaze movements and showed many more torsional terms, mostly related to velocity, although some orientation and acceleration terms were retained. In addition, several simplifying principles were identified. First, bilateral muscle pairs showed similar, but opposite EMG-torsional coupling terms, i.e., a change in torsional kinematics was associated with increased muscle activity on one side and decreased activity on the other side. s, whenever torsional terms were retained in a given muscle, they were independent of the inputs we tested, i.e., INC stimulation vs. corrective motion vs. INC inactivation, and left vs. right INC data. These findings suggest that, despite the complexity of the head-neck system, the brain can use a single, bilaterally coupled inverse model for torsional head control that is valid across different behaviors and movement directions. Combined with our previous data, these new data provide the terms for a more complete three-dimensional model of EMG: head rotation coupling for the muscles and gaze behaviors that we recorded.

Recruitment of a contralateral head turning synergy by stimulation of monkey supplementary eye fields

The supplementary eye fields (SEF) are thought to enable higher-level aspects of oculomotor control. The goal of the present experiment was to learn more about the SEF's role in orienting, specifically by examining neck muscle recruitment evoked by stimulation of the SEF. Neck muscle activity was recorded from multiple muscles in two monkeys during SEF stimulation (100 μA, 150–300 ms, 300 Hz, with the head restrained or unrestrained) delivered 200 ms into a gap period, before a visually guided saccade initiated from a central position (doing so avoids confounds between initial position and prestimulation neck muscle activity). SEF stimulation occasionally evoked overt gaze shifts and/or head movements but almost always evoked a response that invariably consisted of a contralateral head turning synergy by increasing activity on contralateral turning muscles and decreasing activity on ipsilateral turning muscles (when background activity was present). Neck muscle responses began well in advance of evoked gaze shifts (∼30 ms after stimulation onset, leading gaze shifts by ∼40–70 ms on average), started earlier and attained a larger magnitude when accompanied by progressively larger gaze shifts, and persisted on trials without overt gaze shifts. The patterns of evoked neck muscle responses resembled those evoked by frontal eye field (FEF) stimulation, except that response latencies from the SEF were ∼10 ms longer. This basic description of the cephalomotor command evoked by SEF stimulation suggests that this structure, while further removed from the motor periphery than the FEF, accesses premotor orienting circuits in the brain stem and spinal cord in a similar manner.

Cross-validated models of the relationships between neck muscle electromyography and three-dimensional head kinematics during gaze behavior

The object of this study was to model the relationship between neck electromyography (EMG) and three-dimensional (3-D) head kinematics during gaze behavior. In two monkeys, we recorded 3-D gaze, head orientation, and bilateral EMG activity in the sternocleidomastoid, splenius capitis, complexus, biventer cervicis, rectus capitis posterior major, and occipital capitis inferior muscles. Head-unrestrained animals fixated and made gaze saccades between targets within a 60° × 60° grid. We performed a stepwise regression in which polynomial model terms were retained/rejected based on their tendency to increase/decrease a cross-validation-based measure of model generalizability. This revealed several results that could not have been predicted from knowledge of musculoskeletal anatomy. During head holding, EMG activity in most muscles was related to horizontal head orientation, whereas fewer muscles correlated to vertical head orientation and none to small random variations in head torsion. A fourth-order polynomial model, with horizontal head orientation as the only independent variable, generalized nearly as well as higher order models. For head movements, we added time-varying linear and nonlinear perturbations in velocity and acceleration to the previously derived static (head holding) models. The static models still explained most of the EMG variance, but the additional motion terms, which included horizontal, vertical, and torsional contributions, significantly improved the results. Several coordinate systems were used for both static and dynamic analyses, with Fick coordinates showing a marginal (nonsignificant) advantage. Thus, during gaze fixations, recruitment within the neck muscles from which we recorded contributed primarily to position-dependent horizontal orientation terms in our data set, with more complex multidimensional contributions emerging during the head movements that accompany gaze shifts. These are crucial components of the late neuromuscular transformations in a complete model of 3-D head-neck system and should help constrain the study of premotor signals for head control during gaze behaviors.

Electrical stimulation of the frontal eye fields in the head-free macaque evokes kinematically normal 3D gaze shifts

The frontal eye field (FEF) is a region of the primate prefrontal cortex that is central to eye-movement generation and target selection. It has been shown that neurons in this area encode commands for saccadic eye movements. Furthermore, it has been suggested that the FEF may be involved in the generation of gaze commands for the eye and the head. To test this suggestion, we systematically stimulated (with pulses of 300 Hz frequency, 200 ms duration, 30-100 μA intensity) the FEF of two macaques, with the head unrestrained, while recording three-dimensional (3D) eye and head rotations. In a total of 95 sites, the stimulation consistently elicited gaze-orienting movements ranging in amplitude from 2 to 172°, directed contralateral to the stimulation site, and with variable vertical components. These movements were typically a combination of eye-in-head saccades and head-in-space movements. We then performed a comparison between the stimulation-evoked movements and gaze shifts voluntarily made by the animal. The kinematics of the stimulation-evoked movements (i.e., their spatiotemporal properties, their velocity-amplitude relationships, and the relative contributions of the eye and the head as a function of movement amplitude) were very similar to those of natural gaze shifts. Moreover, they obeyed the same 3D constraints as the natural gaze shifts (i.e., modified Listing's law for eye-in-head movements). As in natural gaze shifts, saccade and vestibuloocular reflex torsion during stimulation-evoked movements were coordinated so that at the end of the head movement the eye-in-head ended up in Listing's plane. In summary, movements evoked by stimulation of the FEF closely resembled those of naturally occurring eye-head gaze shifts. Thus we conclude that the FEF explicitly encodes gaze commands and that the kinematic aspects of eye-head coordination are likely specified by downstream mechanisms.