Functional synergies of neck muscles innervated by single medial vestibulospinal axons

Balance Control Mediated by Vestibular Circuits Directing Limb Extension or Antagonist Muscle Co-activation

Maintaining balance after an external perturbation requires modification of ongoing motor plans and the selection of contextually appropriate muscle activation patterns that respect body and limb position. We have used the vestibular system to generate sensory-evoked transitions in motor programming. In the face of a rapid balance perturbation, the lateral vestibular nucleus (LVN) generates exclusive extensor muscle activation and selective early extension of the hindlimb, followed by the co-activation of extensor and flexor muscle groups. The temporal separation in EMG response to balance perturbation reflects two distinct cell types within the LVN that generate different phases of this motor program. Initially, an LVN_extensor population directs an extension movement that reflects connections with extensor, but not flexor, motor neurons. A distinct LVN_{co-activation} population initiates muscle co-activation via the pontine reticular nucleus. Thus, distinct circuits within the LVN generate different elements of a motor program involved in the maintenance of balance.

Neuromusculoskeletal model that walks and runs across a speed range with a few motor control parameter changes based on the muscle synergy hypothesis

Humans walk and run, as well as change their gait speed, through the control of their complicated and redundant musculoskeletal system. These gaits exhibit different locomotor behaviors, such as a double-stance phase in walking and flight phase in running. The complex and redundant nature of the musculoskeletal system and the wide variation in locomotion characteristics lead us to imagine that the motor control strategies for these gaits, which remain unclear, are extremely complex and differ from one another. It has been previously proposed that muscle activations may be generated by linearly combining a small set of basic pulses produced by central pattern generators (muscle synergy hypothesis). This control scheme is simple and thought to be shared between walking and running at different speeds. Demonstrating that this control scheme can generate walking and running and change the speed is critical, as bipedal locomotion is dynamically challenging. Here, we provide such a demonstration by using a motor control model with 69 parameters developed based on the muscle synergy hypothesis. Specifically, we show that it produces both walking and running of a human musculoskeletal model by changing only seven key motor control parameters. Furthermore, we show that the model can walk and run at different speeds by changing only the same seven parameters based on the desired speed. These findings will improve our understanding of human motor control in locomotion and provide guiding principles for the control design of wearable exoskeletons and prostheses.

Adaptive hindlimb split-belt treadmill walking in rats by controlling basic muscle activation patterns via phase resetting

To investigate the adaptive locomotion mechanism in animals, a split-belt treadmill has been used, which has two parallel belts to produce left–right symmetric and asymmetric environments for walking. Spinal cats walking on the treadmill have suggested the contribution of the spinal cord and associated peripheral nervous system to the adaptive locomotion. Physiological studies have shown that phase resetting of locomotor commands involving a phase shift occurs depending on the types of sensory nerves and stimulation timing, and that muscle activation patterns during walking are represented by a linear combination of a few numbers of basic temporal patterns despite the complexity of the activation patterns. Our working hypothesis was that resetting the onset timings of basic temporal patterns based on the sensory information from the leg, especially extension of hip flexors, contributes to adaptive locomotion on the split-belt treadmill. Our hypothesis was examined by conducting forward dynamic simulations using a neuromusculoskeletal model of a rat walking on a split-belt treadmill with its hindlimbs and by comparing the simulated motions with the measured motions of rats.

Animal Models of Vestibular Evoked Myogenic Potentials: The Past, Present, and Future

Vestibular-evoked myogenic potentials (VEMPs) provide a simple and cost-effective means to assess the patency of vestibular reflexes. VEMP testing constitutes a core screening method in a clinical battery that probes vestibular function. The confidence one has in interpreting the results arising from VEMP testing is linked to a fundamental understanding of the underlying functional anatomy and physiology. In this review, we will summarize the key role that studies across a range of animal models have fulfilled in contributing to this understanding, covering key findings regarding the mechanisms of excitation in the sensory periphery, the processing of sensory information in central networks, and the distribution of reflexive output to the motor periphery. Although VEMPs are often touted for their simplicity, work in animals models have emphasized how vestibular reflexes operate within a broader behavioral and functional context, and as such vestibular reflexes are influenced by multisensory integration, governed by task demands, and follow principles of muscle recruitment. We will conclude with considerations of future questions, and the ways in which studies in current and emerging animal models can contribute to further use and refinement of this test for both basic and clinical research purposes.

Neuromusculoskeletal models based on the muscle synergy hypothesis for the investigation of adaptive motor control in locomotion via sensory-motor coordination

Humans and animals walk adaptively in diverse situations by skillfully manipulating their complicated and redundant musculoskeletal systems. From an analysis of measured electromyographic (EMG) data, it appears that despite complicated spatiotemporal properties, muscle activation patterns can be explained by a low dimensional spatiotemporal structure. More specifically, they can be accounted for by the combination of a small number of basic activation patterns. The basic patterns and distribution weights indicate temporal and spatial structures, respectively, and the weights show the muscle sets that are activated synchronously. In addition, various locomotor behaviors have similar low dimensional structures and major differences appear in the basic patterns. These analysis results suggest that neural systems use muscle group combinations to solve motor control redundancy problems (muscle synergy hypothesis) and manipulate those basic patterns to create various locomotor functions. However, it remains unclear how the neural system controls such muscle groups and basic patterns through neuromechanical interactions in order to achieve adaptive locomotor behavior. This paper reviews simulation studies that explored adaptive motor control in locomotion via sensory-motor coordination using neuromusculoskeletal models based on the muscle synergy hypothesis. Herein, the neural mechanism in motor control related to the muscle synergy for adaptive locomotion and a potential muscle synergy analysis method including neuromusculoskeletal modeling for motor impairments and rehabilitation are discussed.

Properties and axonal trajectories of posterior semicircular canal nerve-activated vestibulospinal neurons

We studied the axonal projections of vestibulospinal neurons activated from the posterior semicircular canal. The axonal projection level, axonal pathway, and location of the vestibulospinal neurons originating from the PC were investigated in seven decerebrated cats. Selective electrical stimulation was applied to the PC nerve, and extracellular recordings in the vestibular nuclei were performed. The properties of the PC nerve-activated vestibulospinal neurons were then studied. To estimate the neural pathway in the spinal cord, floating electrodes were placed at the ipsilateral (i) and contralateral (c) lateral vestibulospinal tract (LVST) and medial vestibulospinal tract (MVST) at the C1/C2 junction. To elucidate the projection level, floating electrodes were placed at i-LVST and MVST at the C3, T1, and L3 segments in the spinal cord. Collision block test between orthodromic inputs from the PC nerve and antidromic inputs from the spinal cord verified the existence of the vestibulospinal neurons in the vestibular nuclei. Most (44/47) of the PC nerve-activated vestibulospinal neurons responded to orthodromic stimulation to the PC nerve with a short (<1.4 ms) latency, indicating that they were second-order vestibulospinal neurons. The rest (3/47) responded with a longer (>/=1.4 ms) latency, indicating the existence of polysynaptic connections. In 36/47 PC nerve-activated vestibulospinal neurons, the axonal pathway was histologically verified to lie in the spinal cord. The axons of 17/36 vestibulospinal neurons projected to the i-LVST, whereas 14 neurons projected to the MVST, and 5 to the c-LVST. The spinal segment levels of projection of these neurons elucidated that the axons of most (15/17) of vestibulospinal neurons passing through the i-LVST reached the L3 segment level; none (0/14) of the neurons passing through the MVST extended to the L3 segment level; most (13/14) of them did not descend lower than the C3 segment level. In relation to the latency and the pathway, 33/36 PC nerve-activated vestibulospinal neurons were second-order neurons, whereas the remaining three were polysynaptic neurons. Of these, 33 second-order vestibulospinal neurons, 16 passed through the i-LVST, while 13 and 4 descended through the MVST and c-LVST, respectively. The remaining three were polysynaptic neurons. Histological analysis showed that most of the PC nerve-activated vestibulospinal neurons were located within a specific area in the medial part of the lateral vestibular nucleus and the rostral part of the descending vestibular nucleus. In conclusion, it was suggested that PC nerve-activated vestibulospinal neurons that were located within a focal area of the vestibular nuclei have strong connections with the lower segments of the spinal cord and are related to postural stability that is maintained by the short latency vestibulospinal reflex.

Vestibulospinal control of reflex and voluntary head movement

Secondary canal-related vestibulospinal neurons respond to an externally applied movement of the head in the form of a firing rate modulation that encodes the angular velocity of the movement, and reflects in large part the input "head velocity in space" signal carried by the semicircular canal afferents. In addition to the head velocity signal, the vestibulospinal neurons can carry a more processed signal that includes eye position or eye velocity, or both (see Boyle on ref. list). To understand the control signals used by the central vestibular pathways in the generation of reflex head stabilization, such as the vestibulocollic reflex (VCR), and the maintenance of head posture, it is essential to record directly from identified vestibulospinal neurons projecting to the cervical spinal segments in the alert animal. The present report discusses two key features of the primate vestibulospinal system. First, the termination morphology of vestibulospinal axons in the cervical segments of the spinal cord is described to lay the structural basis of vestibulospinal control of head/neck posture and movement. And second, the head movement signal content carried by the same class of secondary vestibulospinal neurons during the actual execution of the VCR and during self-generated, or active, rapid head movements is presented.

Synaptic organization of the vestibulo-collic pathways from six semicircular canals to motoneurons of different neck muscles

The pattern of inputs from six semicircular canals to neck motoneurons was investigated by stimulating six ampullary nerves electrically and recording intracellular potentials from motoneurons of the rectus capitis dorsalis (RD), the complexus (COMP) and the obliquus capitis caudalis (OCA) muscles at the upper cervical cord of the cat. RD and COMP motoneurons received disynaptic excitation from bilateral anterior and contralateral horizontal ampullary nerves and disynaptic inhibition from bilateral posterior and ipsilateral horizontal ampullary nerves. OCA motoneurons received excitation from ipsilateral vertical and contralateral horizontal ampullary nerves and inhibition from contralateral vertical and ipsilateral horizontal ampullary nerves. Ipsilateral disynaptic inhibitory postsynaptic potentials and contralateral disynaptic excitatory postsynaptic potentials to these motoneurons were mediated by the medial longitudinal fasciculus (MLF) and the other postsynaptic potentials by the extra-MLF pathways. The results indicated that motoneurons of a neck muscle have its own characteristic pattern of inputs from six semicircular canals.